-------
Notes
Introduction to Groundwater InvMtigation*
Orientation and Introduction
pape11
-------
Course Goals
Identify the components of a groundwater system.
List the primary hydrogeological parameters to be considered in a site
investigation.
Construct a flow net and calculate the hydraulic gradient at a site.
Discuss the primary advantages and disadvantages of the most common
geophysical survey methods.
Identify geochemical profiles in contaminated groundwater.
Identify the different types of pumping tests and the information that can
be obtained from each.
Describe monitoring well drilling and sampling techniques.
Introduction to Groundwater lnv«stigaboni
Orientation and Introduction
aee
p*9«12
-------
Notes
introduction to Groundwater Investigation*
Orientation and Introduce on
-------
Course Layout and Agenda
Key Points:
Agenda times are only approximate. Every effort is made to complete units, and
finish the day, at the specified time.
Classes begin promptly at 8:00 am. Please arrive on time to minimize distractions to
fellow students.
Breaks are given between units.
Lunch is 1 hour.
Each student must take the examination given on Thursday.
Direct participation in field or laboratory exercises is optional. Roles are randomly
assigned to ensure fairness.
Attendance at each lecture and exercise is required in order to receive a certificate.
Introduction to Groundwater Investigation* 866
Orientation and Introduction pa0e14
-------
Notes
Introduction to Groundwater Investigation* 095
Orientation and Introduction page 15
-------
Training Evaluation
The Training Evaluation is a tool to collect valuable feedback from YOU
about this course.
We value YOUR comments!! Important modifications have been made to
this course based on comments of previous students.
DO
Write in your comments at the end of
each unit!
Tell us if you feel the content of the
course manual is clear and complete!
Tell us if you feel the activities and
exercises were useful and helpful!
Tell us if you feel the course will help
you perform related duties back on the
job!
Complete the first page at the end of
the course before you leave!
Write comments in ink.
DON'T
Hold back!
Focus exclusively on the presentation
skills of the instructors.
Write your name on the evaluation, if
it will inhibit you from being direct
and honest.
Introduction to Groundwater Invesbgatoni
Orientation and Introduction
ass
-------
Notes
Introduction to Groundwater Investigation 895
Orientation and Introduction
-------
Facility Information
Please put beepers in the vibrate mode and
turn off radios. Be courteous to fellow
students and minimize distractions.
Emergency
Telephone
Numbers
Emergency Exits
Alarms
Sirens
Introduction to Groundwatar Investigations
Orientation and Introduction
age
page IS
-------
Notes
Introduction to Groundwater Investigations 595
Onentaoon and Introduction
-------
Section 2
-------
ROCK CYCLE
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Define the Doctrine of Uniformitarianism
2. Describe the three basic rock types and their textures within
the rock cycle
3. Identify the media responsible for the erosion and transport
of sediments
4. Describe the process of lithification and cementation as
related to sedimentary rocks
5. Describe how sedimentary particles become rounded, sorted,
and stratified.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
ROCK CYCLE
S-1
Doctrine of
Uniformitarianism
S-2
"The Present is the Key
to the Past"
James Mutton, 1785
S-3
8/95
Rock Cvcle
-------
NOTES
/o
Unification
E/o«k>n.
dapoaition
Haat. prauura, and
chamteally activ* lluldt
Cryilalllzatlon
•nd oodlng
S-4
IGNEOUS ROCKS
Solidified from molten liquid (magma)
Volcanic rocks/extrusive rocks
- Obsidian, lava, pumice, tuff
Plutonic rocks/intrusive rocks
- Batholiths, sills, laccoliths
INTRUSIVE IGNEOUS ROCK BODIES
S-5
Rock Cycle
8/95
-------
IGNEOUS ROCKS
Texture
Rocks are composed of interlocking
mineral grains
Minerals form in a liquid or magma
Size of minerals based on cooling rate of
liquid
S-7
IGNEOUS ROCKS
Texture
Intrusive: coarse-grained rock
Visible minerals form in slow cooling liquid
Examples: granite and gabbro
Found in batholiths, laccoliths, and sills
s-a
IGNEOUS ROCKS
Texture
• Extrusive rocks: fine-grained or glassy
rocks
• Small minerals form in fast cooling liquid
• Lava flows, volcanoes
• Examples: basalt and rhyolite
s-e
NOTES
5/P5
-------
NOTES
-f
I/)
r
IGNEOUS ROCKS
Equivalent Chemistry
Coarse-grained
Texture
Gabbro
Granite
Fine-grained
Texture
Basalt
Rhyolite
METAMORPHIC ROCKS
"Changed Form" Rocks
• Heat
• Pressure
• Chemically active fluids
• Recrystallization
METAMORPHIC TEXTURES
• Interlocking crystals; marble
• Layers of platy minerals; schist
(foliation)
S-10
S-11
S-12
Rock Cycle
8/95
-------
NOTES
METAMORPHIC ROCKS
Metamorphism Process
Original Rock
Limestone
Sandstone
Basalt
Siltstone/shale
Granite
o
Metamorphic Rock
Marble
Quartzite
Amphibolite
Slate
Phyllite
Schist
Gneiss
S-13
\
SEDIMENTARY ROCKS
"Most Abundant Surficial Rock Type"
• Derived from preexisting rocks
• Composed of individual grains cemented
together or chemically precipitated
• Most form in water environment
• Make up most rock aquifers
S-14
TYPICAL SEDIMENTARY
ROCKS
Limestone
Shale
Sandstone
Coal
Dolomite
Siltstone
Conglomerate
Evaporite
o°
\
"
»/
c
5/P5
-------
NOTES
RECIPE FOR SEDIMENTARY
ROCKS
• Erosion processes
• Deposition
• Unification
SEDIMENTARY ROCKS
Erosion Process
• Wind
• Water
• Ice
• Gravity
• Biology
S-17
SEDIMENTARY ROCKS
Deposition
• Wind
• Water
• Ice
• Gravity
S-18
Rock Cvcle
8/95
-------
SEDIMENTARY ROCKS
Lithlfication
"Making into stone"
Cementation: natural cements dissolved
in and transported by groundwater
S-10
SEDIMENTARY ROCKS
Types of Cement
Silica (types of quartz)
Iron oxides (hematite/limonite)
Clay mineral groups
- Kaolinite, vermiculite,
montmorillonite, illite
Carbonates (calcite/aragonite)
3-20
SEDIMENTARY ROCKS
Composed of particles of any rock type
- "Pores" form during deposition
• Most aquifers are sedimentary rocks
S-21
NOTES
v>
]ff\
,
A
8/95
Rock Cycle
-------
NOTES
-6
PRIMARY POROSITY
A measure of the total void space
within a rock at the time it was formed
It is generally higher in sedimentary
rocks and lower in igneous and
metamorphic rocks
S-22
SECONDARY POROSITY
Void spaces that form after the rock
has been formed (e.g., faults, joints,
fractures, and conduits)
S-23
PERMEABILITY
The ease with which liquid will
move through a porous medium
S-2X
Rock Cycle
8/95
-------
NOTES
SEDIMENTARY ROCKS
Sphericity
Angular
Rounded
S-25
SEDIMENTARY ROCKS
Sorting
Poor
Well
^ ^
o
S-28
^Y
A
$
S/P5
-------
Section 3
-------
DEPOSITIONAL ENVIRONMENTS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Describe the following depositional environments:
a. Alluvial fans
b. Braided streams
c. Meandering streams
d. Coastal (deltaic and barrier island complexes)
e. Wind-blown deposits
f. Carbonates
g. Evaporites
h. Glaciers (continental and alpine).
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
DEPOSITIONAL
ENVIRONMENTS
s-\
S-2
LONGITUDINAL PROFILE
A Alluvial and landslide
B Braided stream
M Meandering stream
C Coastal
• Stream headwaters
Mouth of
stream
S-3
8/95
Depositional Environments
-------
NOTES
ROCK TYPE
ENVIRONMENT
Conglomerate Landslide, alluvial fan
Sandstone
Clay/shale
Limestone
Rivers, streams, beaches,
deltas, dunes, sand bars
Lagoon, lake, flood plain,
deeper ocean
Coral reef, atoll,
deeper ocean
MEDIAN CHANNEL
Grain Size
Small
RELATIONSHIP OF STREAM VELOCITY
1000
o
100
'o
1.0
0.1
////, :::::;:: EfOSJOn :':: j: '• [y^/-
^^••^sZ///Si
Transportation
Size 0.001 0.01 0.1 1.0 10 100
(mm) Clay Silt Sand Gravel s-a
Depositional Environments
8/95
-------
NOTES
SPHERICITY
s-e
8/P5
Depositional Environments
-------
NOTES
DEPOSITIONAL
ENVIRONMENTS
• Alluvial fan
• Braided stream
• Meandering stream
• Coastal deposits
DEPOSITIONAL
ENVIRONMENTS (cont.)
Wind-blown deposits
Carbonates (Karst)
Evaporites
Glacial deposits
Alluvial Fan
S-10
S-11
S-12
Depositional Environments
8/95
-------
CHARACTERISTICS OF
ALLUVIAL FANS
Depositional environments:
• Poor sorting and rounding
• High gradients
• Shallow and intermittent streams
• Hand-shaped
S-13
S-14
Braided Stream
S-15
NOTES
8/95
Depositional Environments
-------
NOTES
CHARACTERISTICS OF BRAIDED
STREAMS
Depositional environments:
• Resembles braided hair
• High to low gradients
• Shallow streams
• Poor to medium sorting
• Angular to subangular grains
s-ie
S-17
Meandering Stream
S-18
Depositional Environments
8/95
-------
NOTES
CHARACTERISTICS OF
MEANDERING STREAMS
Depositional environments:
• Low gradients
• Deep streams
• Grain size variations
S-18
CHARACTERISTICS OF
MEANDERING STREAMS (cont.)
Depositional environments:
• Oxbow lakes
• Levees and floodplains
• Point bars and cut banks
S-20
S-21
8/95
Depositional Environments
-------
NOTES
STREAM CHANNEL
Sinuosity
Low
High
Coastal Deposits
S-23
TYPICAL COASTAL DEPOSITS
Depositional Environments
• Barrier islands
• Offshore bars
• Deltas
• Spits
• Tidal flats
• Reefs/cays
S-24
Depositional Environments
8/95
-------
BARRIER ISLAND
A
West Bay
Gulf of Mexico
Barrier Island
S-2S
S-27
NOTES
8/95
Depositional Environments
-------
NOTES
Pamet
Monomoy|
NANTUCKET SOUND
S-28
West
Cape Cod Bay
Recharge area
Cape Cod aquifer
T
East
Atlantic Ocean
Unconsolidated
sediments
Bedrock
S-2S
Wind-Blown Deposits
S-30
Deposirional Environments
10
8/95
-------
WIND-BLOWN DEPOSITS
Depositional Environments
• Dunes: continental and coastal
• Volcanic dust and ash
• Glacial till dust (loess)
S-31
S-32
Carbonate Rocks
S-33
NOTES
8/95
11
Depositional Environments
-------
NOTES
CARBONATES
• Limestones
• Dolomites
S-34
KARST TOPOGRAPHY
Depositional Environment
• Soluble rocks at or beneath surface
(carbonates, sulfates, chlorides)
• Chemical solution of soluble rocks
• Closed depressions (sink holes, swallets)
• Little or no surface drainage
• Caves, springs, disappearing streams
S-3S
^
»
Joints -
— >
A
' T "
- -\
\
Master conduit
S-36
Depositional Environments
12
8/95
-------
Evapo rites
EVAPORITES
• Carbonates
• Sulfates
• Chlorides
Glaciation
S-37
S-38
S-39
NOTES
8/95
13
Depositional Environments
-------
NOTES
PROCESSES OF GLACIATION
Erosion
Transportation
Deposition
S-40
GLACIERS/FREEZE-THAW
Weathering and transport
Large-scale changes
Poor to excellent sorting
(e.g., glacial till and outwash)
S-41
GLACIAL DEPOSITS
Depositional Environments
• Outwash and till
• Moraines
• Drumlins
• Eskers
• Kettle holes
• Kames
S-42
^positional Environments
14
8/95
-------
Section 4
-------
SOILS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Discuss the factors that influence soil formation processes
2. Differentiate between physical and chemical weathering
3. Describe the factors that influence soil morphology
4. Define the following physical and chemical properties of
soil:
a. Porosity
b. Permeability
c. Cation exchange capacity
d. Bulk density
e. Capillarity
5. Describe a common soil profile and the interaction of its
component units.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
S v I
xSOILS
A i V
S-1
WHY STUDY SOILS?
First media encountered by spills and
leaks
Contaminant fate and transport
- Interaction with air, water, microbes,
and soil
- Soil variability
S-2
CONTAMINANT FATE IN SOIL
• How much:
- Adsorbed to clay?
- Adsorbed to organic matter?
- Volatilized?
- Consumed by microbes?
- Entered water table?
S-3
\
8/95
Soils
-------
NOTES
/
SOIL
Definition
• Material that supports the growth of plants
• Consists of :
- Rock and mineral fragments
- Organic matter js£^T^^
-rr _jjd5_ ^
SOIL FORMATION
Controls
• Parent rock or sediment
• Climate
• Topography
• Presence/abundance of organisms
• Time
s-s
SOIL FORMATION
Soil from Bedrock
/• Forms in place
• Derived from underlying bedrock
• Retains original bedrock structure
\j
* Example: saprolmc soil
S-6
Soils
8/95
-------
NOTES
SOIL FORMATION
Soil from Sediment
River deposits
Till deposits
Outwash deposits
Loess deposits
\ \ \ \ \ \ \
\ \ \ \ \
T-T^
SOIL FORMATION
Physical Weathering
• Breaks "big rocks" into "small rocks"
• Increases weathered surface
• Influenced by climate and topography
• Time
S-8
SOIL FORMATION
Chemical Weathering
• Influenced by water and dissolved gases
• Acidic water
^
• Minerals are either gained or lost
s-e
V
8/95
Soils
-------
NOTES
SOIL FORMATION
Chemical Reactions in Soil
Hydration/dehydration
Oxidation/reduction (Eh potential)
pH
Ion exchange (calcium for sodium)
Chelation (soil colloids)
S-10
SOIL FORMATION
Humid Climate
• High temperature
- Rapid development of soil profile
- Rapid oxidation and breakdown of
organics
• Cold temperature
- Slow oxidation
- High accumulation of organic materials
S-11
SOIL FORMATION
Arid Climate
High temperature
- No organic horizon
- Slow soil profile development
- Rapid oxidation
Low temperature
- No organic horizon
- Sterile soil
- Slow oxidation
S-12
Soils
8/95
-------
NOTES
SOIL MORPHOLOGY
• Color
• Texture
• Structure
• Consistency
• Horizon boundaries
SOIL MORPHOLOGY
Color
SOIL MORPHOLOGY
Examples of Soil Color
S-13
• Moisture content of soil //'•"
• Parent rock type /^ -^
• Abundance of organic matter
• Degree of oxidation/reduction
S-14
Black-brown: organic material,
Mn-minerals
Reddish: iron oxides, oxidized
Yellow-brown: iron oxides, poorly drained
White: Ca-carbonates, silica/bauxite/clay
S-15
8/95
Soils
-------
NOTES
SOIL MORPHOLOGY
Examples of Soil Color (cont.)
• Greenish or bluish gray: wetlands, gleyed
soil
• Mottled soil: moving water table, oxidized
and reduced
S-16
SOIL MORPHOLOGY
Soil Texture
Percentage of sand, silt, and clay
Water holding capacity
Soil classification systems
S-17
DETERMINATION OF GRAIN SIZES
Particle
Type
Boulder
Cobble
Gravel
Sand
Silts and clay
Particle
Size (mm)
>305
76.0 - 305
4.76 - 76.0
0.074 - 4.76
<0.074
Familiar
Example
Basketball
Grapefruit
Pea to orange
Rock salt to sugar
Talcum powder
^ S-18
Soils
8/95
-------
SOIL MORPHOLOGY
Structure
• Grade
• Shape
• Size
STRUCTURE
Grade
• Structureless
• Weak
• Moderate
• Strong
STRUCTURE
Shape
• Platy
• Prismatic
• Blocky
• Granular
NOTES
s-ie
S-20
S-21
8/95
Soils
-------
NOTES
SOIL MORPHOLOGY
Consistency
Cementation in soil
Plasticity
Strength
Stickiness
S-22
SOIL PROPERTIES
• Infiltration
• Permeability
• Runoff
• Available water capacity
• pH/Eh
S-23
SOIL PROPERTIES (cont.)
• Cation exchange capacity
• Base saturation
• Mineralogy
• Bulk density
S-24
Soils
8/95
-------
SOIL PROPERTIES
Permeability
Ability to transmit water and contaminants
Depends on linkage of pore spaces
S-2S
SOIL PROPERTIES
Cation Exchange Capacity
Negative charge on soil particles
High in clayey soils
Low in sandy soils
S-2S
SOIL PROPERTIES
Bulk Density
• Ratio of the mass to total volume of soil
(g/cm3)
• Volume includes air, liquid, and solid
phases
• Particle density, solid phase only
NOTES
S-27
8/95
Soils
-------
NOTES
SOIL PROPERTIES
Bulk Density Examples
Sandy soil
Silty soil
Clayey soil
2.0 g/cm3
1.9 g/cm3
2.2 g/cm3
S-28
SOIL PROPERTIES
Porosity
Ratio of open space to total volume
Ability to hold or store water
High in sedimentary rocks
Low in crystalline rocks
S-29
SOIL PROPERTIES
Porosity Values (High)
• Styrofoam > 90 %
• Gravel 40 %
• Clay 70%
• Shale < 20 %
• Limestone (karst) 50 %
• Fractured rock 50 %
S-30
Soils
10
8/95
-------
SOIL PROPERTIES
Capillarity
• Capillary fringe
• Height water rises above the water table
• Depends on size of the pores
• Less water content than saturated zone
• Does not yield water
S-31
CAPILLARY FRINGE
Sand
Silt
Clay
S-32
SOIL TYPE VARIABILITY
• Moisture content
• Organic content
• Thickness
• Mineral composition
• Microbe population
• pH and Eh
NOTES
S-33
8/95
11
Soils
-------
NOTES
v~
.JO
SOIL PROFILE
Vertical succession of various soil layers to
bedrock
• O-horizon
• A-horizon
• B-horizon
• C-horizon
Uott ton* hn» forrrmd during th» tat 2 mBlon ytmn
S-34
O-HORIZON
Characteristics
Mainly organic matter (>20%) mixed with
rock and mineral fragments
Contains decaying animal and plant matter
(humus)
S-35
A-HORIZON
Characteristics
Rock/mineral fragments mixed with organic
matter
Commonly known as "topsoil"
Zone of leaching
Contains large-sized pores
S-36
Soils
12
8/95
-------
NOTES
B-HORIZON
Soil Profile
• Zone of accumulation (illuviation)
• Insoluble minerals leached from A-horizon
S-37
C-HORIZON
Soil Profile
Partially decomposed bedrock
Grades into unweathered bedrock
S-M
8/95
13
Soils
-------
Section 5
-------
DRILLING METHODS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Describe the following drilling methods:
a. Cable tool
b. Hollow-stem auger
c. Mud rotary
d. Air rotary
e. Rotasonic
2. List the advantages and disadvantages of the following
drilling methods:
a. Cable tool
b. Hollow-stem auger
c. Mud rotary
d. Air rotary
e. Rotasonic
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
DRILLING METHODS
USES FOR WELLS
Water supply
Monitoring
Remediation
Lithology
"Ground truthing"
Hydraulic properties
SELECTION CRITERIA
8-1
8-2
• Hydrogeologic environment
- Type of formation
- Depth of drilling
• Type of pollutant
• Location
• Availability
• Cost
S-3
8/95
Drilling Methods
-------
NOTES
DRILLING METHODS
Cable tool
Hollow-stem auger
Mud rotary
Air rotary
Rotasonic
S-4
CABLE TOOL DRILLING METHOD
Drilling
Bailing
s-s
CABLE TOOL
Advantages
Good sample recovery
Good delineation of water-bearing zones
during drilling
Highly mobile
Good drilling in most formations
Inexpensive
s-e
Drilling Methods
8/95
-------
NOTES
CABLE TOOL
Disadvantages
Slow
Requires driving casing in unconsolidated
formations
S-7
HOLLOW-STEM
AUGER
DRILLING
s-e
s-e
S/P5
Drilling Methods
-------
NOTES
HOLLOW-STEM AUGER
Advantages
Highly mobile
No drilling fluid required
Problems of hole caving minimized
Soil sampling relatively easy
S-10
HOLLOW-STEM AUGER
Disadvantages
Cannot be used in consolidated formations
Limited depth capability (-150 feet)
Cross contamination of permeable zones is
possible
Limited casing diameter
S-11
MUD ROTARY
DRILLING
S-12
Drilling Methods
8/95
-------
NOTES
MUD ROTARY
DRILLING
Mud
S-13
MUD ROTARY
Advantages
Availability
Satisfactory drilling in most formations
Good depth capability
S-14
MUD ROTARY
Disadvantages
• Requires drilling fluid
- Difficult to remove
- May affect sample integrity
• Circulates contaminants
• Mobility may be limited X
• Poor rock or soil sample recovery
3-15
8/95
Drilling Methods
-------
NOTES
AJR ROTARY
DRILLING
Air
compressor
AIR ROTARY
Advantages
No drilling fluid required
Excellent drilling in hard rock
Good depth capability
Excellent delineation of water-bearing
zones
S-17
AIR ROTARY
Disadvantages
Casing may be required during drilling
Cross contamination of different formations
possible
Mobility may be limited
Difficult formation sampling
S-18
Drilling Methods
8/95
-------
Oscillator
High frequency
sinusoidal force
Drill bit
rotates and
vibrates
Counter-rotating weights
Standing
harmonic
wave in drill
pipe
ROTASONIC DRILLING
S-19
•^
—
Inner drill
pipe and core
bit are
vibrated
and/or
rotated ^
into ground
"-•
Outer drill
pipe and core
bit are
vibrated
down over
winner drill
pipe
*;
I
i
i
. ;
Outer drill
pipe is left
in. place
while inner
drill pipe
is extracted
with core
ROTASONIC DRILLING METHOD
S-20
ROTASONIC
Advantages
Fast (20 shallow boreholes/day)
Versatile (easily penetrates cobbly
materials)
Drills into consolidated and unconsolidated
material
* Clean (cuttings and fluid minimized)
• Excellent sampling (quality cores)
S-21
8/95
Drilling Methods
-------
NOTES
ROTASONIC
Disadvantages
Cost
Availability
Dense or cobbly materials are heated by
vibration (loss of volatiles)
S-22
Drilling Methods
8/95
-------
Sect/on 6
-------
HYDROGEOLOGY
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Describe the hydrologic cycle
2. Differentiate between porosity and permeability
3. Describe the difference between confined and unconfined
aquifers
4. Evaluate the components of Darcy's Law, including
hydraulic conductivity
5. Describe the differences between Darcian velocity and
seepage velocity.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
HYDROGEOLOGY
NOTES
8-1
HYDROGEOLOGY
The study of the interactions of
geologic materials and processes
with water, especially groundwater
S-2
WATER USES
Drinking
Irrigation
Fisheries
Industrial
Transportation
Waste disposal
S-3
8/95
Hydrogeology
-------
NOTES
HYDROLOGIC CYCLE
S-4
HYDROLOGIC
CYCLE
Transpiration /
Precipitation
Evaporation
Water
table
Groundwater
recharge Groundwater runoff
S-5
s-e
Hydrogeology
8/95
-------
NOTES
Water
table
S-7
CONTROLS ON INFILTRATION
• Soil moisture
• Compaction of soil
• Micro- and macrostructures in the soil
• Vegetative cover
• Temperature
• Topographic relief
Ground surface
(
Vadose
zone
Saturated
zone
*
Pore spaces partially
filled with water
Capillary fringe ^
• Groundwater •
s-e
8/95
Hydrogeology
-------
NOTES
/
s
-------
NOTES
LOSING STREAM
Discharge = 10 cf§,
Discharge = 8 cfe
8-13
POROSITY
(NT)
The volumetric ratio between the void
spaces (Vv) and total rock (VJ:
NT.V^ ; NT =
V ^2/
SY = specific yield '
SR = specific retention
S-14
X,
Void space
Percent _
Porosity
Total Volume - Volume Soil Particles
Total Volume
Soil particle
X 100
s-is
8/95
Hydrogeology
-------
NOTES
ROCK AND WATER
CAPACITY
RELATIONSHIPS
VOID SPACE VOLUME
(Porosity)
WATER SATURATION
S-17
s-is
Hydrogeology
8/95
-------
NOTES
WATER RETAINED AFTER GRAVITY
DRAINAGE
(Specific retention)
y
sg
fl§
*£&> f-~^
*^5r 1
^ZJifit (Specific yield)
i/'/j %
iv*i£ ^ -^
\jL* z^±__-2
mA*
'| 1 Cly'--1.-- - I^ s-18
PRIMARY POROSITY
Refers to
the rock
voids formed at the time
or sediment formed
S-20
•**&^k
POROSITY
Clay
Sand
Gravel
Total Porosity Effective Porosity
(NT) (n.)
40-85% 1-10%
25-50% 10-30%
25-45% 15-30%
S-21
8/95
Hydrogeology
-------
NOTES
V
SECONDARY POROSITY
Refers to voids that were formed
after the rock was formed
SECONDARY POROSITY
PERMEABILITY
S-22
S-23
The ease with which liquid will
move through a porous medium
S-24
Hydrogeology
8/95
-------
HYDRAULIC CONDUCTIVITY
The capacity of a porous medium
to transmit water
CONDUCTIVITY
C/lay
Sand Gravel Sandstone
AQUIFER
S-25
S-28
A permeable geologic unit with the
ability to store, transmit, and
yield water in "usable quantities"
S-27
5/P5
Hydrogeology
-------
NOTES
HOMOGENEOUS
Having uniform sediment size and
orientation throughout an aquifer
HETEROGENEOUS
Having a nonuniform sediment size
and orientation throughout an aquifer
ISOTROPIC
Hydraulic conductivity is independent
of the direction of measurement at a
point in a geologic formation
S-2B
S-28
S-30
Hydrogeology
10
8/95
-------
NOTES
ANISOTROPIC
Hydraulic conductivity varies with the
direction of measurement at a point in
a geologic formation
S-31
Homogeneous
Heterogeneous
S-32
AQUITARD
A layer of low permeability that
can store and transmit groundwater
from one aquifer to another
S-33
8/95
11
Hydrogeology
-------
NOTES
AQUICLUDE
An impermeable confining layer
S-34
TOTAL HEAD
(h,)
Combination of elevation (z) and
pressure head (hp)
ht = z + hp
Total head is the energy imparted to a
column of water
S-3S
t
Pressure ,. .
head
1
1
Tj A.
Point of 1
measurement Elevation
head (l)
~f
Hydraulic
or
total '
head
i
(usually sea level)
S-30
Hydrogeology
12
8/95
-------
UNCONFINED AQUIFER
(Water Table)
A permeable geologic unit having the
ability to store, transmit, and yield
water in usable quantities
CONFINED AQUIFER
(Artesian)
S-37
I
JNCONFINED AQUIFER
'•".-.... -:'-' '••
: I 1 1 ,
Water
table
1 I'*
i. .
i i
v/adose
zone "
W-l
iM-
Unconfined aquifer
>XXXX< Confining unit - squitardsXXX/^
S-38
An aquifer overlain by a confining layer
whose water is under sufficient pressure to
rise above the base of the upper confining
layer if it is perforated
S-39
NOTES
8/95
13
Hydrogeology
-------
NOTES
(
DONFINED AQUIFER
Confining
unit
-aquitard
\
Potentiometric
surface
Confined aquifer
Confining unit - aquitard -:
/Base of
upper
confining
unit
S-40
AQUIFERS AND AQUITARDS
$< Vadose zoneQO ^
Unconfined aquifer
Water/
table
Aquitard
Confined aquifer
Aquitard
Confined aquifer
S-41
Recharge Vgdose 2Qne
Water table
Confining layers
(aquitards)
S-42
Hydrogeology
14
8/95
-------
NOTES
POTENTIOMETRIC SURFACE
The level to which water will rise in an
opening (well) if the upper confining
layer of a confined aquifer is perforated
S-43
ARTESIAN GROUNDWATER
Recharge area
•*^~~&-—
•^ \ ^^-
^'o
\ X v.
X ^ «
X *
SYSTEM
Recharge area
Q Potentiometric surface ^^^^"^^
Nb^«T5^r^'-
j^k2- --**$'
^ -^ Aquifer ^ ^ J*
^ ^. •*• _, _,,— ^ ^.'^'•Aquiclude
S-44
ARTESIAN GROUNDWATER
Potentiometr
surface
*
Overburden
pressure ""
SYSTEM
Flowing
c artesian
well ^ _
_ 1 n- - - R" '^
" ^
Aquiclude n U
^ * u /
tessv»'e .S
Hydraulic {] ?
-------
NOTES
DARCY'S LAW
Q = KIA
• Q = discharge
• K = hydraulic conductivity c
• I = hydraulic gradient (^JZJ /
^».
• A = area
S-48
DARCY'S LAW
The flow rate through a porous material is
proportional to the head loss and
inversely proportional to the length
of the flow path
Valid for laminar flow
Assume homogeneous and isotropic
conditions
S-47
HYDRAULIC CONDUCTIVITY
(K)
The volume of flow through a unit cross
section of an aquifer per unit decline
of head
S-48
Hydrogeology
16
8/95
-------
tOTES
S-48
Hydraulic Conductivity
Idll
dh
Q = KIA
K_Q
K~IA
K = hydraulic conductivity
A = cross-sectional area
Q = rate of flow
/dh
I = hydraulic gradient l-r-
s-so
dl
(length of
flow path
DARCY'S LAW
S-51
8/95
17
Hydrogeology
-------
NOTES
Decreasing the
hydraulic head
decreases the
flow rate
S-52
Increasing the
flow path length
decreases the
flow rate
GROUNDWATER VELOCITY
• Darcy's Law Q = KIA or Q_ = Kl
A
• Velocity equation Q = Av or Q
A
By combining, obtain:
• v = Kl Darcian velocity
= v
S-S4
Hydrogeology
18
8/95
-------
NOTES
GROUNDWATER VELOCITY
Because water moves only through pore
spaces that are connected, porosity
is a factor
or NT =S
ne =
effective porosity
^vs = K| \ seepage velocity
S-55
TRANSMISSIVITY
The capacity of the entire thickness of an
aquifer to transmit water
T= Kb
T = transmissivity
K = hydraulic conductivity
b = aquifer thickness
s-se
100m
TRANSMISSIVITY
S-57
8/95
19
Hydrogeology
-------
NOTES
TRANSMISSIVITY
T = Kb
T= (20m/d)(100m)
T = 2000 m2/d
s-se
STORATIVITY
The amount of water available for "use"
in an aquifer (storage coefficient)
"Specific yield" in an unconfined aquifer
S-S9
Hydrogeology
20
8/95
-------
Section 7
-------
WELL INSTALLATION
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. List the materials necessary for the installation of a well
2. Describe the installation of a well in an unconfmed aquifer
3. Describe the installation of a well in a confined aquifer
4. Describe the concept behind nested wells
5. Describe the most common well sampling methods.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
WELL INSTALLATION
S-1
Selection of Filter Pack
and Well Screen
S-2
WELL SCREEN
Surrounded by filter pack
Filter pack consists of:
- Coarser materials
- Uniform grain size
- Higher permeability
S-3
8/95
Well Installation
-------
NOTES
FILTER PACK
Purpose
To allow groundwater to flow freely
Into well
To minimize or eliminate entrance of
fine-grained materials
S-4
FILTER PACK
Selection
Multiply the 70-percent retained grain size
of aquifer materials by 4 or 6
Use 4 if formation is fine and uniform
Use 6 if formation is coarser and
nonuniform
s-s
FILTER PACK
Uniformity Coefficient (UC)
40 percent retained
90 percent retained
= UC
UC should not exceed 2.5
Well Installation
8/95
-------
NOTES
WELL SCREEN
Selection
Select screen slot opening to retain
90 percent of filter pack material
WELL MATERIALS
WELL INSTALLATION
Unconfined aquifer
Confined aquifer
S-7
• Well screen/riser/well points
- Teflon®
- Stainless steel
- PVC
• Gravel/filter pack
• Bentonite
• Grout/cement
S-8
s-8
8/95
Well Installation
-------
.
At.
MONITORING WELL - UNCONFINED AQUIFER
4-Steel cap
Well
MONITORING WELL - CONFINED AQUIFER
Steel cap
.Grout
Well cap
Well screen
Plug
Bentonite
Gravel pack
NOTES
Well Installation
8/95
-------
NESTED WELLS - MULTILEVEL SAMPLING
S-12
WELL AND AQUIFER
DEVELOPMENT
Surge block
Bailer
Pulse pumping
Air surging
S-13
NOTES
8/95
Well Installation
-------
POOR WELL DEVELOPMENT
Muddy water
WELL DEVELOPMENT - SURGE BLOCK
>WQOWWV
-------
NOTES
WELL DEVELOPMENT - BAILER
WELL DEVELOPMENT - PULSE PUMPING
WELL DEVELOPMENT - AIR SURGING
S/P5
Installation
-------
NOTES
s
!?
SAMPLING METHODS
Bladder pump
Submersible pump
Hand pump
Bailer
s-ie
GROUNDWATER SAMPLING
PROTOCOL
Flexible
Written and defensible document to be
used at all sites
S-20
PURGING
Volume Is well specific
Verified by temperature, pH,
specific conductance, and
turbidity
S-21
Well Installation
8/95
-------
DETERMINING WELL VOLUME
V = 0.041 d2h
V = Volume of water in gallons
d = Diameter of well in inches
h = Depth of water in well in feet
S-22
/
)X)
V
v
NOTES
(2-
j, \
8/95
-------
Section 8
-------
VADOSE ZONE
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Describe the vadose zone
2. List three reasons why the vadose zone is important in
groundwater investigations
3. Describe the operation of pressure vacuum lysimeters
4. Characterize the limitations of vacuum lysimeters
5. Describe the principles of soil gas wells
6. Characterize the limitations of soil gas wells.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
VADOSE ZONE
S-1
THE VADOSE ZONE
That part of the geologic profile
between the ground surface and the
water table, including the capillary
fringe
S-2
Ground surface
r
Vadose
zone
Phreatic
zone
Saturated
Water table
S-3
8/95
Vadose Zone
-------
NOTES
THE VADOSE ZONE
• Generally unsaturated
• < 100% water content
• Capillary pressure predominant
S-4
THE VADOSE ZONE
Consists of:
• Solid and participate material
• Vapors in pore spaces
• Liquids on grain surfaces
S-5
THE VADOSE ZONE
\
Solid
particles
Liquid
S-6
Vadose Zone
8/95
-------
CAPILLARY FRINGE
Transition zone between saturated
and unsaturated zones
Result of capillary pressure pulling
water into unsaturated zone
a-7
CAPILLARITY
The result of two forces:
• Attraction of water to the walls
of the pore space (adhesion)
• Attraction of water molecules
to each other (cohesion)
S-8
CAPILLARY FRINGE
Sand
Silt
Clay
S-9
NOTES
8/95
Vadose Zone
-------
NOTES
THE VADOSE ZONE
Physical Properties
Physical properties vary according to:
• Atmospheric conditions
• Hydrogeologic conditions
• Geologic conditions
S-10
THE VADOSE ZONE
Cross Section - Ohio River Valley
VADOSE ZONE ADJACENT
TO STREAMS
Losing stream
Gaining stream
VadOM zon*
S-12
Vadose Zone
8/95
-------
VADOSE ZONE ADJACENT TO
WETLAND
S«tuf«l»d|
THE VADOSE ZONE
Unsaturated Flow
Primarily affected by:
• Matric potential
• Osmotic potential
• Gravitational potential
• Moisture content
THE VADOSE ZONE
Matric Potential
S-14
• Attraction of water to solid
particles
• Responsible for upward flow of
water or capillary pressure
S-15
NOTES
8/95
Vadose Zone
-------
NOTES
THE VADOSE ZONE
Osmotic Potential
Attraction of water to ions
or other solutes in the soil
s-te
THE VADOSE ZONE
Gravitational Potential
Gravitational pull on water
Encourages downward flow of water
or infiltration
S-17
THE VADOSE ZONE
Soil Potential
• Combination of matric and osmotic
potentials
• Impedes or binds the flow of water
in the unsaturated zone
S-18
Vadose Zone
8/95
-------
NOTES
THE VADOSE ZONE
Unsaturated Flow
Occurs when the gravitational
potential is greater than the soil
potential (matric + osmotic)
S-19
THE VADOSE ZONE
Moisture Content
Increased moisture content decreases
soil potential (matric 4- osmotic),
increasing the ability of water to flow
S-20
Devices for Measuring
Moisture Content and
Soil Potential in the
Vadose Zone
S-21
8/95
Vadose Zone
-------
NOTES
MOISTURE CONTENT
Measured by:
• Radioactive devices
• Time domain reflectometry
S-22
MOISTURE CONTENT
Radioactive Devices
• Neutron - Neutron
- Directly measures soil or rock
water content and porosity
• Gamma - Gamma
- Determines soil or rock density
- Indirectly measures water content
and porosity
S-23
MOISTURE CONTENT
Radioactive Devices
Advantages
- In-situ measurements directly or
indirectly related to water content
- Average water content can be
determined at depth
- Accommodates automatic recordings
- Near-surface water content
measurements possible
S-24
Vadose Zone
8/95
-------
MOISTURE CONTENT
Radioactive Devices
Disadvantages:
- Expensive
- Radioactive source requires
special care and license
S-2S
MOISTURE CONTENT
Time Domain Reflectometry
Measures an electromagnetic pulse
emitted from one or more probes
Determines moisture content
3-26
MOISTURE CONTENT
Time Domain Reflectometry
Advantages
- Accurate
- Variable depth placement
- Variety of sensor configurations
- Remote and continual monitoring
S-27
NOTES
8/95
Vadose Zone
-------
NOTES
MOISTURE CONTENT
Time Domain Reflectometry
Disadvantages
- Probes must be placed properly
- Long-term use untested
- Cost of remote monitoring
equipment relatively high
S-26
SOIL POTENTIAL
Measured by:
• Tensiometer
• Electrical resistance block
• Psychrometer
S-29
SOIL POTENTIAL
Tensiometer
Vado»«
zone
Poroutl*^
S-30
Vadose Zone
10
5/P5
-------
NOTES
SOIL POTENTIAL
Tensiometer
Measures the matric potential in soil
Advantages
— Inexpensive
- Durable
- Easy to operate
S-31
SOIL POTENTIAL
Tensiometer
Disadvantages
- Ineffective under very dry
conditions because of air entry
— Sensitive to temperature changes
- Sensitive to atmospheric pressure
changes
S-32
SOIL POTENTIAL
Tensiometer
Disadvantages (cont.)
- Sensitive to air bubbles in lines
- Requires a long time to achieve
equilibrium
S-33
8/95
11
Vadose Zone
-------
NOTES
SOIL POTENTIAL
Electrical Resistance Blocks
c
1
i <
u
Current source
«-»
Water
«->
Water
content
t\
Field calibration
Resistance
S-34
SOIL POTENTIAL
Electrical Resistance Blocks
Advantages
- Suited for general use
- Inexpensive
- Can determine moisture content
or soil potential
- Requires little maintenance
S-35
SOIL POTENTIAL
Electrical Resistance Blocks
Disadvantages
- Ineffective under very dry
conditions
- Sensitive to temperature
- Time-consuming field calibration
- Affected by salinity
- Ineffective in coarse or
swelling/shrinking soils
S-38
Vadose Zone
12
8/95
-------
NOTES
SOIL POTENTIAL
Psychrometer
Measures soil potential under very
dry conditions
a-37
SOIL POTENTIAL
Psychrometer
Advantages
- Continuous recording of pressures
- Variable depth placement
— Remote monitoring
S-M
SOIL POTENTIAL
Psychrometer
Disadvantages
- Very sensitive to temperature
fluctuations
- Expensive
- Complex
- Performs poorly in wet media
3-39
8/95
13
Vadose Zone
-------
NOTES
SAMPLING FLUIDS AND VAPORS
• Fluids
- Pressure-vacuum lysimeter
• Vapors
- Soil-gas probe
S-40
Pressure-Vacuum Lysimeter
S-41
Closad
valves
Collection of Pore Water
S-42
Vadose Zone
14
8/95
-------
NOTES
Transfer to Sample Bottle
8-43
VADOSE ZONE VAPOR SAMPLING
Saturated
S-44
SOIL GAS PROBE
Schematic
• Soil gas
S-45
8/95
15
Vodose Zone
-------
NOTES
Uses for Vadose Zone
Monitoring Equipment
8-4«
VADOSE ZONE MONITORING FOR
NEW TANK FARM
Etrttwn b*rm
Earthan bcrm
Vadose Zone
16
8/95
-------
Section 9
-------
GEOPHYSICAL METHODS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Describe the basic principles of operation of the following
surface geophysical methods:
a. Magnetics
b. Electromagnetics (EM)
c. Electrical resistivity
d. Seismic refraction
e. Ground-penetrating radar
2. Identify the limitations of the following geophysical methods:
a. Magnetics
b. Electromagnetics (EM)
c. Electrical resistivity
d. Seismic refraction
e. Ground-penetrating radar
3. Describe the basic principles of operation of the following
borehole geophysical methods:
a. Spontaneous potential
b. Normal resistivity
c. Natural-gamma
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
STUDENT PERFORMANCE OBJECTIVES (cont.)
d. Gamma-gamma
e. Neutron
f. Caliper
g. Acoustic
h. Temperature
4. Identify the limitations of the following borehole geophysical
methods:
a. Spontaneous potential
b. Normal resistivity
c. Natural-gamma
d. Gamma-gamma
e. Neutron
f. Caliper
g. Acoustic
h. Temperature.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
GEOPHYSICAL METHODS
S-1
GEOPHYSICS
Nonintrusive, investigative tool
Site-specific methods
"Ground truthed" data
Professional interpretation
S-2
RELATIVE SITE COVERAGE
Volume of typical Volume of drilling
geophysical measurement or water sampling
S-3
8/95
Geophysical Methods
-------
NOTES
GROUND TRUTHING
Correlation of physical evidence
(i.e., rock cores) to geophysical
data
S-4
ANOMALY
Significant variation from background
S-5
GEOPHYSICAL TECHNIQUES
Magnetics
Electromagnetics (EM)
Electrical resistivity
Seismic refraction/reflection
Ground-penetrating radar
Borehole geophysics
s-e
Geophysical Methods
8/95
-------
NOTES
MAGNETICS
Measurement of magnetic field strength
in units of gammas
Anomalies are caused by variations
in magnetic field strength in the vicinity of
the sensor
S-7
Amplifier
and
counter
circuits
Magnetometer
S-8
Ground surface
-- 100 S
80 _£
IO 5-
60 i Z
40 | |
20 ~£
0 ?
S-8
8/95
Geophysical Methods
-------
NOTES
MAGNETICS
Advantages
Relatively low cost (cost-effective)
Short time frame required
Little, if any, site preparation needed
Simple survey sufficient (compass and
tape)
S-10
MAGNETICS
Disadvantages
Cultural noise limitations
Difficulty in differentiating between steel
objects (i.e., 55-gallon drums and a
refrigerator)
S-11
ELECTROMAGNETICS
Based on physical principles of inducing
and detecting electrical flow within
geologic strata
Measures bulk conductivity (the inverse
of resistivity) of geologic materials
beneath the transmitter and receiver coils
S-12
Geophysical Methods
8/95
-------
NOTES
Station
measurement
Continuous
measurement
\
S-13
CONTINUOUS MEASUREMENTS VS. STATION MEASUREMENTS
Continuous
measurements
Station
measurements
S-M
SOUTH
NOBTH
Parallel, cottinuoucly recorded EM profiles snowing varioDimy of
conouctivny values and a fracture (rend in underlying rock
S-1£
8/95
Geophysical Methods
-------
NOTES
ELECTROMAGNETICS
Advantages
Rapid data collection with minimum
personnel
Lightweight, portable equipment
Commonly used in groundwater pollution
investigations for determining plume flow
direction
S-18
ELECTROMAGNETICS
Disadvantages
Cultural noise limitations (when used for
hydrogeological purposes)
Limitations in areas where geology varies
laterally (anomalies can be misinterpreted
as plumes)
S-17
ELECTRICAL RESISTIVITY
Measures the bulk resistivity of the
subsurface in ohm-meter units
Current is injected.into the ground
through surface electrodes
S-18
Geophysical Methods
8/95
-------
ELECTRICAL RESISTIVITY
WENNER ARRAY
Currant louro*
Current meter
ELECTRICAL RESISTIVITY
Depth of investigation is equal to
one-fourth of the distance between
electrodes
S-18
S-20
ELECTRICAL RESISTIVITIES OF
GEOLOGIC MATERIALS
Function of:
• Porosity
• Permeability
• Water saturation
• Concentration of dissolved solids in
pore fluids
S-21
NOTES
8/95
Geophysical Methods
-------
NOTES
300
.!2 250
e £ 200
9
Horizontal Distance (meters)
100 200 300 400
500
5 E
I
100
50
gravel
clay
gravel clay gravel I clay
RESISTIVITY PROFILE ACROSS GLACIAL CLAYS AND GRAVELS
S-22
RESISTIVITY SURVEYS
Profiling - lateral contacts using
constant electrode spacing
Sounding - stratigraphic changes
measured with successively larger
electrode spacings
S-23
ELECTRICAL RESISTIVITY
Advantages
Qualitative modeling of data is feasible
Models can be used to estimate depths,
thicknesses, and resistivities of
subsurface layers
S-24
Geophysical Methods
8/95
-------
ELECTRICAL RESISTIVITY (cont.)
Advantages
Layer resistivities can be used to estimate
resistivity of saturating fluid
Extent of groundwater plume can be
approximated
S-25
ELECTRICAL RESISTIVITY
Disadvantages
• Cultural noise limitations
• Large area free from grounded metallic
structures required
• Level of effort/number of operational
personnel
S-26
SEISMIC TECHNIQUES
• Refraction method
• Reflection method
S-27
NOTES
8/95
Geophysical Methods
-------
NOTES
SEISMIC REFRACTION
Cheaper and easier
Determination of velocity and depth of
layers
S-28
SEISMIC REFLECTION
More expensive and complex
Resolution of thin layers
S-29
Geophones
5000
fps
saturated
sand
x/ / /
,/\Refracted wave
SEISMIC WAVE PATHS
S-30
Geophysical Methods
10
8/95
-------
NOTES
SEISMIC REFRACTION
• Measures travel time of acoustic wave
refracted along an interface
• Most commonly used at sites where
bedrock is less than 500 ft below ground
surface
S-31
SEISMOGRAPH FIELD LAYOUT
SHOWING DIRECT AND REFRACTED WAVES
d! < * <"
iii/
Soil
Bedrock
\
Pint arrival wava front*
— k Second arrival wav* fronta
S-32
Slop* ol ralractad way*
(lirtt arrival)
S-33
8/95
11
Geophysical Methods
-------
NOTES
SEISMIC REFRACTION
Advantages
Determine layer velocities
Calculate estimates of depths to different
rock or groundwater interfaces
Obtain subsurface information between
boreholes
Determine depth to water table
S-34
SEISMIC REFRACTION
Assumptions
Velocities of layers increase with depth
Velocity contrast between layers is
sufficient to resolve interface
Geometry of geophones in relation to
refracting layers will permit detection of
thin layers
S-3S
SEISMIC REFRACTION
Disadvantages
Assumptions must be made
Assumptions must be valid
Data collection can be labor intensive
S-38
Geophysical Methods
12
8/95
-------
SEISMIC REFLECTION
• Measures travel time of acoustic wave
reflected along an interface
• Precise depth determination cannot be
made without other methods
S-37
SEISMIC REFLECTION (cont.)
• Magnitude of energy required is limiting
factor
• Requires more complex data review
S-38
GROUND-PENETRATING RADAR
A transmitter emits pulses of high-
frequency electromagnetic waves into
the subsurface which are scattered back
to the receiving antenna on the surface
and recorded as a function of time
S-39
NOTES
8/95
13
Geophysical Methods
-------
NOTES
Recorder
— — — EMctromagnollc source
[ i and antenna
Paper X^
and/or /^**^ *S
electronic Antenna |
•££ ; .
GROUND-PENETRATING RADAR
S-40
GROUND-PENETRATING RADAR
Depth penetration is severely limited by
attenuation of electromagnetic waves into
the ground
S-41
GROUND-PENETRATING RADAR
(cont.)
• Attenuating factors
- Shallow water table
- Increase in clay content in the
subsurface
- Electrical resistivity less than
30 ohm-meters
S-42
Geophysical Methods
14
8/95
-------
GROUND-PENETRATING RADAR
Advantages
• Continuous display of data
• High-resolution data under favorable site
conditions
• Real-time site evaluation possible
S-43
GROUND-PENETRATING RADAR
Disadvantages
• Limitations of site-specific nature of
technique
• Site preparation necessary for survey
• Quality of data can be degraded by
cultural noise and uneven ground surface
S-44
Borehole Geophysics
S-4S
NOTES
8/95
15
Geophysical Methods
-------
potential
7
I
Geologic
log
clay
•and
taw day
layer*
(frath watar)
•ha I*
LMS
•andatone
SH layer*
(brackiah
water)
•hale
few SS
layer*
•and*tone
(•aline water)
(weathered)
den*e rock
probably
granite
I
1
L
COMPARISON OF ELECTRICAL AND
RADIOACTIVE BOREHOLE LOGS
S-46
NOTES
Geophysical Methods
16
8/95
-------
BOREHOLE GEOPHYSICS
• Spontaneous potential
• Normal resistivity
• Natural-gamma
• Gamma-gamma
S-47
BOREHOLE GEOPHYSICS (cont.)
• Neutron
• Caliper
• Acoustic
• Temperature
S-48
SPONTANEOUS POTENTIAL
• Records natural potential between
borehole fluid and fluid in surrounding
materials
• Can only be run in open, fluid-filled
boreholes
S-48
NOTES
8/95
17
Geophysical Methods
-------
NOTES
SPONTANEOUS POTENTIAL (cont)
Primary uses:
• Geologic correlation
• Determination of bed thickness
• Separation of nonporous rocks from
porous rocks (i.e., shale-sandstone
and shale-carbonate)
s-so
RESISTIVITY
Measures apparent resistivity of a volume
of rock or soil surrounding the borehole
Radius of investigation is generally equal to
the distance between the borehole current
and measuring electrodes
Can only be run in open, fluid-filled
boreholes
S-51
GAMMA
• Measures the amount of natural-gamma
radiation emitted by rocks or soils
• Primary use is identification of lithology
and stratigraphic correlation
• Can be run in open or cased and fluid- or
air-filled boreholes
S-52
Geophysical Methods
18
8/95
-------
NOTES
GAMMA-GAMMA
Measures the intensity of gamma
radiation from a source in the probe
after it is backscattered and attenuated
in the rocks or soils surrounding the
borehole
S-S3
GAMMA-GAMMA (cont.)
Primary use is identification of lithology
and measurement of bulk density ana
porosity of rocks or soils
Can be run in open or cased and fluid- or
air-filled boreholes
S-54
NEUTRON
• Measures moisture content in the vadose
zone and total porosity in sediments and
rocks
• Neutron sources and detector are
arranged in logging device so that output
is mainly a function of water within the
borehole walls
s-ss
8/95
19
Geophysical Methods
-------
NOTES
NEUTRON (cont.)
• Can be run in open or cased and fluid- or
air-filled boreholes
s-se
CALIPER
Records borehole diameter and provides
information on fracturing, bedding plane
partings, or openings that may affect
fluid transport
Can be run in open or cased and fluid- or
air-filled boreholes
S-57
ACOUSTIC
• A record of the transit time of an
acoustic pulse emitted into the formation
and received by the logging tool
• Response is indicative of porosity and
fracturing in sediments or rocks
• Can be run in open or cased, fluid-filled
boreholes
S-58
Geophysical Methods
20
8/95
-------
NOTES
TEMPERATURE
A continuous record of the temperature
of the environment immediately
surrounding the borehole
Information can be obtained on the
source and movement of water and the
thermal conductivity of rocks
Can be run in open or cased, fluid-filled
boreholes
S-50
8/95
21
Geophysical Methods
-------
Section 10
-------
GEOCHEMICAL MODELS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. Evaluate the effect organic and inorganic contaminants have
on groundwater chemistry
2. Identify chemical changes in groundwater from petroleum
hydrocarbon contaminants
3. Identify chemical changes in groundwater from sewage and
municipal landfill contaminants
4. Identify chemical changes in groundwater from acid, base,
and ammonia spills and coal fly ash
5. Define the following chemical parameters:
a. Hardness
b. Alkalinity
c. pH
d. Eh
6. Describe how hardness, alkalinity, pH, and Eh affect water
chemistry
7. Describe the effects of the carbonate buffering system on
groundwater
8/95
-------
STUDENT PERFORMANCE OBJECTIVES (cont.)
8. Define dense nonaqueous-phase liquids (DNAPLs) and light
nonaqueous-phase liquids (LNAPLs)
9. Describe gas evolution in uncapped landfills.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
GEOCHEMICAL MODELS
S-1
PRIMARY DRINKING WATER
STANDARDS
• Inorganics
• Microbiological
• Pesticides
• Volatile organic compounds
• Radioactivity
S-2
SECONDARY DRINKING
WATER REGULATIONS
Chloride
Color
Copper
Corrosivity
Fluoride
Foaming agents
Iron
• Manganese
• Odor
• PH
• Sulfate
• Total dissolved solids
• Zinc
S-3
8/95
Geochemical Models
-------
Anatomy of a Plume
S-4
Petroleum Contaminant
S-5
LNAPL
Oxic
Anoxic
Slightly
oxygenated
increase
S-6
Geochemical Models
8/95
-------
Source
LNAPL
Generation of
organic acids
BTEXandDOC
Benzene
Toluene
Xytene
HYDROLYSIS OF ORGANIC
CHEMICALS
O2 + H2O —*HCO3 + organic acids
O
R - C- OH
s-e
{
8/95
Source
LNAPL
Carbonic
acid
generation
Organic acids
Geochemical Models
-------
LNAPL
PH
pH = - log [ H+]
S-10
LNAPL
CARBONATE BUFFERING SYSTEM
Organic chemicals-
HO
H2C03
HCO
CO-
»C02
^ H2CO3
emrtonk tcid
HCO^
co
Alkalinity is HCOJ + COJ
S-11
LNAPL
DISSOLUTION OF LIMESTONE
(CaMg)CO3 + H*
Limestone
Ca** + Mg*
S-12
Geochemical Models
8/95
-------
LNAPL
Source
Alkalinity HCOj. CO3
Hardness Ca**, Mg **
S-13
HARDNESS
Type
Soft
Moderately hard
Hard
Very hard
\ #\Uv^ v nfl&S^W^ w
mg/L
0-60
61-120
121-180
>180
^$L s-14
1 0
> £
^v
MOBILITY OF ALUMINUM (AI)-AND
ZINC (Zn) METALS
I
pH
14
8/95
Geochemical Models
-------
Redox Potential (Eh)
• Oxidizing or reducing environment
• High negative volts - reducing reactions
• High positive volts - oxidizing reactions
• Oxic vs. aerobic
• Anoxic vs. anaerobic
Eh REDUCTION/OXIDATION
Source
LNAPL
Fouling aquMr and w*ll
wWi mlnml practptatn
Geochemical Models
LNAPL
S-1S
8/95
-------
DENSITY-DNAPL
DNAPL
Source
LNAPL
Source
DNAPL
Oxic
Anoxic
lightly
oxygenated
increase
S-20
A
2.i
SEWAGE AND
MUNICIPAL LANDFILLS
Leachate Containment
8/95
Geochemical Models
-------
Source
Sewage and
Municipal Landfills
Oxic
S-22
Source
Sewage and
Municipal Landfills
Carbonic
acid
generation
S-23
Source
Sewage and
Municipal Landfills
Geochemical Models
8/95
-------
Source
Sewage and
Municipal Landfills
SO'
SO.
Sewage and
Municipal Landfills
DISSOLUTION OF SHEET ROCK
CaSO4
(sheet rock)
ION EXCHANGE
f Crater softening')
SULFATE REDUCTION
so=
* s-
S-26
Source
Sewage and
Municipal Landfills
Ammonia forms in anoxic environment
organic material —-*• NHj * HCOj"
•---,. NO;
3
S-27
8/95
Geochemical Models
-------
Sewage and
Municipal Landfill*
GEOCHEMICAL LIFE CYCLE
OF A LANDFILL
100%.
AwnMc
0%
A«obfc
Time
Cellutoee
concentration
S-28
Sewage and
Municipal Landfills
LANDFILL LEACHATE INDICATORS
Excellent:
Ammonia (NHj)
DOC
Low Eh
No 03
High Fe~ and Mn~
Good:
LowpH
High alkalinity (HCOj)
Low sulfate
Cr.Na*. B***
Organic-Poor
Contaminants
Geochemical Models
10
8/95
-------
Coal fly ash landfills; salt storage facilities;
brine disposal; acid/base spills
Source
Organic-poor
contaminants
Low pH Contaminants
S-32
8/95
Source
LowpH
Eh, DO
Noming to
S-M
11
Geochemical Models
-------
LowpH
Source
TDS, cations, anions
Precipitation of Fe, Mn, and
Al
Precipitation of calcite
Sorption of trace metals
Ion exchange s-w
High pH Contaminants
S-35
High pH
Source
Geochemical Models
12
8/95
-------
Source
High pH
IDS, cations
Mn*+
j Precipitation of Fe and Mn
•4 Precipitation of carboiiate (?) plugging pores
Lsorpjion of trace metals
S-37
8/95
Geochemical Models
-------
Section 11
-------
GROUNDWATER MODELS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. List the physical processes that affect groundwater and
contaminant flow
2. List the properties that are included in the retardation factor
3. List the parameters that are included in the basic equation
used in groundwater computer programs
4. List the variables that groundwater models can be used to
predict.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
NOTES
GROUNDWATER MODELS
s-t
GROUNDWATER MODELS
An attempt to simulate groundwater flow
conditions mathematically
• Used to predict groundwater levels
(heads) over time
• Used to predict contaminant transport
S-2
PHYSICAL PROCESSES
• Advection
• Dispersion
• Density
• Immiscible phase
• Fractured media
S-3
8/95
Groundwater Models
-------
NOTES
ADVECTION
Average groundwater velocity
Depends on:
^^Hydraulic conductivity
--Porosity
T? Hydraulic gradient
-)
X-
S-4
Q = Rate of flow
K = Hydraulic conductivity
A = Cross-sectional area of flow
I = Hydraulic gradient
ne = Effective porosity
S-5
Q =
Q =
v =
vs =
* Seepage
KAI
Av
Kl
v —
velocity or
Darcy's Law
Velocity equation
Darcian velocity
IS\
Anvpotivp vplnpitv*
/^*-ivcoiivc vciwOiiy
ne
average linear velocity
Groundwater Models
8/95
-------
NOTES
t
.o
"5
0)
o
o
O
I
Advection
I Distance from source
S-7
DISPERSION
Tendency for solute to spread
Caused by:
- Mechanical mixing
- Molecular diffusion
8-8
PATH LENGTH AND PORE SIZE AS
FACTORS IN CONTAMINANT TRANSPORT
Small pore size
slow movement
Short path
Long path
Large pore size
fast movement
s-e
8/95
Groundwater Models
-------
NOTES
t
c
o
(0
c
0)
o
o
I
Advection
plus
dispersion
Distance from source
S-10
DENSITY - LNAPL
Groundwater flow
S-11
DENSITY - DNAPL
S-12
Groundwater Models
8/95
-------
NOTES
IMMISCIBLE PHASE FLOW
• Mutually insoluble liquids
• Interferes with groundwater flow
• Liquids can become immobile at residual
saturation
S-13
IMMISCIBLE PHASE FLOW
Water
Solid
particle
Immiscible
fluid
S-14
Faulted and Fractured Porous Rock
Poltrtlal grourx/wfttr flow
3-1S
8/95
Groundwater Models
-------
NOTES
CHEMICAL PROCESSES
- • Sorption
- • Hydrolysis
• Cosolvation -"
• lonization
S-18
CHEMICAL PROCESSES (cont.)
Dissolution and precipitation
Complexation reactions
Redox potential
S-17
BIOLOGICAL PROCESSES
Microorganisms
- Bacteria
- Fungi
Transformation of contaminants
- Aerobic conditions
- Anaerobic conditions
S-18
Groundwater Models
8/95
-------
NOTES
RETARDATION FACTOR
Relates groundwater velocity to
contaminant velocity
Current practice: lump chemical and
biological processes into retardation
S-19
RETARDATION
R
R
Pb
Kd
NT
= 1 + pb x Kd
NT
= retardation factor
= bulk density
= distribution coefficient
= total porosity
S-20
t
c
_g
+*
S
"c
0)
o
o
O
1
Advection
plus
retardation
Distance from source I
S-21
8/95
Groundwater Models
-------
NOTES
Groundwater Modeling
S-22
CONCENTRATION
AT DISTANCE "L"
r
Ierfc
L- vt
vL.
vt
DL = longitudinal dispersion coefficient
C0 = solute concentration at source
v = average linear velocity
L = distance
t = time
erfc = complementary error function
S-23
MODELS CAN PREDICT:
• Spatial variation
• Temporal variation
• Parameter variation
S-24
Groundwater Models
8/95
-------
NOTES
MODEL DIMENSIONS
One-dimensional
Two-dimensional
Three-dimensional s*—
S-2S
MODELING PROBLEMS
• Lack of appropriate modeling protocols
and standards
• Insufficient technical support
• Inadequate education and training
• Widely used, but selection and use
inconsistent
S-2C
KEYS TO SUCCESSFUL
USE OF MODELS
• Proper input of data and parameter
estimates
• Effective communication
• Understanding the limitations of the mode!
S-27
V.
8/95
Groundwater Models
-------
NOTES
G.I.G.O.
Garbage in = Garbage out
The first axiom of computer usage
S-26
MOST COMMON EPA MODELS
Name
MODFLOW
HELP
RANDOM WALK
USGS-2D
USGS-MOC
Relative Use
29
24
21
20
19
S-29
Groundwater Models
10
8/95
-------
Section 12
-------
PROBLEM 1
Cross-Section Exercise
-------
PROBLEM 1: CROSS-SECTION EXERCISE
A. Student Performance Objectives
1. Use a topographic map to locate sites for the installation of monitoring wells at
specific elevations.
2. Draw a topographic profile of a specified area.
3. Calculate a vertical exaggeration for a topographic profile.
4. Obtain geological information from monitoring well logs.
5. Use the GSA Munsell color chart and geotechnical gauge to identify rock sample
colors and textures.
6. Given a geologic map, interpret elevations of geologic formations.
7. Draw a geologic cross-section using monitoring well logs and a topographic
profile.
8. Interpret subsurface geology to locate aquifers of concern, identify discontinuities
in geologic formations, and locate potential monitoring/remediation wells.
B. Background Information
Each group of students will have a set of six rock/sediment samples, labeled A through F,
to examine. These samples represent rock/sediment samples from six of the seven
different geologic formations encountered during the installation of monitoring wells at,
and in the vicinity of, the Colbert Landfill site in Spokane, Washington. During the site
investigation, these samples were collected from cuttings generated by mud rotary
drilling. Each sample tube is also oriented with an arrow that indicates the top. DO
NOT attempt to remove the orange caps and open the tubes!
C. Geologic Cross-Section
1. Using the GSA Munsell color chart and sample mask, match the overall color of
the rocks, sand, clay, or gravel within the samples to the color chart. Do not
determine every color if a sample is multicolored, but look for key sediment types
or specific marker colors.
2. Using the geotechnical gauge, generally determine and match the grain size of the
sediments with the written descriptions. For example, actual fine sand or coarse
sand sizes can be found on the chart. Sediments larger than coarse sand, such as
gravel and cobbles, are NOT shown on the geotechnical card. Using the
8/95 1 Cross-Section Exercise
-------
geotechnical gauge and the sediment characteristic diagram depicted in Figure 1,
generally determine the degree of particle rounding and sediment sorting. Well
sorted means most panicles are of similar size and shape, whereas poorly sorted
particles are of no particular size and vary greatly in size and shape, such as sand
mixed with gravel or cobbles.
3. Using the SAMPLES and well log together, match these descriptions and your
visual observations to the official published U.S. Geological Survey geologic
description of the formations. Then identify each formation on the well logs in
the space provided under the "STRATA" column; for example, Kiat, sample F.
START WITH WELL LOG #6 AND PROCEED TO LOG #1. EACH TUBE
REPRESENTS ONLY ONE ROCK FORMATION! Be sure to read the
information written under the "REMARKS" column at the right of the log sheet
for additional sample information. Your instructor will discuss the correct sample
identification at the end of this portion of the exercise.
4. Using the appropriate topographic maps and graph paper provided, locate Wells 6
through 1 along the top of the graph paper from left to right along profile line
A-A'. Determine the respective elevations of the wells (your instructor will
demonstrate this technique).
5. Label the Y-axis of the graph paper to represent the elevation, starting from 2,100
feet at the top to 1,400 feet at the bottom. Each box on the graph represents 20
feet in elevation.
6. Plot the location, depicting the correct surface elevation of each well on the graph.
Also determine and plot the elevations of several easily determined points on the
profile line between each of the wells in order to add more detail to the profile.
This will generate a series of dots representing the elevations of the six wells and
the other elevations you have determined. Make sure to select contour lines that
cross the profile line. The contour interval of these particular topographic maps is
20 feet.
7. After plotting these elevations on the graph, connect them with a smooth curve.
which will represent the shape of the topography from A-A'.
8. Using the well logs previously completed and the colored geologic map, add the
existing geology and formation thickness to each well location. Each formation
thickness is listed on the left side of the well log and is measured from the bottom
of the next overlying formation.
9. Sketch in and interpret the geologic layers of the cross section, starting with the
lowest bedrock formation. Connect all of the same geologic formations, keeping
in mind that some formations have varying thicknesses and areal extent.
10. Using available groundwater information, locate the three aquifers in the cross
section.
Cross-Section Exercise 2 8/95
-------
11. Using the completed cross section, locate potential sites for the installation of
additional monitoring wells or remediation wells and identify formation
discontinuities.
12. Compare your interpretation with the "suggested" interpretation handed out by the
instructor.
D. History of Colbert Landfill
The Colbert Landfill is located 2.5 miles north of the town of Colbert near Spokane,
Washington, and is owned by the Spokane County Utilities Department. This 40-acre
landfill was operated from 1968 to 1986, when it was filled to capacity and closed. It
received both municipal and commercial wastes from many sources. From 1975 to 1980,
a local electronics manufacturing company disposed spent solvents containing methylene
chloride (MC) and 1,1,1-trichloroethane (TCA) into the landfill. A local Air Force base
also disposed of solvents containing acetone and methyl ethyl ketone (MEK). These
solvents were trucked to the landfill in 55-gallon drums and poured down the sides of
open and unlined trenches within the landfill. Approximately 300-400 gallons/month of
MC and 150-200 gallons/month of TCA were disposed. In addition, an unknown volume
of pesticides and tar refinery residues from other sources were dumped into these
trenches.
The original site investigation was prompted by complaints from local residents who
reported TCA contamination of their private wells. The population within 3 miles of the
site is 1,500. In 1981, a Phase 1 investigation was conducted; a Phase 2 was completed
in 1982. Groundwater samples collected from nearby private wells indicated TCA
contamination at 5,600 fig/L, MC contamination at 2,500 /ig/L, and acetone at a
concentration of 445 ng/L. Investigation reports concluded that drinking groundwater
posed the most significant risk to public health. EPA placed the site on the National
Priority List (NPL) in 1983. Bottled water and a connection to the main municipal water
system was supplied to residents with high TCA contamination (above the MCL), and the
cost was underwritten by the potentially responsible parties (PRPs) involved.
Hydrogeological Investigation
The site lies within the drainage basin of the Little Spokane River, and residents with
private wells live on all sides of the landfill. The surficial cover and subsequent lower
strata in the vicinity of the site consist of glacially derived sediments of gravel and sand,
below which lie layers of clay, basaltic lava flows, and granitic bedrock. Beneath the site
there are three aquifers and three aquitards. The stratigraphic sequence beneath the
landfill from the top (youngest) to the bottom (oldest) is:
Qfg upper sand and gravel glacial outwash and Missoula flood deposits which together
form a water table aquifer
Qglf Upper layers of glacial Lake Columbia deposits of impermeable silt and clay that
serve as an aquitard; lower layers of older glaciofluvial and alluvial sand and
gravel deposits that form a confined aquifer
8/95 . 3 Cross-Section Exercise
-------
Mvwp Impermeable but weathered Wanapum basalt flow
Mel Impermeable and unweathered Latah Formation of silt and clay
Kiat Fractured and unfractured granitic bedrock that serves as another confined aquifer
In the upper aquifer (Qfg), which is 8-15 feet thick, groundwater flows from 4 to 13 feet
per day (ft/day). The lower confined sand and gravel aquifer (Qglf) varies from a few
feet thick to 150 feet thick and is hydraulically connected to the Little Spokane River.
Groundwater in this aquifer flows from 2 to 12 ft/day. To the northeast of the landfill,
the upper aquifer is connected to the lower aquifer. Both of these aquifers are classified
as current sources of drinking water according to EPA and are used locally for potable
water. The area impacted by the site includes 6,800 acres and the contamination plume
extends 5 miles toward the town of Colbert. Of the contaminants present, 90 percent
occur as dense, nonaqueous-phase liquids (DNAPLs) at the bottom of the upper aquifer,
and natural DNAPL degradation is slow. It has been estimated that only 10 percent of
the solvents have gone into solution, whereas the remainder occurs in pore spaces and as
pools of pure product above impermeable layers. The TCA plume in the upper aquifer
has extended 9,000 feet in 8-10 years and it moves at a rate of 2-3 ft/day. The flow rate
of the contamination plume in the lower sand and gravel aquifer (Qglf) has not been
calculated because of the complexity and variability of the subsurface geology. However,
TCA and MC have the highest concentrations in the lower sand and gravel aquifer.
Cross-Section Exercise 4 8/95
-------
Sediment Characteristics
O © @
>
O
Well Rounded
A
Poorly Rounded
Well Sorted
I
\
Poorly Sorted
Stratified
FIGURE 1
8/95
Cross-Section Exercise
-------
B
D
Map
View
Cut away
cross-
sect ion
DEVELOPMENT OF CONTOUR LINES
Consider an island in a lake and the patterns made on it when the water level recedes. The
shoreline represents the same elevation all around the island and is thus a contour line (see above
Figure, part A). Suppose that the water levels of the lake drop 10 ft and that the position of the
former shoreline is marked by a gravel beach (Figure B). Now there are two contour lines, the
new lake level and the old stranded beach, each depicting accurately the shape of the new island
at these two elevations. If the water level should continue to drop in increments of 10 ft, with
each shoreline being marked by a beach, additional contour lines would be formed (Figures C
and D). A map of the raised beaches is therefore a contour map (Figure E), which graphically
represents the configuration of the island.
Cross-Section Exercise
8/95
-------
PROBLEM 2
Sediment Analysis
-------
PROBLEM 2: SEDIMENT ANALYSIS
A. Student Performance Objectives
1. Determine the grain size distribution of unconsolidated geologic materials
obtained from an aquifer.
2. Calculate a uniformity coefficient from the data obtained through sample
sieving.
3. Given a formation's sieve analysis, select filter pack and screen slot size.
B. Background Information
1. Keck Field-Sieving Kit
The Keck Field-Sieving kit will be used to provide information on the grain size
distribution of the unconsolidated sediments in the aquifer to be screened by the
monitoring well. It will also be used to determine the correct size of filter pack
material around the screen of a monitoring well, as well as determine the screen size.
Sieving is only done using a dry mixture of unconsolidated sediments such as gravel,
sand, silt, and clay. During the sieving, grain size ranges are retained by each sieve.
The coarsest materials are retained by the top sieve, whereas the finest are collected
by the bottom pan. The amount of sediment retained by each sieve is usually
determined by weighing each fraction on a balance. However, with the Keck Field-
Sieving Kit, this information is gathered by comparing the volume within each
cylinder to the vertical percent scale along the edge of the Keck sieve holder. By
initially using a sample volume that equals one full cylinder (100 percent), the percent
"retained" by each screen after 5 minutes of sieving can be easily obtained. The
"cumulative" percent of sand from each cylinder is calculated and plotted on special
graph paper. The Y or vertical axis on the left side of the graph will represent the
percent sample retained from 0 to 100 percent (the right side of the graph measures
cumulative percent sediment passing), and the X or horizontal axis (along the bottom)
will represent the grain size as measured in either thousandths of an inch or in U.S.
standard sieve sizes (grain size in millimeters is measured along the top of the graph).
Once the data points are plotted, they are connected with a smooth curve.
2. Particle Size Distributions
Because the sample is usually a mixture of sediment types, there is no single way to
describe the range of particle sizes. The Wentworth Scale was developed in 1922 to
classify panicle size from boulders to clay. The Unified Soil Classification System
was adopted by the U.S. Department of Agriculture as an extension of the Wentworth
Scale to further classify fine-grained material. The panicle size distribution can also
be used to determine the size of the filter pack material to be used around the well
screen. This material is mainly used with fine-grained sediments to make the area
8/95 1 Sediment Analysis
-------
around the screen more permeable, while also increasing the hydraulic diameter of
the well. The grain size distribution of this material is selected such that 90 percent
of it is retained by the screen slot opening. This allows the well to produce mostly
sand-free water. Finally, the slope of the curve can also be used to determine the
uniformity of the grain size by calculating the uniformity coefficient.
3. Uniformity Coefficient
The uniformity coefficient (UC) is calculated by dividing the 40 percent retained size
of the sediment by the 90 percent retained size. For example, 40 percent of the
sample was retained by 0.026 inches, while 90 percent was retained by 0.009
inches.
40% retained _ 0.026 in. _ 29
90% retained 0.009 in.
The lower the value, the more uniform the particle size grading; the larger the value,
the less uniform the grading. Values for UC should be less than 5.
C. Determine the Grain Size Distribution of Unconsolidated Geologic Materials
Obtained from an Aquifer
1. Sieve a sample.
a. Get a prepared Keck Field-Sieving kit from the instructors.
b. Remove the cylinder stack from the frame by holding the frame at the TOP
and unscrewing the knob counterclockwise.
c. Fill the beaker with sand to the 100-ml line. This will equal one cylinder
volume (100 percent).
d. Remove the clear cap from the top cylinder and carefully pour approximately
one-half of the sample from the beaker into it. Make sure the box top is
beneath the cylinder to catch any spilled sand.
e. Replace the top cap and carefully replace the cylinder stack into the frame.
f. Slowly tighten the cap by turning the top knob clockwise.
g. Hold the frame by BOTH ends and shake in a circular manner. Add an
occasional vertical shake during this process.
h. Shake for 5 minutes.
i. CAREFULLY remove the cylinders from the holder, add the remaining sand
sample, replace the cylinders into the frame, and continue shaking for another
5 minutes.
j. Tap the cylinders with your fingers until the majority of the sample lies
roughly flat within each cylinder.
k. Using the vertical scale on the side of the frame, visually determine the
percent sample fraction within each cylinder and record the data on the sheets
provided.
1. Carefully remove the cylinders from the frame, invert the stack, and replace
the sand into the bag.
m. Clean out each cylinder by tapping it against your hand. DO NOT TAP
Sediment Analysis 2 8/95
-------
AGAINST THE DESK OR ANY HARD SURFACE! Use the paintbrush
to remove any remaining sand from the screens and gaskets.
n. Replace sieve set into box.
o. Calculate the cumulative percent of each cylinder and record the data on the
data sheets.
p. Using the graph paper provided, plot your data.
2. Determine the grain size distribution of your sediment sample using your data plots
and the unified soil classification scale on the bottom of the graph paper.
3. Calculate the uniformity coefficient for your sample.
8/95 3 Sediment Analysis
-------
s
3'
«i
D
Bottle #
Sediment Sieve Exercise
U.S. Sieve #
Percent Retained
Cumulative Percent
Uniformity Coefficient: UC = 40%/90%
-------
SELECTION OF FILTER
PACK AND WELL SCREEN
S-1
PURPOSE OF FILTER PACK
• Allow groundwater to flow
freely into well
• Minimize or eliminate entrance
of fine-grained materials
S-2
WELL SCREEN
Surrounded by:
• Filter pack coarser than the
aquifer material
• Filter pack of uniform grain size
• Filter pack of higher permeability
than the aquifer material
S-3
NOTES
8/95
Sediment Analysis
-------
NOTES
UNIFORMITY COEFFICIENT (UC)
• Measure of the grading uniformity
of sediment
• 40% retained size divided
by 90% retained size
• UC of filter pack material should
not exceed 2.5
S-4
FILTER PACK SELECTION
• Select by multiplying the 70%
retained grain size of the aquifer
materials by 4 or 6
• Use 4 if aquifer is fine grained
and uniform
• Use 6 if aquifer is coarse grained
and nonuniform
S-5
WELL SCREEN SELECTION
Select screen slot opening to
retain 90% of filter pack material
S-8
Sediment Analysis
8/95
-------
PROBLEM 3
Groundwater Model Demonstration
-------
PROBLEM 4
Hydrogeological Exercises
-------
PROBLEM 4: HYDROGEOLOGICAL EXERCISES
PART 1.
A. General Discussion
Groundwater-level data can be used to determine direction of groundwater flow by
constructing groundwater contour maps and flow nets. To calculate a flow direction, at
least three observation points are needed. First, relate the groundwater field levels to a
common datum—map datum is usually best—and then accurately plot their position on a
scale plan, as in Figure 1. Second, draw a pencil line between each of the observation
points, and divide each line into a number of short, equal lengths in proportion to the
difference in elevation at each end of the line. The third step is to join points of equal
height on each of the lines to form contour lines (lines of equal head). Select a contour
interval that is appropriate to the overall variation in water levels in the study area. The
direction of groundwater flow is at right angles to the contour lines from points of higher
head to points of lower head.
This simple procedure can be applied to a much larger number of water-level values to
construct a groundwater-level contour map such as the one in the example. Locate the
position of each observation point on a base map of suitable scale and write the water
level against each well's position. Study these water-level values to decide which contour
lines would cross the center of the map. Select one or two key contours to draw in first.
Once the contour map is complete, flow lines can be drawn by first dividing a selected
contour line into equal lengths. Flow lines are drawn at right angles from this contour, at
each point marked on it. The flow lines are extended until the next contour line is
intercepted, and are then continued at right angles to this new contour line. Always select
a contour that will enable you to draw the flow lines in a downgradient direction.
B. The Three-Point Problem
Groundwater-flow direction can be determined from water-level measurements made on
three wells at a site (Figure 1).
1. Given:
Well Number Head (meters)
1 26.28
2 26.20
3 26.08
8/95 1 Hydrogeological Exercises
-------
V
v
,
\ ^
V
' ^ \C-
/
WELL 2
( head. 26.20 m )
WELL1
( head. 26.28 m )
WELLS
( head. 26.08 m )
METERS (scale approximate)
FIGURE 1
Procedure:
a. Select water-level elevations (head) for the three wells depicted in
Figure 1.
b. Select the well with water-level elevation between the other wells (Well 2).
c. Draw a line between Wells 1 and 3. Note that somewhere between these
wells is a point, labeled A in Figure 2, where the water-level elevation at
this point is equal to Well 2 (26.20 m).
d. To determine the distance X from Well 1 to point A, solve the following
equation (see Figures 3, 4, and 5):
Distance Y is measured directly from the map (200 m) on Figure 3. H,,
H2, and H3 represent head or water-level elevations from their respectively
numbered wells.
After distance X is calculated, groundwater-flow direction based on the
water-level elevations can be constructed 90* to the line representing
equipotential elevation of 26.20 m (Figure 6).
Hydro geological Exercises
8/95
-------
N
WELL 2
( head, 26.20 m )
WELL1
( head. 26.28 m )
Point A
0 25 SO 100
WELLS
( head. 26.08 m )
METERS (scale approximate)
FIGURE 2
N
rra
WELL 2
( head. 26.20 m )
WELL1
( head, 26.28 m )
26.4° "*
Point A
0 25 50 100
©
WELL 3
( head, 26.08 m )
METERS (scale approximate)
FIGURES
5/P5
Hydrogeological Exercises
-------
V
(26.28 - 26.20) ( 26.28 - 26.08 )
X
200
X = 80
FIGURE 4
N
X = 80m
WELL 2
( head. 26.20 m )
WELL1
( head. 26.28 m )
0 25 50 100
WELL 3
( head, 26.08 m )
METERS (scale approximate)
FIGURES
Hydro geological Exercises
8/95
-------
N
WELL1
( head. 26.28 m )
WELL 2
( head, 26.20 m
0 25 SO
100
Groundwater-Flow
Direction
WELLS
( head, 26.08 m )
METERS (scale approximate)
FIGURE 6
C. Colbert Landfill Three-Point Problem
1. History of Colbert Landfill
The Colbert Landfill is located 2.5 miles north of the town of Colbert near Spokane,
Washington, and is owned by the Spokane County Utilities Department. This 40-acre
landfill was operated from 1968 to 1986, when it was filled to capacity and closed. It
received both municipal and commercial wastes from many sources. From 1975 to 1980,
a local electronics manufacturing company disposed spent solvents containing methylene
chloride (MC) and 1,1,1-trichloroethane (TCA) into the landfill. A local Air Force base
also disposed of solvents containing acetone and methyl ethyl ketone (MEK). These
solvents were trucked to the landfill in 55-gallon drums and poured down the sides of
open and unlined trenches within the landfill. Approximately 300-400 gallons/month of
MC and 150-200 gallons/month of TCA were disposed. In addition, an unknown volume
of pesticides and tar refinery residues from other sources were dumped into these
trenches.
The original site investigation was prompted by complaints from local residents who
reported TCA contamination of their private wells. The population within 3 miles of the
site is 1,500. In 1981, a Phase 1 investigation was conducted; a Phase 2 was completed
in 1982. Groundwater samples collected from nearby private wells indicated TCA
contamination at 5,600 /ig/L, MC contamination at 2,500 ng/L, and acetone at a
concentration of 445 ng/L. Investigation reports concluded that drinking groundwater
posed the most significant risk to public health. EPA placed the site on the National
8/95
Hydro geological Exercises
-------
Priority List (NPL) in 1983. Bottled water and a connection to the main municipal water
system was supplied to residents with high TCA contamination (above the MCL), and the
cost was underwritten by the potentially responsible parties (PRPs) involved.
2. Hydrogeological Investigation
The site lies within the drainage basin of the Little Spokane River, and residents with
private wells live on all sides of the landfill. The surficial cover and subsequent lower
strata in the vicinity of the site consist of glacially derived sediments of gravel and sand,
below which lie layers of clay, basaltic lava flows, and granitic bedrock. Beneath the site
there are three aquifers and three aquitards. The stratigraphic sequence beneath the
landfill from the top (youngest) to the bottom (oldest) is:
Qfg Upper sand and gravel glacial outwash and Missoula flood deposits which together
form a water table aquifer
Qglf Upper layers of glacial Lake Columbia deposits of impermeable silt and clay that
serve as an aquitard; lower layers of older glaciofluvial and alluvial sand and
gravel deposits that form a confined aquifer
Mvwp Impermeable but weathered Wanapum basalt flow
Mel Impermeable and unweathered Latah Formation of silt and clay
Kiat Fractured and unfractured granitic bedrock that serves as another confined aquifer
In the upper aquifer (Qfg), which is 8-15 feet thick, groundwater flows from 4 to 13 feet
per day (ft/day). The lower confined sand and gravel aquifer (Qglf) varies from a few
feet thick to 150 feet thick and is hydraulically connected to the Little Spokane River.
Groundwater in this aquifer flows from 2 to 12 ft/day. To the northeast of the landfill,
the upper aquifer is connected to the lower aquifer. Both of these aquifers are classified
as current sources of drinking water according to EPA and are used locally for potable
water. The area impacted by the site includes 6,800 acres and the contamination plume
extends 5 miles toward the town of Colbert. Of the contaminants present, 90 percent
occur as dense, nonaqueous-phase liquids (DNAPLs) at the bottom of the upper aquifer,
and natural DNAPL degradation is slow. It has been estimated that only 10 percent of
the solvents have gone into solution, whereas the remainder occurs in pore spaces and as
pools of pure product above impermeable layers. The TCA plume in the upper aquifer
has extended 9,000 feet in 8-10 years and it moves at a rate of 2-3 ft/day. The flow rate
of the contamination plume in the lower sand and gravel aquifer (Qglf) has not been
calculated because of the complexity and variability of the subsurface geology. However,
TCA and MC have the highest concentrations in the lower sand and gravel aquifer.
3. Remedial Measures
The remediation goal for this site is to use an extraction and interception system (pump
and treat) for removing groundwater contamination and to completely cap and regrade the
site. A line of 8 groundwater extraction wells of variable depth, located downgradient of
the site, and 10 extraction wells 100 ft deep will be used for site remediation. The wells
in the lower sand and gravel aquifer will pump at a rate of 130 gallons per minute (gpm),
whereas the wells in the water table aquifer will pump at a rate of 20-30 gpm.
Hydrogeological Exercises 6 8/95
-------
Groundwater and soil gas monitoring is scheduled to continue for 30 years to monitor the
location and movement of the groundwater contamination plume.
4. Groundwater Flow-direction Calculations
Using the data in Table 1 from monitoring wells in the vicinity of the Colbert Landfill
(see topographic map from the cross-section exercise), determine the groundwater flow
direction within the shallow and deep aquifers.
Choose three wells that are relatively close together and on the same side of the Little
Spokane River. Assume that north is located at the top of the page. Check your
calculations.
a. Shallow groundwater flow direction:
b. Deep groundwater flow direction:
8/95 7 Hydrogeological Exercises
-------
TABLE 1. CONSTRUCTION DATA ON MONITORING WELLS LOCATED IN THE
VICINITY OF THE COLBERT LANDFILL, SPOKANE, WA
Table 1
B
Well
Number
(MW#)
1
2
•@v
^
5
6
7
8
^
Top of
Casing
Elevation
(ftmsl)
1923.25
1958.45
1929.88
1868.05
1675.50
2003.70
1948.26
1703.20
^
1906.11
Ground
Surface
Elevation
(ft msl)
1920.14
1955.50
1926.94
1865.85
1672.15
2000.79
1945.55
1700.00
1903.60
Ground
Water
Elevation
(ft msl)
1877.14s
1745.50d
1615.94d
1556.85d
variable
1958.79s
1431.21s
1695.00s
1610.75d
Monitoring
Well Depth
(ft below
ground)
105.10
263.50
341.20
340.40
210.50
322.80
75.90
120.90
350.70
Bedrock
Depth
(ft below
ground)
83.40
269.40
344.30
343.50
184.10
321.10
80.10
124.60
350.20
s = shallow aquifer
d = deep aquifer
-H
Hydrogeological Exercises
8/95
-------
PART 2.
A. Groundwater Gradient Calculation
1. Purpose
This part of the exercise uses basic principles defined in the determination of
groundwater-flow directions. Groundwater gradients (slope of the top of the groundwater
table) will be calculated as shown in the three-point problem.
2. Key Terms
• Head—The energy contained in a water mass produced by elevation,
pressure, and/or velocity. It is a measure of the hydraulic potential due to
pressure of the water column above the point of measurement and height
of the measurement point above datum which is generally mean sea level.
Head is usually expressed in feet or meters.
• Contour line—A line that represents the points of equal values (e.g.,
elevation, concentration).
• Equipotential line—A line that represents the points of equal head of
groundwater in an aquifer.
• Flow lines—Lines indicating the flow direction followed by groundwater
toward points of discharge. Flow lines are always perpendicular to
equipotential lines. They also indicate direction of maximum potential
gradient. ,
FEET (scale approximate)
FIGURE 7. WELL LOCATIONS AND HEAD MEASUREMENTS
8/95
Hydro geological Exercises
-------
3. After reviewing Figures 7-9, perform the following:
a. Select an appropriate contour interval that fits the water levels available
and the size of the map on Figure 10. (Twenty-foot contour intervals
should be appropriate for this problem.)
100
101.9
FEET (scale approximate)
88.9
FIGURE 8. EQUIPOTENTIAL LINES WITH WELL HEAD MEASUREMENTS
Hydro geological Exercises
10
8/95
-------
100'/ 95
101.9
I I I I I
FEET (scale approximate)
FIGURE 9. FLOW LINES ADDED TO EQUIPOTENTIAL LINES AND
CALCULATION OF HYDRAULIC GRADIENT
8/95
11
Hydro geological Exercises
-------
Y
Figure 10
Hydro geological Exercises
12
S/P5
-------
b. Draw the equipotential lines on the map (see Figure 8), interpolating
between water-level measurements.
c. Construct flow lines perpendicular to the equipotential lines drawn in step
3 (see Figure 9).
d. Select a distance on your contour map between two contour lines and
compute the gradient. The hydraulic gradient is calculated by measuring
the scale distance between equipotential lines along a flow line that crosses
the site, and dividing that value into the calculated change in head across
the same distance (H2 - HI).
For example, (see Figure 9):
Head at A = 100' (H,)
Head at B = 90' (Hj)
Measured distance between the points is 850' (L)
Head at point A minus head at point B divided by the distance between the points
equals hydraulic gradient (slope from point A to point B).
100 feet - 90 feet 10 m_ . .,.
- - - - — = - = .012 feetlfoot
SSOfeet 850
B. Profile of the Site's Groundwater Surface
After completing the contour map, plot a profile of the sites groundwater surface at Y-Y1
on Figure 10.
8/95 . 13 Hydrogeological Exercises
-------
PART 3.
A. Bakers Quarry Flow Net Construction
1. Site History/Operation
The quarry operation began in 1905 providing local construction-grade granite. The
quarry was closed in 1928 when the volumes of groundwater seeping into the pit made it
economically unfeasible to continue mining (Figure 11). The site was abandoned and the
pit filled with water. The owners of the quarry declared bankruptcy and ownership fell to
the city of Tippersville in lieu of delinquent tax payments.
The quarry was used as a swimming hole and occasional dump site for local citizens until
1958, when several children drowned. The site was fenced and patrolled to prevent
swimming. Uncontrolled dumping by individuals and local industry increased
dramatically with the swimming ban. Dumping took place around the rim of the quarry,
and the bulldozer from the town landfill was periodically used to push material into the
pit. Gradually the pit was filled and several fires forced the town to terminate dumping in
1971. The surface of the site was covered with local material, primarily sand and gravel.
The site gained notoriety when an area-wide survey identified it as a potential industrial
dump site. A preliminary site investigation, started on April 14, 1982, included sampling
a spring located approximately 25 ft from the limits of quarrying. Priority pollutant
analysis of this water sample identified ppm levels of polychlorinated biphenyls and
trichloroethylene. Results from this preliminary investigation were used to justify a more
extensive hydrogeologic study of the site.
2. Elements of the Hydrogeological Investigation
The first step of this investigation was to do a literature review of geologic information.
A discussion with a local amateur geologist revealed a paper from a geologic investigation
performed during active quarrying. Information from this study and observations at an
outcrop onsite provided a geologic background for the investigation. The quarry material
is a slightly gneissoid biotite-muscovite granite. Several dikes were identified in the
quarry wall.
The probable high permeability and infiltration rate of the less-consolidated waste material
compared to that of the granite could cause groundwater mounding in the pit area.
Potential mounding, and inadequate information about groundwater flow direction,
dictated a ringing of the site with monitoring wells.
Twenty-two monitoring wells were planned and installed at the site from October 1 to
November 14, 1982. Eleven monitoring wells were installed in bedrock, and the
unconsolidated zone was sealed with steel casing and grouted. Eleven monitoring wells
were installed in the unconsolidated, heavily weathered bedrock or unconsolidated zones.
For this exercise, use only the data from the 11 wells listed in Table 2. An explanation
of these data is depicted in Figure 12.
Hydrogeological Exercises 14 8/95
-------
400'
800'
1200'
Site Boundary
FIGURE 11. SITE MAP - BAKERS QUARRY, TIPPERSVILLE, MAINE
8/95
15
Hydro geological Exercises
-------
TABLE 2. MONITORING WELL DATA
Well
Number
MW 1
MW2
MW3
MW4
MW5
MW6
MW7
MW8
MW9
MW 10
MW 11
(a)
Top of
Casing
Elevation
(feet)*
87.29
89.94
88.04
82.50
82.50
72.50
80.58
86.03
114.01
108.67
105.07
(b)
Ground
Surface (GS)
Elevation
(feet)*
84.79
87.99
85.44
79.80
80.05
69.50
78.28
83.53
111.21
106.67
103.37
(c)
Groundwater
Elevation
(feet)*
80.49
84.69
75.29
72.40
73.40
67.50
74.78
76.93
92.36
93.97
94.97
(d)
Well Depth
(feet below
GS)
151.9
103.05
103.1
102.3
102.45
99.6
. 99.5
99.2
99.9
98.7
102.1
Bottom of
Well
Elevation
(feet)*
-67.11
-15.06
-17.66
-22.50
-22.40
-30.10
-21.22
-15.67
11.31
7.97
1.27
(e)
Bedrock
Depth
(feet below
GS)
7.5
7.5
2.0
14.0
8.5
9.0
8.0
8.5
10.5
10.8
2.5
Datum: mean sea level
Hydro geological Exercises
16
8/95
-------
MW 1
Caj
Cb)
87.29
84.79
151.9
7.5
Bedrock
80.49
Datum Csea level}
Ca} Top of casing elevation Cfeet)
Cb} Ground surface elevation £feet}
Groundwater elevation Cfeet}
Wei I depth below ground surface
Bedrock depth
FIGURE 12. MONITORING WELL ELEVATIONS
8/95
17
Hydro geological Exercises
-------
B. Site Profile Development
1. Purpose
The development and comparison of topographic profiles across the site will help the
student to understand the variability of the surface terrain usually found on most of the
larger sites. The water-table profile will also be constructed.
2. Procedure
a. To construct cross-section lines, lay the edge of a piece of paper along the
cross-section line selected and draw a straight line. Mark the location of
the monitoring wells along the edge of the paper. (The placement of some
wells may need to be projected because not all of the wells lie along a
straight line.)
A - A' MW9, MW2, and MW4 (in that order)
B - B' MW1, MW8, and MW7
C - C' MW11, MW3, MW7, and MW5
NOTE: Projection of wells to a cross-section line could cause distortions
that might affect interpretation of the distribution of subsurface geology or
soil.
b. Using the graph paper provided, transfer these well locations to the bottom
of the page along the horizontal axis.
c. The vertical axis will represent elevation in feet. Mark off the elevations
in 10-ft increments. Each division of the graph will represent an elevation
increase of 2 ft.
d. Graph the ground surface elevation for each of the chosen monitoring
wells. (This information is found in the monitoring well data, Table 2.)
e. Graph the groundwater elevations for these same locations.
f. Repeat this procedure for the other cross-sections lines.
g. Compare the topographic profile to the water table-profile. Are they
identical? After looking at these data, are there any conclusions that can
be drawn?
Hydrogeological Exercises 18 8/95
-------
PART 4.
A. Student Performance Objectives
1. Perform a falling head test on geologic materials.
2. Calculate total porosity, effective porosity, and estimated hydraulic conductivity.
3. Given groundwater elevations in monitoring wells, determine the equipotential
ground water surface.
4. Given groundwater's equipotential surface, determine groundwater flow direction.
B. Perform a Falling Head Test
1. Set up burets using the stands and tube clamps.
2. Clamp the rubber tube at the bottom of the burets using the hose clamp. Fold the
rubber hose to ensure a good seal before clamping (to help eliminate leaking
water).
3. Position the small, round screen pieces in the bottom of the burets. Use the
tamper to properly position the screens.
4. Measure 250 ml of clean water in the 500-ml plastic beaker.
5. Pour the water slowly into the buret to avoid disturbing the seated screen.
6. Measure 500 ml of gravel or sand material in the 5.00-ml plastic beaker.
7. Pour the gravel or sand material slowly into the water column in the buret to
prevent the disturbance of the screen traps and to allow any trapped air to flow to
the surface of the water in the buret.
8. Add additional measured quantities of water or gravel/sand as needed until both
the water and sediment reach the zero mark on the buret. To calculate the final
total volumes of water and sediment, add the volumes of additional water and
gravel/sand to the initial volumes of 250 and 500 ml of water and sediment. The
total volumes of water and sediment are designated W and S respectively.
9. Measure the static water level in the buret to the base of the buret stand. This is
the total head of the column of water at this elevation. This measurement is
designated h0.
10. Place a plastic, 500-ml graduated beaker below the buret. (The beaker will be
used to collect the water drained from the buret.) The volume of water in the
beaker is designated WD.
8/95 19 Hydro geological Exercises
-------
11. Undo the clamp and simultaneously start the timer to determine the flowrate of
water through the buret. When the drained water front reaches the screen, stop the
timer, clamp the buret hose, and record the elapsed time. Also record the volume
of water drained during this time interval. This time is designated t.
12. Allow the water level in the buret to stabilize. Measure the length from this level
to the base of the buret stand. This is the total head of the water column at this
elevation after drainage has occurred. This measurement is designated h,.
13. Subtract the measurement at h, from the height measurement at h,,. This length is
designated L.
14. The porosity in the sediment of each buret is the volume of water necessary to fill
the column of sediment in the buret to the initial static water mark at ho divided by
the sediment volume (S). This value is total porosity and is designated N.
IS. The effective porosity is estimated by dividing the volume of drained water by the
sediment volume. Effective porosity is designated n.
16. Compare the initial volume of water (W) in the column before draining with the
drained volume (WD). The difference represents the volume of water retained
(Wg), or the specific retention. The volume drained represents specific yield. To
determine the percent effective porosity, divide the volume of drained water by
the volume of total sediment volume.
17. The equation to estimate the hydraulic conductivity (K) of each buret column is
derived from falling head permeameter experiments. The equations for this
exercise are depicted at the bottom of Table 3.
Hydro geological Exercises 20 8/95
-------
TABLE 3. TOTAL HEAD WORKSHEET
Sample
Number
c
E
'S
C/3
|
ater
1
S
Q
0>
a
O)
Oi
o>
3
J2
u
*e3
CJ
^o
13
U
eft
s
ti
£
•s
c
4>
X
is
*2
I
1
*£
•o
>%
I
to
U
260
Total Porosity
N . ^
5
Effective Porosity
"p
S
. Hydraulic Cond.
n -
K =
2.3 * L
n
* In -^
8/95
21
Hydro geological Exercises
-------
PROBLEM 5
Aquifer Stress Tests
-------
AQUIFER STRESS TESTS
STUDENT PERFORMANCE OBJECTIVES
At the conclusion of this unit, students will be able to:
1. List the two factors that control aquifer response during an
aquifer test
2. List four aquifer test methods
3. List the purposes of the step-drawdown test
4. List the advantages and disadvantages of a slug test
5. List the advantages and disadvantages of a distance-
drawdown test
6. List the advantages and disadvantages of a time-drawdown
test
7. Given graph paper, graphically represent groundwater flow
to show the difference between aquifer tests in unconfined
and confined aquifers
8. Given aquifer test data, use the Jacob method to calculate a
hydraulic conductivity for the given conditions.
NOTE: Unless otherwise stated, the conditions for
performance are using all references and materials
provided in the course, and the standards of
performance are without error.
8/95
-------
AQUIFER TESTS
GROUNDWATER AND
CONTAMINANT MOVEMENT
• Position and thickness of aquifers and
aquitards
• Transmissivity and storage coefficient
• Hydraulic characteristics of aquitard
S-1
S-2
GROUNDWATER AND
CONTAMINANT MOVEMENT (cont.)
• Position and nature of boundaries
• Location and amounts of groundwater
withdrawals
• Locations, kinds, and amounts of pollutants
S-3
NOTES
8/95
Aquifer Stress Tests
-------
NOTES
AQUIFER RESPONSE DEPENDS ON:
• Rate of expansion of cone of depression
- Transmissivity of aquifer
- Storage coefficient of aquifer
• Distance to boundaries
- Recharge
- Impermeable
S-4
Limits of cone
of depressiorw/T
Water table
Cone of
depression ' 1
\
Row lines
AquJclude-Confining layer i
Unconfined Aquifer
S-S
Limits of c
of depres
one Land surface
s'°"/^ Potentiometi
'\- "
Drawdown N
' Aquiclude-Conlining layer
ic
Q
t
j
1
surface ,. >v
|x\
\\
1 ''XCone of
depression
4
Aquiclude-Confining layer .'•'•'...'.'.'.'.'.'.
Confined Aquifer
S-6
Aquifer Stress Tests
8/95
-------
NOTES
AQUIFER TEST METHODS
,^
Step-drawdown/well recovery tests
Slug tests
Distance-drawdown tests
Time-drawdown tests
S-7
STEP DRAWDOWN
Well Recovery Tests
Well is pumped at several successively higher
rates and drawdown is recorded
Purpose
- Estimate transmissivity
- Select optimum pump rate for aquifer tests
— Identify hydraulically connected wells
s-e
STEP DRAWDOWN
Well Recovery Tests (cont.)
Advantages
- Short time span
- One well
s-e
8/95
Aquifer Stress Tests
-------
NOTES
SLUG TESTS
Water level is abruptly raised or lowered
Used in low-yield aquifers (<0.01 cm/s)
S-10
SLUG TESTS
Advantages
• Can use small-diameter well
• No pumping - no discharge
• Inexpensive - less equipment required
• Estimates made in situ
• Interpretation/reporting time shortened
S-11
SLUG TESTS
Disadvantages
Very small volume of aquifer tested
Only apply to low conductivities (0.0000001 to
0.01 cm/s)
Transmissivity and conductivity only estimates
S-12
Aquifer Stress Tests
8/95
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SLUG TESTS
Disadvantages (cont.)
Not applicable to large-diameter wells
Large errors if well not properly developed
Do not give storativity
S-13
DISTANCE-DRAWDOWN TESTS
Advantages
Can also use time-drawdown
Results more accurate than single well test
Represent more of aquifer
Can locate boundary effects
S-14
DISTANCE-DRAWDOWN TESTS
Disadvantages
• Requires multiple piezometers or
monitoring wells (at least three wells)
• More expensive than single well test
• Must handle discharge water
• Requires conductivities >0.01 cm/s
S-1S
NOTES
8/95
Aquifer Stress Tests
-------
NOTES
TIME-DRAWDOWN TESTS
Advantages
Only one well required
Tests larger aquifer volume than slug test
Less expensive than multiple-well test
s-ie
TIME-DRAWDOWN TESTS
Disadvantages
Pump turbulence may interfere with
water-level measurements
Tests smaller aquifer volume than
multiple-well test
Must handle discharge water
Requires conductivities above 0.01 cm/s
S-17
THEIS METHOD
First formula for unsteady-state flow
- Time factor
- Storativity
Derived from analogy between
groundwater flow and heat flow
S-18
Aquifer Stress Tests
8/95
-------
THEIS METHOD (cont.)
• Laborious method
- Log-log paper
- Curve matching
• More accurate than Jacob method
S-19
THEIS'S ASSUMPTIONS
• Aquifer is confined
• Aquifer has infinite areal extent
• Aquifer is homogeneous and isotropic
• Piezometric surface is horizontal
S-20
THEIS'S ASSUMPTIONS (cont.)
• Carefully controlled constant pump rate
• Well penetrates aquifer entirely
• Flow to well is in unsteady state
S-21
NOTES
8/95
Aquifer Stress Tests
-------
NOTES
Potentiometric
'surface
Drawdown
I Confining layer-Aquiclude
Confined aquifer
Cone of
depression
Confining layer-Aquiclude
S-22
THEIS EQUATION
s =
QW(u)
4Ts
4Ttu
T = transmissivity
Q = discharge (pumping rate)
W(u) = well function of u
s = drawdown
S = storage coefficient
t = time
r = radial distance
S-23
WELL FUNCTION - W(u)
W(u) = -0.577216-logeu + u-;
and u =
S = storage coefficient
t = time
~4TT
r = distance
T = transmissivity
W(u) is an infinite exponential series and cannot
be solved directly
S-24
Aquifer Stress Tests
8/95
-------
NOTES
JACOB METHOD
• Somewhat more convenient than Theis's
method
- Semilogarithmic paper
- Straight line plot
- Eliminates need to solve well function
W(u)
- No curve matching
JACOB METHOD (cont.)
• Applicable to:
- Zone of steady-shape
- Entire zone if steady-state
JACOB'S FORMULA
T =
264 Q
As
J_
b
T = transmissivity gallons per day per ft (gpd/ft)
Q = pump rate (gpm)
As = change in drawdown (ft/log cycle)
K = hydraulic conductivity in gpd/ft2
b = aquifer thickness in feet
8/95
S-25
S-ZS
S-27
S
^ *
*v
•A
Aquifer Stress Tests
-------
NOTES
cr*"""
c
I]
Land surfacs
Rivsr
Cone of depression
(unsteady shape)
(1)
NONEQUILIBRIUM
NONEQUILIBRIUM
(3)
EQUILIBRIUM
River
S-28
Unsteady shape
Steady shape
S-20
S-30
Aquifer Stress Tests
10
8/95
-------
PERFORMING AN AQUIFER TEST
Jacob Time-drawdown Method
Each student will be given a sheet of four-cycle semilogarithmic graph paper. Then, follow these
directions:
1. Label the long horizontal logarithmic axis (the side with the punched holes) of the graph
paper t-time (minutes). Leave the first numbers (1 through 9) as is. Mark the next series
of heavy lines from 10 to 100 in increments of 10 (10, 20, 30, etc.). Mark the next series
from 100 to 1000 in increments of 100 (100, 200, 300, etc).
2. Label the short vertical arithmetic axis s-drawdown (feet). This will be the drawdown (s)
measured from the top of the casing (provided in Table 1). Mark off the heavy lines by
tens, starting with 0 at the top, then 10, 20, 30, 40, 50, 60, and 70 (the bottom line). Each
individual mark represents 1 foot.
3. Plot the data in Table 1 on the semilogarithmic paper with the values for drawdown on the
arithmetic scale and corresponding pumping times on the logarithmic scale.
4. Draw a best-fit straight line through the data points.
5. Compute the change in drawdown over one log cycle where the data plot as a straight line.
6. Using the information given in Table 1 (Q = 109 gpm and b = 30 feet) and Jacob's formula
(provided in the manual on slide 27), calculate the value for hydraulic conductivity.
8/95 . 11 Aquifer Stress Tests
-------
TABLE 1. PUMPING TEST DATA
Pumping Time (t)
(minutes)
Q = 109 gpm
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
18
20
22
24
26
28
32
35
40
45
50
55
60
90
120
Drawdown (s)
Measured from Top of Casing
(ft)
b = 20 ft
6.
6.
7.
8.
8.
9.
10
11
12
13
14
15
17
18
19
20
23
25
26
28
29
30
32
34
36
38
40
42
43
1
5
5
0
6
5
.5
.2
.0
.0
.0
.5
.0
.0
.3
.5
.5
.2
.7
.2
.5
.5
.0
.5
.6
.5
.5
.0
.5
50.1
54
.8
Aquifer Stress Tests
12
8/95
-------
PROBLEM 6
Groundwater Investigation Problem
-------
PROBLEM 6: GROUNDWATER INVESTIGATION (BETTENDORF, IOWA)
A. Student Performance Objectives
1. Determine the source(s) of hydrocarbon contamination at a contaminated site.
2. Perform a Phase 1 field investigation using soil gas surveys, soil borings, and
monitoring wells.
3. Present the results of the field investigation to the class.
4. Justify the conclusions of the field investigation.
B. Background Information
Task
Your environmental consulting firm has been retained by the attorney representing the
Leavings to:
• Determine the source of the hydrocarbon contamination. This is not an emergency
response action.
• Provide a brief report that includes the names of the source(s) of contamination, the
total cost of investigation, and a drawing of a representative cross section through the
contaminant plume.
• Justify the data that are obtained and the conclusions of the report.
Leavings Residence
On October 12, 1982, the Bettendorf, Iowa, fire department was called to the Leavings
residence with complaints of gasoline vapors in the basement of the home.
On October 16, 1982, the Leavings were required to evacuate their home for an indefinite
period of time until the residence could be made safe for habitation. The gasoline vapors
were very strong, so electrical service to the home was turned off. Basement windows were
opened to reduce the explosion potential.
08/95 . 1 Groundwoter Investigation
-------
Pertinent Known Facts
The contaminated site is in a residential neighborhood in Bettendorf, Iowa. It backs on
commercially zoned property, which has only been partially developed to date. The
residential area is about 10 years old and contains homes in the $40,000 to $70,000 range.
There was apparently some cutting and filling activity at the time the area was developed.
Within 1/4 mile to the northwest and southwest, 11 reported underground storage tanks
(USTs) are in use or have only recently been abandoned:
• Two tanks owned and operated by the Iowa Department of Transportation (IDOT)
are located 1000 ft northwest of the site.
• Three in-place tanks initially owned by Continental Oil, and now by U-Haul, are
located 700 ft southwest of the site. According to the Bettendorf Fire Department
(BFD), on of the three tanks reportedly leaked.
• Three tanks owned and operated by an Amoco service station are located 1200 ft
southwest of the site. BFD reports no leaks.
• Three tanks owned and operated by a Mobil Oil service station are located 1200 ft
southwest of the site. BFD reports no leaks.
Neighbors that own lots 8 and 10, which adjoin the Leavings residence (Lot 9), have
complained about several trees dying at the back of their property. No previous occurences
of gasoline vapors have been reported at these locations.
The general geologic setting is Wisconsin loess soils mantling Kansan and Nebraskan glacial
till. Valleys may expose the till surface on the side slope. Valley soils typically consist of
the colluvial and alluvial silts.
Previous experience by your environmental consulting firm in this area includes a
geotechnical investigation of the hotel complex located west of Utica Ridge Road and
northwest of the Amoco service station. Loess soils ranged from 22 ft thick on the higher
elevations of the property (western half) to 10 ft thick on the side slope. Some silt fill (5-7
ft) was noted at the east end of the hotel property. Loess soils were underlain by a gray,
lean clay glacial till which apparently had groundwater perched on it. Groundwater was
typically within 10-15 ft of ground surface. This investigation was performed 8 years ago
and nothing in the boring logs indicated the observation of hydrocarbon vapors. However,
this type of observation was not routinely reported at that time.
Other projects in the area included a maintenance yard pavement design and construction
phase testing project at the IDOT facility located northwest of the Leavings residence. Loess
soils were also encountered in the shallow pavement subgrade project completed 3 years ago.
Consulting firm records indicated that the facility manager reported a minor gasoline spill a
year before and that the spill had been cleaned up when the leaking tank was removed and
replaced with a new steel tank. The second tank at the IDOT facility apparently was not
replaced at that time.
Groundwater Investigation 2 8/95
-------
Budget
The budget for implementing the field investigation is $25,000.
Interviews
• Lot 9 (the Leavings residence): Observations outside the residence indicate that the
trees are in relatively good condition. The house was vacant. Six inches of free
product that looks and smells like gasoline was observed in the open sump pit in the
basement. The power to the residence was turned off, so the water level in the sump
was allowed to rise. The fluid level in the sump was about 3 feet below the level of
the basement floor.
• Neighbors (Lots 8 and 10): These property owners reported that several trees in
their back yards died during the past spring. They contacted the developer of the
area (who also owns the commercial property that adjoins their lots) and complained
that the fill that was placed there several years ago killed some of their trees. No
action was taken by the developer. Both neighbors said that when the source of the
gas was located, they wanted to be notified so they could file their own lawsuits.
The neighbors also noted that this past September and October were unusually wet
(lots of rainfall).
• IDOT: The manager remembers employess of your firm testing his parking pad.
He reported that one UST was replaced in 1979, whereas the other tank was installed
when the facility was built in 1967. Both of the original tanks were bare metal tanks.
The older tank has always contained gasoline, but the newer one contains diesel fuel.
No inventory records or leak testing records are available. The manager stated that
he has never had any water in his tanks. He will check with his supervisor to have
the USTs precision leak tested.
• U-Haul: The manager said that the station used to be a Continental Oil station with
three USTs. The three USTs were installed by Continental in 1970 when the station
was built. Currently, only one 6000-gal UST (unleaded) remains in service for the
U-Haul fleet, this tank was found to be leaking a month ago, but the manager does
not know how much fuel spilled.
• Mobil: The manager was pleasant until he found out the purpose of the interview.
He did state that he built the station in 1970 and installed three USTs at that time.
He would not answer any additional questions.
• Amoco: The manager was not in, but an assistant provided his telephone number.
In a telephone interview, the manager said he was aware of the leaking tank at the
U-Haul facility and was anxious to prove the product was not from his station. He
said they installed three USTs for unleaded, premium, and regular gasoline in 1972.
An additional diesel UST was installed in 1978. The tanks are tested every 2 years
using the Petrotite test method. The tanks have always tested tight. No inventory
08/95 3 Groundwater Investigation
-------
control system is being used at present. He stated that if monitoring wells were
needed on his property, he would be happy to cooperate.
Developer (Mr. M. Forester): Mr. Forester bought the property in question in the
1960s. He developed the residential area first and some of the commercial
development followed. About 40 acres remain undeveloped to date. He plans to
build a shopping center on the remaining 40 acres in the future.
Mr. Forester obtained a lot of cheap dirt and fill when the interstate cut went through
about 1/2 mile west of the property in the late 1960s. He filled in a couple of good-
sized valleys at that time. He has a topographic map of the area after it was filled.
He stated that he will cooperate fully with any investigation. If any wells are needed
on the property, he would like to be notified in advance. There are no buried utilities
on the property except behind the residential neighborhood.
Review of Bettendorf City Hall Records
An existing topographic map and scaled land use map are available.
Ownership records indicate the land was previously owned by Mr. and Mrs. Ralph Luckless.
The city hall clerk stated that she had known them prior to the sale of the farm in 1964.
Zoning at that time was agricultural only. The section of the farm now in question was
primarily used for grazing cattle because it was too steep for crops. The clerk remembered
a couple of wooded valleys in that same field. She also remembered a muddy stream that
used to run where Golden Valley Drive is now and that children used to swim in it. She also
stated that one valley was between Golden Valley Drive and where all the fill is now (near
U-Haul and Amoco).
The current owner of the undeveloped property is Mr. M. Forester, a developer with an
Iowa City, Iowa address.
There is no record of storm or sanitary sewer lines along Utica Ridge Road south of Golden
Valley Drive. Storm and sanitary sewer lines run along Spruce Hills Drive.
Iowa Geological Survey
There are no records of any wells in the section.
Adjoining section wells indicate top of bedrock at about 650 feet mean sea level (MSL). The
uppermost usable aquifer is the Mississippian for elevations from 350 feet to 570 feet MSL.
The materials overlying the Mississippian are Pennsylvanian shales and limestone.
Groundwater Investigation 4 8/95
-------
Soil Conservation Survey maps
The 1974 edition indicates "Made Land" over nearly all of the area not designated as
commercial zone. Made Land normally indicates areas of cut or fill.
08/95 5 Groundwoter Investigation
-------
ASSIGNMENT: PHASE 1 FIELD INVESTIGATION
TABULATION OF FEES FOR PHASE 1 FIELD INVESTIGATION GROUP
WORK SHEET #1 || # UNITS
Recommendation for making residence
habitable
Soil gas survey - mobilization fee
Soil gas survey
Soil boring - mobilization fee
Soil boring with photo ionization detector -
25 feet deep max - grouted shut
Convert soil boring to PVC monitoring well
(additional cost for each conversion)
Convert soil boring to stainless steel
monitoring well (additional cost for each
conversion)
Monitoring wells - mobilization fee
2" PVC
1 5 ft screen - 25 ft deep
2" stainless steel
1 5 ft screen - 25 ft deep
Well security - locking protector pipe
Field investigation engineering analysis and
report
1 ea
1 ea
1 ea
1 ea
1 ea
COST
$500 LS
(lump sum)
$500 LS
$1500/ac
$500 LS
$500 ea
$800 ea
conversion
$1300ea
conversion
$500 LS
$1200ea
$1700ea
$300 ea
15%
$2000 min
TOTAL COST:
TOTAL
$
$
$
$
$
$
$
$
$
$
$
$
Groundwater Investigation
8/95
-------
GROUP
MONITORING WELLS
WELL NUMBER || 1
GRID LOCATION
FILL
LOESS
ALLUVIUM
TILL
NON DETECTED
DISSOLVED PRODUCT
FREE PRODUCT
WATER ELEVATION
2
3
4
5
6
7
8
9
10
SOIL BORINGS
SOIL BORING
GRID LOCATION
FILL
LOESS
ALLUVIUM
TILL
HIT { + )
MISS (-)
A
B
C
D
E
F
G
H
I
J
A
2o
S
08/95
Groundwater Investigation
-------
5
o
8
o
o
o
o
o
o
o
a
5
§
o
o
o
o
CD
o
s
o
o
o
§
I K 1 L
600 N
nzj
GROUP
+
Soil gas survey
hit" " miss "—"
<3?L
Monitoring well
fill *
loess 0^°^
till ^
alluvium
nondetection
free product
gw elevation
Soil borings
fill
loess
till
alluvium
hit "+"
miss "-"
1
500 N
400 N
300 N
200 N
100 N
100 S
200 S
300 S
400 S
11
500 S
600 S
700 S
800 S
900 S
1000 S
Groundwater Investigation
8/95
-------
' I J
GROUP 3
L
en
en
11
" CD
05/P5
Groundwater Investigation
-------
8 K g L
CNJ i- O
Iowa DOT
Maintenance
Facility
Groundwater Investigation
10
8/P5
-------
600 N
O I O
o ^ o > o
co c\j ••-
Predevelopment
Topographic" Map
1000 S
08/95
11
Groundwater Investigation
-------
Existing Topographic Map
Groundwater Investigation
12
8/95
-------
Section 13
-------
APPENDIX A
Checklist for a Hydrogeological Investigation
-------
CHECKLIST FOR A HYDROGEOLOGICAL INVESTIGATION
HAZARDOUS WASTE SITES INFORMATION LIST
When evaluating activities at sites where hazardous wastes may be causing or contributing to
groundwater contamination, it is important to gather as much information as possible. The
development of as much site information as possible can often provide valuable insight about site
history, waste disposal practices, regional and local geology, and the potential for impacts to the
environment in the site vicinity.
To make your information-gathering efforts easier, the following checklist includes some of the types
of questions that could helpful to ask during a site investigation. Although these questions are
oriented more toward field activities, they may also prove to be helpful to those people responsible
for evaluating the adequacy of other site assessment documents.
Sources
National Water Well Association. 1991. Groundwater and Unsaturated Zone
Monitoring and Sampling. 45 pp. In: Practical Handbook of Groundwater
Monitoring.
U.S. EPA. 1986. RCRA Ground Water Monitoring Technical Enforcement
Guidance Document. 208 pp.
Stropes, D.F. 1987. Unpublished Research: Technical Review of Hazardous
Wastes Disposal Sites. 25 pp.
I. SITE/FACILITY HISTORY
A. Waste disposal history of the site.
1. Is this a material spill or other emergency response activity not at a Toxic
Substances Storage and Disposal Facility (TSSDF)?
2. What hazardous wastes are being manufactured, stored, treated, or disposed
of at the site?
3. For active manufacturing operations, what industrial processes are being used
and what raw materials are used in the industrial processes?
4. Are the raw materials altered or transformed in any way during industrial
processes to result in waste materials that are different from the raw
materials?
5. How long has the facility been in operation?
8/95 1 The Hydro geological Investigation
-------
6. Have the types of hazardous wastes manufactured, stored, treated, or
disposed of at the site changed during the history of the site?
7. Have the industrial processes used at the site changed over the history of the
site?
8. If the industrial processes are different, what previous industrial processes
were used in the past, how long were they used, and what types of wastes
were end products of the processes?
9. What environmental media (i.e., air, land, or water) have been or are being
affected by the facility/site activities?
10. What is the form of the site wastes (e.g., sludge, slurry, liquid, powder,
containerized, bulk storage)?
11. How much waste is generated or disposed of at the location daily?
12. What is the history of aboveground and underground storage tank use at the
site?
13. What types of regulated manufacturing or pollution control units exist at the
facility?
14. What governmental agencies are responsible for the regulated units?
15. Do any historical records about the site exist? If so, where are these records?
16. Has a check of any existing historical maps or aerial photos been performed
to provide further insight about past site activities?
17. Is there any history of groundwater contamination as a result of the site
activities?
B. Details of the site disposal activities.
1. Are the site disposal units currently in compliance with all applicable rules,
regulations, and standards?
2. Are disposal areas isolated from the subsurface by the use of liners,
impermeable material, etc.?
3. What type of isolating material is in use?
4. Are multiple isolation systems in use?
5. Is a leachate/contaminant collection system in use?
The Hydro geological Investigation 2 8/95
-------
6. Are any monitoring wells installed adjacent to the disposal/collection system
units?
7. Is there a surface water runoff control system?
8. Are any parts of the site/facility capped with an impermeable cover material?
9. What is the condition of the cap?
10. Are areas of previous waste disposal well defined?
C. What is the nature of groundwater usage from aquifers beneath the site or in adjacent
areas?
1. Do any water supply wells exist in the aquifers beneath the site and adjacent
areas?
2. Are water supply wells used for potable water supplies or for industrial
process water?
3. Is the groundwater treated prior to use?
4. What are the pump rates of the water supply wells? Daily? Monthly?
Annually?
5. What are the depths of the wells' screened intervals?
6. What other well drilling, well construction, or well completion information
is available?
7. Do subsurface geologic well logs exist for the wells?
8. Are the wells upgradient, at, or downgradient of the site/facility?
9. Does pumping from these wells modify the regional groundwater table or
potentiometric surface?
II. HYDROGEOLOG1C CHARACTERIZATION
A. Has the purpose of the hydrogeologic investigation been clearly and adequately
defined?
1. Characterize the hydrogeoiogic system at the site.
2. Determine whether there has been downgradient degradation of water quality
from a potential source of contamination.
8/95 3 The Hydro geological Investigation
-------
3. Determine the upgradient source of contamination at a known downgradient
contamination receptor (well, spring, or surface water body).
B. Has the site location and all major site features been shown on a map?
1. Has the site been located on a state map?
2. Has the site been located on a USGS 7-1/2 minute topographic quadrangle
map published at a scale of 1:25,000?
3. Have coordinates for further site identification (latitude, longitude, degrees,
minutes, seconds, or a site-specific grid system) been provided?
C. Has a base map of the site been prepared?
1. What is the map source?
2. Are aerial photos available?
3. Are all components of a map (north arrow, scale, map legend) shown and
defined on the base map?
4. Does the map show elevations and contours?
5. Is the scale of the map adequate to delineate dimensions of onsite features
adequately?
6. Does the map show features adjacent to the site that may be pertinent to the
hydrogeologic investigation?
7. Are all natural physical features (e.g., topography, surface waters, surface
water flow divides) shown on the map?
D. Has the subsurface geology been identified?
1. Is the geologic interpretation based on soil borings and well drilling logs?
2. Have any other reference materials been used?
3. Are aquifers present beneath the site?
4. Is the first aquifer encountered confined or unconfined?
5. Are all aquifers and confining units continuous across the site?
6. Have all geologic strata been described (e.g., thickness, rock type,
unconsolidated/consolidated materials, depth)?
The Hydrogeological Investigation 4 8/95
-------
7. Do multiple aquifers exist at the site?
8. Have any porous vs. fractured flow media been described?
E. Do the driller's logs of the deepest borings at each well cluster show that soil
material samples were collected at 5-ft intervals? If not, at what intervals were
samples taken?
1. Is there a stratigraphic log of the deepest boreholes?
2. Were the borings extended to a depth below any confining beds beneath the
shallowest aquifer?
3. Have enough borings of the area been done to adequately define the
continuity and thickness of any confining beds?
4. Have ail logs been prepared by a qualified geologist, soil scientist, or
engineer using a standardized classification system?
5. Were any laboratory tests conducted on the soil and soil material samples?
What types of tests were performed?
6. Were grain size distributions used to determine the size of the gravel pack or
was sand filter placed in the annular space opposite the well screen?
F. Have field and/or laboratory permeability tests been performed to identify variations
in aquifer and confining bed properties?
1. What type(s) of tests were performed?
2. What was the range of hydraulic conductivity values found in the aquifer?
What was the arithmetic mean value?
3. What was the range of hydraulic conductivity values found in the confining
bed? What was the arithmetic mean value?
4. Where are the most permeable subsurface zones located relative to the waste
disposal facility?
5. Have geologic cross sections been constructed?
G. Have field and/or laboratory tests been performed to determine the specific yield,
storativity, or effective porosity of the aquifer?
1. What type(s) of tests were performed?
2. What is the range of specific yield, storativity, or effective porosity values?
8/95 5 The Hydrogeological Investigation
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3. What are the average values?
H. Has the horizontal groundwater flow direction been determined?
1. Have a minimum of three piezometers been installed to determine the
direction of flow in the aquifer?
2. Do any water-level readings show local variations of the water table caused
by mounds or sinks?
3. Do any identified mounds or sinks result in alterations of the regional or local
horizontal groundwater direction of flow?
4. Do any surface features which may have an effect on the horizontal flow
exist?
5. Have all piezometer installations in the uppermost aquifer been screened at
approximately the same depth below the water table?
6. Do any discernible seasonal variations in water levels exist?
7. Do any short-term variations in water levels exist? If so, what possible
causes may explain these variations?
I. Has the magnitude of the horizontal hydraulic gradient been determined at various
locations across the site?
1. What is the average horizontal hydraulic gradient at the site?
2. Where is the horizontal hydraulic gradient the steepest?
3. Does this location correlate to a known area of lower hydraulic conductivity
in the aquifer?
4. Does this location correlate to a known area of lower aquifer thickness?
5. Where is the horizontal hydraulic gradient the lowest (flat)?
6. Does this location correlate to a known area of greater hydraulic conductivity
in the aquifer?
7. Does this location correlate to a known area of greater aquifer thickness?
J. If multiple aquifers exist, have wells been installed in each aquifer to determine the
vertical component of groundwater flow?
1. Have the wells in each aquifer been installed in a single borehole or in
separate boreholes?
The Hydrogeological Investigation 6 8/95
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2. If a single borehole was used, what tests were conducted to ensure that no
leakage between the upper and lower aquifers exist?
3. If a single borehole was used, what well installation, construction, and
development techniques were used to ensure that no leakage between the
upper and lower aquifers exist?
4. Based on the difference in hydraulic head between upper and lower aquifers,
can the site be described as:
a. Predominantly a recharge area?
b. Predominantly a discharge area?
c. Predominantly an area of horizontal flow?
5. If recharge, discharge, or horizontal flow areas exist, have these locations
been shown on a hydrogeplogic map (including supporting cross sections) of
the site?
K. Has the magnitude of the vertical hydraulic gradients been determined at various
locations across the site?
1. What is the average vertical hydraulic gradient at the site?
2. Where is the vertical hydraulic gradient the steepest?
3. Can this location be correlated to any known areas of lower hydraulic
conductivity?
4. Where is the vertical hydraulic gradient the flattest?
5. Based on the vertical hydraulic gradient, what relationship exists between the
shallow and deeper aquifers?
6. Is there any regional or offsite vertical hydraulic gradient information that
may support or conflict with the site's vertical hydraulic gradient data?
L. Determination of seepage velocities and travel times.
1. What is the average seepage velocity of water moving from the waste facility
to the downgradient site boundary?
2. What is the average travel time of water to move from the waste facility to
the nearest downgradient monitoring wells?
3. What is the basis for the seepage velocity and travel time determinations?
8/95 7 The Hydro geological Investigation
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M. Have potentiometric maps, flow nets, geologic maps, and cross sections been
prepared to show the direction of groundwater flow at the site?
Horizontal Flow Components (plan view)
1. Do the contours and contour intervals between the equipotential lines
adequately describe the flow regime?
Suggested Contour Intervals:
a. 0.1-0.5 ft if the horizontal flow component is relatively flat.
b. 0.5-1.0 ft if the horizontal flow component is moderately steep.
c. 1.0-5.0 ft if the horizontal flow component is extremely steep.
2. Have the equipotential lines been accurately drawn:
a. With respect to the elevations of water levels in the wells or
piezometers?
b. With respect to nearby or onsite rivers, lakes, wells, or other
boundary conditions?
c. With respect to other naturally occurring or man-made physical
features that might cause groundwater mounds or sinks in the area?
3. Do variations in the spacing of equipotential lines correspond to known areas
with relative transmissivity variations?
4. Do the constructed groundwater-flow lines cross equipotential lines at right
angles?
5. Can conclusions about the aquifer(s) relative homogeneity and isotropy be
made based on variations in the flow lines?
Vertical Flow Components (cross sections)
1. Is the transect for the vertical flow component cross section(s) laid out along
the line of a groundwater flow path as seen in the plan view?
2. Is the variation in land-surface topography accurately represented on the cross
section(s)?
3. Have both vertical and horizontal scales been provided?
4. What differences exist between the vertical and horizontal scales?
The Hydrogeological Investigation g 8/95
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5. Are all monitoring wells, piezometers, and screened intervals accurately
shown?
N. What is the site water quality and geochemistry?
1. What are the upgradient groundwater quality conditions?
2. What are the downgradient groundwater quality conditions?
3. What water-quality parameters have been determined downhole?
4. What water-quality parameters have been determined at the well head?
5. Have all appropriate field water-quality determinations, equipment selections,
and procedures been followed?
6. Does an adequate QA/QC procedure exist?
7. What if any, relationship exists between the site water-quality conditions and
the past and/or present activities at the site?
III. DETECTION MONITORING SYSTEM
A. Are the facility upgradient and downgradient monitoring wells properly located to
detect any water-quality degradation from the waste source(s)?
Horizontal Flow
1. Will groundwater from the upgradient well locations flow through or under
the waste source in an unconfined aquifer?
2. Will groundwater from the upgradient well locations flow beneath the waste
source and under an overlying confining bed in a confined aquifer?
3. Will groundwater from the upgradient well locations flow beneath the waste
source in an unconfined aquifer separated from the waste source by an
impervious liner?
4. Will groundwater from the waste source area flow toward downgradient
wells?
Vertical Flow
1. Are the monitoring wells correctly screened to intercept a possible
contaminant plume from the waste source based on an accurate interpretation
of the vertical flow regime (recharge area, discharge area, or area of
horizontal flow)?
8/95 9 The Hydro geological Investigation
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B. Are the monitoring wells located adequately to provide sufficient groundwater flow
information?
1. Are regional water levels unaffected by local groundwater mounds or sinks?
2. Are additional monitoring wells located to provide water-level information
from local groundwater mounds or sinks?
3. Do upgradient and downgradient monitoring wells provide representative
samples?
C. Monitoring Well Construction
1. Were precautions taken during the drilling of the borehole and installation of
the well to prevent introduction of contaminants into the well?
2. Is the well casing and screen material inert to the probable major
contaminants of interest?
3. What type of well casing and screen material was used?
4. Does the casing and screen material manufacturer have any available
information about possible leaching of contaminants from the casing and
screen material?
5. How are the well casing and screen segments connected?
6. If cement or glue has been used, what is the potential for contaminants to
leach into the groundwater?
7. Were all downhole well components steam cleaned prior to installation?
8. If another cleaning technique was used, what materials were used?
D. Are there as-built drawings or details of each monitoring well nest or cluster showing
information such as depth of well, screen intervals, type and size of screen, length
of screen and riser, filter packs, seals, and protective casings?
1. Do the figures show design details of as-built wells as opposed to details of
proposed wells?
2. Are the well depth(s) and diameter(s) shown?
3. Are the screened intervals and type and size of screen openings shown?
4. Is the length of the screen shown?
5. Is the length of the riser pipe and stick-up above the land surface shown?
The Hydro geological Investigation 10 8/95
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6. Is the filter pack around the screened interval shown?
7. Does the filter pack extend at least 1 ft above and below the screened
intervals?
8. What type of sealant was placed in the annulus above the filter pack?
9. What is the thickness of the seal?
10. How was the seal put in place?
11. Will a protective casing or reinforced posts be necessary to protect the
monitoring well from damage?
12. Is any manufacturer's information available to verify that all materials used
in the well construction do not represent potential sources of water
contamination?
13. Have samples of well construction materials been kept for future analysis to
verify that the materials do not represent sources of water contamination?
E. Are the screened intervals appropriate to the geologic setting and the sampling of a
potential problem?
1. Is the screen set opposite a stratigraphic layer with relatively high hydraulic
conductivity?
2. Is the screened interval set sufficiently below the water table so that water-
level measurements can be taken and water samples can be collected during
periods of low water level?
3. Is the screened interval placed in the aquifer(s) of concern?
4. If a single long screen was installed over the entire saturated thickness of the
aquifer, what effect will this have on analytical data from this monitoring
well?
5. Has the entire aquifer thickness been penetrated and screened?
6. Have piezometers been installed to determine vertical and horizontal flow
directions?
7. Is the base of the waste disposal unit above the seasonal high water table?
8. What is the thickness of the unsaturated zone between the base of the waste
disposal unit and the seasonal high water table?
8/95 11 The Hydro geological Investigation
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F. Has a professional survey been conducted to determine the elevation and location of
the measuring point at each well with reference to a common datum?
1. Is the survey at each monitoring well accurate to ±0.01 ft?
2. Is each surveyed measuring point located at the top of the well casing?
3. What benchmark was used as a starting point for the survey?
4. Are the elevations of the measuring point at each well referenced to mean sea
level and not to some local datum?
G. Has an adequate sampling and analysis program been written for the site?
1. Are the major contaminants inorganic compounds?
2. Will field filtering and preservation be done in the field?
3. Are the major contaminants organic coumpounds?
4. Where would you expect to find the contaminants in the aquifer?
a. Floating at the top of the aquifer (LNAPLs)?
b. Dissolved in the groundwater and flowing with it?
c. Concentrated at the bottom of the aquifer (DNAPLs)?
5. What are the possible degradation end products of the original organic
contaminants?
6. Is the sampling method adequate to prevent any loss of volatile constituents?
7. Are field measurements such as pH, Eh, specific conductance, dissolved
oxygen, and temperature taken in the field?
8. Does a generic sampling and analysis protocol exist?
9. Does the sampling and analysis protocol address sample preservation, storage,
transport, container identification, and chain-of-custody procedures?
10. Is the analytical laboratory certified by EPA for the analyses to be
performed?
11. Did the laboratory provide input to the sampling and analysis program?
12. Is the sampling and analysis program written, clear, concise, understandable
and site specific?
The Hydro geological Investigation \2 8/95
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H. Has a QA/QC plan been written for the groundwater monitoring program?
Water-Level Measurements
1. Have worksheets containing relevant fixed data, some of which are indicated
below, been prepared for use by the person taking water-level readings?
a. Well identification number?
b. Location of measuring point of each well?
c. Elevation of measuring point at each well relative to mean sea level?
d. Elevations of screened interval at each well?
e. Type of measuring instrument to be used?
2. Do the worksheets for use by the person taking water-level readings have
columns for computation of:
a. Depth to the water table?
b. Measuring point data to be added to or subtracted from readings of
measuring instrument?
c. Adjusted depth to water surface?
d. Conversion of depth to water surface?
3. Do the worksheets have a space for pertinent comments?
Sample Collection
1. Are well purging procedures prior to sampling described as written
procedures?
2. Is the method of purging specified?
3. Is the sample collection technique specified?
4. Is the sample storage vessel described?
5. Is the sample volume specified?
6. Is the sample identification system described?
7. Are there provisions for:
8/95 13 The Hydro geological Investigation
-------
a. Trip blanks?
b. Spiked samples?
c. Duplicate, replicate, or blind samples?
8. Is the sampling frequency specified?
Sample Analysis
1. Does the laboratory have a QA/QC program for all samples?
2. Is the QA/QC program written?
3. Does the laboratory provide information on the accuracy and precision of the
analytical results?
4. Did the laboratory participate in the development of the sampling and analysis
plan for the groundwater monitoring program and the QA/QC plan for the
nonlaboratory portion of the sampling program?
I. Is the QA/QC plan being followed during implementation of the sampling and
analysis program?
1. Is the same consultant who prepared the QA/QC plan responsible for its
implementation?
2. How many copies are there of the QA/QC plan?
3. Who has copies and where are they located?
4. Does the field person taking water-level measurements and collecting samples
have a copy?
5. Does the field person understand the importance of following the QA/QC
plan explicitly each time?
6. What safeguards and checks are there to ensure there will be no deviation
from the QA/QC plan in the field and the laboratory?
J. If any more field work or data will be necessary to meet the objectives of the
hydrogeologic investigation, what types of additional field installations and data will
be needed?
The Hydro geological Investigation 14 8/95
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APPENDIX B
Sampling Protocols
-------
GENERALIZED GROUNDWATER SAMPLING PROTOCOL
Step
Goal
Recommendations
Hydrologic measurements Establish nonpumping water level
Well purging
Sample collection
Filtration/preservation
Field determinations
Field blanks/standards
Sample storage,
transportation, and chain
of custody (COC)
Remove or isolate stagnant H20,
which would otherwise bias
representative sample
Collect samples at land surface or
in well bore with minimal
disturbance of sample chemistry
Filtration permits determination of
soluble constituents and is a form
of preservation; it should be done
in the field as soon as possible
after sample collection
Field analyses of samples will
effectively avoid bias in
determining
parameters/constituents that do
not store well (e.g., gases,
alkalinity, and pH)
These blanks and standards will
permit the correction of analytical
results for changes that may
occur after sample collection.
Preserve, store, and transport
with other samples.
Refrigerate and protect samples to
minimize their chemical alteration
prior to analysis. Document
movement of samples from
collector to laboratory.
Measure the water level to
±0.3 cm (±0.01 ft)
Pump water until well purging
parameters (e.g., pH, T, Q'1,
Eh) stabilize to ± 10% over at
least two successive well
volumes pumped
Pumping rates should be
limited to ~ 100 mL/min for
volatile organics and gas-
sensitive parameters
For trace metals, inorganic
anions/cations, and alkalinity.
Do not filter TOC, TOX, or
other volatile organic
compound samples; filter other
organic compound samples
only when required
Samples for determining gases,
alkalinity, and pH should be
analyzed in the field if at all
possible
At least one blank and one
standard for each sensitive
parameter should be made up
in the field on each day of
sampling. Spiked samples are
also recommended for good
QA/QC.
Observe maximum sample
holding or storage periods
recommended by EPA.
Documentation of actual
holding periods should be
carefully performed. Establish
COC forms, which must
accompany all samples during
shipment.
Adapted from: U.S. EPA. 1985. Practical Guide for Ground-Water Sampling. EPA/600/2-85/104.
Robert S. Kerr Environmental Research Laboratory, Ada, OK.
8/95
Sampling Protocols
-------
APPENDIX C
References
-------
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U.S. EPA. 1988. Selection Criteria for Mathematical Models Used in Exposure Assessments:
Ground-Water Models. EPA/600/8-88/075. U.S. Environmental Protection Agency.
U.S. EPA. 1988. Superfund Exposure Assessment Manual. EPA/540/1-88/001. U.S.
Environmental Protection Agency.
U.S. EPA. 1988. Technology Screening Guide for Treatment of CERCLA Soils and Sludges.
EPA/540/2-88/004. U.S. Environmental Protection Agency.
U.S. EPA. 1989. Ground-Water Monitoring in Karst Terranes: Recommended Protocols &
Implicit Assumptions. EPA/600/X-89/050. U.S. Environmental Protection Agency.
U.S. EPA. 1990. Basics of Pump-and-Treat Ground-Water Remediation Technology.
EPA/600/8-90/003. U.S. Environmental Protection Agency.
U.S. EPA. 1990. Catalog of Superfund Program Publications. EPA/540/8-90/015. U.S.
Environmental Protection Agency.
U.S. EPA. 1990. Handbook: Ground Water Volume I: Ground Water and Contamination.
EPA/625/6-90/016a. U.S. Environmental Protection Agency.
U.S. EPA. 1990. Quality Assurance Project Plan. U.S. Environmental Protection Agency,
Emergency Response Branch, Region VIII.
U.S. EPA. 1990. Subsurface Contamination Reference Guide. EPA/540/2-90/001. U.S.
Environmental Protection Agency.
U.S. EPA. 1991. Compendium of ERT Ground Water Sampling Procedures. EPA/540/P-91/007.
U.S. Environmental Protection Agency.
U.S. EPA. 1991. Compendium of ERT Soil Sampling and Surface Geophysics Procedures.
EPA/540/P-91/006. U.S. Environmental Protection Agency.
References 6 8/95
-------
U.S. EPA. 1991. Ground-Water Monitoring (Chapter 11 of SW-846). Final Draft. U.S.
Environmental Protection Agency, Office of Solid Waste.
U.S. EPA. 1991. Handbook Ground Water Volume II: Methodology. EPA/625/6-90/016b. U.S.
Environmental Protection Agency.
U.S. EPA. 1993. Subsurface Characterization and Monitoring Techniques: A Desk Reference
Guide. Volume I: Solids and Ground Water, Appendices A and B. EPA/625/R-93/003a. U.S.
Environmental Protection Agency, Office of Research and Development, Washington, DC.
U.S. EPA. 1993. Subsurface Characterization and Monitoring Techniques: A Desk Reference
Guide. Volume II: The Vadose Zone, Field Screening and Analytical Methods, Appendices C and
D. EPA/625/R-93/003b. U.S. Environmental Protection Agency, Office of Research and
Development, Washington DC.
Van Der Leeden, F., F.L. Troise, and D.K. Todd. 1990. The Water Encyclopedia. Second
Edition. Lewis Publishers, Inc., Chelsea, MI.
Practical Applications of Ground Water Models. National Conference August 19-20,1985. National
Water Well Association, Dublin, OH.
Verruijt, A. 1970. Theory of Groundwater Flow. Gordon & Breach Sciences Publishing, Inc.,
New York, NY.
Walton, W.C. 1962. Selected Analytical Methods for Well and Aquifer Evaluation. Bulletin 49,
Illinois State Water Survey.
Walton, W.C. 1970. Groundwater Resource Evaluation. McGraw-Hill, New York, NY.
Walton, W.C. 1984. Practical Aspects of Ground Water Modeling. National Water Well
Association, Dublin, OH.
Walton, W.C. 1989. Analytical Groundwater Modeling. Lewis Publishers, Inc., Chelsea, MI.
Walton, W.C. 1989. Numerical Groundwater Modeling: Flow and Contaminant Migration. Lewis
Publishers, Inc., Chelsea, MI.
Wang, H.F., and M.P. Anderson. 1982. Introduction to Groundwater Modeling. W.H. Freeman
Co., San Francisco, CA.
Ward, C.H., W. Giger, and P.L. McCarty (eds). 1985. Groundwater Quality. John Wiley &
Sons, Somerset, NJ.
Wilson, J.L., and P.J. Miller. 1978. Two-Dimensional Plume in Uniform Ground-Water Flow.
Journal of Hydraulics Div. A. Soc. of Civil Eng. Paper No 13665. HY4, pp. 503-514.
8/95 1 References
-------
APPENDIX D
Sources of Information
-------
SOURCES OF INFORMATION
SOURCES OF U.S. ENVIRONMENTAL PROTECTION AGENCY DOCUMENTS
Center for Environmental Research Information (CERI) (no charge for documents)
Center for Environmental Research Information (CERI)
ORD Publications
26 West Martin Luther King Drive
Cincinnati, OH 45268
513 569-7562
FTS 8-684-7562
Public Information Center (PIC) (no charge for public domain documents)
Public Information Center (PIC)
U.S. Environmental Protection Agency
PM-211B
401 M Street, S.W.
Washington, DC 20460
202 382-2080
FTS 8-382-2080
Superfund Docket and Information Center (SDIC)
U.S. Environmental Protection Agency
Superfund Docket and Information Center (SDIC)
OS-245
401 M Street, S.W.
Washington, DC 20460
202 260-6940
FTS 8-382-6940
National Technical Information Services (NTIS) (cost varies)
National Technical Information Services (NTIS)
U.S. Department of Commerce
5285 Port Royal Road
Springfield, VA 22161
703 487-4650
l-800-553-NTIS(6847)
Superintendent of Documents
Government Printing Office
202 783-3238
8/95 1 Sources of Information
-------
SOURCES OF MODELS AND MODEL INFORMATION
Superfund Exposure Assessment Manual
EPA/540/1-88/001, April 1988
Chapter 3 "Contaminant Fate Analysis" - 35 models
National Ground Water Association
National Ground Water Association
6375 Riverside Dr.
Dublin, OH 43017
614 761-1711
International Groundwater Modeling Center (IGWMC)
Paul K. M. van der Heijde, Director IGWMC
Institute for Ground-Water Research and Education
Colorado School of Mines
Golden, CO 80401-1887
303 273-3103
303 273-3278 (fax)
Groundwater Flow Model
Groundwater Education of Michigan (GEM) Regional Center
Institute for Water Sciences
1024 Trimpe Hall
Western Michigan University
Kalamazoo, MI 49008
616 387-4986
Cost (as of 3/95): $275.00 (including shipping)
UST Video: Groundwater Cleanup
Industrial Training Systems Corp.
20 West Stow Road
Marlton, NJ 08053
609 983-7300
Cost: $595.00
Sources of Information 2 8/95
-------
GEOPHYSICS ADVISOR EXPERT SYSTEM VERSION 2.0
Gary R. Olhoeft, Jeff Lucius, Cathy Sanders
U.S. Geological Survey
Box 25046 DFC - Mail Stop 964
Denver, CO 80225
303 236-1413/1200
U.S. Geological Survey preliminary computer program for Geophysics Advisor Expert
System. Distributed on 3.5" disk and written in True BASIC 2.01 to run under Microsoft
MS-DOS 2.0 or later on IBM-PC or true compatible computers with 640k or greater memory
available to the program. No source code is available.
This expert system program was created for the U.S. Environmental Protection Agency,
Environmental Monitoring Systems Laboratory, Las Vegas, Nevada. The expert system is
designed to assist and educate non-geophysicists in the use of geophysics at hazardous waste
sites. It is not meant to replace the expert advice of competent geophysicists.
COMPREHENSIVE LISTING OF AERIAL PHOTOGRAPHY
U.S. Department of Agriculture, ASCS
Aerial Photography Field Office
2222 West 2300 South
P.O. Box 30010
Salt Lake City, UT 84130-0010
801 524-5856
8/95 3 Sources of Information
-------
APPENDIX E
Soil Profiles
-------
SOIL PROFILE DEVELOPED ON ALLUVIAL FAN DEPOSITS
Gravelly sandy clay
Sand, sandy clay
Interbedded (stratified) silt,
sand, and gravel
:-:::-:::::-
Clay
Silt
£'£&&
ODD
000
°00
Sand Gravel
S-1
-------
SOIL PROFILE DEVELOPED ON VALLEY FILL DEPOSITS
Horizon jtyjjlM^
A
B
O1
Sandy clay
££ Sandy clay
Interbedded (stratified)
fine sand and silt
Interbedded (stratified) silt,
sand, and gravel
•v
?$£•$$?&
Silt
Sand
C
00 0
Ooo
L>00
travel
S-2
-------
SOIL PROFILE DEVELOPED ON ALLUVIUM
Horizon
A
B
_~_ ~_~_ ~_~_ ~
Clay
Silt
O1
Silty clay
Clay
Clay
4'
S-3
-------
SOIL PROFILE DEVELOPED ON COASTAL PLAIN
DEPOSITS IN A HUMID CLIMATE
Horizon
B
0'
Clayey sand and silty clay;
unsorted and unstratified
Silty sandy clay;
stratified and sloping
toward ocean
Silty sandy clay;
stratified
6'
-i-:-:-:-:-:
ttpy
Silt
V-vVvVoi-V-v
Sand
S-4
-------
SOIL PROFILE DEVELOPED ON COASTAL BEACH DEPOSITS
IN A HUMID CLIMATE
Horizon
A
:-:-:-:-:-:-
--_-_-_---_-
::::::::::::
£#$£££:
':'•&.*'•&•!?'•}••
BBI
''»",•*.' •'.''''.'*•".*'
''•.'•'••.'•'••.'•'.'•.'•'.•'
••;^V-;.V-v:-;.;;
Clay Silt Sand
•:•••:•••:•••:•••:•••:••:•••:•••:•• •••••••••••••I 6'
0'
Silty sandy clay
Stratified quartz sands;
poorly graded with little or
no fines
S-5
-------
SOIL PROFILE DEVELOPED ON SAND DUNE DEPOSITS IN
A SEMI-ARID CLIMATE
Horizon
6'
Clayey fine sand and silty
clay
Fine to coarse sand;
poorly graded sediment
Silt Sand
S-6
-------
SOIL PROFILE DEVELOPED ON LIMESTONE IN A HUMID
CLIMATE
Horizon
B
F»Y• • • +*t++r' •, • • /**
*-• •%• »j •.•*•• ,*f *j*V* _*-'
* ""•" "' F-jI*-*f m* '' **•* *'*',* J-'Vf V "• *".
• •".• *• **.*j **.**•'*.' "• v jjjfij-f *"•* "• ""
$f&''ff£*f£*s£*'fif&f&r
•*»; .*»• /!•/»• •*•* At;/*;/.; .*.
•^^Vv'v^W^.-Vv •^*.**1
ff *VM *• *V "• *1* vjj/-'*.' *• •"/ **f-* ft'-*
<•••<••••:•• •.<»*»>«• •.•••.*»>r
••'••/••'..'.'••.'••'•'.'••'••.'••'.•.'.•'.'.'.•'.v.v.
. .• .'.• .*;•..:..: .".:.•;•..:..
•••.•'.••^yrt**v-v.vrt*^>vV'V
'.'.''.''''.•'.•'.•'.''.'.'.'.''.•'.•i.'V+rr'
•..•.•.^4ZaV.\/.vl4^.'.-./
0'
Silty clay
Silty clay
Clay
Limestone bedrock
10'
sgs
Clay
•//.V/.'/.v//.'//
- :".' •//.'//.'//."/:
• !*.*".* s'.v :'/.: !'/.*//
Silt
Sand
Lir
nesto
ne
S-7
-------
SOIL PROFILE DEVELOPED ON SANDSTONE IN
A HUMID CLIMATE
Horizon
B
y Sand Gravel
m
o-e
0'
Gravelly clay
Very gravelly clay
Very gravelly clay
Sandstone bedrock
51
S-8
-------
SOIL PROFILE DEVELOPED ON SHALE IN A HUMID CLIMATE
Horizon
B
O1
Clay
Shaly clay
Platy clay shale
8
i Shale bedrock
Clay
S-9
-------
SOIL PROFILE DEVELOPED ON GNEISS IN A HUMID CLIMATE
Horizon
B
xvxvxvxvxvxvxvxvxvxvxvxvxv
xvxvxvxv xvxvxvxvxvxvxvxvxv
xvxyxyxyxyxyxyxyxyxyxyxyxy
is»t;»i\»;\»t»»;\>t»»i\»i;»i;>i»»i<'i»»ti»i»»i»»;;>f»>t»»s\'»ic»ii»s»i«»
XVXVXVXVXVXVXVXVXVXVXVXVXV
xvxvxvxvxvxvxvxvxvxvxvxvxv
xvxvxvxvxvxvxvxvxvxvxvxvxv
0'
Silt; unsorted and
unstratified
Clayey silt; stratified
Clay with angular bedrock
fragments
Gneiss
10'
Sand Gneiss
0MO
-------
SOIL PROFILE DEVELOPED ON SCHIST IN A HUMID CLIMATE
Horizon
A
B
v^-'/A'^V^jy/^
o1
Fine sandy silt
Silty clay
Sandy silty clay
Schist bedrock
4'
::-:::::-:-:
Clay
'•$:$:0:$:i':
Silt
V^v^v;
Sand
i
\* *\'\ -
%* ***\-
*' *%*%•
\^ *\ ' •
\* '** -
\* /%/ ,
\' *\* .
%* *% * -
»* *** •
% * *\ * -
\* *\* •
\* *\* •
Schist
S-11
-------
SOIL PROFILE DEVELOPED ON SLATE IN A HUMID CLIMATE
Horizon
B
•>':&•
•AS--.-
•yj^si'
0'
Slaty clay
Slaty silty clay
Slaty silty clay
Slate bedrock
6'
ClagBlate Silt
-12
-------
SOIL PROFILE DEVELOPED ON GRANITE IN A HUMID
CLIMATE
Horizon
A
B
0'
Silty clay
Sandy clay
Sandy clay; bedrock
fragments
Granite bedrock
6'
I-I-I-I-I-I-
Clay
*.*•*•*.* •) •*.*•* •*.* •) •*.* •*
Silt
•::::-:^:-:-;:-:-:
»• + •*• + +
«• + + •»• +
f + + •»• +
f + + + +
i- + + + +
Sand Granite
S-13
-------
SOIL PROFILE DEVELOPED ON BASALT IN A
SUBTROPICAL CLIMATE
Horizon
A
B
..
'/~'fir.•;."'•"«" • • .•.•""•"•""•'•"'•'•"'•"/"•"« • «Ji»"/"«"j
-------
SOIL PROFILE DEVELOPED ON GLACIAL TILL
IN A HUMID CLIMATE
Horizon
A
B
->>>:::::
~_-_-_~_~_~-
Clay
•*«-.•.'.',•'.•.';•.•. |
•}::.•::•• •,!.v.!.;
$•••'•£&••'•''••*
Silt
VvVv^\^V*«X*v-^
3'
Clayey silt
Silty clay
Clayey silt
S-15
-------
SOIL PROFILE DEVELOPED ON GLACIAL TILL
IN A HUMID CLIMATE
Horizon
A
B
Silt
1 0'
3'
Fine sandy clay
Fine sandy clay
Fine sandy clay
>-16
-------
SOIL PROFILE DEVELOPED ON GLACIAL DRUMLIN DEPOSITS
Horizon
B
Gravelly, fine sandy clay
Gravelly, fine sandy clay;
poorly sorted
Gravelly clay; unsorted
and unstratified
>:;:-::::::
Clay
,• •*.* •*.* •*.* **«* •'
•;-:-\;:-;.yv.v;.;;
000
Poo
Poo
xSand Gravel
S-17
-------
SOIL PROFILE DEVELOPED ON
GLACIAL LAKE SEDIMENTS
Horizon
A
B
- -— -— -— -— -—__ Clay
Silt Gravel
Silty clay with varve layers
Stratified silty clay with
varve layers
-------
SOIL PROFILE DEVELOPED ON GLACIAL OUTWASH
DEPOSITS
Horizon
B
0'
«i.:-A.-* • • ^i-j^r
-~_~_-_~_-_~
Clav
"silt"
.;.;V.V.;.V.V-;.V-
000
Ooo
°00
Sand Gravel
Unsorted and unstratified
gravelly silty clay
Stratified gravelly clay silt
Interbedded (stratified)
well-graded, sand and
gravel
S-19
-------
VOL.1
-------
Unneci States
Agency
Office oi Researcn and
Development
Washington DC 20460
- 199C
Handbook
Ground Water
Volume
Ground Water and
Contamination
-------
EPA/625/6-90/016a
September 1990
Handbook
Ground Water
Volume I: Ground Water and Contamination
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Cincinnati, OH 45268
-------
NOTICE
This document has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
This document is not intended to be a guidance or support document for specific regulatory program. Guid-
ance documents are available from EPA and must be consulted to address specific regulatory issues.
-------
Contents
Page
Chapter 1. '-Basic Geology 1
Chaper2. Classification of Ground-Water Regions 18
Chapter 3. Ground Water-Surface Water Relationship 50
Chapter 4. Basic Hydrogeology 74
Chapter 5. Ground-Water Contamination 94
Chapter 6. Ground-Water Investigations 114
Chapter 7. Ground-Water Restoration 128
in
-------
Acknowledgments
Many individuals contributed to the preparation and review of this handbook. The document was prepared by
Eastern Research Group, Inc., for EPA's Center for Environmental Research Information, Cincinnati. OH.
Contract administration was provided by the Center for Environmental Research Information.
Volume I. Ground Water and Contamination, will be followed by Volume II, Methodology. Although extensively
revised, Volume I was obtained from previous publications, "Handbook: Ground Water (EPA/625/6-87/016) and
"Protection of Public Water Supplies from Ground-Water Contamination" (EPA/625/4-85/016).
Authors and Reviewers
Michael J. Barcelona - Western Michigan University, Kalamazoo, Ml
Russell Boulding - Eastern Research Group, Inc., Arlington, MA
Ralph C. Heath - Private Consultant. Raleigh, NC
Jack Keely - Private Consultant, Ada, OK
Wayne A. Pettyjohn - Oklahoma State University, Stillwater. OK
Contract Management
Carol Grove - EPA-CERI, Cincinnati, OH
Heidi Schultz, ERG, Inc., Arlington, MA
IV
-------
Preface
The subsurface environment of ground water is characterized by a complex interplay of physical, geochemical
and biological forces that govern the release, transport and fate of a variety of chemical substances. There are
literally as many varied hydrogeologic settings as there a^e types and numbers of contaminant sources. In
situations where ground-water investigations are most necessary, there are frequently many variables of land
and ground-water use and contaminant source characteristics which cannot be fully characterized.
The impact of natural ground-water recharge and discharge processes on distributions of chemical constituents
is understood for only a few types of chemical species. Also, these processes may be modified by both natural
phenomena and man's activities so as to further complicate apparent spatial or temporal trends in water quality.
Since so many climatic, demographic and hydrogeologic factors may vary from place to place, or even small areas
within specific sites, there can be no single "standard" approach for assessing and protecting the quality of ground
water that will be applicable in all cases.
Despite these uncertainties, investigations are under way and they are used as a basis for making decisions about
the need for, and usefulness of, alternative corrective and preventive actions. Decision makers, therefore, need
some assurance that elements of uncertainty are minimized and that hydrogeologic investigations provide reliable
results.
A purpose of this document is to discuss measures that can be taken to ensure that uncertainties do not undermine
our ability to make reliable predictions about the response of contamination to various corrective or preventive
measures.
EPA conducts considerable research in ground water to support its regulatory needs. In recent years, scientific
knowledge about ground-water systems has been increasing rapidly. Researchers in the Office of Research and
Development have made improvements in technology forassessing the subsurface, in adapting techniques from
other disciplines to successfully identify specific contaminants in ground water, in assessing the behavior of
certain chemicals in some geologic materials and in advancing the state-of-the-art of remedial technologies.
An important part of EPA's ground-water research program is to transmit research information to decision makers,
field managers and the scientific community. This publication has been developed to assist that effort and,
additionally, to help satisfy an immediate Agency need to promote the transfer of technology that is applicable to
ground-water contamination control and prevention.
The need exists for a resource document that brings together available technical information in a form convenient
for ground-water personnel within EPA and state and local governments on whom EPA ultimately depends for
proper ground-water management. The information contained in this handbook is intended to meet that need. It
is applicable to many programs that deal with the ground-water resource. However, it is not intended as a guidance
or support document for a specific regulatory program.
GUIDANCE DOCUMENTS ARE AVAILABLE FROM EPA AND MUST BE CONSULTED TO ADDRESS
SPECIFIC REGULATORY ISSUES.
-------
Chapter 1
BASIC GEOLOGY
Introduction
Geology, the study of the earth, includes the investigation
of earth materials, the processes that act on these
materials, the products that are formed, the history of
the earth, and the origin and development of life
forms. There are several subfields of geology.
Physical geology deals with all aspects of the earth
and includes most earth science specialities.
Historical geology is the study of the origin of the
earth, continents and ocean basins, and life forms,
while economic geology is an applied approach
involved in the search and exploitation of mineral
resources, such as metallic ores, fuels, and water.
Structural geology deals with the various structures of
the earth and the forces that produce them.
Geophysics is the examination of the physical properties
of the earth and includes the study of earthquakes
and methods to evaluate the subsurface.
From the perspective of ground water,. all of the
subfields of geology are used, some more than
others. Probably the most difficult concept to
comprehend by individuals with little or no geological
training is the complexity of the subsurface, which is
hidden from view and, at least presently, cannot be
adequately sampled. In geologic or hydrogeologic
studies, it is best to always keep in mind a
fundamental principle of geology, that is, the present
is the key to the past. This means that the processes
that are occurring today are the same processes that
occurred throughout the geologic past—only the
magnitude has changed from one time to the next.
Consider, for example, the channel and flood plain of
a modern day river or stream. The watercourse
constantly meanders from one side of the flood plain to
another, eroding the banks and carrying the sediments
farther downstream. The channel changes in size and
position, giving rise to deposits of differing grain size
and, perhaps, composition. The changes may be
abrupt or gradual, both vertically and horizontally, as
is evident from an examination of the walls of a gravel
pitorthebluftsalonga river. Because of the dynamic
nature of streams and deltas, one will find a geologic
situation that is perplexing, not only to the individual
involved in a ground-water investigation, but to the
geologist as well. Each change ingrain size will cause
a difference in permeability and ground-water velocity,
while changes in mineral composition can lead to
variances in water quality. At the other end of the
depositional spectrum are deposits collected in lakes,
seas, and the oceans, which are likely to be much more
widespread and uniform in thickness, grain size, and
composition.
As one walks from the sandy beach of a lake into the
water, the sediments become finer and more widely
distributed as the action of waves and currents sort the
material brought into the lake by streams. Fartherfrom
shore, the bottom of the lake may consist of mud.
which is a mixture of silt, clay, and organic matter. In
some situations the earthy mud grades laterally into a
lime ooze or mud. In geologic time these sediments
become lithified or changed into rock...the sand to
sandstone, the mud to shale, and the limy mud to.
limestone. It is important to note, however, that the
sand, mud, and lime were all deposited at the same
time, although with lithification each sediment type
produced a different sedimentary rock.
Minerals
The earth, some 7,926 miles in diameter at the
equator, consists of a core, mantle, and crust, which.
have been defined by the analysis of seismic or
earthquake waves. Only a thin layer of the crust has
been examined by humans. It consists of a variety of
rocks, each of which is made up of one or more
minerals.
Most minerals contain two or more elements, but of all
of the elements known, only eight account for nearly 98
percent of the rocks and minerals:
-------
Oxygen 46%
Silicon 27.72%
Aluminum 8.13%
Iron 5%
Calcium 3.63%
Sodium 2.83%
Potassium 2.59%
Magnesium 2.09%
Without detailed study, it is usually difficult to
distinguish one mineral from another, except for a few
common varieties, such as quartz, pyrite, mica, and
some gemstones. On the other hand, it is important
to have at least a general understanding of mineralogy
because it is the mineral make-up of rocks that, to a
large extent, controls the type of water that a rock
will contain under natural conditions and the way it will
react to contaminants or naturally occurring
substances.
The most common rock-forming minerals are relatively
few and deserve at least a mention. They can be
divided into three broad groups: (1) the carbonates,
sulfates, and oxides, (2) the rock-forming silicate
minerals, and (3) the common ore minerals.
Carbonates, Sulfates, and Oxides
Calcite, a calcium carbonate (CaCOs), is the major
mineral in limestone. It is quite soluble, which accounts
for its usual presence in water. The most common
mineral is quartz. It is silicon dioxide (SiO2), hard, and
resistant to both chemical and mechanical weathering.
In sedimentary rocks it generally occurs as sand-size
grains (sandstone) or even finer, such as silt or clay
size, and it may also appear as a cement. Because of
the low solubility of silicon, silica generally appears in
concentrations less than 25 mg/L in water. Limonite is
actually a group name for the hydrated ferric oxide
minerals (F62O3H2O), which occur so commonly in
many types of rocks. Limonite is generally rusty or
blackish with a dull, earthy luster and a yellow-brown
streak. It is a common weathering product of other iron
minerals. Because limonite and other iron-bearing
minerals are nearly universal, dissolved iron is a
very common constituent in water and causes
staining of clothing and plumbing fixtures. Gypsum,
a hydrated calcium sulfate (CaSO4-2H20). occurs
as a sedimentary evaporite deposit and as crystals in
shale and some clay deposits. Quite soluble, it is the
major source of sulfate in ground water.
Rock-Forming Silicates
The most common rock-forming silicate minerals include
the feldspars, micas, pyroxenes, amphiboles, and
olivine. Except in certain igneous and metamorphic
rocks, these minerals are quite small and commonly
require a microscope for identification. The feldspars
are alumino-silicates of potassium or sodium and
calcium. Most of the minerals in this group are white,
gray, or pink. Upon weathering they turn to clay and
release the remaining chemical elements to water. The
micas, called muscovite and biotite, are platy alumino-
silicate minerals that are common and easily recognized
in igneous, metamorphic, and sedimentary rocks.
The pyroxenes, a group of silicates of calcium,
magnesium, and iron, as well as the amphiboles,
which are complex hydrated silicates of calcium,
magnesium, iron, and aluminum, are common inmost
igneous and metamorphic rocks. They appear as
small, dark crystals of accessory minerals. Olivine,
a magnesium-iron silicate, is generally green or yellow
and is common in certain igneous and metamorphic
rocks. None of the rock-forming silicate minerals have
a major impact on water quality in most situations.
Next to organic matter, clay minerals are the most
chemically active materials in soil and unconsolidated
materials. Both consolidated rocks and unconsolidated
sediments that have a high clay mineral content tend to
have low permeabilities and, consequently, water
movement through them is very slow. The two broad
groups of clay minerals commonly recognized are the
silicate clays and the hydrous oxide clays. Silicate clays
form from the weathering of primary silicate minerals,
such as feldspars and olivine. They have a sheet-like
lattice structure and a strong adsorptive capacity. Silicate
clays are classified according to different stacking
arrangements of the lattice layers and their tendency to
expand in water. The stacking type strongly affects
certain properties of clays, including (I) surface area, (2)
the tendency to swell during hydration, and (3) cation
exchange capacity (CEC), which is a quantitative
measure of the ability of a mineral surface to adsorb
ions.
Table1-1 summarizes some properties of silicate clay
minerals, which are listed from the most reactive
(montmorillonite and vermiculite) to least reactive
(kaolinite). The montmorillonite group is most sensitive
to swelling and has a high CEC. The structure in
kaolinite results in both a low surface area and CEC.
Illite and chlorite have intermediate surface areas,
CEC, and sensitivities to swelling. Clay minerals in
sedimentary rocks are usually mixtures of different
groups. In addition, mixed-layer clay minerals can form
and these have properties and compositions that are
intermediate between two well-defined clay types (e.g.,
chlorite-illrte, illite-montmorillonite). Hydrous oxide clays,
which are less well understood than silicate clays, are
oxides of iron, magnesium, and aluminum that are
associated with water molecules. Compared to silicate
clays, CEC is lower in hydrous oxide clays.
-------
Type ol Clay3
Property
Lattice typec
Expanding?
Specific surface
(m2/g)
External surface
Internal surface
Swelling capacity
Cation exchange
capacity (meq/iOOg)
Other similar
Clays
Montmorillonite Verriculite
(Smetite)
2:1
Yes
700-800
High
Very High
High
80-150
Beidellite
Nontrorite
Saporite
Bentonited
2:1
Slightly
700-800
High
High
Med-High
100-150+
Illrte
2:1
No
65-120
Medium
Medium
Medium
10-40
Chlorite
2:2
No
25-40
Medium
Medium
Low
10-40
Kaolinite
1:1
No
7-30
Low
None
Low
3-15
Halloysite
Anauxite
Dickit
aClays are arranged from most reactive (montmorillonite) to least reactive (kaolinite).
b The term smectite is now used to refer to the montmorillonite group of clays (Soil Science Society of
America, 1987)
c Tetrahedral:octahedral layers.
^Bentonite is a clay formed from weathering of volcanic ash and is made up mostly of montmorillonite and
beidellite.
e Upper range occurs with smaller particle size.
Sources: Adapted from Grim (1968), Brady (1974), and Ahlrichs (1972).
Table 1-1. Important Characteristics of Silicate Clay Minerals
Ores
The three most common ore minerals are galena,
sphalerite, and pyrite. Galena, a lead sulfide (PbS), is
heavy, brittle, and breaks into cubes. Sphalerite is
a zinc sulfide (ZnS) mineral that is brownish, yellowish,
or black. It ordinarily occurs with galena and is a major
ore of zinc. The iron sulfide pyrite (FeS), which is also
called fool's gold, is common in nearly all types of rocks.
It is the weathering of this mineral that leads to acid-
mine drainage, which is common in many coal fields
and metal sulfide mining regions.
Rocks, Their Origin and Properties
Three types of rock comprise the crust of the earth.
Igneous rocks solidified from molten material either
within the earth (intrusive) or on or near the surface
(extrusive). Metamorphic rocks were originally igneous
or sedimentary rocks that were modified by
temperature, pressure, and chemically active fluids.
Sedimentary rocks are the result of the weathering of
preexisting rocks, erosion, and deposition. Geologists
have developed elaborate systems of nomenclature
and classification of rocks, but these are of little value
in hydrogeologic studies and, therefore, only the most
basic descriptions will be presented.
Igneous Rocks
Igneous rocks are classified on the basis of their
composition and grain size. Most consist of feldspar
and a variety of dark minerals; several others also
contain quartz. If the parent molten material cools
slowly deep below the surface, minerals will have
an opportunity to grow and the rock will be coarse
grained. Magma that cools rapidly, such as that
derived from volcanic activity, is so fine grained thai
individual minerals generally cannot be seen even with
a hand lens. In some cases the molten material began
to cool slowly, allowing some minerals to grow, and
then the rate increased dramatically so that the
-------
remainder formed a fine groundmass. This texture,
consisting of large crystals in a fine-grained matrix, is
called porphyritic.
Intrusive igneous rocks can only be seen where they
have been exposed by erosion. They are concordant
if they more or less parallel the bedding of the
enclosing rocks and discordant if they cut across the
bedding. The largest discordant igneous masses are
called batholiths and they occur in the eroded centers of
many ancient mountains. Their dimensions are in the
range of tens of miles. Batholiths usually consist largely
of granite, which is surrounded by metamorphic rocks.
Discordant igneous rocks also include dikes that
range in width from a few inches to thousands of feet.
Many are several miles long. Sills are concordant
bodies that have invaded sedimentary rocks along
bedding planes. They are relatively thin. Both sills
and dikes tend to cool quite rapidly and, as a result, are
fine grained.
Extrusive rocks include lava flows or other types
associated with volcanic activity, such as the glassy
rock, pumice, and the consolidated ash called tuff.
These are fine grained or even glassy.
With some exceptions, igneous rocks are dense and
have very little porosity or permeability. Most,
however, are fractured to some degree and can store
and transmit a modest amount of water. Some lava
flows are notable exceptions because they contain
large diameter tubes or a permeable zone at the top of
the flow where gas bubbles migrated to the surface
before the rock solidified. These rocks are called
scoria.
Metamorphic Rocks
Metamorphism is a process that changes preexisting
rocks into new forms because of increases in
temperature, pressure, and chemically active fluids.
Metamorphism may affect igneous, sedimentary, or
other metamorphic rocks. The changes brought about
include the formation of new minerals, increase in grain
size, and modification of rock structure or texture, all of
which depend on the original rock's composition and
the intensity of the metamorphism.
Some of the most obvious changes are in texture,
which serves as a means of classifying metamorphic
rocks into two broad groups, the foliated and non-
foliated rocks. Foliated metamorphic rocks typify
regions that have undergone severe deformation,
such as mountain ranges. Shale, which consists
mainly of silt and clay, is transformed into slate by the
change of clay to mica. Mica, being a platy mineral,
grows with its long axis perpendicular to the principal
direction of stress, forming a preferred orientation.
This orientation, such as the development of cleavage
in slate, may differ greatly from the original bedding.
With increasing degrees of metamorphism, the grains
of mica grow to a larger size so that the rock has a
distinct foliation, which is characteristic of the
metamorphic rock, schist. At even higher grades of
metamorphism, the mica may be transformed to a
much coarser-grained feldspar, producing the strongly
banded texture of gneiss.
Non-foliated rocks include the hornfels and another
group formed from rocks that consist mainly of a single
mineral. The hornfels occur around an intrusive body
and were changed by "baking" during intrusion. The
second group includes marble and quartzite, as well
as several other forms. Marble is metamorphosed
limestone and quartzite is metamorphosed quartz
sandstone.
There are many different types of metamorphic rocks,
but from a hydrogeologic viewpoint they normally
neither store nor transmit much water and are of only
minor importance as aquifers. Their primary permeability
is notably small, if it exists at all, and fluids are forced to
migrate through secondary openings, such as faults,
joints, or other types of fractures.
Sedimentary Rocks
Sedimentary rocks are deposited, either in a body of
water or on the land, by running water, by wind, and by
glaciers. Eachdepositional agent leaves a characteristic
stamp on the material it deposits. The sediments
carried by these agents were first derived by the
weathering and erosion of preexisting rocks. The most
common sedimentary rocks are shale, siltstone,
sandstone, limestone, and glacial till. The change
from a loose, unconsolidated sediment to a rock is
the process of (Unification. Although sedimentary
rocks appear to be the dominant type, in reality they
make up but a small percentage of the earth. They do,
however, form a thin crust over much of the earth's
surface, are the type most readily evident, and serve as
the primary source of ground water.
The major characteristics of sedimentary rocks are
sorting, rounding, and stratification. A sediment is well
sorted if the grains are nearly all the same size. Wind
is the most effective agent of sorting and this is followed
by water. Glacial till is unsorted and consists of a wide
mixture of material that ranges from large boulders to
clay.
While being transported, sedimentary material loses
-------
its sharp, angular configuration as it develops some
degree of rounding. The amount of rounding depends
on the original shape, composition, transporting medium,
and the distance traveled.
Sorting and rounding are important features of both
consolidated and unconsolidated material because
they have a major control on permeability and
porosity. The greater the degree of sorting and
rounding the higher will be the water-transmitting and
storage properties. This is why a deposit of sand, in
contrast to glacial till, can be such a productive aquifer.
Most sedimentary rocks are deposited in a sequence of
layers or strata. Each layer or stratum is separated
by a bedding plane, which probably reflects variations
in sediment supply or some type of short-term erosion.
Commonly bedding planes represent changes in grain
size. Stratification provides many clues in our attempt
to unravel geologic history. The correlation of strata
between wells or outcrops is called stratigraphy.
Sedimentary rocks are classified on the basis of
texture (grain size and shape) and composition. Clastic
rocks consist of particles of broken or worn material
and include shale, siltstone, sandstone. and
conglomerate. These rocks are lithified by compaction,
in the case of shale, and by cementation. The most
common cements are clay, calcite, quartz, and limonite.
The last three, carried by ground water, precipitate
in the unconsolidated material under specific
geochemical conditions.
The organic or chemical sedimentary rocks consist of
strata formed from or by organisms and by chemical
precipitates from sea water or other solutions. Most
have a crystalline texture. Some consist of well preserved
organic remains, such as reef deposits and coal seams.
Chemical sediments include, in addition to some
limestones, the evaporites, such as halite (sodium
chloride), gypsum, and anhydrite. Anhydrite is an
anhydrous calcium sulfate.
i
Geologists also have developed an elaborate
classification of sedimentary rocks, which is of little
importance to the purpose of this introduction. In fact,
most sedimentary rocks are mixtures of clastic debris,
organic material, and chemical precipitates. One
should keep in mind not the various classifications,
but rather the texture, composition, and other features
that can be used to understand the origin and history of
the rock.
The term texture has different meanings in geology and
soil science. In soil science it is simply the relative
proportions of clay-, silt-, and sand-sized particles in soil
or unconsolidated material. The term fabric applies to
the total of all physical features of a rock or soil that can
be observed. Soil fabric analysis involves the study of
distinctive physical features resulting from soil-forming
processes, which also strongly influence the location
and rate of water movement in soil.
A variety of scales are available for the classification of
materials based on particle-size distribution. I n geology.
the Wentworth-Udden scale is most widely used: boulder
(>256 mm), cobble (64-256 mm), pebble (4-64 mm),
granule or gravel (2-4 mm), sand (1/16-2 mm), silt (1/
256-1/16 mm), and clay (<1/256 mm). The U.S.
Department of Agriculture (USDA) soil textural
classification system is most widely used by soil
scientists, and engineers usually use the Unified soil
classification system. The hydrologic properties of soils
are strongly related to particle-size distribution.
Weathering
Generally speaking, a rock is stable only in the
environment in which it was formed. Once removed
from that environment, it begins to change, rapidly in a
few cases, but more often slowly, by weathering. The
two major processes of weathering are mechanical and
chemical, but they usually proceed in concert.
Mechanical Weathering
Mechanical weathering is the physical breakdown of
rocks and minerals. Some is the result of fracturing due
to the volumetric increase when water in a crack turns
to ice, some is the result of abrasion during transport
by water, ice, or wind, and a large part is the result of
gravity causing rocks to fall and shatter. Mechanical
weathering alone only reduces the size of the rock; its
chemical composition is not changed. The weathered
material formed ranges in size from boulders to silt.
Chemical Weathering
Chemical weathering, on the other hand, is an actual
change in composition as minerals are modified from
one type to another. Many, if not most of the changes
are accompanied by a volumetric increase or decrease,
which in itself further promotes additional chemical
weathering. The rate depends on temperature, surface
area, and available water.
The major reactions involved in chemical weathering
are oxidation, hydrolysis, and carbonation. Oxidation
is a reaction with oxygen to form an oxide, hydrolysis is
reaction with water, and carbonation is a reaction with
CO2 to form a carbonate. In these reactions the total
volume increases and. since chemical weathering is
most effective on grain surfaces, disintegration of a
rock occurs.
-------
Quartz, whether vein deposits or individual grains,
undergoes practically no chemical weathering; the end
product is quartz sand. Some of the feldspars weather
to clay and release calcium, sodium, silica, and many
other elements that are transported in water. The iron-
bearing minerals provide, in addition to iron and
magnesium, weathering products that are similar to
the feldspars.
Basic Soil Concepts
Although the term soil is often loosely used to refer to
any unconsolidated material, soil scientists distinguish
it from other unconsolidated geologic materials by
observable features, such as accumulation of organic
matter, formation of soil structure, and leaching, that
result from soil-forming processes.
The soil at a particular location is the result of the
interaction of five factors: (I) parent material, (2)
topography, (3) climate, (4) biota, and (5) time. The
interaction of these factors results in the formation of a
soil profile, the description of which forms the basis for
classifying a soil. Specific soil-forming processes that
influence soil profile development include (I) organic
matter accumulation; (2) weathering of minerals to
clays; (3) the depletion of clay and other sesquioxide
minerals from upper horizons (eluviation), with
subsequent enrichment in lower horizon (illuviation); (4)
leaching or accumulation of soluble salts; (5) the
formation of soil structure by the aggregation of soil
particles into larger units called peds; and (6) the
formation of slowly permeable layers called fragipans.
Perhaps the most distinctive features of a soil profile are
its major horizons. The O horizon, if present, is a layer
of partially decomposed organic material. The A horizon,
which lies at the surface or near surface, is a mineral
horizon characterized by maximum accumulation of
organic matter; it usually has a distinctly darker color
than lower horizons. The B horizon, the zone of most
active weathering, is commonly enriched in clays, and
has a well-defined soil structure. The C horizon is
unconsolidated material that has experienced little or
no weathering. The R horizon is solid rock.
Soil physical properties, such as texture, structure, and
pore size distribution, are the major determinants in
water movement in soil. Depending on the specific soil,
water movement may be enhanced or retarded
compared to unweathered geologic materials. Organic
matter enhances water-holding capacity and infiltration.
The formation of soil structure also enhances
permeability, particularly in clayey soils. On the other
hand, the formation of restrictive layers, such as
fragipans, may substantially reduce infiltration compared
to unweathered materials. Micromorphological and
general fabric analysis of soil is used infrequently in the
study of ground-water contamination, more because of
unfamiliarity with the methods than their lack of value.
Minerals in the soil are the chemical signature of the
bedrock from which they originated. Rainfall and
temperature are two significant factors that dictate the
rate and extent to which mineral solids in the soil react
with water. Organic matter and clay content are major
parameters of importance in studying the transport and
fate of contaminants in soil.
Erosion and Deposition
Once a rock begins to weather, the by-products await
erosion or transportation, which must be followed by
deposition. The major agents involved in this part of the
rock cycle are running water, wind, and glacial ice.
Waterborne Deposits
Mass wasting is the downslope movement of large
amounts of detrital material by gravity. Through this
process, sediments are made available to streams that
carry them away to a temporary or permanent site of
deposition. During transportation some sorting occurs
and the finer silt and clay are carried farther downstream.
The streams, constantly filling, eroding, and widening
their channels, leave materials in their valleys that
indicate much of the history of the region. Stream
valley deposits, called alluvium, are shown on geologic
maps by the symbol Qal, meaning Quaternary age
alluvium. Alluvial deposits are distinct, but highly
variable in grain size, composition, and thickness.
Where they consist of glacially derived sand and
gravel, called outwash, they form some of the most
productive water-bearing units in the world.
Sediments, either clastic or chemical/organic,
transported to past and present seas and ocean basins
spread out to form, after lithification, extensive
formations of sandstone, siltstone, shale, and limestone.
In the geologic past, these marine deposits covered
vast areas and when uplifted they formed the land
surface, where they again began to weather in
anticipation of the next trip to the ocean.
The major features of marine sedimentary rocks are
their widespread occurrence and rather uniform
thickness and composition, although extreme changes
exist in many places. If not disturbed by some type of
earth movement, they are stratified and horizontal.
Furthermore, each lithologic type is unique relative to
adjacent units. The bedding planes or contacts that
-------
divide them represent distinct differences in texture
or composition. From a hydrologic perspective,
differences in texture from one rock type to another
produce boundaries that strongly influence ground-
water flow. Consequently, ground water tends to flow
parallel to these boundaries, that is, within a particular
geologic formation, rather than across them.
Windborne Deposits
Wind-laid or eolian deposits are relatively rare in the
geologic record. The massively cross-bedded
sandstone of the Navajo Sandstone in Utah's Zion
National Park and surrounding areas is a classic
example in the United States. Other deposits are more
or less local and represent dunes formed along beaches
of large water bodies or streams. Their major
characteristic is the high degree of sorting. Dunes,
being relatively free of silt and clay, are very permeable
and porous, unless the openings have been filled by
cement. They allow rapid infiltration of water and can
form major water-bearing units, if the topographic and
geologic conditions are such that the water does not
rapidly drain.
Another wind-deposited sediment is loess, which
consists largely of silt. It lacks bedding but is typified
by vertical jointing. The silt is transported by wind from
deserts, flood plains, and glacial deposits. Loess
weathers to a fertile soil and is very porous. It is
common along the major rivers in the glaciated parts of
the United States and in China, parts of Europe, and
adjacent to deserts and deposits of glacial outwash.
Glacial Deposits
Glaciers erode, transport, and deposit sediments that
range from clay to huge boulders. They subdue the land
surface over which they flow and bury former river
systems. The areas covered by glaciers during the last
Ice Age in the United States are described in Chapter
2, but the deposits extend far beyond the former
margins of the ice. The two major types of glaciers
include valley or mountain glaciers and the far more
extensive continental glaciers. The deposits they leave
are similar and differ, for the most part, only in scale.
As a glacier slowly passes over the land surface, it
incorporates material from the underlying rocks into
the ice mass, only to deposit that material elsewhere
when the ice melts. During this process, it modifies the
land surface, both through erosion and deposition. The
debris associated with glacial activity is collectively
termed glacial drift. Unstratified drift, usually deposited
directly by the ice, is glacial till, a heterogeneous
mixture of boulders, gravel, sand, silt, and clay. Glacial
debris reworked by streams and in lakes is stratified
drift. Although stratified drift may range widely in grain
size, the sorting far surpasses that of glacial till.
Glacial lake clays are particularly well sorted.
Glacial geologists usually map not on the basis of
texture, but rather the type of landform that was
developed, such as moraines, outwash, drumlins, and
so on. The various kinds of moraines and associated
landforms are composed largely of unstratified drift
with incorporated layers of sand and gravel. Stratified
drift is found along existing or former stream valleys or
lakes that were either in the glacier or extended
downgradient from it. Meltwater stream deposits are
mixtures of sand and gravel. In places, some have
coalesced to develop extensive outwash plains.
Glaciers advanced and retreated many times.
reworking, overriding, and incorporating sediments
from previous advances into the ice, subsequently
redepositing them elsewhere. There was a constant
inversion of topography as buried ice melted causing
adjacent, waterlogged till to slump into the low areas.
During advances, the ice might have overridden older
outwash layers so that upon melting these sand and
gravel deposits were covered by a younger layer of till.
Regardless of the cause, the final effect is one of
complexity of origin, history, and stratigraphy. When
working with glacial till deposits, it is nearly always
impossible to predict the lateral extent or thickness
of a particular lithology in the subsurface. Surficial
stratified drift is more uniform than till in thickness,
extent, and texture.
Geologic Structure
A general law of geology is that in any sequence of
sedimentary rocks that has not been disturbed by
folding or faulting, the youngest unit is on the top. A
second general law is that sedimentary rocks are
deposited in a horizontal or nearly horizontal position.
The fact that rocks are found overturned, displaced
vertically or laterally, and squeezed into open or tight
folds, clearly indicates that the crust of the earth is a
dynamic system. There is a constant battle between the
forces of destruction (erosion) and construction (earth
movements).
Folding
Rocks, folded by compressional forces, are common
in and adjacent to former or existing mountain ranges.
The folds range from a few inches to 50 miles or so
across. Anticlines are rocks folded upward into an arch.
Their counterpart, synclines, are folded downward like
a valley (fig. 1-1). A monocline is a fle'cture in which the
rocks are horizontal, or nearly so, on either side of the
f lecture.
-------
Map View
Anticline
Syndine
Cross Section
The arrow indicates the direction of dip. In an anticline.
the rocks dip away from the craat and in a tyncflne they
dip toward the center.
Figure 1-1. Dip and Strike Symbols Commonly
Shown on Geologic Maps
Although many rocks have been folded into various
structures, this does not mean that these same
structures form similar topographic features. As the
folding takes place over eons, the forces of erosion
attempt to maintain a low profile. As uplift continues,
erosion removes weathering products from the rising
mass, carrying them to other places of deposition. The
final topography is related to the credibility of the rocks,
with resistant strata, such as sandstone, forming ridges,
and the less resistant material, such as shale, forming
valleys (fig. 1-2). Consequently, the geologic structure
of an area may bear little resemblance to its topography.
The structure of an area can be determined from field
studies or a geologic map, if one exists. Various types
of folds and their dimensions appear as unusual patterns
on geologic maps. An anticline, for example, will be
depicted as a series of rock units in which the oldest is
in the middle, while a syncline is represented by the
youngest rock in the center. More or less
equidimensional anticlines and synclines are termed
domes and basins, respectively.
The inclination of the top of a fold is the plunge. Folds
may be symmetrical, asymmetrical, overturned, or
recumbent. The inclination of the rocks is indicated by
dip and strike symbols. The strike is perpendicular to
the dip and the degree of dip is commonly shown by a
number. The dip may range from less than a degree to
vertical.
Unconformities
An unconformity is a break in the geologic record. It is
caused by a cessation in deposition that is followed by
erosion and subsequent deposition. The geologic
record is lost by the period of erosion because the
rocks that contained the record were removed.
If a sequence of strata is horizontal but the contact
between two rock groups in the sequence represents
an erosional surface, that surface is said to be a
disconformity (fig. 1-3). Where a sequence of strata
has been tilted and eroded and then younger, horizontal
rocks are deposited overthem, the contact is an angular
unconformity. A nonconformity occurs where eroded
Projected position of rocks had they
not been removed by erosion
"~
Figure 1-2. Geologic Structure May Influence Surface Topography
-------
Nonconformity An9V'ar Unconformity
Disconformity
/. I . I . 1 .1
/III 1 I
Figure 1-3. An Unconformity Represents a Break In the Geologic Record
igneous or metamorphic rocks are overlain by
sedimentary rocks.
Fractures
Fractures in rocks are either joints or faults. A joint is
a fracture along which no movement has taken place; a
fault implies movement. Movement along faults is as
little as a few inches to tens of miles. Probably all
consolidated rocks and a good share of the
unconsolidated deposits contain joints. Joints may
exert a major control on water movement and
chemical quality. Characteristically joints are open and
serve as major conduits or pipes. Water can move
through them quickly, perhaps carrying contaminants,
and, being open, the filtration effect is lost. It is a
good possibility that the outbreak of many waterbome
diseases that can be tied to ground-water supplies is
the result of the transmission of infectious agents
through fractures to wells and springs.
Faults are most common in the deformed rocks of
mountain ranges, suggesting either lengthening or
shortening of the crust. Movement along a fault may
be horizontal, vertical, or a combination. The most
common types of faults are called normal, reverse, and
lateral (fig. 1-4). A normal fault, which indicates stretching
of the crust, is one in which the upper or hanging
wall has moved down relative to the lower or foot wall.
The Red Sea, Dead Sea, and the large lake basins
in the east African highlands, among many others, lie
in grabens, which are blocks bounded by normal faults
(fig. 1-4). A reverse or thrust fault implies compression
and shortening of the crust. It is distinguished by
the fact that the hanging wall has moved up relative to
the foot wall. A lateral fault is one in which the
movement has been largely horizontal. The San
Andreas Fault, extending some 600 miles from San
Francisco Bay to the Gulf of California, is the most
notable lateral fault in the United States. It was
movement along this fault the produced the 1906 San
Francisco earthquake.
Geologic Time
Geologic time deals with the relation between the
emplacement or disturbance of rocks and time. In
order to provide some standard classification, the
geologic time scale was developed (table 1-2). It is
based on a sequence of rocks that were deposited
during a particulartime interval. Commonly the divisions
are based on some type of unconformity. In
considering geologic time, three types of units are
defined. These are rock units, time-rock units, and time
units.
\
fe-%;«W'~rwvv
V
fault_/N^.-..-
Cross- Section
of
Normal Fault
r
811 \m^:
V
^ -
aneinc Wai jAv
Foot Wall -^ A\
Cross Section
of
Reverse Fauft
J
~ '
Hanging Wall
Flan
0
Lateral
F
view
f
Fault
*
Normal Fauh •
Graben
Ntaa/V
Normal Fault
Figure 1-4. Cross Sections of Normal, Reverse and Lateral Faults
-------
Millions of
EH
Cenozoic
Mesozoic
Paleozoic
EflrjQd
Quaternary
Tertiary
Cretaceous
Jurassc
Triassic
Permian
Pennsylvanian
Mssissppian
Devonian
Silunan .
Ordoviaan
Cambrian
Esxsti
Recent
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Yaars Ago
0-2
2-13
13-25
25-36
36-58
58-63
63-135
135-181
181-230
230-280
280-310
310-345
345-405
405-425
425-500
500-600
Precambrian Lasted at leatt 2.5 Billion years
Table 1-2. Geologic Time Scale
Rock Units
A rock unit refers to some particular lithology. These
maybe further divided into geologic formations, which
are of sufficient size and uniformity to be mapped in the
field. The Pierre Shale, for example, is a widespread
and, in places, thick geologic formation that extends
over much of the Northern Great Plains. Formations
can also be divided into smaller units called members.
Formations have a geographic name that may be
coupled with a term that describes the major rock type.
Two or more formations comprise a group.
Time and Time-Rock Units
Time-rock units refer to the rock that was deposited
during a certain period of time. These units are divided
into system, series, and stage. Time units refer to the
time during which a sequence of rocks were deposited.
The time-rock term, system, has the equivalent time
term, period. That is, during the Cretaceous Period
rocks of the Cretaceous System were deposited and
they consist of many groups and formations. Time units
are named in such a way that the eras reflect the
complexity of life forms that existed, such as the
Mesozoic or "middle life." System or period
nomenclature largely is based on the geographic
location in which the rocks were first described, such
as Jurassic, which relates to the Jura Mountains of
Europe.
The terms used by geologists to describe rocks relative
to geologic time are useful to the. ground-water
investigator in that they allow one to better perceive a
regional geologic situation. The terms alone have no
significance as far as water-bearing properties are
concerned.
Geologic Maps And Cross Sections
Geologists use a number of techniques to graphically
represent surface and subsurface conditions. These
include surficial geologic maps, columnar sections,
cross-sections of the subsurface, maps that show the
configuration of the surface of a geologic unit, such as
the bedrock beneath glacial deposits, maps that indicate
the thickness or grain size of a particular unit, a variety
of contour maps, and a whole host of others.
A surficial geologic map depicts the geographic extent
of formations and their structure. Columnar sections
describe the vertical distribution of rock units, their
lithology, and thickness. Geologic cross sections
attempt to illustrate the subsurface distribution of rock
units between points of control, such as outcrops or
well bores. An isopach map shows the geographic
range in thickness of a unit. These maps and cross-
sections are based largely or entirely on well logs, which
are descriptions of earth material penetrated during the
drilling of a well or test hole.
Whatever the type of graphical representation, it must
be remembered that maps of the subsurface and
cross-sections represent only interpretations, most of
which are based on scanty data. In reality, they are
merely graphical renditions that are presumably based
on scientific thought, a knowledge of depositional
characteristics of rock units, and a data base that
provides some control. They are not exact because the
features they attempt to show are complex, nearly
always hidden from view, and difficult to sample.
All things considered, graphical representations are
exceedingly useful, if not essential, to subsurface
studies. On the other hand, a particular drawing that
is prepared for one purpose may not be suitable for
another purpose even though the same units are
involved. This is largely due to scale and
generalizations.
A geologic map of a glaciated area is shown in fig. 1-
5. The upland area is mantled by glacial till (Qgm)
and the surficial material covering the relatively flat
flood plain has been mapped as alluvium (Qal). Beneath
the alluvial cover are other deposits of glacial origin that
consist of glacial till, outwash, and glacial lake deposits.
A water well drillers log of a boring in the valley states
"this well is just like all of the others in the valley" and
that the upper 70 feet of the valley fill consists of a
"mixture of clay, sand, silt, and boulders." This is
underlain by 30 feet of "water sand," which is the
aquifer. The aquifer overlies "slate, jingle rock, and
coal." The terminology may be quaint, but it is
nonetheless a vocabulary that must be interpreted.
10
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Qgm
Scale (miles)
Qal = aluvium
Qgm - ground monine
Qkt = terrace deposits
Figure 1-5. Generalized Geologic Map of a
Glaciated Area Along the Souris River Valley In
Central North Dakota
Examination of the local geology, as evidenced by
strata that crop out along the hill sides, indicates that
the bedrock or older material that underlies the glacial
drift consists of shale, sandstone cemented by calcite,
and lignite, which is an immature coal. These are the
geologic terms, at least in this area, for "slate, jingle
rock, and coal," respectively.
For generalized purposes, it is possible to use the
drillers log to construct a cross section across or along
the stream valley (fig. 1-6). In this case, one would
assume for the sake of simplicity, the existence of an
Water Well
Figure 1-6. Generalized Geologic Cross Section
of the Souris River Valley Based on Driller's Log
aquifer that is rather uniform in composition and
thickness. A second generation cross section, shown
in fig. 1-7, is based on several bore-hole logs described
by a geologist who collected samples as the holes were
being drilled. Notice in this figure that the subsurface
appears to be much more complex, consisting of several
isolated permeable units that are incorporated within
the fine-grained glacial deposits that fill the valley In
addition, the aquifer does not consist of a uniform
thickness of sand, but rather a unit that ranges from 30
to 105 feet in thickness and from sand to a mixture of
sand and gravel. The water-bearing characteristics o!
each of these units are all different. This cross section
too is quite generalized, which becomes evident as
one examines an actual log of one of the bore holes
(table 1-3).
In additionto showing more accurately the composition
of the subsurface, well logs also can provide some
interesting clues concerning the relative permeabilities
of the water-bearing units. Referring to Table 1-4, a
generalized log of well 1 describing the depth interval
ranging from 62 to 92 feet, contains the remark "losing
water" and in well 5, at a depths of 80 to 120 feet, is
the notation, "3 bags of bentonite." In the first case
"losing water" means that the material being penetrated
by the drill bit from 62 to 92 feet was more permeable
thantheannulusof the cutting-filled borehole. Some
of the water used for drilling, which is pumped down the
hole through the drill pipe to remove the cuttings, found
it easierto move out into the formation than to flow back
up the hole. The remark is a good indication of a
permeability that is higher than that present in those
sections where water was not being lost.
In the case of well 5, the material extending from 80 to
120 feet was so permeable that much of the drilling fluid
was moving into the formation and there was no return
of the cuttings. To regain circulation, bentonite, or to
use the field term, "mud," was added to the drilling
fluid to seal the permeable zone. Even though the
geologist described the aquifer materials from both
zones similarly, the section in well 5 is more permeable
than the one in well 1, which in turn is more permeable
than the other coarse-grained units penetrated where
there was no fluid loss.
The three most important points to be remembered
here are. first, graphical representations of the surf ace
or subsurface geology are merely guesses of what
might actually exist, and even these depend to some
extent on the original intended usage. Secondly, the
subsurface is far more complex .than is usually
anticipated, particularly in regard to unconsolidated
deposits. Finally, evaluating the original data, such as
well logs, might lead to a better appraisal of the
11
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Test
Hole
Test Test
Hole Hole
• .. . . . . -o .
••:••'.••'••••••!"••«"••.• ••
.••.'•.•••••";:*;i.':l:''r'/.'•" ••
•'•:•.' v.'/. ;.-. '.•••'.» o •••.•
• »-•••'• -.«•*• ••'•
scale(feet)
/>-'^°r :^/. ' S,' •>'• ^.•V?0; '>; •'.•.\VVC>'-'^-
Figure 1-7. Geologic Cross Section of the Souris River Valley Based on Detailed Logs of Test Holes
Sample Description
and Drilling Condition
Depth (ft!
Topsoil. silty clay, black 0-1
Clay, ailty, yellow brown, poorly con»ollclated 1-5
Clay, silty, yellow any, toft, moderately compacted B-10
Clay, ailty, as above, aHty layer*, soft 10-16
Silty, clayey, gray, aoft, uniform drilling 15-20
Clay, silty, some fine to medium sand, gray 30-30
Clay, gray to black, soft, very tight 30-40
Clay, at above, gravelly near top 40-60
'• Clay, as above, no gravel 60-00
Clay, at above, very silty in spots, gray 60-70
Clay and silt, very eesy drilling 70-80
Clay, as above to gravel, fine to coarse.
sandy, thin clay layers, taking lott of water 80-90
Gravel, as above, some clay near top, very
rough drilling, mixed three bags of mud. lott
of lignite chips 90-100
Gravel, as above, cobbles and boulders 100-120
Gravel, as above, to sand, fine to coarse,
lots of lignite, much easier drilling 120-130
Clay, gravelly and rocky, rough drilling, poor
sample return 130-140
Sandy day, gravelly and rocky, rough drilling.
poor sample return (tHl) 140-150
Sandy clay, as above, poor sample return 150-160
Clay, sandy, gray, soft, plastic, noncalcareous 160-170
Clay, sandy, as above, tight, uniform drilling 170-180
Clay, as above, much less sand, gray, soft,
tight, plastic 160-190
Clay, as above, no sand, good sample return 190-200
Clay, aa above 200-210
Table 1-3. Geologist's Log of a Test Hole, Souris
River Valley, North Dakota
subsurface, an appraisal that far surpasses the use of
generalized lithologic logs alone.
Ground Water In Igneous And Metamorphlc
Rocks
Nearly all of the porosity and permeability of igneous
and metamorphic rocks are the result of secondary
openings, such as fractures, faults, andthedissolution
of certain minerals. A few notable exceptions include
large lava tunnels present in some flows, interflow or
coarse sedimentary layers between individual lava flows,
and deposits of selected pyroclastic materials.
Because the openings in igneous and metamorphic
rocks are, volumetrically speaking, quite small, rocks of
thistypearepoorsuppliers of ground water. Moreover,
the supplies that are available commonly drain rapidly
after a period of recharge by infiltration of precipitation.
In addition they are subject to contamination from the
surface where these rocks crop out.
The width, spacing, and depth of fractures ranges
widely, as do their origin. Fracture widths vary from
about .0008 inches the surface to .003 inches at a
depth of 200 feet, while spacing increased from 5 to 10
feet near the surface to 15 to 35 feet at depth in the Front
Range of the Rocky Mountains (Snow, 1968). In the
same area porosity decreased from below 300 feet or
so, but there are many recorded exceptions. Exfoliation
fractures in the crystalline rocks of'the Piedmont near
Atlanta, GA range from 1 to 8 inches in width (Cressler
and others, 1983).
12
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Material
Depth (ft)
Test Hole 1
FBI 0-3
Silt, olive-gray 3-14
Sand, fine-medium 14-21
Silt, sandy, gray 21-25
Clay, gray 25-29
Sand. fine-coarse 29-47
Clay, gray 47-62
Gravel, fine to coarae, losing water 62-92
Silt, eartdy, gray 92-100
Observation well depth 80 feet
Teat Hole 2
Fill 0-2
Clay, silty and sandy, gray 2-17
Clay, very sandy, gray 17-19
Sand, fine-medium 19-60
Sand, fine-coarse with gravel 60-80
Gravel, coaree, 2 bagi bentonhe and bran 80-100
Observation well depth 88 feet
Teat Hole 3
Silt, yellow 0-5
Clay, silly, black 6-15
Sand, fine to coarse 15 29
Clay, sllty, gray 29-65
Sand, medium-coarse, some gravel 65-69.
Gravel, sandy, taking water 69-88
Sand, fine to medium, abundant chips of lignite 88-170
Observation well depth 84 feet
Test Hole 4
Fill 0-5
Silt, brown 5.12
Sand, fine-medium 12-28
Clay, silty and sandy, gray 28-37
Sand, fine 37-49
Clay, dark gray 49-55
Send, fine 55.61
Clay, sandy, gray 61-66
Sand, fine-coarse, some gravel 66-103
Silt, gray 103-120
Observation well depth 96 feet
Test Hole 5
Clay, sllty, brown 0-10
Silt, clayey, gray 10-80
Gravel, fine-coarse, sandy, taking loti of water
3 bags bentonite 80-120
Sand, fine to coarse, gravelly 120-130
Clay, gravelly and rocky (tni) 130-150
Sand, fine. Fort Union Group 160-180
Observation well depth 100 feet
Table 1-4. Generalized Geologic Logs of Five Test
Holes, Souris River Valley, North Dakota
The difficulty of evaluating water and contaminant
movement in fractured rocks is that the actual direction
of movement may not be in the direction of decreasing
head, but rather in some different though related
direction. The problem is further compounded by the
difficulty in locating the fractures. Because of these
characteristics, evaluation of water availability,
direction of movement, and velocity is exceedingly
difficult. As a general rule in the eastern part of the
United States, well yields, and therefore fractures.
permeability, and porosity, are greater in valleys and
broad ravines than on flat uplands, which in turn is
higher than on hill slopes and hill crests.
Unlesssomespecialcircumstanceexists.waterobtained
from igneous and metamorphic rocks is nearly always
of excellent chemical quality. Dissolved solids are
commonly less than 100 mg/L. Water from
metamorphosed carbonate rocks may have moderate
to high concentrations of hardness.
Ground Water In Sedimentary Rocks
Usable supplies of ground water can be obtained from
all types of sedimentary rocks, but the fine-grained
strata, such as shale and siltstone, may only provide
a few gallons per day and even this can be highly
mineralized. Even though fine-grained rocks may
have relatively high porosities, the primary
permeability is very low. On the other hand, shale is
likely to contain a great number of joints that are both
closely spaced and extend to depths of several tens of
feet. Therefore, rather than being impermeable, they
can be quite transmissive. This is of considerable
importance in waste disposal schemes because
insufficient attention might be paid during engineering
design to the potential for flow through fractures. In
addition, the leachate that is formed as water infiltrates
through waste might be small in quantity but highly
mineralized. Because of the low bulk permeability, it
would be difficult to remove the contaminated water or
even to properly locate monitoring wells.
From another perspective, fine-grained sedimentary
rocks, owing to their high porosity, can store huge
quantities of water. Some of this water can be released
to adjacent aquifers when a head difference is
developed by pumping. No doubt fine-grained
confining units provide, on a regional scale, a great
deal of water to aquifer systems. The porosity,
however, decreases with depth because of compaction
brought about by the weight of overlying sediments.
The porosity of sandstones range from less than 1
percent to a maximum of about 30 percent. This is a
function of sorting, grain shape, and cementation.
Cementation can be variable both in space and time
and on outcrops can differ greatly from that in the
subsurface.
Fractures also play an important role in the movement
of fluids through sandstones and transmissivities may
be as much as two orders of magnitude greater in a
fractured rock than in an unfractured part of the same
geologic formation.
13
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Sandstone units that were deposited in a marine or
near marine environment can be very wide spread,
covering tens of thousands of square miles, such as
the St. Peter Sandstone of Cambrian age. Those
representing ancient alluvial channel fills, deltas, and
related environments of deposition are more likely to be
discontinuous and erratic in thickness. Individual units
are exceedingly difficult to trace in the subsurface.
Regional ground-water flow and storage may be strongly
influenced by the geologic structure.
Carbonate rocks are formed in many different
environments and the original porosity and permeability
are modified rapidly afterburial. Some special carbonate
rocks, such as coquina and some breccias, may
remain very porous and permeable, but these are the
exception.
It is the presence of fractures and other secondary
openings that develop high yielding carbonate aquifers.
One important aspect is the change from calcite to
dolomite (CaMgfCOsJ), which results in a volumetric
reduction of 13 percent and the creation of considerable
pore space. Of particular importance and also concern
in many of the carbonate regions of the world, is the
dissolution of carbonates along fractures and bedding
planes by circulating ground water. This is the manner
in which caves and sinkholes are formed. As
dissolution progresses upward in a cave, the overlying
rocks may collapse to form a sinkhole that contains
water if the cavity extends below the water table.
Regions in which there has been extensive
dissolution of carbonates leading to the formation of
•caves, underground rivers, and sinkholes, are called
karst. Notable examples include parts of Missouri,
Indiana, and Kentucky.
Karst areas are particularly troublesome, even though
they can provide large quantities of water to wells and
springs. They are easily contaminated, and it is
commonly difficult to trace the contaminant because
the water can flow very rapidly, and there is no filtering
action to degrade the waste. Not uncommonly a well
owner may be unaware that he is consuming unsafe
water. An individual in Kentucky became concerned
because his well yield had declined. The well, which
drew water from a relatively shallow cave below the
water table, was cased with a pipe, on the end of which
was a screen. When the screen was pulled, it was
found to be completely coated with fibrous material.
The owner was disconcerted to learn that the fibrous
covering was derived from toilet paper.
Ground Water In Unconsolidated Sediments
Unconsolidated sediments accumulate in many
different environments, all of which leave their
trademark on the characteristics of the deposit. Some
are thick and areally extensive, as the alluvial fill in the
Basin and Range Province, others 'are exceedingly
long and narrow, such as the alluvial deposits along
streams and rivers, and others may cover only a few
hundred square feet, like some glacial forms. In addition
to serving as majoraquifers, Unconsolidated sediments
are also important as sources of raw materials for
construction.
Although closely related to sorting, the porosities of
Unconsolidated materials range from less than 1 to
more than 90 percent, the latter representing
uncompacted mud. Permeabilities also range widely.
Cementing of some type and degree is probably
universal, but not obvious, with silt and clay being
the predominant form.
Most Unconsolidated sediments owe their
emplacement to running water and, consequently,
some sorting is expected. On the other hand, water as
an agent of transportation will vary in both volume and
velocity, which is climate dependent, and this will leave
an imprint on the sediments. It is to be expected that
stream related material, which most Unconsolidated
material is, will be variable in extent, thickness, and
grain size. Other than this, one can draw no general
guidelines; therefore, it is essential to develop some
knowledge of the resulting stratigraphy that is
characteristic of the most common environments of
deposition. The water-bearing properties of glacial
drift, of course, are exceedingly variable, but stratified
drift is more uniform and better sorted than glacial till
Relation Between Geology, Climate, and Ground-
Water Quality
The availability of ground-water supplies and their
chemical quality are closely related to precipitation. As
a general rule, the least mineralized water, both in
streams and underground, occurs in areas of the
greatest amount of rainfall. Inland, precipitation
decreases, water supplies diminish, and the quality
deteriorates. The mineral composition of water-bearing
rocks exerts a strong influence on ground-water quality
and thus, the solubility of the rocks may override the role
of precipitation.
Where precipitation exceeds 40 inches per year,
shallow ground water usually contains less than
500mg/Land commonly less than 250 mg/L of dissolved
solids. Where precipitation ranges between 20 and
40 inches, dissolved solids may range between 400
and 1,000 mg/L, and in drier regions they commonly
exceed 1,000 mg/L.
14
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Dissolvnc) solids concentrations, ing/I
2f,OGOO
Q 500 1000
§U>1000
o too 20)
Scale (mites)
Figure 1-8. Dissolved Solids Concentrations In Ground Water Used for Drinking In the United States
(from Pettyjohn and others, 1979)
The dissolved solids concentration of ground water
increases toward the interior of the continent. The
increase is closely related to precipitation and the
solubility of the aquifer framework. The least
mineralized ground water is found in abroad belt that
extends southwardfromthe New England states, along
the Atlantic Coast to Florida, and then continues to
parallel much of the Gulf Coast. Similarly, along the
Pacific Coast from Washington to central California, the
mineral content is also very low. Throughout this belt,
dissolved solids concentrations generally are less than
250 mg/L and commonly less than 100 mg/L (fig. 1-8).
The Appalachian region consists of a sequence of
strata that range from nearly flat-lying to complexly
folded and faulted. Likewise, ground-water quality in
this region also is highly variable, being generally
harder and containing more dissolved minerals than
does water along the coastal belt. Much of the
difference in quality, however, is related to the
abundance of carbonate aquifers, which provide
waters rich in calcium and magnesium.
Westward from the Appalachian Mountains to about
the position of the 20-inch precipitation line (eastern
North Dakota to Texas), dissolved solids in ground
water progressively increase. They are generally less
than 1,000 mg/L and are most commonly in the 250 to
750 mg/L range. The water is moderately to very hard,
and in some areas concentrations of sulf ate and chloride
are excessive.
From the 20-inch precipitation line westward to the
northern Rocky Mountains, dissolved solids are in the
500 to 1.500 mg/L range. Much of the water from
glacial drift and bedrock formations is very hard and
15
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contains significant concentrations of calcium sulfate.
Other bedrock formations may contain soft sodium
bicarbonate, sodium sulfate, or sodium chloride water.
Throughout much of the Rocky Mountains, ground-
water quality is variable, although the dissolved
solids concentrations commonly range between 250
and750mg/L. Stretching southward from Washington
to southern California, Arizona, and New Mexico is a
vast desert region. Here the difference in quality is wide
and dissolved solids generally exceed 750 mg/L In the
central parts of some desert basins the ground water
is highly mineralized, but along the mountain flanks
the mineral content may be quite low.
Extremely hard water is found over much of the Interior
Lowlands, Great Plains, Colorado Plateau, and Great
Basin, isolated areas of high hardness are present in
northwestern New York, eastern North Carolina, the
southern tip of Florida, northern Ohio, and parts of
southern California. In general, the hardness is of the
carbonate type.
On a regional level, chloride does not appear to be
a significant problem, although it is troublesome
locally due largely to industrial activities, the intrusion
of seawater caused by overpumping coastal aquifers,
or interaquifer leakage related to pressure declines
brought about by withdrawals.
In many locations, sulfate levels exceed the federal
recommended limit of 250 mg/L; regionally sulfate
may be a problem only in the Great Plains, eastern
Colorado Plateau, Ohio, and Indiana. Iron problems
are ubiquitous because concentrations exceeding only
.3 mg/L will cause staining of clothing and fixtures.
Fluoride is abnormally high in several areas,
particularly parts of western Texas, Iowa, Illinois.
Indiana. Ohio, New Mexico, Wyoming, Utah, Nevada,
Kansas, New Hampshire, Arizona, Colorado, North
and South Dakota, and Louisiana.
Awater-qualityproblemofgrowingconcem. particularly
in irrigated regions, is nitrate, which is derived from
fertilizers, sewage, and through natural causes. When
consumed by infants less than six months old for a
period of time, high nitrate concentrations can cause
a disease known as "blue babies." This occurs because
the child's blood cannot carry sufficient oxygen; the
disease is easily overcome by using low nitrate water
for formula preparation. Despite the fact that nitrate
concentrations in ground water appear to have been
increasing in many areas during the last 30 years or
so. there have been no reported incidences of "blue
babies" for more than 20 years, at least in the states that
comprise the Great Plains.
Conclusions
In detail, the study of geology is complex, but the
principles outlined above should be sufficient for a
general understanding of the topic, particularly as it
relates to ground water. If interested in a more
definitive treatment, the reader should examine the
references at the end of the chapter.
References
Baver, L.D., W.H. Gardner, and W.R. Gardner, 1972,
Soil physics, 4th ed.: John Wiley & Sons, New York.
Birkeland, P. W., 1984, Soils andgeomorphology: Oxford
University Press, New York.
Birkeland, P.W and E.E. Larson, 1989, Putnam's
geology, 5th ed.: Oxford University Press, New York.
Blatt, H. G. Middleton, and R. Murray. 1980. Origin
of sedimentary rocks, 2nd ed.: Prentice-Hall Publ.
Co.. Inc., Englewood Cliffs, NJ.
Butler, B.E., 1980, Soil classification for soil survey:
Oxford University Press, New York.
Catt, J.A., 1988, Quaternary geology for scientists and
engineers: Halstead Press. New York.
Chorley. R.J., S.A. Schumm, and D.E. Sugden. 1984,
Geomorphology: Methuen. New York.
Davis, S.N. and R.J.M. DeWiest. 1966, Hydrogeology:
John Wiley & Sons, New York.
Dercourt, J. and J. Pacquet, 1985, Geology, principles
and methods: Gulf Publishing. Houston. TX.
Eicher, D.L., 1976, Geologic time: Prentice-Hall,
Englewood Cliffs, NJ.
Ernst. W.G..1969, Earth materials: Prentice-Hall Publ.
Co., Inc., Englewood Cliffs, NJ.
Eyles, N. (ed.), 1983, Glacial geology, an introduction
for engineers and earth scientists: Pergamon Press,
New York.
Field, M.S., 1989, The vulnerability of karst aquifers to
chemical contamination:in Recent Advances in Ground-
Water Hydrology. American Institute of Hydrology,
Minneapolis, MN, pp. 130-142:
Flint, K.F. and B.J. Skinner.1977, Physical geology, 2nd
ed: John Wiley & Sons, New York.
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Flint. R.F., 1971, Glacial and Quaternary geology:
John Wiley & Sons, New York.
Foster. R.J., 1971, Geology: Charles E. Merrill Publ.
Co., Columbus, OH.
Grim, R.E., 1968, Clay mineralogy, 2nd ed: McGraw-
Hill, New York.
Heath, R.C.. 1984, Ground-water regions of the
United States: U.S. Geol. Survey Water-Supply Paper-
2242.
Hunt, C.B.,.1972, Geology of soils: their evolution,
classification and uses: W.H. Freeman, San Francisco.
LaMoreaux, P.E., B.M. Wilson, and B.A. Mermon
(eds.), 1984, Guide to the hydrology of carbonate rocks:
UNESCO, Studies and Reports in Hydrology No. 41.
Pettyjohn, W.A., J.R.J. Studlick. and R.C. Bain. 1979,
Oualiiy of drinking water in rural America: Water
Technology, July-Aug.
Press, F. and R. Siever, 1982, Earth, 3rd ed: W.H.
Freeman, San Francisco.
Sawkins. F.J., C.G. Chase, D.G. Darby, and George
Rapp, Jr., 1978, The evolving earth, a text in physical
geology: Macmillan Publ. Co., Inc., New York.
Selby, M.J., 1986, Earth's changing surtace, an
introduction to geomorphology: Oxford University Press,
New York.
Sparks, B.W., 1986., Geomorphology, 3rd ed: Longman,
New York.
Spencer, E.W., 1977, Introduction to the structure of
the earth, 2nd ed: McGraw-Hill Book Co., Inc., New
York.
Tarbuck, E.J. and F.K. Lutgens, 1984, The earth, an
introduction to physical geology: Charles E. Merrill
Publ. Co., Inc., Columbus, OH.
Tolman, C.F., 1937, Ground water: McGraw-Hill Book
Co., Inc., New York.
17
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Chapter 2
CLASSIFICATION OF GROUND-WATER REGIONS
To describe concisely ground-water conditions in the
United States, it is necessary to divide the country into
regions in which these conditions are generally similar.
Because the presence and availability of ground water
depends primarily on geologic conditions, ground-water
regions also are areas in which the composition,
arrangement, and structure of rock units are similar
(Heath, 1982).
To divide the country into ground-water regions, it is
necessary to develop a classification that identifies
features of ground-water systems that affect the
occurrence and availability of ground water. The five
features pertinent to such a classification are: (I) the
components of the system and their arrangement, (2)
the nature of the water-bearing openings of the dominant
aquifer or aquifers with respect to whether they are of
primary or secondary origin, (3) the mineral composition
of the rock matrix of the dominant aquifers with respect
to whether it is soluble orinsoluble, (4) the water storage
and transmission characteristics of the dominant aquifer
or aquifers, and (5) the nature and location of recharge
and discharge areas.
The first two of these features are primary criteria used
in all delineations of ground-water regions. The remaining
three are secondary criteria that are useful in subdividing
what might otherwise be large and unwieldy regions
into areas that are more homogeneous and, therefore,
more convenient for descriptive purposes. Table 2-1
lists each of the five features together with explanatory
information. The fact that most of the features are more
or less interrelated is readily apparent from the comments
in the column headed "Significance of Feature."
Ground-Water Regions of the United States
On the basis of the criteria listed above the United
States, exclusive of Alaska and Hawaii, can be
divided into 11 ground-water regions.
Figure 2-1 shows the boundaries of these 11 regions.
A special area, region 12, which consists of those
segments of the valleys of perennial streams that are
underlain by sand and gravel thick enough to be
hydrologically significant (thicknesses generally more
than about 26 feet), is shown in Figure 2-2.
The nature and extent of the dominant aquifers and their
relations to other units of the ground-water system are
the primary criteria used in delineating the regions.
Consequently, the boundaries of the regions generally
coincide with major geologic boundaries and at most
places do not coincide with drainage divides. Although
this lack of coincidence emphasizes that the physical
characteristics of ground-water systems and stream
systems are controlled by different factors, it does not
mean that the two systems are not related. Ground-
water systems and stream systems are intimately related,
as shown in the following discussions of each of the
ground-water regions.
1. Western Mountain Ranges
(Mountains with thin soils overfractured rocks, alternating
with narrow alluvial and, in pad, glaciated valleys)
The Western Mountain Ranges, shown in Figure 2-3,
encompass three areastotaling 278,000 mi2-The largest
area extends in an arc from the Sierra Nevada in
California, norththrough the Coast Ranges and Cascade
Mountains in Oregon and Washington, and east and
south through the Rocky Mountains in Idaho and
Montana into the Bighorn Mountains in Wyoming and
the Wasatch and Uinta Mountains in Utah. The second
area includes the southern Rocky Mountains, which
extend from the Laramie Range in southeastern
Wyoming through central Colorado into the Sangre de
Cristo Range in northern New Mexico. The smallest
area includes the part of the Black Hills of South Dakota
in which Precambrian rocks are exposed.
As would be expected in such a large region, both the
origin of the mountains and the rocks that form them are
complex. Most of the mountain ranges are underlain by
18
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Feature
Aspect
Range In Condition*
Signlllonce ol Feature
Component of the
system
Unconfirmed aquifer
Confining beds
Confined aquifers
Presence and
arrangements of
components
Thin, discontinuous, hydrologically
insignificant.
Minior aquifer, serves primarily as a storage
reservoir and recharge conduit tor under-
lying aquifer.
The dominant aquifer.
Not present, or hydrologically insignificant.
Thin, markedly discontinuous, or very leaky.
Thick, extensive, and Impermeable.
Complexly interbedded with aquifers or
productive zones.
Not present, or hydrologicaily insignificant.
Thin or not highly productive.
Multiple thin aquifers Interbedded with
nonproductive zones.
The dominant aquifer—thick and productive.
A single, unconfirmed aquifer.
Two interconnected aquifers ol essentially
equal hydrologic importance.
A three-unit system consisting of an
unconfined aquifer, a confining bed, and
confined aquifer.
A complexly interbedded sequence of
aquifers and confining beds.
Affect response of the system to
pumpage and other stresses.
Affect recharge and discharge
conditions. Determine suscept-
ibility to pollution.
Water-bearing
openings of
dominant aquifer
Primary openings
Secondary openings
Pcxes in unconsolidateo deposits.
Pores in semiconsolidated rocks.
Pores, tubes, and cooling fractures In
volcanic (extrusive-igneous) rocks.
Fractures and faults in crystalline and
consolidated sedimentary rocks.
Solution-enlarged openings in limestones
and other soluble rocks.
Control water-storage and trans-
mission characteristics. Alfecl
disperson and dilution ol
wastes.
. Composition of rock
matrix of
dominant aquifer
Insoluble
Soluble
Essentially insoluble.
Both relatively Insoluble and soluble
constituents.
Relatively soluble.
Affects water-storage and trans-
mission characteristics. Has
major influence on water
quality.
Storage and
transmission
characteristics of
dominant aquifer
Porosity
Transmlssivity
Large, as in well-sorted, unconsolidated
deposits.
Moderate, as in poorly-sorted unconsolidated
deposits and semiconsolidated rocks.
Small, as in fractured crystalline and
consolidated sedimentary rocks.
Large, as In cavernous limestones, some
lava flows, and clean gravels.
Moderate, as in well-sorted, coarse-grained
sands, and semiconsolidated limestones.
Small, as in poorly-sorted, fine-grained
deposits and most fractured rocks.
Very small, as In confining beds.
Control response to pumpage and
other stresses. Determine yield
ol wells. Atfect long-term yield
of system. Affect rate at which
pollutants move.
Recharge and
discharge
conditions of
dominant aquifer
Recharge
Discharge
In upland areas between streams, particu-
larly In humid regions.
Through channels ol losing streams.
Largely or entirely by leakage across
confining beds from adjacent aquifers.
Through springs or by seepage to stream
channels, estuaries, or the ocean.
By evaporation on Hood plains and in basin
"sinks."
By seepage across confining beds into
adjacent aquifers.
Affect response to stress and
long-term yields. Determine
susceptibility to pollution.
Affect.water quality.
Table 2-1. Features of Ground-Water Systems Useful In the Delineation of Ground-Water Regions
19
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2. Alluvial BMin
Nongl*ciNOS
•~. •.. •••• ^. ~ \
• *-l^ Yo\.
^\
^
500 Miles
800 Kilometers
Figure 2-1. Ground-Water Regions Used In This Report [The Alluvial Valleys Region (region 12) Is
shown on figure 2-2]
Figure 2-2. Alluvial Valleys Ground-Water Region
20
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CANADA
UNITED STATES
WASHINGTON
OREGON
San Juan Mountains
COLO.
3
I
I I
3 100
100
1
1
200
200
I
300
1
400
300
|
1
500
400
1
1
600
I
700
500 Miles
1
1
800
Kilometers
Figure 2-3. Western Mountain Ranges Region
21
-------
granitic and metamorphic rocks flanked by consolidated
sedimentary rocks of Paleozoic to Cenozoic age. The
other ranges, including the San Juan Mountains in
southwestern Colorado and the Cascade Mountains in
Washington and Oregon, are underlain by lavas and
other igneous rocks.
The summits and slopes of most of the mountains
consist of bedrock exposures or of bedrock covered by
a layer of boulders and other rock fragments produced
by frost action and other weathering processes acting
on the bedrock. This layer is generally only a few feet
thick on the upper slopes but forms a relatively thick
apron along the base of the mountains. The narrow
valleys are underlain by relatively thin, coarse, bouldery
alluvium washed from the higher slopes. The large
synclinal valleys and those that occupy downfaulted
structural troughs are underlain by moderately thick
deposits of coarse-grained alluvium transported by
streams from the adjacent mountains, as shown in
Figure 2-4.
The Western Mountain Ranges and the mountain ranges
in adjacent regions are the principal sources of water
supplies developed at lower altitudes in the western half
of the conterminous United States. As McGuinness
(1963) noted, the mountains of the West are moist
"islands" in a sea of desert or semidesert that covers the
western half of the Nation. The heaviest precipitation
falls on the western slopes; thus, these slopes are the
major source of runoff and are also the most densely
•vegetated. Much of the precipitation falls as snow
during the winter.
The Western Mountain Ranges are sparsely populated
and have relatively small water needs. The region is an
exporterofwaterto adjacent "have-not" areas. Numerous
surface reservoirs have been constructed in the region.
Many such impoundments have been developed on
streams that drain the western flank of the Sierra
Nevada in California and the Rocky Mountains in
Colorado.
Melting snow and rainfall at the higher altitudes in the
region provide abundant water for ground-water
recharge. However, the thin soils and bedrock fractures
in areas underlain by crystalline rocks fill quickly, and
the remaining water runs off overland to streams.
Because of theirsmall storage capacity, the underground
openings provide limited base runoff to the streams,
which at the higher altitudes flow only during rains or
snowmelt periods. Thus, at the higher altitudes in this
Synclinal valley
Consolidated
sedimentary rock» C\
Water-bearing
fractures
Alluvial
deposits
Granitic and metamorphi
rocks
Figure 2-4. Topographic and Geologic Features In the Southern Rocky Mountains Part of the Western
Mountain Ranges Region
22
-------
region underlain by crystalline rocks, relatively little
opportunity exists for development of ground-water
supplies. The best opportunities exist in the valleys that
contain at least moderate thicknesses of saturated
alluvium or in areas underlain by permeable sedimentary
or volcanic rocks. Ground-water supplies in the valleys
are obtained both from wells drawing from the alluvium
and from wells drawing from the underlying rocks. The
yieldsof wells in crystalline bedrock and from small, thin
deposits of alluvium are generally adequate only for
domestic and stock needs. Large yields can be obtained
from the alluvial deposits that overlie the major lowlands
and from wells completed in permeable sedimentary or
volcanic rocks.
2. Alluvial Basins
(Thick alluvial deposits in basins and valleys
bordered by mountains and locally of glacial origin)
The Alluvial Basins region occupies a discontinuous
area of 396.000 mi2extending from the Puget Sound-
Williamette Valley area of Washington and Oregon to
west Texas. The region consists of an irregular
alternation of basins or valleys and mountain ranges. In
the Alluvial Basins region, basins and valleys are the
dominant feature. The principal exception is the Coast
Ranges of southern California which topographically
more closely resemble the Western Mountain Ranges.
Most of the Nevada and all of the Utah pans of this
region are an area of internal drainage referred to as the
Great Basin. No surface or subsurface flow leaves this
part of the region and all water reaching it from adjacent
areas and from precipitation is evaporated ortranspired.
The basins and valleys range from about 280 ft below
sea level in Death Valley in California to 6,550 ft above
sea level in the San Luis Valley in Colorado. The basins
range in size from a few hundred feet in width and a mile
or two in length to, for the Central Valley of California,
as much as 50 mi in width and 400 mi in length. The
crests of the mountains are commonly 3,300 to 4,900 ft
above the adjacent valley floors.
The surrounding mountains, and the bedrock beneath
the basins, consist of granite and metamorphic rocks of
Precambrian to Tertiary age and consolidated
sedimentary rocks of Paleozoic to Cenozoic age. The
rocks are broken along fractures and faults that may
serve as water-bearing openings. However, the openings
' in the granitic and metamorphic rocks in the mountainous
areas have a relatively small capacity to store and to
transmit ground water.
The dominant element in the hydrology of the region is
the thick (several hundred to several thousand feet)
layer of generally unconsolidated alluvial material that
partially fills the basins. Figures 2-5, 2-6, and 2-7
illustrate this dominant element. Generally, the coarsest
material occurs adjacent to the mountains; the material
gets progressively f inertoward the centers of the basins.
However, as Figure 2-6 shows, in most alluvial fans
there are layers of sand and gravel that extend into the
central parts of the basins. In time, the fans formed by
adjacent streams coalesced to form a continuous and
thick deposit of alluvium that slopes gently from the
Partly drained
tributary area
Figure 2-5. Common Ground-Water Flow Systems In the Alluvial Basins Region (From U.S. Geological
urvey Professional Paper 813-G)
23
-------
Figure 2-6. Common Relationships between Ground Water and Surface Water In the Alluvial
Basins Region (Modified from U.S. Geological Survey Professional Paper 813-G)
mountains toward the center of the basins. These
alluvial-fan deposits are overlain by or grade into fine-
grained flood plain, lake, or playa deposits in the central
part of most basins. The fine-grained deposits are
especially suited to large-scale cultivation.
The Puget Sound and Williamette Valley areas differ
geologically from the remainder of the region. The
Puget Sound area is underlain by thick and very
permeable deposits of gravel and sand laid down by
glacial meltwater. The gravel and sand are interbedded
with clay in parts of the area. The Williamette Valley is
mostly underlain by interbedded sand, silt, and clay
deposited on floodplains by the Williamette River and
other streams.
The Alluvial Basins region is the driest area in the United
States, with large parts of it being classified as semiarid
and arid. Annual precipitation in the valleys in Nevada
and Arizona ranges from about 4 to 6 in. However, in the
mountainous areas throughout the region, in the northern
part of the Central Valley of California, and in the
Washington-Oregon area, annual precipitation ranges
from about 16 in to more than 31 in. The region also
receives runoff from streams that originate in the
mountains of the Western Mountain Ranges region.
Because of the very thin cover of unconsolidated material
on the mountains, precipitation runs off rapidly down the
valleys and out onto the fans, where it infiltrates. The
water moves through the sand and gravel layers toward
the centers of the basins. The centers of many basins
consist of flat-floored, vegetation-free areas onto which
ground water may discharge and on which overland
runoff may collect during intense storms. The waterthat
collects in these areas (playas), evaporates relatively
quickly, leaving both a thin deposit of clay and other
sediment and a crust of the soluble salts that were
dissolved in the water, as Figure 2-5 illustrates.
Studies in the region have shown that the hydrology of
the alluvial basins is more complex than that described
in the preceding paragraph, which applies only to what
has been described as "undrained closed basins." As
Figure 2-5 shows, water may move through permeable
bedrock from one basin to another, arriving, ultimately,
at a large playa referred to as a "sink." Waterdischarges
from sinks not by "sinking" into the ground, but by
evaporating. In those parts of the region drained by
perennial watercourses ground water discharges to the
streams from the alluvial deposits. However, before
entering the streams, water may move down some
valleys through the alluvial deposits for tens of miles. A
reversal of this situation occurs along the lower Colorado
River and atthe upstream end of the valleys of some of
the other perennial streams; in these areas, water
moves from the streams into the alluvium to supply the
needs of the adjacent vegetated zones.
24
-------
I I 1 I I I I I [
0 100 200 300 400 500 600 700 800 Kilometers
Figure 2-7. Areas Underlain by Sand and Gravel In the Alluvial Basins Region
25
-------
Ground water is the major source of water in the Alluvial
Basins region. Because of the dry climate, agriculture
requires intensive irrigation. Most of the ground water is
obtained from the sand and gravel deposits in the valley
alluvium. These deposits are interbedded with finer
grained layers of silt and clay that are also saturated
with water. When hydraulic heads in the sand and
gravel layers are lowered by withdrawals, the water in
the silt and clay begins to move slowly into the sand and
gravel. The movement, which in some areas takes
decades to become significant, is accompanied by
compaction of the silt and clay and subsidence of the
land surface. Subsidence is most severe in parts of the
Central Valley, where it exceeds 30 ft in one area, and
in southern Arizona, where subsidence of more than
13 ft has been observed.
3. Columbia Lava Plateau
(Thick sequence of lava flows irregularly interbedded
with thin unconsolidated deposits and overlain by thin
soils)
As Figure 2-8 shows, the Columbia Lava Plateau
occupies an area of 141,000 mi2 in northeastern
California, eastern Washington and Oregon, southern
Idaho, and northern Nevada. As its name implies, it is
basically a plateau, standing generally between 1,640
and 5,900 ft above sea level, that is underlain by a great
thickness of lava flows irregularly interbedded with silt,
sand, and other unconsolidated deposits.
The great sequence of lava flows, which ranges in
thickness from less thanl 60 ft adjacent to the bordering
mountain ranges to more than 3,300 ft in south-central
Washington and southern Idaho, is the principal water-
bearing unit in the region. As Figure 2-9 shows, the water-
bearing lava is underlain by granite, metamorphic rocks,
older lava flows, and sedimentary rocks, none of which
are very permeable. Individual lava flows in the water-
bearing zone range in thickness from several feet to
more than 160 ft and average about 50 ft. The volcanic
rocks yield water mainly from permeable zones that
occur at or near the contacts between some flow layers.
Parts of some flows are separated by soil zones and, at
places, by sand, silt, and clay. These sedimentary
layers, where they occur between lava flows, are
commonly referred to as "interflow sediments." Gravel,
sand, silt, and clay cover the volcanic rocks and the
older exposed bedrock in parts of the area.
From the standpoint of the hydraulic characteristics of
the volcanic rocks, it is useful to divide the Columbia
Lava Plateau region into two parts: (I) the area in
southeastern Washington, northeastern Oregon, and
the Lewiston area of Idaho, part of which is underlain by
volcanic rocks of the Columbia River Group; and (2) the
remainder of the area shown on Figure 2-8, which also
includes the Snake River Plain. The basalt underlying
the Snake River Plain is referred to as the Snake River
Basalt; that underlying southeastern Oregon and the
remainder of this area has been divided into several
• units, to which names of local origin are applied
(Hampton. 1964).
The Columbia River Group is of Miocene to Pliocene (?)
age and consists of relatively thick flows that have been
deformed into a series of broad folds and offset locally
along normal faults. Movement of ground water occurs
primarily through the interflow zones near the top of
flows and, to a much smaller extent, through fault zones
and through joints developed in the dense central and
tower parts of the flows. The axes of sharp folds and the
offset of the interflow zones along faults form subsurface
dams that affect the movement of ground water. Water
reaching the interflow zones tends to move down the dip
of the flows from fold axes and to collect updip behind
faults that are transverse to the direction of movement
(Newcomb, 1962). As a result, the basalt in parts of the
area is divided into a series of barrier-controlled
reservoirs, which are only poorly connectedhydraulically
to adjacent reservoirs.
The water-bearing basalt underlying California, Nevada.
southeastern Oregon, and southern Idaho is of Pliocene
to Holocene age and consists of small, relatively thin
flows that have been affected to a much smaller extent
by folding and faulting than has the Columbia River
Group. The thin flows contain extensive, highly
permeable interflow zones that are relatively effectively
interconnected through a dense network of cooling
fractures. Structural barriers to ground-water movement
are of minor importance. This is demonstrated by
conditions in the 17,000 mi2 area of the Snake River
Plain east of Bliss. Idaho.
The interflow zones form a complex sequence of
relatively horizontal aquifers that are separated vertically
by the dense central and lower parts of the lava flows
and by interlayered clay and silt. Hydrologists estimate
that the interflow zones, which range in thickness from
about 3 ft to about 26 ft, account for about 10 percent
of the basalt. MacNish and Barker (1976) have estimated
that the hydraulic conductivity along the flow-contac!
zones may be a billion times higher than the hydraulic
conductivity across the dense zones. The lateral extent
of individual aquifers is highly variable.
The large differences in hydraulic conductivity between
the aquifers and the intervening "confining zones" result
in significant differences in hydraulic heads between
different aquifers. These differences reflect the head
tosses that occur as water moves vertically through the
26
-------
125'
120°
Explanation
Chiefly sedimentary rocks
Chiefly volcanic rocks
Sedimentary and volcanic rocks
Major aquifers thin or absent
igure 2-8. Generalized Distribution and Types of Major Aquifers of the Columbia Lava Plateau Region
edified from U.S. Geological Survey Professional Paper 813-S)
27
-------
Older mountains
^ A f\
River canyon
Explanation
Lava {
Flows |
_ _
x «. * x
111 III
^-Interflow zone
Silt and clay
' Cooling fractures
Figure 2-9. Topographic and Geologic Features of the Columbia Lava Plateau Region
system. As a result, heads decrease with increasing
depth in recharge areas and increase with increasing
depth near the streams that serve as major lines of
ground-water discharge. As Figure 2-10 shows, the
difference in heads between different aquifers can
result in the movement of large volumes of water
between aquifers through the ppenhole (uncased)
sections of wells.
Much of the Columbia Lava Plateau region is in the "rain
shadow" east of the Cascades and, as a result, receives
only 8 to 47 in of precipitation annually. The areas that
receive the least precipitation include the plateau area
immediately east of the Cascades and the Snake River
Plain. Recharge to the ground-water system depends
on several factors, including the amount and seasonal
distribution of precipitation and the permeability of the
surficial materials. Most precipitation occurs in the
winter and thus coincides with the cooler, nongrowing
season when conditions are mostfavorableforrecharge.
The Columbia-North Pacific Technical Staff (1970)
estimates that recharge may amount to 24 in in areas
underlain by highly permeable young lavas that receive
abundant precipitation. Considerable recharge also
occurs by infiltration of water from streams that flow
onto the plateau from the adjoining mountains. These
sources of natural recharge are supplemented in
agricultural areas by the infiltration of irrigation water.
Discharge from the ground-water system occurs as
seepage to streams, as spring flow, and by
evapotranspiration in areas where the water table is at
or nearthe land surface. The famous Thousand Springs
and other springs along the Snake River canyon in
southern Idaho are, in fact, among the most spectacular
displays of ground-water discharge in the world.
The large withdrawal of water in the Columbia Lava
Plateau for irrigation, industrial, and' other uses has
resulted in declines in ground-water levels of as much
as 100 to 200 ft in several areas. In most of these areas,
the declines have been slowed or stopped through
28
-------
so-
~n ET
Jit £
.ins
^VY,
Wtte- level <
S
I
8
200-
750
500 750 0 2SO 500
Hole Radius in Millimeters
750
Figure 2-10. Well In a Recharge Area In the
Columbia River Group (Modified from Luzler and
Burt, 1974)
regulatory restrictions or other changes that have
reduced withdrawals. Declines are still occurring, at
rates as much as a few teet per year, in a few areas.
4. Colorado Plateau and Wyoming Basin
(Thin soils over consolidated sedimentary rocks)
The Colorado Plateau and Wyoming Basin region
occupies an area of 160,000 mi^ in Arizona, Colorado,
New Mexico, Utah, and Wyoming. It is a region of
canyons and cliffs of thin, patchy, rocky soils, and of
sparse vegetation adapted to the arid and semiarid
climate. The large-scale structure of the region is that of
a broad plateau standing at an altitude of 8,200 to
11,500 ft and underlain by horizontal to gently dipping
layers of consolidated sedimentary rocks. As Figure 2-
11 shows, the plateau structure has been modified by
an irregular alternation of basins and domes, in some of
which major faults have caused significant offset of the
rock layers. The region is bordered on the east, north,
and west by mountain ranges that tend to obscure its
plateau structure. It also contains rather widely scattered
extinct volcanoes and lava fields.
The rocks that underlie the region consist principally of
sandstone, shale, and limestone of Paleozoic to
Cenozoic age. In parts of the region these rock units
include significant amounts of gypsum (calcium suit ate).
In the Paradox Basin in western Colorado the rock units
include thick deposits of sodium- and potassium-bearing
Canyon
Extinct volcanoes
Ridges
Dome
Fault scarp
Cliff
Fault
Sandstone
Limestone
Metamorphic
rocks
Figure 2-11. Topographic and Geologic Features of the Colorado Plateau and Wyoming Basin Region
29
-------
minerals, principally halite (sodium chloride). The
sandstones and shales are most prevalent and most
extensive. The sandstones are the principal sources of
ground water and contain water in fractures developed
both along bedding planes and across the beds and in
interconnected pores. The most productive sandstones
are those that are only partially cemented and retain
significant primary porosity.
Unconsolidated deposits are of relatively minor
importance in this region. Thin deposits of alluvium
capable of yielding small to moderate suppliesof ground
water occur along parts of the valleys of major streams.
especially adjacent to the mountain ranges in the
northern and eastern parts of the region. In most of the
remainder of the region there are large expanses of
exposed bedrock, and the soils, where present, are thin
and rocky.
Recharge of the sandstone aquifers occurs where they
are exposed above the cliffs and in the ridges. Average
precipitations ranges from about 6 in in the lower areas
to about 39 in in the higher mountains. The heaviest
rainfall occurs in the summer in isolated, intense
thunderstorms during which some recharge occurs
where intermittent streams flow across sandstone
outcrops. However, most recharge occurs in the winter
during snowmelt periods. Water moves down the dip of
the beds away from the recharge areas to discharge
along the channels of major streams through seeps and
springs and along the walls of canyons cut by the
streams.
The quantity of water available for recharge is small, but
so are the porosity and the transmissivity of most of the
sandstone aquifers. The water in the sandstone aquifers
is unconfined in the recharge areas and is confined
downdip. Because most of the sandstones are
consolidated, the storage coefficient in the confined
parts of the aquifers is very small. Even small rates of
withdrawal cause extensive cones of depression around
pumping wells.
The Colorado Plateau and Wyoming Basin is a dry,
sparsely populated region in which most water supplies
are obtained from the perennial streams. Less than 5
percent of the water needs are supplied by ground
water, and the development of even small ground-water
supplies requires the application of considerable
knowledge of the occurrence of both rock units and their
structure, and of the chemical quality of the water. Also,
because of the large surface relief and the dip of the
aquifers, wells even for domestic or small livestock
supplies must penetrate to depths of a few hundred
feet in much of the area. Thus, the development of
ground-water supplies is far more expensive than in
most other parts of the country. These negative aspects
notwithstanding, ground waterin the region can support
a substantial increase over the present withdrawals.
As in most other areas of the country underlain by
consolidated sedimentary rock, mineralized (saline)
water—that is, water containing more than 1,000 mg/L
of dissolved solids—is widespread. Most of the shales
and siltstonescontain mineralized waterthroughout the
region and below altitudes of about 6,500 ft.
Freshwater—water containing less than 1,000 mg/L of
dissolved solids—occurs only in the most permeable
sandstones and limestones. Much of the mineralized
water is due to the solution of gypsum and halite.
Although the aquifers that contain mineralized water
are commonly overlain by aquifers containing freshwater,
this situation is reversed in a few places where aquifers
containing mineralized water are underlain by more
permeable aquifers containing freshwater.
5. High Plains
(Thick alluvial deposits over fractured sedimentary
rock)
The High Plains region occupies an area of 174,000 mi^
extending from South Dakota to Texas. The plains are
a remnant of a great alluvial plain built in Miocene time
by streams that flowed east from the Rocky Mountains.
Erosion has removed a large part of the once extensive
plain, including all of the part adjacent to the mountains,
except in a small area in southeastern Wyoming.
The original depositional surface of the alluvial plain is
still almost unmodified in large areas, especially in
Texas and New Mexico, and forms a flat, imperceptibly
eastward-sloping tableland that ranges in altitude from
about 6,500 ft near the Rocky Mountains to about
1,600 ft along its eastern edge. The surface of the
southern High Plains contains numerous shallow circular
depressions, called playas, that intermittently contain
water following heavy rains. As Figure 2-12 shows,
other significant topographic features include sand
dunes, which are especially prevalent in central and
northern Nebraska, and wide, downcut valleys of streams
that flow eastward across the area from the Rocky
Mountains.
The High Plains region is underlain by one of the most
productive and most extensively developed aquifers in
the United States. The alluvial materials derived from
the Rocky Mountains, which are referred to as the
Ogallala Formation, are the dominant geologic unit of
the High Plains aquifer. The Ogallala ranges in thickness
from a few tens of feet to more than 650 ft and consists
of poorly sorted and generally unconsolidated clay, silt,
sand and gravel.
30
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Plstte River
Explanation
[.-: :V-.y/. j Sand j^"—"^ Clay
Sandstone
Figure 2-12. Topographic and Geologic Features of the High Plains Region
Younger alluvial materials of Quaternary age overlie the
Ogallala Formation of late Tertiary age in most parts of
the High Plains. Where these deposits are saturated,
they form a part of the High Plains aquifer; in parts of
south-central Nebraska and central Kansas, where the
Ogallala is absent, they comprise the entire aquifer. The
Quartemary deposits are composed largely of material
derived from the Ogallala and consist of gravel, sand,
silt, and clay. An extensive area of dune sand occurs in
the Sand Hills area north of the Platte River in Nebraska.
Other, older geologic units that are hydrologically
connected to the Ogallala include the Arikaree Group of
Miocene age and a small part of the underlying Brule
Formation. The Arikaree Group is predominantly a
massive, very fine to fine-grained sandstone that locally
contains beds of volcanic ash, silly sand, and sandy
clay. The maximum thickness of the Arikaree is about
1000 ft, in western Nebraska. The Brule Formation of
Oligocene age underlies the Arikaree. In most of the
area in which it occurs, the Brule forms the base of the
High Plains aquifer. However, in the southeastern comer
of Wyoming and the adjacent parts of Colorado and
Nebraska, the Brule contains fractured sandstones
hydraulically interconnected to the overlying Arikaree
Group; in this area the Brule is considered to be a part
of the High Plains aquifer.
In the remainder of the region, the High Plains aquifer
is underlain by several formations, ranging in age from
Cretaceous to Permian and composed principally of
shale, limestone, and sandstone. The oldest of these
underlies parts of northeastern Texas, western
Oklahoma, and central Kansas and contains layers of
relatively soluble minerals including gypsum, anhydrite,
and halite (common salt), which are dissolved by
circulating ground water.
Prior to the erosion that removed most of the western
part of the Ogallala, the High Plains aquifer was
recharged by the streams that flowed onto the plain
from the mountains to the west as well as by local
precipitation. The only source of recharge now is local
precipitation, which ranges from about 16 in along the
western boundary of the region to about 24 in along the
eastern boundary. Precipitation and ground-water
recharge on the High Plains vary in an east-west
direction, but recharge to the High Plains aquifer also
varies in a north-south direction. The average annual
rate of recharge has been determined to range from
about 0.2 in in Texas and New Mexico to about 4 in in
the Sand Hills in Nebraska This large difference is
explained by differences in evaporation and transpiration
and by differences in the permeability of surficial
materials.
-------
In some parts of the High Plains, especially in the
southern part, the near-surface layers of the Ogallala
have been cemented with lime (calcium carbonate) to
form a material of relatively low permeability called
caliche. Precipitation on areas underlain by caliche
soaks slowly into the ground. Much of this precipitation
collects in playas that are underlain by silt and clay, with
the result that most of the water evaporates. It is only
during years of excessive precipitation that significant
recharge occurs and this, as noted above, averages
only aboutO.2 in per year in the southern part of the High
Plains. In the Sand Hills area about 20 percent of the
precipitation (or about4 in annually) reaches the water
table as recharge.
Figure 2-13 shows that the water-table of the High
Plains aquifer has a general slope toward the east.
Gutentag and Weeks (1980) estimate that, on the basis
of the average hydraulic gradient and aquifer
characteristics, that water moves through the aquifer at
a rate of about 1 ft per day.
Natural discharge fromtheaquiferoccursto streams, to
springs and seeps along the eastern boundary of the
plains, and by evaporation and transpiration in areas
where the water table is within a few feet of the land
surface. However, at present the largest discharge is
probably through wells. The widespread occurrence o'
permeable layers of sand and gravel, which permit the
construction of large-yield wells almost any place in the
region, has led to the development of an extensive
agricultural economy largely dependent on irrigation.
Most of this water is derived from ground-water storage,
resulting in a long-term continuing decline in ground-
water levels in parts of the region of as much as 3 ft per
year.
The depletion of ground-water storage in the High
Plains is a matter of increasing concern in the region.
However, from the standpoint of the region as a whole,
the depletion does not yet represent a large part of the
storage that is available for use. Weeks and Gutentag
(1981) estimate, on the basis of a specific yield of 15
percent of the total volume of saturated material, that
the available (usable) storage in 1980 was about
3.3 billion acre-ft. Luckey, Gutentag, and Weeks (1981)
estimate that this is only about 5 percent less than the
storage that was available at the start of withdrawals.
However, in areas where intense irrigation has long
been practiced, depletion of storage is severe.
6. Nonglaclated Central Region
(Thin regolith over fractured sedimentary rocks)
As Figure 2-14 shows, the Nonglaciated Central region
is an area of about 671,000 mi2 extending from the
Appalachian Mountains on the east to the Rocky
Mountains on the west. The part of the region in eastern
Colorado and northeastern New Mexico is separated
from the remainder of the region by the High Plains
region. The Nonglaciated Central region also includes
the Triassic Basins in Virginia and North Carolina and
the "driftless" area in Wisconsin, Minnesota, Iowa, and
Illinois where glacial deposits, if present, are thin and of
no hydrologic importance.
The region is geologically complex. Most of it is underlain
by consolidated sedimentary rocks that range in age
from Paleozoic to Tertiary and consist largely of
sandstone, shale, limestone, dolomite, and
conglomerate. A small area in Texas and western
Oklahoma is underlain by gypsum. Figure 2-15 shows
that throughout most of the region the rock layers are
horizontal or gently dipping. Principal exceptions are
the Valley and Ridge section, the Wichita and Arbuckle
Mountains in Oklahoma, and the Ouachita Mountains in
Oklahoma and Arkansas, in all of which the rocks have
been folded and extensively faulted. As Figure 2-16
shows, around the Black Hills and along the eastern
side of the Rocky Mountains the rock layers have been
bent up sharply toward the mountains and truncated by
erosion. The Triassic Basins in Virginia and North
Carolina are underlain by moderate to gently dipping
beds of shale and sandstone that have been extensively
faulted and invaded by narrow bodies of igneous rock.
The land surface in most of the region is underlain by
regolith formed by chemical and mechanical breakdown
of the bedrock. In the western part of the Great Plains
the residual soils are overlain by or intermixed with
wind-laid deposits. In areas underlain by relatively pure
limestone, the regolith consists mostly of clay and is
generally only a few feet thick. Where the limestones
contain chert and in the areas underlain by shale and
sandstone, the regolith is thicker, up to 100 ft or more
in some areas. The chert and sand form moderately
permeable soils, whereas the soils developed on shale
are finer grained and less permeable.
As Figure 2-15 shows, the principal water-bearing
openings in the bedrock are fractures, which generally
occur in three sets. The first set, and the one that is
probably of greatest importance from the standpoint of
groundwater as well yields, consists of fractures
developed along bedding planes. The two remaining
sets are essentially vertical and thus cross the bedding
planes at a steep angle. The primary difference between
the sets of vertical fractures is in the orientation of the
fractures in each set. The vertical fractures facilitate
movement of water across the rock layers and thus
serve as the principal hydraulic connection between the
bedding-plane fractures.
32
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100"
SOUTH DAKOTA
OKLAHOMA
Explanation
900— Altitude of the water table in
meters, winter 1978
Contour interval 300 meters
Datum is National Geodetic
Vertical Datum of 1929
200 Kilometers
Figure 2-13. Altitude of the Water Table of the High Plains Aquifer
33
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r^
/ >
Catskill - -0 X
Mountains /y \ • \ ^
BigSpriteV0
X7> ! "»-,r""7£j(
>=: \ Ozark > / Jt*
^ ^.u:., t^r.-.rr^'A^
Wichita IFa'teaus Ag. „
Mountains I /S^ Sf
Figure 2-14. Location of Geographic Features Mentioned In the Discussions or Regions Covering the
Central and Eastern Parts of the United States
Regolith
?n^
V.1S"5
J-rr-
si.:->r^rr.: •-• • t^r^: '^^ •
-l::--i-1::?--~-~-4- - - - -' - - '^rr- '_" -Um^st-on.e. T-
B»^^ ^ • T ^T7" • ;W''-;'-':5>V'tf:;i;Kf' • -
Beddmg^iane/-^ • JL. • • •«.'
fractures
Shale
»M*»
«*J
Sandstone
'-~tt^^-:''A'-'
?'~^--~££^fc
Fresh water
^^'^ Salty water
Figure 2-15. Topographic and Geologic Features of the Nonglaciated Central Region
34
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Explanation
F I Fresh water (TJ '• '• -j Sandstone
--3 Shale
Metamorphic rocks
Figure 2-16. Topographic and Geologic Features Along the Western Boundary of the Nonglaclated
Central Region
In the parts of the region in which the bedrock has been
folded or bent, fractures range from horizontal to vertical.
Fractures parallel to the land surface, where present,
are probably less numerous and of more limited extent
than in areas of flat-lying rocks.
The openings developed along most fractures are less
than aO.04 in wide. The principal exception occurs in
limestones and dolomites. Water moving through these
rocks gradually enlarges the fractures to form, in time,
extensive cavernous openings or cave systems. Many
large springs emerge from these openings.
Recharge of the ground-water system in this region
occurs primarily in the outcrop areas of the bedrock
aquifers in the uplands between streams. Precipitation
in the region ranges from about 16 in per year in the
western part to more than 47 in in the eastern part. This
wide difference in precipitation is reflected in recharge
rates, which range from about 0.2 in per year in west
Texas and New Mexico to as much as 20 in per year in
Pennsylvania and eastern Tennessee.
Discharge from the ground-water system is by springs
and seepage into streams and by evaporation and
transpiration.
The yield of wells depends on: (I) the number and size
of fractures that are penetrated and the extent to which
they have been enlarged by solution, (2) the rate of
recharge, and (3) the storage capacity of the bedrock
and regolith. Yields of wells in most of the region are
small, in the range of about 2.5 to about 250 gallons per
minute, making the Nonglaciated Central region one of
the least favorable ground-water regions in the country.
Even in parts of the areas underlain by cavernous
limestone, yields are moderately low because of both
the absence of a thick regolith and the large water-
transmitting capacity of the cavernous openings, which
quickly discharge the water that reaches them during
periods of recharge.
The exceptions to the small well yields are the
cavernous limestones of the Edwards Plateau, the
Ozark Plateaus, and the Ridge and Valley section.
Figure 2-14 shows the location of these areas. The
Edwards Plateau in Texas is bounded on the south by
the Balcones Fault Zone, in which limestone and dolomite
up to 500 ft in thickness has been extensively faulted,
which facilitates the development of solution openings.
This zone forms one of the most productive aquifers in
the country. Wells of the City of San Antonio are located
in this zone; individually, they have yields of more than
16,000 gallons per minute.
As Figures 2-15 and 2-16 show, another feature that
makes much of this region unfavorable for ground-
water development is the occurrence o! salty water at
relatively shallow depths. In most of 4he Nonglaciated
Central region, except the Ozark Plateaus, the Ouachita
and Arbuckle Mountains, and the Ridge and Valley
section, the water in the bedrock contains more than
35
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1,000 mg/L of dissolved solids at depths less than
500 ft.
7. Glaciated Central Region
(Glacial deposits over fractured sedimentary rocks)
Figure 2-14 shows the Glaciated Central region, which
occupies an area of 500,000 mi2 extending from the
Triassic Basin in Connecticut and Massachusetts and
the Catskill Mountains in New York on the east to the
northern part of the Great Plains in Montana on the
west. Figure 2-17 shows that the Glaciated Central
region is underlain by relatively flat-lying consolidated
sedimentary rocks that range in age from Paleozoic to
Tertiary. The bedrock is overlain by glacial deposits
that, in most of the area, consist chiefly of till, an
unsorted mixture of rock particles deposited directly by
the ice sheets. The till is interbedded with and overlain
by sand and gravel deposited by meltwater streams, by
silt and clay deposited in glacial lakes, and, in large
parts of the North-Central States, by loess, a well-
sorted silt believed to have been deposited primarily by
the wind.
On the Catskill Mountains and other uplands in the
eastern part of the region, the glacial deposits are
typically only a few to several feet thick. In much of the
central and western parts of the region, the glacial
deposits exceed 330 ft in thickness. The principal
exception is the "driftless"areain Wisconsin, Minnesota,
Iowa, and Illinois where the bedrock is overlain by thin
soils. This area, both geologically and hydrologically,
resembles the Nonglaciated Central region and is,
therefore, included as part of that region.
The glacial deposits are thickest in valleys in the bedrock
surface. In most of the region westward from Ohio to the
Dakotas, the thickness of the glacial deposits exceeds
the relief on the preglacial surface, with the result that
the locations of valleys and stream channels in the
preglacial surface are no longer discernible from the
land surface. Figure 2-17 shows that the glacial deposits
in buried valleys include, in addition to till and lacustrine
silts and clays, substantial thicknesses of highly
permeable sand and gravel.
Ground water occurs both in the glacial deposits and in
the bedrock. Water occurs in the glacial deposits in
pores between the rock particles and in the bedrock
primarily along fractures.
Large parts of the region are underlain by limestones
and dolomites in which fractures have been e nlarged by
solution. On the whole, caves and other large solution
openings are much less numerous and hydrologically
much less important in the Glaciated Central region.
The glacial deposits are recharged by precipitation on
the interstream areas and serve both as a source of
water to shallow wells and as a reservoir for recharge to
the underlying bedrock. Precipitation ranges from about
16 in per year in the western part of the region to about
Morai
Loess
j [ Fresh water
HHB Salty water
Figure 2-17. Topographic and Geologic Features of the Glaciated Central Region
36
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39 in n the east. On sloping hillsides underlain by clay-
rich till, the annual rate of recharge, even in the humid
eastern part of the region, probably does not exceed
2 in. In contrast, relatively flat areas underlain by sand
and gravel may receive as much as 12 in of recharge
annually in the eastern part of the region.
Ground water in small to moderate amounts can be
obtained any place in the region, both from the glacial
deposits and from the bedrock. Large to very large
amounts of ground water are obtained from the sand
and gravel deposits and from some of the limestones,
dolomites, and sandstones. The shales are the least
productive bedrock formations in the region.
Because of the widespread occurrence of limestone
and dolomite, water from both the glacial deposits and
the bedrock contains as much as several hundred
milligrams per liter of dissolved minerals and is
moderately hard. Concentrations of iron in excess of
0.3 mg/L are a problem in water from some of the
sandstone aquifers in Wisconsin and Illinois and locally
in glacial deposits throughout the region. Sulfate in
excess of 250 mg/L is a problem in water both from the
glacial deposits and from the bedrock in parts of New
York, Ohio, Indiana, and Michigan.
As is the case in the Nonglaciated Central region
mineralized water occurs at relatively shallow depth in
bedrock in large parts of this region. The thickness of
the freshwater zone in the bedrock depends on the
vertical hydraulic conductivity of both the bedrock and
the glacial deposits and on the effectiveness of the
hydraulic connection between them. Boththe freshwater
and the underlying saline water move toward the valleys
of perennial streams to discharge. As a result, the depth
to saline water is less under valleys than under uplands.
At depths of 1,600 to 3,300 ft in much of the region, the
mineral content of the water approaches that of seawater
(about 35,000 mg/L). At greater depths, the mineral
content may reach concentrations several times that of
seawater.
8. Piedmont Blue Ridge Region
(Thick regolith over fractured crystalline and
metamorphosed sedimentary rocks)
The Piedmont and Blue Ridge region is an area of about
95,000 mi2 extending from Alabama on the south to
Pennsylvania on the north. The Piedmont part of the
region consists of low, rounded hills and long, rolling,
northeast-southwest trending ridges. The Blue Ridge is
mountainous and includes the highest peaks east of the
Mississippi.
The Piedmont and Blue Ridge region is underlain by
bedrock of Precambrian and Paleozoic age consisting
of igneous, and metamorphosed igneous, and
sedimentary rocks. The land surface in the Piedmont
and Blue Ridge is underlain by clay-rich, unconsolidated
material derived from in situ weathering ot the underlying
bedrock. This material, which averages about 33 to
65 ft in thickness and may be as much as 330 ft thick
on some ridges, is referred to as saprolite. In many
valleys, especially those of larger streams, flood plains
are underlain by thin, moderately well-sorted alluvium
deposited by the streams. Whilethe distinction between
saprolite and alluvium is not important, the term regolith
is used to refer to the layer of unconsolidated deposits.
As Figure 2-18 shows the regolith contains water in pore
spaces between rock particles. The bedrock, on the
other hand, does not have any significant intergranular
porosity. It contains water, instead, in sheetlike openings
formed along fractures. The hydraulic conductivities of
the regolith and the bedrock are similar and range from
about 0.003 to 3 ft perday.The major difference in their
water-bearing characteristics is their porosities, the
porosity of regolith being about 20 to 30 percent and the
porosity of the bedrock about 0.01 to 2 percent. Small
supplies of water adequate for domestic needs can be
obtained from the regolith through large-diameters bored
or dug wells. However, most wells, especially those
where moderate supplies of water are needed, are
relatively small in diameter and are cased through the
regolith and finished with open holes in the bedrock.
Although, the hydraulic conductivity of the bedrock is
similar to that of the regolith, bedrock wells generally
have much larger yields than regolith wells because,
being deeper, they have a much larger available
drawdown.
AH ground-water systems function both as reservoirs
that store water and as pipelines that transmit water
from recharge areas to discharge areas. The yield of
bedrock wells in the Piedmont and Blue Ridge region
depends on the number and size of fractures penetrated
by the open hole and on the replenishment of the
fractures by seepage into them from the overlying
regolith. Thus, the ground-water system in this region
can be viewed, from the standpoint of ground-water
development, as a terrain in which the reservoir and
pipeline functions are effectively separated. Because of
its larger porosity, the regolith functions as a reservoir
that slowly feeds water downward into the fractures in
the bedrock. The fractures serve as an intricate
interconnected network of pipelines that transmit water
either to springs or streams or to wells.
Recharge of the ground-water system occurs on the
areas above the flood plains of streams, and natural
discharge occurs as seepage springs that are common
37
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Bedrock outcrop*
Best well tiles indicated with X'»
Figure 2-18. Topographic and Geologic Features of the Piedmont and Blue Ridge Region
near the bases of slopes and as seepage into streams.
With respect to recharge conditions, it is important to
note that forested areas, which include most of the Blue
Ridge and much of the Piedmont, have thick and very
permeable soils overlain by a thick layer of forest litter.
In these areas, even on steep slopes, most of the
precipitation seeps into the soil zone, and most of this
moves laterally through the soil and a thin, temporary,
saturated zone to surface depressions or streams to
discharge. The remainder seeps into the regolith below
the soil zone, and much of this ultimately seeps into the
underlying bedrock.
The Piedmont and Blue Ridge region has long been
known as an area generally unfavorable for ground-
water development. This reputation seems to have
resulted both from the small reported yields of the
numerous domestic wells in use in the region that were,
generally, sited as a matter of convenience and from a
failure to apply existing technology to the careful selection
of well sites where moderate yields are needed. As
water needs in the region increase and as reservoir
sites on streams become increasingly more difficult to
obtain, it will be necessary to make intensive use of
ground water.
9. Northeast and Superior Uplands
(Glacial deposits over fractured crystalline rocks)
The Northeast and Superior Uplands region is made up
of two separate areas totaling about 160,000 mi2 .The
Northeast Upland encompasses the Adirondack
Mountains, the Lake Champlain valley, and nearly all of
New England. The Superior Upland encompasses most
of the northern parts of Minnesota and Wisconsin
adjacent to the western end of Lake Superior.
Bedrock in the region ranges in age from Precambrian
to Paleozoic, and as Figure 2-19 shows, consists mostly
of intrusive igneous rocks and metamorphosed
sedimentary rocks. Most have been intensively folded
and cut by numerous faults.
As Figures 2-19 and 2-20 show, the bedrock is overlain
by unconsolidated glacial deposits including till and
gravel, sand, silt, and clay. The thickness of the glacial
deposits ranges from a few feet on the higfier mountains,
which also have large expanses of barren rock, to more
than300 ft in some valleys. The most extensive glacial
deposit is till. In most of the valleys and other low areas,
the till is covered by glacial outwash consisting of
38
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Figure 2-19. Topographic and Geologic Features of the Northeast and Superior Uplands Region.
interlayered sand and gravel, ranging in thickness from
a few feet to more than 65 ft.
Ground-water supplies are obtained in the region from
both the glacial deposits and the underlying bedrock.
The largest yields come from the sand and gravel
deposits, which in parts of the valleys of large streams
are as much as 200ft thick. Water occurs in the bedrock
in fractures similar in origin, occurrence, and hydraulic
characteristics to those in the Piedmont and Blue Ridge
region.
Recharge from precipitation generally begins in the fall
after plant growth stops. It continues intermittently over
the winter during thaws and culminates during the
period between the spring thaw and the start of the
growing season. Precipitation on the Northeast Upland,
about 47 in per year, is twice that on the Superior
Upland, with the result that recharge is largest in the
Northeast. The glacial deposits in the region serve as a
storage reservoir for the fractures in the underlying
bedrock.
Water supplies in the Northeast and Superior Uplands
region are obtained from open-hole drilled wells in
bedrock, from drilled and screened or openend wells in
sand and gravel, and from large-diameter bored or dug
wells in till. The development of water supplies from
bedrock, especially in the Superior Upland, is more
uncertain than from the fractured rocks in the Piedmont
and Blue Ridge region because the ice sheets that
advanced across the region removed the upper, more
fractured part of the rock and also tended to obscure
many of the fracture-caused depressions in the rock
surface with the layer of glacial till.
Most of the rocks that underlie the Northeast and
Superior Uplands are relatively insoluble, and
consequently, the ground water in both the glacial
deposits and the bedrock generally contains less than
500 mg/L of dissolved solids. Two of the most significant
water-quality problems confronting the region, especially
the Northeast Upland section, are acid precipitation and
pollution caused by salts used to de-ice highways.
Much of the precipitation falling on the Northeast in
1982 had a pH in the range of 4 to 6 units. Because of
the low buffering capacity of the soils derived from rocks
underlying the area, there is relatively little opportunity
for the pH to be increased. One ot the results of this is
the gradual elimination of living organisms from many
lakes and streams. The effect on ground-water quality,
which will develop much more slowly, has not yet been
39
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n
57
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tu
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5
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determined. The second problem—that of de-icing
salts—affects ground-water quality adjacent to streets
and roads maintained for winter travel.
10. Atlantic and Gulf Coastal Plain
(Complexly interbedded sand, silt and clay)
The Atlantic and Gulf Coastal Plain region is an area of
about 326,000 mi2 extending from Cape Cod,
Massachusetts, to the Rio Grande in Texas. This region
does not include Florida and parts of the adjacent
states.
The topography of the region ranges from extensive,
flat, coastal swamps and marshes, 3 to 6 ft above sea
level, to rolling uplands, 300 to 800 ft above sea level,
along the inner margin of the region.
The region is underlain by unconsolidated sediments
that consist principally of sand, silt, and clay. These
sediments, which range in age from Jurassic to the
present, range in thickness from less than a foot near
the inner edge of the region to more than 39,000 ft in
southern Louisiana. The sediments are complexly
interbedded to the extent that most of the named
geologic units into which they have been divided contain
layers of the different types of sediment that underlie the
region. These named geologic units dip toward the
coast or toward the axis of the Mississippi embayment,
with the result that those that crop out at the surface
form a series of bands roughly parallel to the coast or to
the axis of the embayment, as shown in Figure 2-21.
Although sand, silt, and clay are the principal types of
material underlying the Atlantic and Gulf Coastal Plain,
there are also small amounts of gravel interbedded with
the sand, a few beds composed of mollusk shells, and
small amounts of limestone present in the region. The
most important limestone is the semi-consolidated Castle
Hayne Limestone of Eocene age, which underlies an
area of about 10,000 mi2 in eastern North Carolina, is
more than 650 ft thick in much of the area, and is the
most productive aquifer in North Carolina. A soft, clayey
limestone (the chalk of the Selma Group) of Late
Cretaceous age underlies parts of eastern Mississippi
and western Alabama, but instead of being an aquifer,
it is an important confining bed.
From the standpoint of well yields and ground-water
use, the Atlantic and Gulf Coastal Plain is one of the
most important regions in the country. Recharge to the
ground-water system occurs in the interstream areas,
both where sand layers crop out and by percolation
downward across the interbedded clay and silt layers.
Discharge from the system occurs by seepage to
streams, estuaries, and the ocean.
Wells that yield moderate to large quantities of water
can be constructed almost anywhere in the region.
Because most of the aquifers consist of unconsolidated
sand, wells require screens: where the sand is fine-
grained and well sorted, the common practice is to
Figure 2-21. Topographic and Geologic Features of the Gulf Coastal Plain
41
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surround the screens with a coarse sand or gravel
envelope.
Withdrawals near the outcrop areas of aquifers are
rather quickly balanced by increases in recharge and
(or) reductions in natural discharge. Withdrawals at
significant distances downdip do not appreciably affect
conditions in the outcrop area and thus must be partly
or largely supplied from water in storage in the aquifers
and confining beds.
If withdrawals are continued for long periods in areas
underlain by thick sequences of unconsoiidated deposits,
the lowered ground-water levels in the aquifer may
result in drainage of water from layers of silt and clay.
The depletion of storage in fine-grained beds results in
subsidence of the land surface. Subsidence in parts of
the Houston area totaled about 30 ft as of 1978.
Subsidence near pumping centers in the Atlantic Coastal
Plain has not yet been confirmed but is believed to be
occurring at a slower rate than along the Texas Gulf
Coast.
Depletion of storage in the aquifers underlying large
areas of the Atlantic and Gulf Coastal Plain is reflected
in long-term declines in ground-water levels. These
declines suggest that withdrawals in these areas are
exceeding the long-term yield of the aquifers.
Another problem that affects ground-waterdevelopment
in the region concerns the presence of saline water in
the deeper parts of most aquifers. In some of the deeper
aquifers, the interface between freshwater and saltwater
is inshore, but in parts of the region, including parts of
Long Island, New Jersey, and Mississippi, the interface
in the most intensively developed aquifers is a significant
distance offshore. Pumping near the interfaces has
resulted in local problems of saltwater encroachment.
Another significant feature of the ground-water system
in this region is the presence of "geopressured" zones
at depths of 5,900 to 20,000 ft in Texas and Louisiana.
which contain water at a temperature of 80°C to more
than 273°C. Water in these zones contains significant
concentrations of natural gas, and the water in some
zones is under pressures sufficient to support a column
of water more thani3,000 ft above land surface.
Because the elevated temperature, natural gas, and
high pressure are all potential energy sources, these
zones are under intensive investigation.
11. Southeast Coastal Plain
(Thick layers of sand and clay over semiconsolidated
carbonate rocks)
Figure 2-22 shows the Southeast Coastal Plain, an area
o
of abou 182,000 mi in Alabama, Florida, Georgia, and
South Carolina. It is a relatively flat, low-lying area.
Much of the area, including the Everglades in southern
Florida, is a nearly flat plain less than 30 ft above sea
level.
The land surface of the Southeast Coastal Plain is
underlain by unconsoiidated deposits of Pleistocene
age consisting of sand, gravel, clay, and shell beds and.
in southeastern Florida, by semi-consolidated limestone.
In most of the region, the suriicial deposits rest on
formations, primarily of middle to late Miocene age,
composed of interbedded clay, sand, and limestone.
The formations of middle to late Miocene age or su rf icial
deposits overlie semi-consolidated limestones and
dolomites that are as much as 5,000 ft thick.
The Tertiary limestone that underlies the Southeast
Coastal Plain constitutes one of the most productive
aquifers in the United States and is the feature that
justifies treatment of the region separately from the
remainder of the Atlantic and Gulf Coastal Plain. The
aquifer, which is known as the Floridan aquifer, underlies
all of Florida and southeast Georgia and small areas in
Alabama and South Carolina. The Floridan aquifer
consists of layers several feet thick composed largely of
loose aggregations of shells and fragments of marine
organisms interbedded with much thinner layers of
cement and cherty limestone. The Floridan, one of the
most productive aquifers in the world, is the principal
source of ground-water supplies in the Southeast Coastal
Plain region.
In southern Florida, south of Lake Okeechobee, and in
a belt about 18 mi wide northward along the east coast
of Florida to the vicinity of St. Augustine, the water in the
Floridan aquifer contains more than 100 mg/L of
chloride. In this area, most water supplies are obtained
from surficial aquifers. The most notable of these aquifers
underlies the southeastern part of Florida and, in the
Miami area, consists of 100 to 330 ft of cavernous
limestone and sand and is referred to as the Biscayne
aquifer. The Biscayne is an unconf ined aquifer, which is
recharged by local precipitation and by infiltration of
water from canals that drain water from impoundments
developed in the Everglades. It is the principal source of
water for municipal, industrial, and irrigation uses and
can yield as much as 1,300 gal per min to small-
diameter wells less than 80 ft deep finished with open
holes only 3 to 6 ft long.
The surficial aquifers in the remainder of the region are
composed primarily of sand, except in the coastal
zones of Florida where the sand is interbedded with
shells and thin limestones. These suriicial aquifers
serve as sources of small ground-water supplies
42
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GEORGIA \
FLORIDA^ ••
WJ
Explanation
• JO— Altitude of the water level in well« in meter* above
aea level. May 1980, Contour Interval 10 metera
Principal recharge areas
Figure 2-22. Potentlometric Surface for the Florldan Aquifer (Adapted from Johnston, Healy, and
Hayes, 1981)
43
-------
throughout the region and are the primary sources of
ground water where the water in the Floridan aquifer
contains more than about 250 mg/L of chloride.
The Floridan aquifer is the principal source of ground
water in the region. Ground water in the upper part of the
aquifer is unconfined in the principal recharge areas in
Georgia and in west-central Florida, which are shown in
Figure 2-22. In the remainder of the region, water in the
aquifer is confined by clay in the Hawthorn Formation
and in other beds that overlie the aquifer.
Recharge occurs where the potentiometric surface of
the Floridan aquifer is lower than the water table in the
overlying surficial aquifer. As Figure 2-22 shows, the
principal recharge areas include a broad area along the
west side of Florida extending from the central part of
the peninsula to south-central Georgia and an area
extending from west-central Florida through southeast
Alabama into southwest Georgia. In these areas,
recharge rates are estimated to exceed 5 in. per yr.
Recharge occurs by infiltration of precipitation directly
into the limestone, where it is exposed at the land
surface, and by seepage through the permeable soils
that partly mantle the limestone in the outcrop areas.
Considerable recharge also occurs in the higher parts
of the recharge areas through permeable openings in
the confining beds, where these beds have been
breached by the collapse of caverns in the limestone
during the process of sinkhole formation. Figure 2-23
illustrates this sinkhole formation. Thus, the land surface
in most of Florida north of Lake Okeechobee is marked
by thousands of closed depressions ranging in diameter
from a few feet to several miles. The larger depressions
are occupied by lakes generally referred to as sinkhole
lakes.
Discharge from the Floridan aquifer occurs through
springs and by seepage to streams. Considerable
discharge also occurs by diffuse seepage across the
overlying confining beds in areas where the
potentiometric surface of the aquifer stands at a higher
altitude than the water table. In most of these areas
wells open to the aquifer will flow at the land surface.
Recharge area-
-I— Discharge-^-
area
Figure 2-23. Topographic and Geologic Features of the Southeast Coastal Plain Region
44
-------
The mostspectaculardischargefromthe Ftoridan aquifer
is through sinkholes exposed along streams and
offshore.
Water supplies are obtained from the Floridan aquifer
by installing casing through the overlying formations
and drilling an open hole in the limestones and dolomites
comprising the aquifer. Total withdrawals from the
aquifer are estimated to have been about 3.5 billion
gallons per day in 1978. Large withdrawals also occur
from the other aquifers in the region.
12. Alluvial Valleys
(Thick sand and gravel deposits beneath floodplains
and terraces of streams)
In the preceding discussions of ground-water regions,
streams and other bodies of surface water were
mentioned as places of ground-water discharge. In
most areas ground-water systems and surface streams
form a water system so intimately interconnected that a
change in one causes a change in other. For example,
withdrawals from ground-water systems reduce
discharge to streams and thereby reduce streamflow.
The movement of waterf rom streams into ground-water
systems in response to withdrawals is not a significant
feature in most areas because ground-water withdrawals
are dispersed overthe uplands between streams rather
than concentrated near them. An exception to this
occurs where stream channels and floodplains are
underlain by highly permeable deposits of sand and
gravel. The large yields of these deposits, as well as the
variability and availability of streamflow. encourage the
development of these sand and gravel deposits as
sources of ground water, and thus, encourage the
concentration of withdrawals near streams. From the
standpoint of ground-water hydrology, three criteria are
used to differentiate alluvial valleys from other valleys.
These criteria are as follows:
1. The alluvial valleys contain sand and gravel deposits
thick enough to supply water to wells at moderate to
large rates. [Commonly, the water-transmitting capacity
of the sand and gravel is at least 10 times larger than
that of the adjacent (enclosing) rocks.]
2. The sand and gravel deposits are in hydraulic contact
with a perennial stream that serves as a source of
recharge and whose flow normally far exceeds that
demand from any typical well field.
3. The sand and gravel deposit occurs in a clearly
defined band ("channel") that normally does not extend
beyond the fioodpiain and adjacent terraces. !n other
words, the width of the deposit is small or very small
compared with its length.
According to these criteria, tne valleys of streams tna;
were not affected by glacial mehwater are not considerec
alluvial valleys. The floodpiams in these valleys are
commonly underlain only by thin deposits of fine-grained
alluvium. These criteria also eliminate the "buried"
valleys of the glaciated area Although the water-
transmitting capacity of the sand and gravel in buried
valleys may be large, the yield to wells in most of them
is small because of the limited opportunity for recharge
through the surrounding, less-permeable materials.
The alluvial valleys are commonly underlain, in addition
to sand and gravel, by deposits of silt and clay. In many
of the glaciated valleys in New York and New England
the land surface is underlain by a layer of sand and
gravel that ranges in thickness from 3 to 6 ft to more
than 30 ft. The bottom of this deposit ranges, from one
part of a valley to another, from a position above the
watertable to several feet belowthe bottom of streams.
This surficial deposit of sand and gravel is commonly
underlain by interbedded sill and clay which is, in turn,
underlain by a discontinuous "basal" layer of sand and
gravel.
The sequence of deposits in the alluvial valleys depends,
of course, on the history of deposition in the valleys.
Figure 2-24 shows that the sand and gravel in the
valleys of major streams, such as those of the Mississippi,
Missouri, and Ohio, are commonly overlain by deposits
of clay and otherf ine-grained alluvium deposited during
floods since the end of the glacial period.
Under natural conditions the alluvial deposits are
recharged by precipitation on the valleys, by ground
water moving fromthe adjacent and underlying aquifers,
by overbank flooding of the streams, and, in some
glacial valleys, by infiltration from tributary streams.
Water in the alluvial deposits discharges to the streams
in the valleys.
The layers of sand and gravel in the alluvial valleys are
among the most productive aquifers in the country.
They have been extensively developed as sources of
waterfor municipalities, industries, and irrigation. Some
of the gravel layers have hydraulic conductivities nearly
as large as those of cavernous limestone. The large
yields of the sand and gravel depend not only on their
large water-transmitting capacity but also on their
hydraulic connection to the streams flowing in the
valleys. Large withdrawals from the deposits result in a
reduction in ground-water discharge to the streams
and, if large enough, cause infiltration of water from the
streams into the deposits.
45
-------
Mississippi River
I - ;i <*
I V —_
Flood plain ->x-xxv
Figure 2-24. Topographic and Geologic Features of a Section of the Aluvlal Valley of the Mississippi
River
References
Columbia-North Pacific Technical Staff, 1970, "Water
Resources" in Columbia-North Pacific comprehensive
framework study of water and related lands: Vancouver,
Washington, Pacific Northwest River Basins Comm.,
app. 5.
Gutentag, E.D. and J.B. Weeks, 1980, Water table in
the High Plains Aquifer in 1978 in parts of Colorado.
Kansas, Nebraska. New Mexico, Oklahoma, South
Dakota, Texas, and Wyoming: U.S. Geological Survey
Hydrologic Investigation Atlas 642.
Hampton, E.R., 1964, Geologic factors that control the
occurrence and availability of ground water in the Forth
Rock basin, Lake County, Oregon: U.S. Geological
Survey Professional Paper 383-B
Heath, R.C.,1982, Classification of ground-water
systems of the United States: Ground Water, V. 20, no.
4. July-August 1982.
Johnston. R.H., H.G. Healy, and L.R. Hayes, 1981,
Potentiometric surface of the tertiary limestone aquifer
system, southeastern United States, May 1980: U.S,
Geological Survey Open-File Report 81-486.
Luckey, R.R., E.D. Gutentag, and J.B. Weeks, 1981.
Water-level and saturated-thickness sharges
predevelopment to 1980, in the high plains aquifer in
parts of Colorado, Kansas, Nebraska, New Mexico,
Oklahoma, South Dakota, Texas, and Wyoming: U.S.
Geological Survey Hydrologic Investigations Atlas 652.
Luzier, J.E. and R.J. Burl, 1974, Hydrology of basalt
aquifers and depletion of ground water in east-central
Washington: Washington Department of Ecology, Water-
Supply Bulletin 33.
MacNish, R.D.and R.A. Barker, 1976, Digital simulation
of a basalt aquifer system, Walla Walla River Basin,
Washington and Oregon: Washington Department of
Ecology, Water-Supply Bulletin 44.
46
-------
McGuiness, C.L., 1963. The role of ground water in the
national water situation: U.S. Geological Survey Water-
Supply Paper 1800.
Newcomb, R.C., 1962, Storage of ground water behind
subsurface dams in the Columbia River basalt,
Washington, Oregon, and Idaho: U.S. Geological Survey
Professional Paper 383-A.
U.S. Geological Survey, Professional Paper 813,
"Summary appraisals of the nation's ground-water
resources". Published as a series of chapters based on
the boundaries established by the United States Water
Resources Council for Water-Resources Regions in
the United States.
U.S. Geological Survey, 1970, The National atlas of the
United States of America.
Weeks. J.B. and E.D. Gutentag, 1981. Bedrock geology,
altitude of base, and 1980 saturated thickness of the
high plains aquifer in parts of Colorado, Kansas,
Nebraska, New Mexico, Oklahoma, South Dakota.
Texas, and Wyoming: U.S. Geological Survey
Hydrologic Investigations Atlas 648.
Bibliography
A large number of publications were consulted, for both
general and specific information, in the preparation of
this paper. Specific reference to these publications
generally is omitted in the text, both to avoid interruption
of the discussions and to save space. Publications that
served as primary references are listed below, under
the categories of general references and references to
regional discussions. General references include
publications that were used both for background
information on the classification of ground-water systems
and for general information on the regions. References
to the regional discussions include publications that
served as a source of additional information on the
individual regions.
General Bibliography
Fenneman, N.M.. 1931, Physiography of Western
United States. McGraw-Hill, New York.
systems of the United States:
no. 4, July-August 1982.
Ground Water, v. 20,
-, 1938, Physiography of Eastern United
States.McGraw-Hill, New York.
Fuller, M.L., 1905, "Underground waters of eastern
United States." U.S. Geological Survey Water-Supply
Paper 114.
Heath, R.C., 1982, Classification of ground-water
Mann, W.B., IV, and others. 19831, Estimated wateruse
in the United States, 1980: U.S. Geological Survey
Circular 1001.
McGuiness. C.L., 1963, The role of ground water in the
national water situation." U.S. Geological Survey Water-
Supply Paper 1800.
Meinzer, O.E., 1923, The occurrence of ground water
in the United States, with a discussion of principles."
U.S. Geological Survey Water-Supply Paper 489.
Shimer, J.A., 1972, Field guide to landforms in the
United States. Macmillan. New York.
Thomas, H.E., 195l,Theconservationofgroundwater.
McGraw-Hill, New York.
, 1952, Ground-water regions of the United
States—their storage facilities," v. 3 of The Physical and
Economic Foundation of Natural Resources. U.S. 83d
Cong. House Committee on Interior and Insular Affairs,
pp3-78.
U.S. Geological Survey, 1970, The National atlas of the
United States of America.
, Professional Paper813, Summary
appraisals of the nation's ground-water resources:
Published as a series of chapters based on the
boundaries established by the United States Water-
Resources Council for Water-Resources Regions in
the United States.
Bibliographies for Regional Discussions
2. Alluvial Basins
Harshbarger, J.W., D.D. Lewis. H.E. Skibitzke, W.L
Heckler, and L.R. Kister, 1966, Arizona water": (rev. by
H.L. Baldwin) U.S. Geological Survey Water-Supply
Paper 1648.
Robinson. T.W., 1953. Big Smoky Valley, Nevada,"
chap. 8 of subsurface Facilities of Water Management
and Patterns of Supply-Type Area Studies, v. 4 of The
Physical and Economic Foundation of Natural
Resources. U.S. 83d Cong. House Committee of Interior
and Insular Affairs, pp 132-146.
3. Columbia Lava Plateau
Columbia-North Pacific Technical Staff,1970, Water
resources: in Columbia-North Pacific Comprehensive
47
-------
Framework Study of Water and Related Lands. Pacific
Northwest River Basins Comm., Vancouver,
Washington, app. 5.
Hampton, E.R.,1964, Geologic factors that control the
occurrence and availability of ground water in the Fort
Rock Basin, Lake County, Oregon: U.S. Geological
Survey Professional Paper 383-B.
Luzier, J.E. and R.J. Burt, 1974, Hydrology of basalt
aquifers and depletion of ground water in east-central
Washington: Washington Department of Ecology,
Water-Supply Bulletin 33.
MacNish, R.D.andR.A. Barker.1976, Digital simulation
of a basalt aquifer system, Walla Walla River Basin,
Washington and Oregon: Washington Department of
Ecology, Water-Supply Bulletin 44.
Nace, R.L., 1958, Hydrology of the Snake River basalt:
Washington Academy of Science Journal, v. 48, no. 4,
pp. 136-138.
Newcomb, R.C., 1962, Storage of ground water behind
subsurface dams in the Columbia River basalt,
Washington, Oregon, and Idaho: U.S. Geological
Survey Professional Paper 383-A.
,1965, Geology and ground-water
resources of the Walla Walla River Basin, Washington-
Oregon: Washington Division of Water Resources.
Water Supply Bulletin 21.
4. Colorado Plateau and Wyoming Basin
Harshbarger, J.W.. C.A. Repenning, and J.T.
Callahan.1953, The Navajo Country, Arizona-Utah-
New Mexico: chap. 7 of Subsurface Facilities of Water
Management and Patterns of Supply-Type Area Studies.
v. 4 of The Physical and Economic Foundation of
Natural Resources. U.S. 83d Cong. House Committee
on Interior and Insular Affairs, pp 105-129.
Lohman, S. W.( 1965, Geology and artesian watersupply
of the Grand Junction Area, Colorado: U.S. Geological
Survey Professional Paper 451.
Gaum, C.H.,1953, High Plains, or Llano Estacado,
Texas-New Mexico: chap. 6 of Subsurface Facilities of
Water Management and Patterns of Supply-Type Area
Studies, v. 4 of The Physical and Economic Foundation
of Natural Resources. U.S. 83d Cong. House Committee
on Interior and Insular Affairs, pp 94-104.
Gutentag, E.D., and J.B. Weeks, 1980, Water table in
the High Plains aquifer in 1978 in parts of Colorado,
Kansas, Nebraska, New Mexico, Oklahoma, South
Dakota, Texas, and Wyoming." U.S. Geological Survey
Hydrologic Investigations Atlas 642.
Lohman, S.W., 1953, High Plains of West-Central
United States, general aspects: chap. 4 of Subsurface
Facilities of Water Management of Patterns of Supply-
Type Area Studies, v. 4 of The Physical and Economic
Foundation of Natural Resources. U.S. 83d Cong. House
Committee on Interior and Insular Affairs, pp 70 78.
Luckey, R.R.. E.D. Gutentag, and J.B. Weeks,1981,
Water-level and saturated-thickness changes,
predevelopment to 1980, in the High Plains aquifer in
parts of Colorado, Kansas, Nebraska, New Mexico,
Oklahoma, South Dakota, Texas, and Wyoming: U.S.
Geological Survey Hydrologic Atlas 652.
Weeks, J.B., and E.D. Gutentag.1981. Bedrock geology,
altitude of base, and 1980 saturated thickness of the
High Plains aquifer in parts of Colorado, Kansas,
Nebraska, New Mexico, Oklahoma, South Dakota,
Texas, and Wyoming: U.S. Geological Survey
Hydrologic Investigations Atlas 648.
7. Glaciated Central Region
Feth, J.H., and others, 1965, Preliminary map of the
conterminous United States showing depth to and
quality of shallowest ground watercontaining more than
1,000 Parts per million dissolved solids: U.S. Geological
Survey Hydrologic Atlas 199.
Todd, O.K., 1980, Groundwater hydrology: 2d ed. John
Wiley, New York.
8. Piedmont and Blue Ridge Region
LeGrand, H.E.,1967, Ground water of the Piedmont
and Blue Ridge provinces in the southeastern states."
U.S. Geological Survey Circular 538.
Le Grand, H.E., and M.J. Mundorff ,1952, Geology and
ground water in the Charlotte Area, North Carolina."
North Carolina Department of Conservation and
Development, Bulletin 63.
Stewart, J.W., 1964, Infiltration and permeability of
weathered crystalline rocks, Georgia Nuclear
Laboratory, Dawson County, Georgia: U.S. Geological
Survey Bulletin 1133-D.
9. Northeast and Superior Uplands
Delaney. D.F., and A. Maevsky.1980. Distribution of
aquifers, liquid-waste impoundments, and municipal
water-supply sources, Massachusetts: U.S. Geological
Survey Water-Resources Investigations Open-File
Report 80-431.
48
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10. Atlantic and Gulf Coastal Plain
Back, W..1966, Hydro-chemical facies and ground-
water flow patterns in the northern part of Atlantic
Coastal Plain: U.S. Geological Survey Professional
Paper 498-A.
Brown, G.A., and O.J. Cosner.1975. Ground-water
conditions in the Franklin area, southeastern Virginia:
U.S. Geological Survey Hydrologic Atlas 538.
Cohen, P., O.L. Franke, and B.L Foxworthy, 1968, An
atlas of Long Island's water resources: New York
Water Resources Commission Bulletin 62.
Gabrysch, R.K.,1980, Approximate land-surface
subsidence in the Houston-Galveston Region, Texas,
1906-78. 1943-78, and 1973-78: U.S. Geological
Survey Open-File Report 80-338.
LeGrand, H.E., and W.A. Pettyjohn.1981. Regional
hydrogeologic concepts of homoclinal flanks: Ground
Water, v. 19. no. 3. May-June.
11. Southeast Coastal Plain
Cooper. H.H.. Jr.. and W.E. Kenner, 1953. Central and
northern Florida: chap. 9 of Subsurface Facilities of
Water Management and Patterns of Supply-Type Area
Studies, v. 4 of The Physical and Economic Foundation
of Natural Resources. U.S. 83d Cong. House Committee
on Interior and Insular Affairs, pp 147-161.
Heath. R.C., and C.S. Conover, 1981, Hydrologic
almanac of Florida: U.S. Geological Survey Open-File
Report 81-1107.
Johnston, R.H., H.G. Healy, and L.R. Hayes, 1981.
Potentiometric surface of the Tertiary limestone aquifer
system, southeastern United States. May 1980: U.S
Geological Survey Open-Fill Report 81-486.
Stringfield, V.T., 1967, Artesian water in Tertiary
limestone in the southeastern states: U.S. Geological
Survey Professional Paper 517.
12. Alluvial Valleys
Boswell, E.H., E.M. Gushing, and R.L. Hosman, 1968.
Quartemary aquifers in the Mississippi Embayment:
U.S. Geological Survey Professional Paper 448-E.
Rorabaugh, M.I., 1956. Ground water in northeastern
Louisville. Kentucky: U.S. Geological Survey Water-
Supply Paper 1360-B. pp 101-169.
49
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Chapter 3
GROUND WATER-SURFACE WATER RELATIONSHIP
Introduction
The interrelations between ground water and surface
water are of great importance in both regional and local
hydrologic investigations and a wide variety of
information can be obtained by analyzing streamflow
data. Most commonly the surface water investigator
deals withstreamhydrographs.channelcharacteristics.
geomorphology, or flood routing. Although the
hydrogeologist may evaluate induced infiltration into a
streamside aquifer, he is generally more interested in
aquifer characteristics, such as hydraulic conductivity,
thickness, boundaries, and well yields. Many
hydrologists tend to ignore the fact that, at least in humid
areas, ground-water runoff accounts for a significant
part of a stream's total flow.
Evaluation of the ground-watercomponent of runoff can
provide important and useful information regarding
regional recharge rates, aquifer characteristics, and
ground-water quality, and can indicate areas of high
potential yield to wells. The purpose of this chapter is to
describe a number of techniques that can be used to
evaluate runoff to obtain a better understanding and
evaluation of ground-water resources. In particular, the
following will be examined:
1. Ground-water runoff
2. Surface runoff
3. Regional ground-water recharge rates
4. Determination of areas of relatively high
permeability or water-yielding characteristics
5. Determination of the background concentration of
ground-water quality
6. Estimation of evapotranspiration
7. Determination of the percentage of precipitation
that is evaportranspired, becomes ground-water
runoff, or becomes surface-water runoff.
The approaches taken, admittedly some highly
subjective, are based on: (I) short-term runoff events,
(2) long-term hydrographs, and (3) dry-weather flow
measurements. In the first approach a single event,
such as a flood wave of a few hours or few days
duration, can be analyzed, while the latter two
approaches are based on annual stream hydrographs,
flow-duration curves, or seepage runs. Short-term events
may provide a considerable amount of information for a
local area, while long-term events are most useful for
regional studies. Streamflow may consist of several
components including ground-water runoff, surface
runoff, effluent, and precipitation that falls directly into
the channel.
The volume of water that is added by precipitation
directly into the channel is relatively small compared to
the stream's total flow. The contribution by waste effluent
may or may not be significant, since it depends on the
activities that are occurring in the basin. In permeable
basins in humid regions, ground-water runoff may
account for 70 or 80 percent of the stream's annual
discharge. The remainder is surface runoff, which
originates as precipitation or snow melt that flows
directly into the stream channel. This chapter is
concerned largely with ground-water runoff and surf ace
runoff and the separation of these two components.
In order to fully appreciate the origin and significance of
ground-water runoff, it is first necessary to examine the
regional ground-water flow system. Figure 3-1 illustrates
a typical flow pattern. Particularly in humid and semi-
arid regions, the water table generally conforms with the
surface topography. Consequently, the hydraulic
gradient or water table slopes away from divides and
topographically high areas toward adjacent low areas,
such as streams and rivers. Topographic highs and
lows, therefore, serve as recharge and discharge areas,
respectively.
Ground-water flow systems may be local, intermediate,
or regional. As these terms imply, ground-water flow
paths may be short, amounting to a few yards at one
50
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Figure 3-1. Approximate Flow Pattern In Uniformly Permeable Material between the Sources
Distributed over the Air-Water Interface and the Valley Sinks (After Hubbert, 1940)
extreme to many miles in the regional case. Individual
flow lines are, of course, influenced by the stratigraphy
and, in particular, are controlled by hydraulic conductivity.
As water infiltrates a recharge area, the mineral content
is relatively low. The quality changes, however, along
the flow path and dissolved solids, as well as several
other constituents, generally increase with increasing
distances traveled in the subsurface. It is for this reason
that even nearby streams may be typified by different
chemical quality. A stream, seep, or spring m a local
discharge area may be less mineralized than that
issuing from a regional discharge zone because of the
increase in mineralization that takes place along longer
flow paths. It must be remembered, however, that other
conditions, such as soil type, solubility of the enclosing
rocks, surface drainage characteristics, and waste
disposal practices, may have a profou nd effect on water
quality at any particular site.
Even streams in close proximity may differ considerably
in discharge even though the size of the drainage area
and climatic conditions are similar. Figure 3-2 gives the
superimposed hydrographs of White River in
southwestern South Dakota and the Middle Loup River
in northwestern Nebraska, which are good examples.
White River has a low discharge throughout most of the
year, but from May to September, flash floods are
common. The wide extreme in discharge is characteristic
of a flashy stream.
The flow of Middle Loup River is nearly constant,
although from late spring to early fall higher flows may
occur. These peaks, however, differ considerably from
those found in White River because the increase in
discharge takes place over a longer interval, the stage
does not range widely, and the recession occurs more
slowly. The differences in hydrographs of these two
nearby rivers is puzzling, until the geology and
topography of their respective basins are examined.
White Riverf lows through the Badlands of South Dakota,
an area of abrupt changes in relief, steep slopes, little
vegetative cover, and rocks that consist largely of silt
and clay, both of which may contain an abundance of
bentonite. When wet. bentonite, a swelling clay,
increases greatly in volume. As a result of these features,
rainfall in the White River basin tends to quickly run off
and there is little opportunity for infiltration and ground-
water recharge to occur. Thus, intense rainstorms cause
flash floods, such as those that occurred in June,
August, and September.
The Middle Loup basin is carved into the undulating
grassland topography of the Sandhills of Nebraska,
where surficial materials consist of wind-blown sand.
Since the low relief, grass-covered surface promotes
infiltration, precipitation is readily absorbed by the
underlying sand. As a result, there is very little surface
runoff and a great amount of infiltration and ground-
water recharge. The ground water slowly migrates to
the river channel, thus providing a high sustained flow.
In a comparison of the hydrographs of these two rivers,
it is evident that the geologic framework of the basin
51
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MAM
Middle Loup River
Figure 3-2. Hydrographs of Two Nearby Streams
serves as a major control on runoff. This further implies
that in any regional hydrologic study, the investigation
should begin with an examination of geologic maps.
Gaining and Losing Streams
Although the discharge of most streams increases
downstream, the flow of some streams diminishes.
These streams are referred to as gaining or losing,
respectively. The hydrologic system, however, is even
more complex, because a stream that may be gaining
in one season, may be losing during another.
Furthermore, various human activities may also affect
a stream's discharge.
Under natural conditions a gaining stream is one where
the water table is above the base of the stream channel.
Of course the position of the water table fluctuates
throughout the year in response to differences in ground-
water recharge and discharge. Normally the watertable
is highest in the spring, which is the annual major period
of ground-water recharge. From spring to fall, very little
recharge occurs and the amount of ground water in
storage is slowly depleted as it seeps into streams.
Eventually, the water table may decline to the same
elevation as a stream bottom, or even below it, at which
time st reamf low ceases except during periods of surface
runoff. Following a period of recharge, caused either by
infiltration of rainfall or seepage from a flood wave, the
water table may again rise and temporarily contribute
ground-water runoff.
Figure 3-3 shows a generalized diagram of the hydrology
of a stream during two seasons of the year. During the
spring, the water table is high and the gradient dips
steeply towards the stream. If streamflow was measured
at selected points, it would be found that the discharge
increases downstream because of the addition of
ground-water runoff. That is, it is a gaining stream. In the
fall when the water table lies at or below the stream
bottom, however, the same stream might become a
losing stream. During a major runoff event the stage in
the stream would be higher than the adjacent water
table and water would migrate from the stream into the
ground. The stream would continue to lose water until
the water table and river stage were equal. When the
stage declined, ground-water runoff would begin again.
In this case the stream changed from gaining to losing
and back again to gaining. Similar situations may occur
over longer intervals, such as during droughts. As a
drought continues, the water table slowly declines as
ground-water storage is depleted. A period of high
flows, such as release from a dam, may cause
tremendous amounts of water to flow from the stream
channel into the ground, thus saturating the depleted
streamside deposits. It may require weeks of high flow
to replenish the ground-water reservoir, and until this is
accomplished, the stream will be losing.
Some streams, particularly in arid and karst regions, are
nearly always losing. Examples include those channels
that cross coarse-grained alluvial fans. Even during
52
-------
Losing stream
(A-A')
Gaining in spring
Losing in fall
(B-B'I
Gaining stream
(C-C-)
Perennial
C
Land surface
Water table
(S) in spring
(F) in (all
Figure 3-3. The Relation between the Water Table and Stream Types
flash floods, the great mass of flood water soon spreads
out over the fan or adjacent desert to infiltrate or
evaporate.
Because of the extensive network of solution openings
in karst terrain, the water table may consistently lie
below the bottom of all the streams. During a period of
runoff, the water may rapidly flow into sink holes and
solution openings or simply disappear into a swallow
hole in a stream channel, only to appear again perhaps
several miles downstream.
Gaining and losing streams also can be created
artificially. Where well fields lie along stream channels
and induce water to flow from the stream to the well,
streaflow is diminished. In some cases stream depletion
by pumping wells has proceeded to such an extent that
the stream channels are dry throughout the year.
Conversely, in some irrigated regions, so much infiltration
occurs that the water table rises to near land surface.
The underlying soil and ground water may become
highly mineralized by the leaching of soluble salts.
These highly mineralized waters may discharge into a
stream, increasing its flow but deteriorating the chemical
quality. In other places, municipal or industrial wastes
may add considerably to a stream's flow, also
deteriorating its quality. In fact, at certain times of the
year, the entire flow may consist of waste water.
Bank Storage
Figure 3-4 shows that, as a flood wave passes a
particular stream cross section, the watertable may rise
in the adjacent streamside deposits. The rise is caused
by two phenomena. First, the stream stage, which is
higherthan the watertable, will temporarily block ground-
water runoff, thus increasing the amount of ground
water in storage. Secondly, because of the increased
head in the stream, water will flow from the stream
channel into the ground, thus providing another
component of water added to storage.
Once the flood wave begins to recede, which may occur
quite rapidly, the newly added ground water will begin
to flow back into the channel, rapidly at first and then
more slowly as the hydraulic gradient decreases. This
temporary storage of water in the near vicinity of the
stream channel is called bank storage.
The rising and recession limbs of a hydrograph of a
flood wave should provide clues concerning bank
storage and streamside permeability. For example,
where streamside deposits are of low permeability,
such as clay or shale, the rising limb should be quite
steep, but more gradual where the deposits are
permeable. Since there would be little or no bank
storage in the first case, recession curves also should
53
-------
13
o>
I
o>
o>
O 7
200 400 600 800
Horizontal Distance, in Feet
Land Surface
1000
1200
Silt and clay
Sand
200 400 600 800
Horizontal Distance, in Feet
1000
1200
Figure 3-4. Movement of Water Into and Out of Bank Storage Along a Stream In Indiana
be steep, but the release from bank storage in a
permeable basin should reduce the slope ol the
recession curve.
Effect of the Geologic Framework on Stream
Hydrographs
Unfortunately, the discharge of ground water into a
stream is not always as simple as has been implied from
the above examples. As Figure 3-5 shows, an
examination of the aquifer framework and its effect on
a stream hydrograph is enlightening. Notice in Figure 3-
5a that the streamchannel is deeply cut into a shale that
is overlain by sand. Ground water flows into the stream
along a series of springs and seeps issuing at the sand-
shale contact. During a runoff event the stream stage
rises, but even at its peak, the stage remains below the
top of the shale. In this case, the contribution of ground
water remains constant despite the rise in stage. To
separate the ground-water runoff component from the
stream hydrograph. one merely needs to draw a straight
line from the inflection points of the rising and falling
limbs.
In Figure 3-5b the stream channel is cut into a deposit
of sand that is underlain by shale. Ground water flows
into the stream, but as the stage rises, ground-water
runoff decreases and eventually stops. Surface water
then begins to flow into the ground where it is retained
as bank storage. As the stage declines, ground water
again starts to discharge into the channel eventually
providing the entire flow. This is the classic case of bank
storage. Hydrograph separation is more difficult in this
case.
Figure 3-5c is a combination 6f the previous two
54
-------
01
en
mnnmmfflmiM
Figure 3-5. The Aquifer Framework In the Vicinity of a Stream Plays a Major Role In Ground-Water Runoff and Mydrograph Separation
-------
examples. Ground water from a perched aquifer
contributes a steady flow, while bank storage is gained
and then released from the streamside aquifer.
Hydrograph separation is even more difficult in this
situation because of the contribution from both aquifers.
The final example, Figure 3-5d, consists of three
aquifers—one perched, one in direct contact with the
stream, and one deeper, confined aquifer. As the stream
rises, there is a decrease in the head difference between
the stream and the confined aquifer. The decrease in
head difference will reduce upward leakage from the
artesian aquifer, the amount depending on the thickness
and vertical permeability of the confining bed and the
head difference.
Single-Event Hydrograph Separation Techniques
Following a runoff event, the water held as bank storage
begins to discharge into the channel. In the beginning
the rate of bank storage discharge is high because of
the steep water-level gradient, but as the gradient
decreases so does ground-water runoff. The recession
segment of the stream hydrograph gradually tapers off
into what is called a depletion curve. To a large extent,
the shape of the depletion curve is controlled by the
permeability of the .streamside deposits, although soil
moisture and evapotranspiration may play important
roles.
Depletion Curves
Intervals between surface runoff events are generally
short and forthis reason, depletion curves are plotted as
. a combination of several arcs of the hydrograph with the
arcs overlapping in their lower parts, as shown in Figure
3-6. To plot a depletion curve, tracing paper is placed
over a hydrograph of daily flows and, using the same
horizontal and vertical scales, the lowest arcs of the
hydrographs are traced, working backward in time from
the lowest discharge to a period of surface runoff. The
tracing paper is moved horizontally until the arc of
another runoff event coincides in its lower part with the
arc already traced; this arc is plotted on top of the first.
The process is continued until all the available arcs are
plotted on top of one another.
The upward curving parts of the individual arcs are
disregarded because, presumably, they are affected by
channel storage or surface runoff, or both. The resulting
continuous arc is a mean or normal depletion curve that
represents the trend that the hydrograph would have
followed during a protracted dry period.
Even for the same stream, there may be appreciable
differences in the shape of the depletion curve at
different times of the year. This is largely due to
evaporation, transpiration, and temperature effects. In
cases such as these, a family of depletion curves may
be constructed. One curve should represent winter
when there is little or no evapotranspiration. another
curve should represent the summer when
evapotranspiration is at its maximum, and perhaps a
third curve should be prepared to represent intermediate
conditions.
Depletion curves are the basis for estimating ground-
water runoff during periods of surface runoff. They also
shed a great deal of light on the characteristics of a
ground-water reservoir.
Hydrograph Separation
A flood hydrograph is a composite hydrograph consisting
of surface runoff superimposed on ground-water runoff.
When attempting to separate these two components of
flow, however, some problems generally occur.
Whatever method is employed, there is always some
question as to the accuracy of the division. One can only
say that, in any given case, ground-water runoff is
probably not less than about "x" or more than about "y."
Keeping in mind the complexities of a stream hydrograph
brought about by variable parameters, and particularly
the geology of the basin, an attempt will be made to
develop some logical methods for hydrograph
separation.
Using the flood hydrograph in Figure 3-7a, we can see
that point A represents the start of surface runoff. Using
a previously prepared depletion curve, the original
recession can be extended to B. The area below AB
represents the ground-water runoff that would have
occurred had there been no surface runoff. Point D
represents the end of surface runoff. A depletion curve
can be matched with the recession limb, extending it
from D to C. A partial envelope has now been formed
that shows the upper and lower limits between which a
line may reasonably be drawn to separate the two
components of runoff. This assumption ignores possible
effects brought about by difference in the geologic
framework. This envelope forms a basis for the most
commonly used separation methods which are described
below.
Method 1. Using a depletion curve and starting at D in
Figure 3-7b, extend the recession curve back to a line
drawn vertically through the peak of the hydrograph (C).
A second line is then extended from A. the start of
surface runoff, to C. This method is more likely to be
valid where ground-water runoff is relatively large and
reaches the stream quickly.
Not uncommonly, the end of surface runoff is difficult to
56
-------
Time, in Days
Figure 3-6 Ground-Water Depletion Curves Have
Different Shapes that Reflect the Seasons
determine, but point D can be estimated by means of
the equation
N=A2 (1)
where N = the number of days after a peak when
surface runoff ceases and A = the basin area, in square
miles. The distance N is measured directly on the
hydrograph.
Method2. In this example in Figure 3-7b, separation is
accomplished merely by extending a straight line,
originating at the start of surface runoff (A), to a point on
the recession curve representing the end of surface
runoff (D). This method of separation is certainly the
simplest and is justifiable if little is known about the
iquifer framework.
Method 3. In this example, also in Figure 3-7b. the
prerunoff recession line is extended from A to a point
directly under the hydrograph peak (B). From this point
a second line is projected to D, the end of surface runoff.
The separation technique to be employed should be
based on knowledge of the hydrogeotogy of the basin,
keeping in mind the effect of the geologic framework on
the hydrograph.
Separation of Complex Hydrographs
Commonly runoff events occur at closely spaced
intervals and there is insufficient time for the recession
curve to develop before runoff again increases. This
complicates hydrograph separation.
Figure 3-7c shows two methods that can be used to
determine ground-water runoff under a complex
hydrograph. which represents two storms.
Method 1. The recession curve preceding the first runoff
event is continued to its intersection with a line drawn
through the first peak (A-B). The distance N is calculated
and measured. The recession limb of the first event is
continued to its intersection with the N-days line (C-D).
Line B-D is then constructed. The first recession trend
is continued to its intersection with a line drawn through
the peak of the second runoff event (CD-E). From this
point (E), the line is extended N days.
Method2. As Figure 3-7c shows, the easiest method is
to project a straight line from A to F. Although by far the
simplest, this technique is not necessarily any less
accurate than Method 1.
Hydrograph Separation by Chemical Techniques
Generally ground water is more highly mineralized than
surface runoff. During baseftow the stream's natural
quality is at or near its maximum concentration of
dissolved solids, but as surface runoff reaches the
channel and provides an increasing percentage of the
flow, the mineral content is diluted. Following the
discharge peak, surface runoff diminishes, ground-
water runoff increases, and the mineral content again
increases.
Several investigators have used the relation between
runoff and water quality to calculate the ground-water
contribution from one or more aquifers or to measure
streamflow. This method of hydrograph separation,
which requires the solution of a series of simultaneous
equations, is based on the concentration of a selected
chemical parameter that is characteristics of ground-
water and surface runoff.
57
-------
Peak
b.
Figure 3-7. Separation of the Stream Hydrograph
The basic equations, which may take several forms, are
as follows:
Qg + Qs = Q
CgQg + CsQs = CQ (2)
where Qg. Qs, and Q are ground-water runoff, surface
runoff, and total runoff, respectively; and Cg, Cs, and C
represent the concentration of dissolved mineral species
or specific conductance of ground water, surface runoff,
and total runoff, respectively. Usually specific
conductance is used as the C parameter because of
the relative ease of obtaining it.
If Cg, Cs, C, and Q are known we can determine the
quantity of ground-water runoff as follows:
Qg = Q (C—Cs)/(Cg—Cs) (3)
C is determined by measuring the specific conductance
in a well, in a series of wells, or during baseflow. The
quality of surface runoff (Cs) is obtained from analysis
of overland flow or, possibly in the case of small
streams, at the period of peak discharge when the
entire flow consists of surface runoff. It is assumed Cg
and Cs are constant. C and Q are measured directly.
Visocky (1970) used continuous recording equipment
to measure specific conductance and stage (water
level) in the Panther Creek Basin in north-central Illinois.
By using the equations given above, he calculated the
ground-water runoff component of the stream on the
basis of the relationship between discharge and specific
conductance. He also calculated and compared ground-
water runoff as determined from a ground-water rating
curve and found that the chemical method provided a
lower estimate under normal conditions than did the
rating curve technique. On the other hand, the chemical
method indicated more ground-water runoff following
storms that were preceded by extended dry periods,
which had caused considerable declines in water level
in nearby observation wells.
During baseflow, the quantity of ground-waterdischarge
from surficial sand and from limestone in the Floridan
artesian aquifer into Econf ina Creek in northwest Florida
was distinguished by Toler (1965). In this case, as
Figure 3-8 shows, the artesian water had a dissolved
solidscontent of 50-68 mg/L, while thatfromthe surficial
sand was only 10-20 mg/L. The artesian water
discharged through a series of springs along the central
part of the basin and amounted to 70 to 75 percent of
the stream's baseflow. The equation used for this
analysis is as follows:
Qa - (C-Csd)/(Ca—Csd) Q (4)
where Qa = artesian runoff, Q = runoff and Csd, Ca. and
C represent the dissolved solids in water from the sand,
the artesian aquifer, and during any instant in the
stream, respectively. Of course,
Q-Qa = Qsd (5)
58
-------
CO. -i- CO.,, = CO
0. = ? C. = 50
O=18 C =43
Q + Q = 18
50 Q. + 10 Q^, = 43 x 18
-100. - 100^ = - 180
50O. + 100^ = 774
400.
= 594
O. = 14.85 fs
OR
C. ~
= 18(43- 10) = 594 =
50-10 40
Figure 3-8. Contribution to Econf Ina Creek During a Period of Dry Weather Flow When the
Stream Discharge Was 18 cfs and the Dissolved Solids Concentration Was 43 mg/L: From
Sand Aquifer = 3.15 cfs, From Limestone Aquifer = 14.85 cfs
Continuous streamf low and conductivity measurements
were collected at a gaging station on Four Mile Creek in
east-central Iowa by Kunkle (1965). The basin above
the gage, which contains 19.5 m 2- consists largely of till
that is capped on the uplands by loess. As Figure 3-9
shows, the stream lies in a valley that contains as much
as 30 feet of permeable alluvium. Ground water from
the alluvium and loess, as well as the stream during low
flow, has an average specific conductance of 520
micromhos (Cg) while surface runoff is about 160
micromhos (Cs).
Figure 3-10 shows continuous record of discharge and
conductivity representing a storm in September 1963.
Instantaneous ground-water runoff during this event
was calculated for several points under the hydrograph
by using the following formulas
Qg + Qs = Q
CgQg + CsQs = CQ (6)
where Qg = ground-water runoff, Qs = surface runoff,
Q = runoff, and Cg. Cs, and C = specific conductance of
ground-water runoff, surface runoff, and runoff,
respectively. As determined from the graphs in Figure
3-10, where Q=2.3cfs, C=410; Cg = 520 andCs= 160,
then Qg is 1.6 cfs. Therefore, when the stream's
discharge (Q) was 2.3 cfs, ground-water runoff was 1.6
cfs. This calculation provides one point under the
hydrograph. Several other points need to be determined
so that a separation line can be drawn.
Computer Separation Programs
Various methods of hydrograph separation have been
described, all of which are laborious, time consuming,
and, commonly, open to questions of accuracy and
interpretation. In each case a mechanical technique is
used to provide a number of points on a hydrograph
through which a line can be drawn that separates
ground-water runoff from surface runoff. Once this line
is determined, one must measure, directly on the
hydrograph, the daily components of streamflow and
then sum the results.
59
-------
Surface runoff
Loess
Cg = 520
Glacial
30' ±
Figure 3-9. Four Mile Creek, Iowa
1000
Ground-walef runoll
computed from
conductivity
22 23 24 25 26 27 28 29 30 t 23
SEPTEMBER OCTOBER
Figure 3-10. Hydrographs Showing Water Dis-
charge, Specific Conductance, and Computed
Ground-Water Runoff in Four Mile Creek near
Traer, Iowa, September and October 1963
Annual ground-water runoff divided by total discharge
provides the percentage of streamflow that consists of
ground water. Effective ground-water recharge is that
quantity of precipitation that infiltrates, is not removed
by evapotranspiration, and eventually discharges into a
stream.
Effective ground-water recharge rates can be easily
estimated with a computer program described by
Pettyjohnand Henning (1979). This program separates
the hydrograph by three different methods, provides
monthly recharge rates and an annual rate, produces a
flow-duration table, and gives the operator the option of
generating the separated hydrograph and a flow-
duration curve with a line printer; as illustrated in Figure
3-11. The data base is obtained from annual streamflow
records, which are published by the U.S. Geological
Survey. The computer program will operate on a
mainframe or microcomputer.
Ground-Water Rating Curve
A widely used technique to measure streamflow is the
surface-water rating curve, which shows the relation
between stage and discharge. Figure 3-12 shows a
similar curve, called a ground-water rating curve, that
illustrates the relation between the water table and
streamflow. Prepared forthose aquifer-stream systems
that are hydrologically connected, the ground-water
rating curve can be used to separate ground-water
runoff from a stream hydrograph.
To prepare the curve, synchronous water table and
stream discharge measurements are required. Ground-
water levels are obtained either from: (1) a series of
wells spread throughout the basin, (2) a series of wells,
each of which represents an area of similar geology, or
(3) a single near-stream well. Wells influenced by
pumping should not be used. If more than one well is
used, water levels, referred to some datum, such as sea
level, must be averaged to form a composite curve.
Furthermore, measurements of both ground water and
stream stage should be made only during rainless
60
-------
neuioioo ecn cwo «r neum triune, xio
i MI run INU*MI
»B.O M.m.
;
3
let*. OI3OMMU I. Hit » (' 0« 17.1} l«CNt»
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C*«M> MIC* fti-nerr «.«isc I cr cm «.» i»cr[;
MIC* u r u.o
ft ««
-------
JO M «0
Base Flow, In Cubic Feet per Second
Figure 3-12. Rating Curve of Mean Ground-Water Stage Compared with Base Flow of Beaverdam
Creek, Maryland
the elliptical shape of the data on the rating curve is
controlled by evapotranspiration.
Using the rating curve, Olmsted and Hely (1962)
separated the Brandywine Creek hydrograph, shown in
Figure 3-16. and found that over a six year period,
ground-water runoff accounted for 67 percent of the
total flow. This compares favorably with the 64 percent
determined for North Branch Rancocas Creek in the
coastal plain of New Jersey; 74 percent for Beaverdam
Creek in the coastal plain of Maryland (Rasmussen and
Andreason, 1959}; 42 percent for Perkiomen Creek, a
flashy stream in the Triassic Lowland of Pennsylvania;
and 44 percent for the Pomperaug River Basin, a small
stream in Connecticut (Meinzer and Steams, 1928).
During certain times of the year, when the water table
lies at a shallow depth and large quantities of water are
lost by evapotranspiration, a single rating curve cannot
be used to separate a hydrograph with any degree of
accuracy. As Figure 3-17 shows, Schicht and Walton
(1961), in their study of Panther Creek basin in Illinois,
developed two rating curves. One is used when
evapotranspiration is very high and the other when
evapotranspiration is small. Double rating curves also
can be used to estimate evapotranspiration losses.
Evapotranspiration can also be calculated from the
graph used by Olmsted and Hely (1962) in the case
cited above. For example, when the grou nd-water stage
was 80 inches, streamflow was expected to be about
550 cfs in February and March but only 400 cfs in June.
In this case, the difference, about 150 cfs, is due to
evapotranspiration.
Seepage or Dry Weather Measurements
Seepage or dry-weather measurements consist of flow
determinations made at several locations along a stream
during a short time interval. It is essential that there be
no surface runoff during these measurements. Many
investigators prefer to conduct seepage runs during the
stream's 90 percent flow, that is, when the flow is so low
that it is equaled or exceeded 90 percent of the time.
It is often implied that the 90 percent flow is the only time
the flow consists entirely of ground-water runoff. This is
not necessarily the case. The 9.0 percent flow-duration
period, depending on geographic location and climate,
commonly occurs during the late summer and fall when
62
-------
^o
Brandywine |
Creek Basin
Gneiss and granitic
to ultramalic rocks
Schist
Geology generalized from Bixom
and Siese (1932, 1938)
Contact approximately
located
Stream-gaging station
Precipitation-gaging
station
Temperature-measuring
station
• Ch-3
Observation well
-^-Ch-12
Index well
Generalized boundary
ot basin
Figure 3-13. Sketch Map of Brandywine Creek Basin, Showing Generalized Geology and Location of
Hydrologlc and Meteorologlc Stations Used In Report
63
-------
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Apr. o
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May o Mar. (
June O
July 4-
0 100 200 300 400 500 600 700 800
Monthly average base (low. In cubic feet per second (Og)
Figure 3-15. Relation of Monthly Average Base
Flow to Ground-Water Stage In the Brandywlne
Creek Basin
soil moisture is depleted, there is little or no ground-
water recharge, and the water table, having declined to
its lowest level, has a low gradient. Under these
conditions, ground-water runoff is minimal. However
the physical aspect of the system may change following
a recharge period and ground-water runoff may account
for a substantial portion of the stream's flow. Hydrograph
analyses, using techniques already described, may
readily show that ground water provides 50 to 70
percent or more of the runoff. Therefore, the 90 percent
flow may reflect only a small fraction of the total quantity
of ground-water runoff.
Seepage measurements permit an evaluation of ground-
water runoff (how much there is and where it originates)
and provides clues to the geology of the basin as well.
The flow of some streams increases substantially in a
short distance. Under natural conditions this increase
probably indicates deposits orzones of high permeability
in or adjacent to the stream channel. These zones may
consist of deposits of sand and gravel, fracture zones,
solution openings in limestone or merely by local fades
changes that increase permeability. In gaining stretches,
ground water may discharge through a number of
springs and seeps, along valley walls or the stream
channel, or seep upward directly into the stream. Areas
of significant ground-water discharge may cause the
formation of quicksand.
In areas where the geology and ground-water systems
are not well known, streamflow data can provide a
means of testing estimates of the ground-water system.
If the streamflow data do not conform to the estimates,
then the geology must be more closely examined. For
example, the northwest comer of Ohio is crossed by the
Wabash and Fort Wayne moraines between which lies
the St. Joseph River. As indicated by the Glacial Map of
Ohio (Goldthwait and others, 1961), the St. Joseph
basin consists mainly of till. However, low-flow
measurements show that the discharge of the river
increases more than 14 cfs along its reach in Ohio,
64
-------
Daily discharge, affected
by regulation el low Mow
Estimated base flow, not including
•fleets of tegulalior
1953
Figure 3-16. Hydrograph of Brandywlne Creek at Chadds Ford, Pennsylvania, 1952-53
2 i-
in
T>
c
u.
c
I
•D
C
D
O
O
c
6 -
10 -
12 f-
6 -
Explanation
• Data for Periods When Evapotranspiration is Very Small
O Data for Periods When Evapotranspiration Is Great '
40
80 120 160
Ground-Water Runoff In Cubic Feet per Second
200
240
260
Figure 3-17. Rating Curves of Mean Ground-Water Stage vs. Ground-Water Runoff at Gaging Station
1, Panther Creek Basin, Illinois
65
-------
indicating that the basin contains a considerable amount
of outwash. Thus, hydrologic studies indicate the need
for geologic map modification.
On the other hand, geologic maps may indicate
reasonable locations for constructing stream gaging
stations for hydrologic monitoring networks. The
Auglaize River in northwestern Ohio rises from a mass
of outwash that lies along the front of the Wabash
moraine. The southwest-flowing river breaches the
moraine near Wapakoneta and then flows generally
north to its confluence with the Maumee River at
Defiance. A gaging station is near Ft. Jennings in a till
plain area and slightly above a reservoiron the Auglaize.
In reality this gage measures, at a single point, the flow
resulting as an end product of all causative hydrologic
factors upbasin (ground-water runoff, surface runoff,
slope, precipitation, use patterns, eta)—it shows merely
inflow into the reservoir. Low-flow measurements,
however, indicate that nearly all of the baseflow is
derived from outwash along the distal side of the Wabash
moraine; there is no grain across the wide till plain
downstream. It would seem that the most logical stream
gage site for hydrologic evaluations would be at the
breach in the Wabash moraine just downstream from
the till-outwash contact.
Figure 3-18 shows a numberof discharge measurements
made in the Scioto River basin, which lies in a glaciated
part of central Ohio. The flow measurements themselves
are important in that they show the actual discharge, in
this case at about 90 percent flow. In this case the
discharge is reported as millions of gallons per day,
instead of the usual cubic feet per second. The discharge
at succeeding downstream sites on the Scioto River are
greaterthan the flow immediately upstream. This shows
that the river is gaining and that water is being added to
it by ground-water runoff from the adjacent outwash
deposits.
A-particularly useful method for evaluating streamflow
consists of relating the discharge to the size of the
drainage basin (cfs/mi2 or mgd/mi2 of drainage basin).
As Figure 3-18 shows, this technique can be used to
relate the flow index (cfs/mi2 or mgd/mi2.) to the geology
and hydrology of the area. A cursory examination of the
data shows that the flow indices can be conveniently
separated into three distinctive units. These units are
arbitrarily called Unit I (0.01 to 0.020 mgd/mi2), Unit 2
(0.021 to 0.035 mgd/mi2) and Unit 3 (0.036 to 0.05 mgd/
mi2). The Olentangy River and Alum and Big Walnut
Creeks fall into Unit I, Big Darby and Deer Creeks into
Unit 2, and the Scioto River, Walnut Creek, and the
lower part of Big Walnut Creek into Unit 3. Notice that
even though the latter watercourses fall into Unit 3, the
actual discharge ranges widely—from 3.07 to 181 mgd.
Logs of wells drilled along the streams of Unit I show a
preponderance of fine-grained material that contains
only a few layers of sand and gravel, and wells generally
yield less than 3 gpm. Along Big Darby and Deer Creek,
however, logs of wells and test holes indicate that
several feet of sand and gravel underlie fine-grained
alluvial material, the latter of which ranges in thickness
from 5 to about 25 feet. Adequately designed and
constructed wells that tap these outwash deposits can
produce as much as 500 gpm. Glacial outwash, much
of it coarse grained, forms an extensive deposit through
which the streams and river of Unit 3 flow. The outwash
extends from the surface to depths that exceed 200
feet. Industrial wells constructed in these deposits,
most of which rely on induced infiltration, can produce
more than 1,000 gpm. Formed by combining the seepage
data and well yields with a map showing the area! extent
of the deposits that are characteristic of each stream
valley, the map in Figure 3-18 indicates potential well
yields in the area. The potential ground-water yield map
relies heavily on streamflow measurement, but
nonetheless, provides, with some geologic data, a good
first-cut approximation of ground-water availability.
Stream reaches characterized by significant increases
in flow due to ground-water runoff, may also have
unusual quality characteristics. In northern Ohio the
discharge of a small stream, shown in Figure 3-19, that
drains into Lake Erie increases over a 3-mile stretch
from about 1 to more than 28 cfs and remains relatively
constant thereafter. The increase begins at an area of
springs where limestone, which has an abundance of
solution openings, approaches land surface and actually
crops out in the stream bottom. The till-limestone contact
declines downstream eventually exceeding 90 feet in
depth.
In the upper reaches of a stream, baseflow is provided
by ground water that discharges from the adjacent till.
Since this water has been in the ground but a short time,
the mineral content is low. Electrical conductivity is
probably in the range of 640 umhos. Where streamflow
begins to increase significantly, the limestone aquifer
provides the largest increment. Furthermore, the bedrock
water contains excessive concentrations of dissolved
solids (electrical conductivity of about 2,400 umhos),
hardness, and sulfate, and in this stretch calcite
precipitates on rocks in the stream channel. The fish
population in the upper reaches is quite abundant until
the stream reaches the limestone discharge zone. At
this point, the population quickly diminishes and remains
66
-------
Columbus
167 Upper number is low flow, mgd.
.0500 Lower number is low flow, mgd/sq mi
Area of surficial outwash; well yields
may exceed 1000 gpm.
Area ol ouiwash covered by a few reel
of alluvium; well yields commonly
between 500 and 1000 gpm.
ChiBeome
Generally line-grained alluvium along
flood plain; well yields usually less than
10
i
15
!
20
I
Scale (miles)
Figure 3-18. Discharge Measurements In the Scioto River Basin, Ohio
67
-------
Station Number
No. species
No. Indiv
0.0.
Q
Temp C
PH
AIK
CO,
Cond.
Limestone with solution openings
pH t as CO2 t & CaCO3 I (travertine)
1 inch = 1 mile
Figure 3-19. Green Creek Drainage Basin (Seneca and Sandusky Countys, Ohio)
-------
in a reduced state throughout the remaining length of
the stream. No doubt the reduction in fish population is
related to the quality of the water.
In describing the hydrology of Wolf Creek in east-
central Iowa, Kunkle (1965) used seepage
measurements and water-quality data to determine the
amount of ground-water runoff provided by alluvium
and limestone. As Figure 3-20 shows, the 325 mi2 basin
is mantled by till and underlain by limestone and shale,
but the valley itself contains about 40 feet of permeable
alluvium. Well data show that the stream is hydraulically
connected with the limestone aquifer along a 5-mile
stretch and basef low is provided by discharge from both
the limestone and the alluvium. On either side of this
reach the limestone potentiometric surface is below the
stream bed.
Measurements were made at three stations during low-
flow conditions. The discharge 8 miles upstream from
the limestone discharge area was 16.4 cfs, midway
along the reach was 29.8 cfs, and 7 miles downstream
was 37.0 cfs. Water from the limestone has an average
conductivity of 1.330 umhos. while that from the alluvium
and upstream-derived baseflow average 475 u,mhos.
After mixing, the surface water had a conductivity of
550 u,mhos.
Using a slight modification of the equations given
previously, it is possible to calculate the amount of
ground-water runoff from the limestone in this reach
under the given conditions.
Ci Qi + CaQa + CbQb =CQo (7)
Qi + Qa -f Qb = QO
where Qi, Qa, Qb, QO are the discharge from upstream
(inflow), from the alluvium, from the limestone, and from
the outflow respectively, and Ci, Ca, Cb and C represent
the conductivity of the inflow from upstream, from the
alluvium, from the limestone, and from the outflow
water. Substituting:
475 Qi + 475 Qa + 1,330 Qb = 20,350
-475Qi - 475 Qa — 475 Qb = -17,575
855 Qb = 2.775 and Qb = 3-2cfs (8)
Thus in this particular stretch, the limestone was
providing about 3.2 cfs of the stream's total flow of
37 ds.
Carrying the analyses a bit further, we could assume
that since the limestone provides 3 to 4 cfs during
baseflow, wells tapping the limestone in this stretch
could provide a like amount without dewatering the
system. Since 1 cfs = 450 gpm, wells could produce a
total yield of 1,350 to 1,800 gpm. Using a similar
approach we could predict the minimum yield of wells
C = 475
Q = 16.4 CfS
C
Q
550
29.6 cfs
C = 550
Q = 37.0 cfs
6 miles
5 miles
7 miles
C 4
e O
> % 6
Oa . 40 ft •
' ft o
o *
o Qa •
,^*"o « 0
o ^
— .^^ Gravel
J~*~^ « e
O
o
A
Till
Potent Iometric surface
In limestone
Conductivity of alluvium = 475:
limestone = 1330
Umestone C = 1330
Q| = Inflow
Ob = bedrock Inflow
475 0, «• 475Q. + 1330Qb = C Q0
O; * Q. + Q6 = Q0
Q8 = alluvium Inflow
O0 = outflow
Figure 3-20. Generalized Hydrogeology of Wolf Creek, Iowa
69
-------
tapping the alluvium, assuming that they would capture
only the ground-water runoff.
Temperature Surveys
The temperature of shallow ground water is nearly
uniform, reflecting the me an annual daily airtemperature
of the region. The temperature of shallow ground water
ranges from a low of about 37 degrees in the north-
central part of the U.S. to more than 77 degrees in
southern Florida. Of course, at any particular site the
temperature of ground water remains nearly constant.
Surface-water temperatures, however, range within
wide extremes—freezing in the winter in northern regions
and exceeding 100 degrees during hot summerdays in
the south. Mean monthly stream temperatures during
July and August range from a low of 55 in the northwest
to more then 85 degrees in the southeast.
During the summer where ground water provides a
significant increment of flow, the temperature of a
stream in a gaining reach will decline. Conversely,
during winter the ground water will be warmer than that
on the surface and although ice will normally form, parts
of a stream may remain open because of the inflow of
the warmer ground water. In central Iowa, for example,
winter temperatures commonly drop below zero and ice
quickly forms on streams, ponds, and lakes. The ground-
water temperature in this region, however, is about 52
degrees and, if a sufficient amount is discharging into a
surface-waterbody, the temperature may remain above
32 degrees and the water will not freeze. In the summer,
the relatively cold ground water (52 degrees) mixes with
the warm surface water (more than 79 degrees)
producing a mixture of water colder than that in non-
gaining reaches.
Examination of winter aerial photography may show
places where ice is either absent or thin. In the summer
it is possible to float down a river, periodically measuring
the temperature. Ground-water discharge areas are
detected by temperature decrease. A third method of
detection is by means of an aircraft-mounted thermal
scanner. This sophisticated instrument is able to detect
slight differences in temperature and would probably be
more accurate than thermometry or low attitude aerial
photography.
Flow-Duration Curves
A flow-duration curve shows the frequency of occurrence
of various rates of flow. It is a cumulative frequency
curve prepared by arranging all discharges of record in
order of magnitude and subdividing them according to
the percentages of time during which specific flows are
equaled or exceeded: all chronologic orderor sequence
is lost (Cross and Hedges, 1959). Flow-duration curves
may be plotted on either probability or semilog paper. In
either case, the shape of the curve is an index of natural
storage in a basin, including ground water. Since dry-
weather flow consists entirely of ground-water runoff,
the lower end of the curve indicates the general
characteristics of shallow aquifers.
Figure 3-21 shows several flow-duration curves for
Ohio streams. During low-flow conditions, the curves
for several of the streams, such as the Mad, Hocking,
and Scioto Rivers, and Little Beaver Creek, trend
toward the horizontal, while Grand River, and Whiteoak
and Home Creeks all remain very steep.
Mad River flows through a broad valley that is filled with
very permeable sand and gravel. The basin has a large
ground-water storage capacity and, consequently, the
river maintains a high sustained flow. The Hocking Rver
locally contains outwash in and along its floodplain,
which provides a substantial amount of grourjd-water
runoff. Above Columbus, the Scioto River crosses thin
layers of limestone that crop out along the stream
valley, and the adjacent uplands are covered with
glacialtill;ground-waterrunofffromthis reach is relatively
small. Immediately south of Columbus, however, the
Scioto Valley widens and is filled with coarse outwash
(see figure 3-18). The reason that Mad River has a
higher low-flow index than the Scioto River at Chillicothe
is because the Mad River receives ground-water runoff
throughout its entire length, while the flow of the Scioto
River increases significantly only in the southern part of
the basin, that is, in the area of outwash south of
Columbus.
Whiteoak and Home Creeks originate in bedrock areas
where relatively thin alternating layers of sandstone,
shale, and limestone crop out along the hill sides. The
greater relief in these basins promotes surface runoff
and the rocks are not very permeable. Obviously the
ground-waterstorage characteristics and potential yield
of these basins are far less than those filled or partly
filled with outwash.
Figure 3-22 shows a geologic map of a part of southern
Mississippi and northern Louisiana. Notice that gaging
stations 1, 2, and 3 record the drainage from the
Citronelle Formation, while stations 4,5, and 6 represent
the drainage from the older rocks. Respective flow-
duration curves, illustrated in Figure 3-23, show that
stations 1 and 2 have high low-flow indices, with station
3 a relatively close third. The high flow-duration indices
indicate that the Citronelle Formation has a greater
ground-water storage capacity, a higher rate of natural
recharge, and presumably would provide larger yields
to wells than the underlying strata. This formation
70
-------
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Figure 3-21. Flow-Duration Curves for Selected Ohio Streams
71
-------
LOUSUNA ','''/''>
Figure 3-22. Geologic Map of Area In Southern
Mississippi Having Approximately Uniform
Climate and Altitude
8°ESH888 8
-
^8888
888
8 8
it • • t
Percent of Tim* Indicated Discharge Was Equaled or Exceeded
Figure 3-23. Flow-Duration Curves for Selected
Mississippi Streams, 1939-48
consists of sand, gravel, and clay, while the other strata
are generally composed of finer materials. Thus it would
appear that streamflow data can be used as an aid to a
better understanding of the permeability and infiltration
capacity, as well as facies changes, of geologic units.
'Flow Ratios
Walton (1970) reported that grain-size frequency
distribution curves are somewhat analogous to flow-
duration curves in that their shapes are indicative of
water-yielding properties of deposits. He pointed out
that a measure of the degree to which all of the grains
approach one size, and therefore, the slope of the grain-
size frequency distribution curve, is the sorting. One
parameter of sorting is obtained by the ratio
(D25/D75)1/2. Walton modified this equation by
replacing the 25 and 75 percent grain-size diameters
with the 25 and 75 percent flow. In this case a low ratio
is indicative of a permeable basin or one that has a large
ground-water storage capacity.
The Q25 and Q75 data are easily obtainable from flow-
duration curves. Using the data from Figure 113, Mad
River has a flow ratio of 1.58 and the Scioto River's ratio
is 2.58, while Home Creek, typifying a basin of low
permeability, has the highest ratio which is 5.16.
References
Trainer, F.W. and F.A. Watkins, 1975, Geohydrologic
reconnaissance of the Upper Potomac River Basin:
U.S. Geological Survey Water-Supply Paper 2035.
Johnstone, D. and W.P. Cross, 1949, Elements of
applied hydrology: Ronald Press, New York.
Gray, D.M. (editor), 1970, Handbook of the principles
of hydrology: Water Information Center, Inc.
Visocky, A.P., 1970, Estimating the ground-water
contribution to storm runoff by the electrical conductance
method. Ground Water, v. 8, no. 2.
Toler, L.G., 1965, Use of specific conductance to
distinguish two base-flow co mponentsinEconfina Creek,
Florida. U.S. Geological Survey Professional Paper
525-C.
Kunkle. G.R., 1965, Computation of ground-water
discharge to streams during floods, or to individual
reaches during base flow, by use of specific conductance:
U.S. Geological Survey Professional Paper 525-D.
72
-------
Pettyjohn, W.A. and R.J. Henning. 1979, Preliminary
estimate of ground-water recharge rates, related
streamftow and water quality in Ohio: Ohio State
University Water Resources Center. Project Completion
Report 552.
Rasmussen, W.C. and G.E. Andreason, 1959,
Hydrologic budget of the Beaverdam Creek Basin,
Maryland: U.S. Geological Survey Water-Supply Paper
1472.
Olmsted, F.H. and A.G. Hely, 1962. Relation between
ground water and surface water in Brand/wine Creek
Basin, Pennsylvania: U.S. Geological Survey
Professional Paper 417-A.
Meinzer, O.E. and N.D. Stearns, 1928, A study of
ground water in the Pomerang Basin: U.S. Geological
Survey Water-Supply Paper 597-B.
Schicht. R.J. and W.C. Walton, 1961, Hydrologic budgets
for three small watersheds in Illinois: Illinois State
Water Survey Report of Investigations 40.
LaSala, A.M., 1967, New approaches to water resources
investigations in upstate New York: Ground Water, vol.
5. no. 4.
Goktthwait, R.P., G.W. White, and J.L. Forsyth. 1961,
Glacial map of Ohio: U.S. Geological Survey,
Miscellaneous Geological Investigations Map 1-316.
Cross, W.P. and R.E. Hedges. 1959. Flow duration of
Ohio streams: Ohio Division of Water Bulletin 31.
Walton. W.C., 1970,Groundwater resource evaluation:
McGraw-Hill Publ. Co.. New York.
73
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Chapter 4
BASIC HYDROGEOLOGY
Introduction
Hydrogeology is the study of ground water, its origin,
occurrence, movement, and quality. Ground water is a
part of the hydrologic cycle and it reacts in concert with
all of the other parts. Therefore, it is essential to have
some knowledge of the components, particularly
precipitation, infiltration, and the relation between ground
water and streams, as well as the impact of the
geologic framework on water resources. This chapter
provides a brief outline of these topics and interactions.
Precipitation
Much precipitation never reaches the ground; it
evaporates in the air and from trees and buildings. That
which reaches the land surface is variable in time, areal
extent, and intensity. The variability has a direct
impact on streamflow, evaporation, transpiration, soil
moisture, ground-water recharge, ground water, and
ground-water quality. Therefore, precipitation should
be examined first in any hydrogeoiogic study in order
to determine how much is available, its probable
distribution, and when and under what conditions it is
most likely to occur. In addition, a determination of the
amount of precipitation is the first step in a water-
balance calculation.
Seasonal Variations In Precipitation
Throughout much of the United States, the spring
months are most likely to be the wettest owing to the
general occurrence of rains of low intensity that often
continue for several days at a time. The rain, in
combination with springtime snowmett, will saturate the
soil, and streamflow is generally at its peak over a
period.of several weeks or months. Because the soil is
saturated, this is the major period of ground-water
recharge. In addition, since much of the total runoff
consists of precipitation and snowmett (surface runoff),
streams most likely will contain less dissolved mineral
matter than at any other time during the year.
Not uncommonly, the fall also is a wet period,
although precipitation is not as great or prolonged as
during the spring. Because ground-water recharge
can occur over wide areas during spring and fall, one
should expect some natural changes in the chemical
quality of ground water in surficial or shallow
aquifers.
During the winter in northern states, the ground is
frozen, largely prohibiting infiltration and ground-water
recharge. An early spring thaw coupled with widespread
precipitation may lead to severe flooding over large
areas.
Types of Precipitation
Precipitation is classified by the conditions that produce
a rising column of unsaturated air, which is
antecedent to precipitation. The major conditions are
convective, orographic, and cyclonic.
Convectional precipitation is the result of uneven heating
of the ground, which causes the air to rise, expand,
the vapor to condense, and precipitation to occur.
Much of the summer precipitation is convective, that is,
high intensity, short duration storms that are usually of
small areal extent. They often cause flash floods in
small basins. Most of the rain does not infiltrate, usually
there is a soil-moisture deficiency, and ground-water
recharge is likely to be of a local nature. On the other
hand, these typically small, local showers can have a
significant impact on shallow ground-water quality
because some of the water flows quickly through
fractures or other macropores, carrying water-soluble
compounds leached from the dry soil to the water table.
In cases such as these, the quality of shallow ground
water may be impacted as certain chemical constituents,
and perhaps microbes as well, may increase
dramatically within hours (see Chapter 5).
Orographic precipitation is caused by topographic
barriers that force the moisture-laden air to rise and
cool. This occurs, for example, in the Pacific Northwest,
74
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where precipitation exceeds 100 inches per year, and
in Bangladesh, which receives more than 425 inches
per year, nearly all of which falls during the monsoon
season. In this vast alluvial plain, rainfall commonly
averages 106 inches during June for a daily average
exceeding 3.5 inches.
Cyclonic precipitation is related to large low pressure
systems that require 5 or 6 days to cross the United
States from the northwest or Gulf of Mexico. These
systems are the major source of winter precipitation.
During the spring, summer, and fall, they lead to rainy
periods that may last 2 or 3 days or more. They are
characterized by low intensity and long duration, and
cover a wide area. They probably have a major impact
on natural recharge to shallow ground-water systems
during the summer and fall, and influence ground-water
quality as well.
Recording Precipitation
Precipitation is measured by recording and
nonrecording rain gages. Many are located throughout
the country but because of their inadequate density,
estimates of annual, and particularly summer,
precipitation probably are too low. Records can be
obtainedfrom Climatological Data, which are published
by the National Oceanic and Atmospheric
Administration (NOAA). Precipitation is highly variable,
both in time and space. The area! extent is evaluated
by means of contour or isohyet maps (fig. 4-1).
A rain gage should be installed in the vicinity of a site
under investigation in order to know exactly when
precipitation occurred, how much fell, and its intensity.
Data such as these are essential to the interpretation of
hydrographs of both wells and streams, and they
provide considerable insight into the causes of
fluctuations in shallow ground-water quality.
Infiltration
The variability of streamflow depends on the source
of the supply. If the source of streamflow is from
surface runoff, the stream will be characterized by short
periods of high flow and long periods of low flow or no
flow at all. Streams of this type are known as "flashy."
If the basin is permeable, there will be little surface
runoff and ground water will provide the stream with a
high sustained, uniform flow. These streams are
known as "steady." Whether a stream is steady or
flashy depends on the infiltration of precipitation and
snowmelt.
When it rains, some of the water is intercepted by trees
or buildings, some is held in low places on the ground
(depression storage), some flows over the ground to a
stream (surface runoff), some is evaporated, and some
infiltrates. Of the water that infiltrates, a part replenishes
the soil-moisture deficiency, if any, while the remainder
percolates deeper, perhaps becoming ground water.
The depletion of soil moisture begins immediately after
a rain due to evaporation and transpiration.
Infiltration capacity (f) is the maximum rate at which a
soil is capable of absorbing water in a given condition.
Several factors control infiltration capacity.
---- 16. _ '8 _ 20 22 .24 26 28 30 32 34
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Unas of Bquol irocpitation
(inches)
L_J_..\_. LLJL
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~ -^4,
36 38 N,-
40
Figure 4-1. Distribution of Annual Average Preclpltalton In Oklahoma, 1970-79 (from Pettyjohn and
others, 1983)
75
-------
o Antecedent rainfall and soil-moisture conditions. Soil
moisture fluctuates seasonally, usually being high
during winter and spring and low during the summer
and fall. If the soil is dry, wetting the top of it will create
a strong capillary potential just under the surface,
supplementing gravity. When wetted, the clays forming
the soil swell, which reduces the infiltration capacity
shortly after a rain starts.
o Compaction of the soil due to raindrop impact.
o Inwash of fine material into soil openings, which
reduces infiltration capacity. This is especially important
if the soil is dry.
o Compaction of the soil by animals, roads, trails, urban
development, etc.
o Certain microstructures in the soil will promote
infiltration, such as soil structure, openings caused by
burrowing animals, insects, decaying rootlets and
other vegetative matter, frost heaving, desiccation
cracks, and other macropores.
o Vegetative cover, which tends to increase infiltration
because it promotes populations of burrowing
organisms and retards surface runoff, erosion, and
compaction by raindrops.
o Decreasing temperature, which increases water
viscosity, reducing infiltration.
o Entrapped air in the unsaturated zone, which tends
to reduce infiltration.
o Surface gradient.
Infiltration capacity is usually greater at the start of a
rain that follows a dry period, but it decreases rapidly
(fig. 4- 2). After several hours it is nearly constant
because the soil becomes clogged by particles and
swelling clays. A sandy soil, as opposed to a clay-rich
soil, may maintain a high infiltration capacity for a
considerable time.
As the duration of rainfall increases, infiltration
capacity continues to decrease. This is partly due to
the increasing resistance to flow as the moisture front
moves downward; that is, the resistance is a result of
frictional increases due to the increasing length of flow
channels and the general decrease in permeability
owing to swelling clays. If precipitation isgreaterthan
infiltration capacity, surface runoff occurs. If precipitation
is less than the infiltration capacity, all moisture is
absorbed.
Coarse leiiura
Pino Tcu
Figure 4-2. Infiltration Capacity Decreases with
Time During a Rainfall Event
When a soil has been saturated by water then allowed
to drain by gravity, the soil is said to be holding its field
capacity of water. (Many investigators are opposed to
the use and definition of the term field capacity because
it does not account for the rapid flow of water through
preferred paths, such as macropores.) Drainage
generally requires no more than two or three days and
most occurs within one day. A sandy soil has a low field
capacity that is reached quickly; clay-rich soils are
characterized by a high field capacity that is reached
slowly (fig. 4-3).
The water that moves down becomes ground-water
recharge. Since recharge occurs even when field
Average inches depth 0(4
water per foot depth of
soil in plant root zone
3
Figure 4-3. Relation Between Grain Size and
Field Capacity and Wilting Point
76
-------
capacity is not reached, there must be a rapid transfer
of water through the unsaturated zone. This probably
occurs through macropores (Pettyjohn. 1982). Figure
4-4 is a graph of the water table following a storm that
provided slightly more than three inches of rain in about
an hour in mid-July in north-central Oklahoma. At that
time the water table in a very fine-grained aquifer was
about 7.5 feet below land surface. Notice that the water
table began to rise within a half hour of the start of the
rain despite the very low soil-moisture content. The
velocity of the infiltrating water through the unsaturated
zone was about 15 feet per hour, and this only could
have occurred by flow through fractures and other
macropores. Clearly field capacity could not have been
reached in this short period of time.
Surface Water
Streamflow, runoff, discharge, and yield of drainage
basin are all nearly synonymous terms. Channel
storage refers to all of the water contained at any
instant within the permanent stream channel. Runoff
includes all of the water in a stream channel flowing
past a cross section; this water may consist of
precipitation that falls directly into the channel,
surface runoff, ground-water runoff, and effluent.
Although the total quantity of precipitation that falls
directly into the channel may be large, it is quite small in
comparison to the total flow. Surface runoff, including
interflow or stormflow, is the only source of water in
ephemeral streams and intermittent streams during
part of the year. It is the major cause of flooding.
During dry weather, ground-water runoff may account
for the entire flow of a stream. It is the major source of
water to streams from late summer to winter; at this
time streams also are most highly mineralized under
natural conditions. Ground water moves slowly to the
stream, depending on the hydraulic gradient and
permeability; the contribution is slow but the supply is
steady. When ground-water runoff provides a stream's
entire discharge, the flow is called dry-weather or base
flow. Other sources of runoff include the discharge of
industrial or municipal effluent or irrigation return flow.
Rates of Flow
Water courses are generally classified on the basis of
their length, size of drainage basin, or discharge; the
latter is probably the most significant index of a stream's
utility in a productive society. Rates of flow generally
are reported as cubic feet per second (cfs), millions of
gallons per day (mgd), acre-feet per day. month, or
year, cfs per square mile of drainage basin (cfs/mi2),
or inches depth on drainage basin per day, month, or
year. In the United States, the most common unit of
measurement is cfs. The discharge (Q) is determined
by measuring the cross-sectional area of the channel
(A), in square feet, and the average velocity of the
water (v). in feet per second, so that:
Q=vA (9)
Stream Discharge Measurements and Records
At a stream gaging site the discharge is measured
periodically at different rates of flow, which are plotted
against the elevation of the water level in the stream
(stage or gage-height). This forms a rating curve (fig.
4-5). At a gaging station the stage is continuously
c
£
Jo
"5. ,
y 3
| 880
TJ
5 2
a 879
1
fc 1
| 878
"5
o>
I °
877
""TT""1
• — —
1
y.
\-
••.
Well A-4
— •••s^
/*>'' Well r>5
//
f Barometric
•-. pressure. ...-••
12 6
July 14
12
l
July 15
Figure 4-4. Response of the Water Table In a
Fine-Grained, Unconflned Aquifer to a High
Intensity Rain
Diwtwp* (cubic
-------
measured and this record is converted, by means of
the rating curve, into a discharge hydrograph. The
terminology used to describe the various parts of a
stream hydrograph are shown in Figure 4-6.
CrMt
Figure 4-6. Stream Hydrograph Showing
Definition of Terms
Discharge, water quality, and ground-water level
records are published annually by the U. S. Geological
Survey for each state. An example of the annual
record of a stream is shown in Figure 4-7. Notice that
these data are reported in "water years." The water
year is designated by the calendar year in which it ends,
which includes 9 of the 12 months. Thus, water year
1990 extends from October 1,1989. to September 30,
1990.
The Relation between Ground Water and Surface
Water
There are many tools for learning about ground water
without basing estimates on the ground-water system
itself, and one approach is the use of streamflow data
(See Chapter 3). Analyses of streamflow data permit
an evaluation of the basin geology, permeability, the
amount of ground-water contribution, and the major
areas of discharge. In addition, if chemical quality data
are available or collected for a specific stream, they
can be used to determine background concentrations
of various parameters and locate areas of ground-
water contamination as well.
Ground Water
The greatest difficulty in working with ground water is
that it is hidden from view, cannot be adequately tested,
and occurs in a complex environment. On the other
hand, the general principles governing ground-water
occurrence, movement, and quality are quite well
known, which permits the investigator to develop a
reasonable degree of confidence in his predictions.
The experienced investigator is well aware, however,
that these predictions are only estimates of the manner
in which the system functions. Ground-water hydrology
is not an exact science, but it is possible to develop
a good understanding of a particular system if one pays
attention to fundamental principles.
The Water Table
Water under the surface of the ground occurs in two
zones, an upper unsaturated zone and the deeper
saturated zone (fig. 4-8). The boundary between the
two zones is the water table. In the unsaturated zone,
most of the open spaces are filled with air, but water
occurs as soil moisture and in a capillary fringe that
extends upward from the water table. Water in the
unsaturated zone is undera negative hydraulic pressure,
that is, it is less than atmospheric. Ground water occurs
below the water table and all of the pores and other
openings are filled with fluid that is under pressure
greater than atmospheric.
In a general way, the water table conforms to the
surface topography, but it lies at a greater depth under
hills than it does under valleys (fig. 4-8). In general, in
humid and semiarid regions the water table lies at
depths ranging from 0 to about 20 feet or so. but its
depth exceeds hundreds of feet in some desert
environments.
The elevation and configuration of the water table must
be determined with care, and many such
measurements have been incorrectly taken. The
position of the watertable can be determined from the
water level in swamps, flooded excavations (abandoned
gravel pits, highway borrow pits, etc.), sumps in
basements, lakes, ponds, streams, and shallow wells.
In some cases there may be no water table at all or it
may be seasonal.
Measurement of the water level in drilled wells,
particularly if they are of various depths, will more likely
reflect the pressure head of one or more aquifers that
are confined than the actual water table.
Figure 4-9 illustrates the difference in water levels in
several wells, each of which is of a different depth.
Purposely no scale has been applied to the sketch
because the drawing is relative. That is, the same
principle exists regardless of scale, and individual zones
could be only a few inches or feet thick, or they might
exceed several tens of feet. Notice that each well has
a different water level but the water table can be
determined only in Well 2. Wells 1. and 3-5, which tap
confined aquifers, are deeper and each is screened in
78
-------
ARKANSAS RIVER RA^'N
07176)00 BIND CREEK NEAR AVANT. OK
LOCATION.—Lit 36'29'I2-, long 96'03'SO". In SW l/» » '/» sec.7 (revised), 1.2) N.. R.I2 E. , Osege County,
HydrolooJc Unit 11070107, IX) ft upstreM fro* county road bridge at Avant, t.S «1 upitrta* fre» Candy Creek,
«nd «l BMC »k.2.
DRAINAGE AREA.-.Mk Hi*.
PERIOD or RECORD.--August IMS to current r«ar.
CACE."*ater-slage recorder. Datu» of gage I> 651.2< Ft above National Orodetle Vertical Oitun of 1929.
REMARK S.»R*eerd.i fair. Several untxihllshed observations of «ater temperature, spoclflc conductance, and pH «ere
•vide durino. the year and art available at the District Office, Mo. slightly regulated since I'M by Blueste*
Lake. So»e regulation since Narci 1977 by Birch Lak« (station 07176*60), located on Birch Creek, 12.1 «1
upitrea*. Saiall inversions upstrea* for aunlelpal water supply 'or the cities of Paifcuska and Barnsdall.
AVERAGE DISCHARGE.—*) years, 221 ft'/«, 160,10O acre-rt/yr.
EXTREMES TOR PERIOD or RECORD.--Ma»l«M dUeharoe. )J,kOO rt'/s. Oct. 2. 1959, gage height. 31.kO fti *a»l«w» gage
height, )2.0) ft. Mar. 11, 197k; no flo. at tl«ej.
EXTREMES FOR CURRENT VEAR.--Pee discharge of «,000 ft'/i and M>l«ui («)i
Date Tlae Discharge Cage Height Date TIM
Discharge
(ftj/s)
Cage Height
Ift)
Nov. 2k
Dec. 1»
1615 8,270 11.25
Mlnleu. dally discharge, ).1 rt'/s. Oct. 17. 18.
Mar. )
Apr. 1
1215
20*>
Discharge Cage Height
(ftj/s) (ft)
•1*,200
DISCHARGE, CICIC FEE1 PIR SECOM), VATER TEAR OCTOBER 1W7 TO SEPTEHBER 19S8
HtAN VALUES
DAY
1
2
3
k
5
c
7
8
' 9
10
11
.12
13
Ik
15
16
17
18
19
20
21
22
23
2k
»
26
27
28
29
30
31
TOTAL
ME AM
MAX
MIN
AC-FT
CAL »R
tTR YR
OCT
kit
)32
30)
283
277
268
23k
132
21
7.0
«.3
t!2
5.8
5.6
k.8
).a
). i
3.1
3. )
).6
k.1
k.7
8.5
9.6
8.3
7.6
7.k
R.k
9.6
9.S
1)
2k11.k
77.8
kl*
J.I
k780
1987
1988
MOV
Ik
13
12
12
12
10
10
10
10
10
10
9.8
9.6
9.6
80
,j
9k
51
36
26
U
10
8.2
3)70
1450
3)6
207
727
2kO
200
...
«596.2
220
3J70
8.2
13080
TOTAL 123598
TOTAL 11 Ik 32
DEC
157
111
115
92
»*
to
)6
29
26
2)
20
1)
H
20
JO
17
52
HI
SkOO
k770
9*)
778
595
525
87<
1110
21 HO
15*0
758
752
9)8
22178
715
SkOO
It
k3990
.2 MEAN
.0 MEAN
3AM
k57
)k1
297
257
16)
1)6
1)2
120
81
120
120
120
ik2
Ul
163
923
1>*O
519
lore
78 1
»97
>**
25)
>21
191
129
91
170
120
Ik6
120
9737
31k
15*0
81
19)10
339 MAI
30k MAX
FEB
101
101
120
81
118
107
105
106
11k
116
111
10k
102
67
k»
kl
kl
•2
m
/12
IkT
117
•4
•0.
7)
70
69
69
69
...
...
2792
96.)
212
kl
55*0
71*0
1)200
MAR
(.
322
k*80
Ik 60
I860
1690
1320
898
638
53k
k76
3k9
322
2*8
1S6
1)7
170
355
k05
)))
272
2)2
209
196
188
175
170
U6
55)
528
1170
2110)
Ml
k«80
69
k18«0
Mni ).i
MIN 3.1
.APR
13200
7000
9?2
*))
813
711
662
629
608
2180
1090
6k3
kOk
311
252
152
1k5
2570
8)1
k8k
38)
)17
166
157
200
220
200
166
151
1k6
...
)«*oe
121k
1)200
US
72220
AC-FI 2k 5200
AC-FT 221000
HAY
1k6
Ikk
1)7
1))
129
125
120
120
120
108
91
5k
)5
28
22
21
2)
2k
2k
2k
2k
2k
29
J3
59
kl
27
20
IB
17
15
19)7
62.5
1k«
15
38*0
3UN
12
1k2
t?
55
k<
2*
2)
21
19
17
16
16
16
16
16
15
12
11
11
11
11
11
11
11
11
11
10
10
10
9.6
658.6
22.0
Ik2
9.6
1)10
3UL
8.6
9.6
.'0
22
2k
2*
22
18
U
16
22
112
62
3*
26
22
j*
26
23k
1S6
SO
kl
31
27
26
2k
22
27
277
12)
56
Utk.k
S2.1
277
8.8
3200
AUC
)*
)0
27
21
20
16
U
16
16
16
16
15
15
15
13
15
15
15
15
15
15
15
IS
15
16
16
16
16
16
16
16
5)7
17.)
36
15
1070
SEP
16
17
20
17
16
16
Ik
11
9.
9.
f
.
.
•
U6
208
U10
ISkO
kSO
219
160
327
266
168
108
80
66
6f
5k
...
J»59.k
182
15*0
9.6
10830
Figure 4-7. Stream Discharge Record for Bird Creek near Avant, Oklahoma (From U.S. Geological
urvey Water Resources Data for Oklahoma Water Year 1987, p. 90)
79
-------
Wrf
Flooded BM*mant
Stream
UnMtuntcd Zone
(SoiMovtun)
J } I M I
Saturated Zon*
(Ground Watarl
Openings Urge*
• fiied with Air
ary Fringe
•WntBT Table
OpenineaFetodwith
•Water
Figure 4-8. The Water Table Generally Conforms to the Surface Topography
>^s-s«> .
.•N'%«S».%\%'.1 r*V*V«%\V*'
» -
•%•%•%•%•%'!• 4" %•%•%•%
f t f f t ft ft f f f t'
Water Table
a I
LS\S\%\S^y-%»_%iSj,V
'•<.••*••*••*••*••*••*•••••
•_% •S • ^ • % •S •L% ^.% •_*• • % \*
•%•%•%•%•%*•%•%•%*•%•%
s«^WW
t;f!tifi(ifif;f;(;fi
\ «%•_%•%•
pack
screen
o w tr ts 1 1
•*• »*«^» •*••*• •*•* »
:
Figure 4-9. The Water Level In a Well Indicates the Pressure that Exists In the Aquifer that it Taps
80
-------
a particular permeable zone that is bounded above and
below by less permeable confining units. The water
level in each well reflects the pressure that exists in the
individual zone that is tapped by the well. A different
situation occurs in Well 1, because the gravel pack
surrounding the well casing and screen provides a high
permeability conduit that .connects all of the water-
bearing zones. The water level in Well 1 is a composite
of the pressure in all of the zones.
Because hydraulic head generally differs with depth, it
is exceedingly important to pay attention to well depth
and construction details when preparing water-level
maps and determining hydraulic gradient and flow
direction.
For example, water-level measurements in wells 1 and
2 or 5 and 4 would suggest a gradient to the left, but
wells 2 and 3 or 3 and 4 would allude a gradient to the
right of the drawing. In addition, the apparent slope of
the gradient would depend on the wells being measured.
Accurately determining the position of the water table
is important not only because of the need to determine
the direction and magnitude of the hydraulic gradient,
but, in addition, the thickness, permeability, and
composition of the unsaturated zone exert a major
control on ground-water recharge and the movement
of contaminants from land surface to an underlying
aquifer. Attempting to ascertain the position of the
water table by measuring the water level in drilled
wells nearly always will incorrectly suggest an
unsaturated zone that is substantially thicker than
actually is the case, and thus may provide a false sense
of security.
Ground water has many origins, however, all fresh
ground water originated from precipitation that
infiltrated. Magmatic or juvenile water is "new" water
that has been released from molten igneous rocks.
The steam that is so commonly given off during
volcanic eruptions is probably not magmatic, but rather
shallow ground water heated by the molten magma.
Connate water is defined as that entrapped within
sediments when they were deposited. Ground water,
however, is dynamic and probably in only rare
circumstances does connate water meet this definition.
Rather, the brines that underlie all or nearly all fresh
ground water have changed substantially through
time because of chemical reactions with the geologic
framework.
Aquifers and Confining Units
In the subsurface, rocks serve either as confining units
or aquifers. A confining unit or aquitard is characterized
by low permeability that does not readily permit water
to pass through it. Confining units do, however, store
large quantities of water. Examples include shale,
clay, and silt. An aquifer has sufficient permeability to
permit water to flow through it with relative ease and,
therefore, it will provide a usable quantity to a well or
spring.
Water occurs in aquifers under two different
conditions—unconfined and confined (fig. 4-10). An
unconfined or water-table aquifer has a free water
surface that rises and falls in response to differences
between recharge and discharge. A confined or
artesian aquifer is overlain and underlain by aquitards
and the water is under sufficient pressure to rise above
the base of the confining bed, if it is perforated. In some
cases, the water is under sufficient pressure to rise
above land surface. These are called flowing or
artesian wells. Thewaterlevelin an unconfined aquifer
is referred to as the water table; in confined aquifers
the water level is called the potentiometric surface.
Figure 4-10. Aquifer A Is Unconfined and Aquifers
B and C are Confined, but Water May Leak
Through Confining Units to Recharge Adjacent
Water-Bearing Zones
Water will arrive at some point in an aquifer through
one or several means. The major source is direct
infiltration of precipitation, which occurs nearly
everywhere. Where the watertable lies below a stream
or canal, the surface water will infiltrate. This source is
important part of the year in some places (intermittent
streams) and is a continuous source in others
(ephemeral or losing streams). Interaquifer leakage, or
flow from one aquifer to another, is probably the most
significant source in deeper, confined aquifers.
Likewise, leakage from aquitards is very important
where pumping from adjacent aquifers has lowered
the head orpotentiometric surface sufficiently for leakage
to occur. Underflow, which is the normal movement
81
-------
of water through an aquifer, also will transmit ground
water to a specific point. Additionally, water can reach
an aquifer through artificial means, such as leakage
from ponds, pits, and lagoons, from sewer lines, and
from dry wells, among others.
An aquifer serves two functions, one as a conduit
through which flows occurs, and the other as a storage
reservoir. This is accomplished by means of openings
in the rock. The openings include those between
individual grains and those present in joints, fractures,
tunnels, and solution openings. There are also artificial
openings, such as engineering works, abandoned
wells, and mines. The openings are primary if they
were formed at the tine the rock was deposited and
secondary if they developed after lithificat ion. Examples
of the latter include fractures and solution openings.
Porosity and Hydraulic Conductivity
Porosity, expressed as a percentage or decimalfraction,
is the ratio between the openings and the total rock
volume. It defines the amount of water a saturated rock
volume can store. If a unit volume of saturated rock is
allowed to drain by gravity, not all of the water it
contains will be released. The volume drained is the
specific yield, a percentage, and the volume retained
is the specific retention. Related to the attraction
between water and earth materials, specific retention
generally increases as sorting and grain size decrease.
Porosity determines the total volume of water that a rock
unit can store, while specific yield defines the amount
that is available to wells. Porosity is equal to the sum
of specific yield and specific retention. Typical values
for various rock types are listed in Table 4-1.
Permeability (P) is used in a qualitative sense, while
hydraulic conductivity (K) is a quantitative term They
are expressed in a variety of units gpdft2 (gallons per
day per square foot) will be used in this section; see
Table 4-2 for conversion factors) and both reler to the
ease with which water can pass through a rock unit. It
M«IW«J
Soil
City
Sand
Grtval
Umecione
SandMon*. aamiconaolidated
Granita
Bault. young
Paratify
55
so
25
20
20
It
0.1
11
Seacific v«*d
1* by voO
40
2
22
19
16
6
0.08
8
Scwttc
RMcntan
15
48
3
1
2
5
0.01
3
Table 4-1. Average Porosity, Specific Yield, and
Specific Retention Values for Selected Earth
Materials
is the hydraulic conductivity that allows an aquifer to
serve as a conduit. Hydraulic conductivity values
range widely from one rock type to another and even
within the same rock. It is related to grain size, sorting,
cementation, and the amount of secondary openings,
among others. Typical ranges in values of hydraulic
conductivity for most common water-bearing rocks are
'shown in Table 4-3 and Figure 4-11.
Those rocks or aquifers in which the hydraulic
conductivity is nearly uniform are called homogeneous
and those in which it is variable are heterogeneous or
nonhomogeneous. Hydraulic conductivity also can
vary horizontally in which case the aquifer is anisotropic.
If uniform in all directions, which is rare, it is isotropic.
The fact that both unconsolidated and consolidated
sedimentary strata are deposited in horizontal units is
the reason that hydraulic conductivity is generally
greater horizontally than vertically, commonly by
several orders of magnitude.
Hydraulic Gradient
The hydraulic gradient (I) is the slope of the water table
or potentiometric surface, that is, the change in water
level perunit of distance along the direction of maximum
head decrease. It is determined by measuring the
water level in several wells. The water level in a well
(fig. 4-12), usually expressed as feet above sea level, is
the total head (ht), which consists of elevation head (z)
and pressure head (hp).
ht=z+hp (10)
The hydraulic gradient is the driving force that causes
ground water to move in the direction of maximum
decreasing total head. It is generally expressed in
consistent units, such as feet per foot. For example, if
the difference in water level in two wells 1000 feet apart
is 2 feet, the gradient is 2/1,000 or 0.002 (fig. 4-13).
Since the water table or potentiometric surface is a
plane, the direction of ground-water movement and the
hydraulic gradient must be determined by information
from three wells (fig. 4-14). The wells must tap the same
aquifer, and should be of similar depth and screened
interval.
Using the three point method, water-level elevations
are determined for each well, and their locations are
plotted on a map. Lines are drawn to connect the wells
in such a way that a triangle is formed. Using the
elevations of the end points, each line is divided into a
number of equal elevation segments. Selecting points
of equal elevation on two of the lines, equipotential or
potentiometric contours are drawn through the points.
A flow line is then constructed so that it intersects the
82
-------
***** Cl.*.Llllll, t»
Mdtara par day
Jmd-»)
1
• MX 10*
3*5x10-'
4.1x10-* .
CantlRMlar* par taoond
1
3,11*10-'
4.73x10-*
Faat par day
P 1 d~ *)
243 x 10*
1
1.34x10-'
GaKew paroay
par aquara loot
jpald- '»!-*)
34&X101
112x10*
1
Sowar* malar* par day
laat par day
PI* a-1)
Gallon* par d*r
POT loot
(g* t -' II -')
ana
to.n
i
•0.9
741
1
Vdurn*
(In mllllnwtwt)
(In Inclw*)
Z.7
70
251
1.S74
47.746
(m> into -')
.0313
J0047a
.000063
•0.
1
1.7O
au
.00378
JOI67
.0023
35.3
•0
1
.134
11.800
2C4
449
Table 4-2. Conversion Factors for Hydraulic Conductivity, Recharge Rates, and Flow Rates
Material
Hydraulic conductivity (rounded values)
Coarse sand
Medium sand
Silt
Clay
Limestone (Castle Hayne)
Saprotite
Granite and gneiss
Slate
. ... 200
130
1
0 001
300
. ... s
5
3
1500
1000
5
001
2000
50
50
25
60
40
0 2
00004
80
2
2
•j
Table 4-3. Hydraulic Conductivity of Selected Rocks (Heath, 1980)
equipotential contours at a right angle. Ground water surface map is a graphical representation of the
flows in the direction of decreasing head or water level.
Potentiometrlc Surface Maps and Flow Nets
Potentiometric surface or water-level maps are an
essential part of any ground-water investigation
because they indicate the direction in which ground
water is moving and provide an estimate of the hydraulic
gradient, which controls velocity. A potentiometric
hydraulic gradient. They are prepared by plotting water-
level measurements onabasemapandthencontouring
them. The map should be drawn so that it actually
reflects the hydrogeological conditions. Sample map is
shown in Figure 4-15.
The water-level contours are called potentiometric lines,
indicating that the water has the potential to rise to that
83
-------
Igneous and Metamorphic Rocks
Unfroctured Fractured
Basalt
Unfroctured Froctured Lovo flow
Sandstone
Fractured Semiconsolidoted
Shale
Unfra ct ured Fractured
Carbonate Rocks
Froctured Cavernous
Clay Silt. Loess
Sllty Sand
Clean Sand
Fine Coarse
Glacial Till Gravel
10'* IO'7 10"' IO"5 I0~4 tO"s IO"2 10"' I 10 10 z I05 10*
m d-'
i i
IO"7 10'' I0~5 10'* I0"s I0~2 10"' I 10 10 2 10 * 10 4 10
ftd '
i i i i _ i i i i i i • i i i
IO"7 IO"6 I0"s IO"4 I0"s IO"2 10"' I 10 10 * 10 s 10 4 10 s
gal d" ft-'
Figure 4-11. General Range In Hydraulic Conductivity for Various Rock Types
elevation. In the case of a confined aquifer, however, flow lines, the former are constructed with uniform
the water may have the potential to rise to a certain differences in elevation between them and the latter so
elevation, but it cannot actually do so until the confining that they form, in combination with equipotential lines,
unit is perforated by a well. Therefore, a potentiometric a series of squares. A carefully prepared flow net in
surface map of a confined aquifer represents an conjunction with Darcy's Law (discussed below) can
imaginary surface. be used to estimate the quantity of water flowing
through an area.
A potentiometric surface map can be developed into a
flow net by constructing flow lines that intersect the A plan view flow net of an unconfined aquifer is shown
equipotential lines at right angles. Flow lines are in Figure 4-16. Notice that all of the water-table contours
imaginary paths that would be followed by particles of point upstream, and that the flow lines originate in the
water as they move through the aquifer. Although central part of the interstream divide (recharge area)
there is an infinite number of both equipotential and and terminate at the streams (discharge line). A vertical
84
-------
i
Total
head
(ht)
1
i
^ Measuring
*~ point
Potentiometric
t
\
l
\
L surface
Pressure
head (hp)
r
' Elevation
, head (z)
Datum (Sea Level)
Figure 4-12. Relationship Between Total Head,
Pressure Head, and Elevation Head
Figure 4-13. The Hydraulic Gradient Is Defined
by the Decline In Water Level In Wells a Defined
Distance Apart
flow net, representing the line A-A' in Figure 4-16, is
shown in Figure 4-17. In this case, the curved flow lines
illustrate that the ground water is moving in the same
direction but not in the same manner as implied from the
plan view.
Af low net that represents a different hydrologic situation
is shown in Figure 4-18. In this case, the streams are
gaining in the upper part of the map, while below their
confluence the water-table contours begin to point
downstream. This indicates that the water table is
below the channel, the stream is losing water to the
subsurface, and the flow lines are diverging from the
line source of recharge. A vertical flow net is shown in
Figure 4-19.
Direction of GrounO-
Water Movement
27.5 —
Sagmentsol
Wtter Tat* Contour*
W«t«r Tabfe Akituto
77.2
27.0 -t /
268
Figure 4-14. The Generalized Direction of
Ground-Water Movement Can Be Determined by
Means of the Water Level In Three Wells of
Similar Depth (From Heath and Trainer, 1981)
Ground water Hows not only through aquifers, but
across confining units as well. Owing to the great
differences in hydraulic conductivity between aquifers
and confining units, most of the flow occurs through
aquifers where the head loss per unit of distance is far
less than in a confining unit. As a result, flow lines tend
to parallel aquifer boundaries; they are less dense and
trend nearly perpendicular through confining units (fig.
4-20). Consequently, lateral flow in units of low hydraulic
conductivity is small compared to aquifers, but vertical
leakage through them can be significant. Where an
aquifer flow line intersects a confining unit the flow line
is refracted to produce the shortest path. The degree of
refraction is proportional to the differences in hydraulic
conductivity.
Calculating Ground-Water Flow
Darcy's Law, expressed in many different forms, is
used to calculate the quantity of underflow or vertical
leakage. One means of expressing it is:
Q = KIA (11)
where:
Q
A
K
I
= quantity of flow, ingpd
= cross-sectional area through which the
flow occurs, in ft2
= hydraulic conductivity, in gpd/ft2
= hydraulic gradient, in ft/ft
The flow rate is directly proportional to the gradient
and therefore the flow is laminar, which means the
water will follow distinct flow lines rather than mix with
other flow lines. Where laminar flow does not occur, as
85
-------
\
Aquifer Boundary
^~~~N
Bluffe along River Valley
. 838 Wei Location and Altitude of Water Laval (feet)
Figure 4-15. A Potentlometrlc Surface Map Representing the Hydraulic Gradient in an Aquifer that
Crops Out Along the Bluffs of a River Valley
Flow li
„ - - NEquipotential line
Land surface
v >* X^ I / '"\ •'
>is ^^L(y\\''
- 20
10
- 0
-•10
--20
Horizontal Scale, in (eet
9 4q°°
Figure 4-17. Vertical Flow Net of an Unconflned
Aquifer (Modified from Heath, 1983)
in the case of unusually high velocity, which might be
found in fractures, solution openings, or adjacent to
some pumping wells, the flow is turbulent.
As an example of Darcy's Law, notice in Figure 4-21A
that a certain quantity (Q) of fluid enters the sand- filled
Gaining stream tube, with a cross section of A, and the same amount
Figure 4-16. Plan View Flow He\ of an Unconflned exits Tne water level declines along the length of the
Aquifer (Modified from Heath, 1983) flow Path (L) and the nead is n'9ner in tne manometer
86
-------
Gaining stream
Gaining stream
B
Losing stream
Figure 4-18. Plan View Flow Net of an Unconflned
Aquifer Where Streams Change from Gaining to
Losing (Modified from Heath, 1983)
at the beginning of the flow path than it is at the other
end. The difference in head (H) along the flow path (L)
is the hydraulic gradient (H/L or I). The head loss
reflects the energy required to move the fluid this
distance. If Q and A remain constant but K is increased.
then the head loss decreases. H is important to keep in
mind the fact that the head loss occurs in the direction
of flow.
Water table - ' '>ro?
3000
60,00
9000
i
Horizontal scale, in feet
Figure 4-19. Vertical Flow Net of an Unconflned
Aquifer with a Losing Stream (Modified from
Heath, 1983)
In Figure 4-21 B, the flow tube has been inverted and the
water is flowing from bottom to top or top to bottom. Q.
K, A, and I all remain the same. This illustrates an
important concept when the manometers are considered
as wells. Notice that the deeper well has a head that is
higher than the shallow well when the water is moving
upward, while the opposite is the case when the flow is
downward.
Where nearby wells of different depths and water levels
occur in the field, as shown in Figure 4-21C, it clearly
indicates the existence of recharge and discharge areas.
In recharge areas, shallow wells have a higher head
than deeper wells; the difference indicates the energy
required to vertically move the water the distance
Land surface
'
_Wjrter table
Unconfined —
aquifer
Confined -/--
aquifer
Figure 4-20. Flow Lines In Aquifers Tend to Parallel Boundaries but in Confining Units They are Nearly
erpendicularto Boundaries (Modified from Heath, 1983)
87
-------
A. Horizontal Mod-filed tube.
Gradiant - H/L - I. tha anarwnquM
to mow tha watar dtaanoa L
Q - Quantity o* flow, gpd
A - CiOMMCtionalMof fow.ff
K • Hydraulic cooductMty - gpd/ff
B. Vvtical tuba with flow
IfWTV LkHUJTri CO lOp.
Vertical tuba wfth flow
rfom top to bottom.
"i:
T
Q
{
K t
L
1
-
C.
Watar Laval
How
Otocharv*
Area
t| 1'~
Figure 4-21. Graphical Explanation of Darcy's
Law. Notice That the Flow In a Tube can be
Horizontal or Vertical In the Direction of
Decreasing Head
between the screens of the two wells. Where the flow
is horizontal, there should be no difference in head. In
discharge areas, the deeper well will have the higher
head. Waste disposal in recharge areas might lead
to the vertical migration of leachate to deeper aquifers
and, from this perspective, disposal sites should be
located in discharge areas.
An example of the use of Darcy's Law, consider a sand
aquifer, about 30 feet thick, that lies within a mile wide
flood plain of a river. The aquifer is covered by a
confining unit of glacial till, the bottom of which is about
45 feet below land surface. The difference in water
level in two wells a mile apart is 10 feet. The hydraulic
conductivity of the sand is 500 gpd/ft2. The quantity
of underflow passing through a cross-section of the
river valley is:
Q =KIA
= 500 gpd/ft2 * (10 ft/5280 ft) * (5280 ft*30 ft)
= 150,000 gpd (12)
Thequantrtyof flowfromone aquifer to anotherthrough
a confining unit can be calculated by a slightly modified
form of Darcy's Law.
where
K' =
m' =
A =
H =
Q|_= (KVm')AH (13)
quantity of leakage, in gpd
vertical hydraulic conductivity of the confining
unit, gpd/ft2
thickness of the confining unit, ft
cross-sectional area through which leakage
is occurring, ft2
difference in head between the two wells
tapping the upper and lower aquifers, ft
As illustrated in Figure 4-22, assume two aquifers are
separated by a layer of silt. The silty confining unit is
10 feet thick and has a vertical hydraulic conductivity
of 2 gpd/ft2. The difference in water level in wells
tapping the upper and lower aquifers is 2 feet. Let us
also assume that these hydrogeologic conditions exist
in an area that is a mile long and 2000 feet wide. The
daily quantity of leakage that occurs within this area
from the deep aquifer to the shallow aquifer is
Q= (2gpd/ft2/10ft)*(5,280ft*2,000ft)*2ft (14)
- 4.224,000 gpd
This calculation clearly shows that the quantity of
leakage, either upward or downward, can be immense
even if the hydraulic conductivity of the aquitard is small.
Interstitial Velocity
The interstitial velocity of ground water is of particular
importance in contamination studies. It can be estimated
by the following equation.
v=KI/7.48n (15)
where:
v = average velocity, in ft per day
n = effective porosity
Other terms are as previously defined. '
As an example, assume there is a spill that consists
of a conservative substance, such as chloride. The
88
-------
Figure 4-22. Example of Interaqulfer Leakage
liquid waste infiltrates through the unsaturated zone
and quickly reaches a water-table aquifer that
consists of sand and gravel with a hydraulic conductivity
of 2,000 gpd/ft2 and an effective porosity of 0.20. The
water level in a well at the spill lies at an altitude of
1,525 feet and at a well a mile directly downgradient it
is at 1,515 feet (fig. 4-23). What is the velocity of the
water and contaminant and how long will it be before the
second well is contaminated by chloride?
Time
(2,000 gpd/ft2 *(10 ft/5,280 ft))/7.48
2.5 ft/day (16)
5,280 ft/2.5 ft/day
2,112 days or 5.8 years
.20
This velocity value is crude at best and can only be used
as an estimate. Hydrodynamic dispersion, for
Spill
15251
1 mile
1515'
A'/••'/>•/ ••'•••'.•^'; •••;••':'•.••:' ^
~j:.::?JX^X:J;t;^
'$£ K - 2000 gpd/sq.ft
'\-/:'; Unconfined aquifer
Confining unit
Figure 4-23. Using Ground-Water Velocity
Calculations, It Would Require Nearly 6 Years for
the Center of Mass of the Spill to Reach the
Downgradient Well
example; is not considered in the equation. This
phenomenon causes particles of water to spread
transverse to the major direction of flow and move
downgradient at a rate faster than expected. It is
caused by an intermingling of streamlines due to
differences in interstitial velocity brought about by the
.•irregular pore space and interconnections.
Furthermore, most chemical species are retarded in
their movement by reactions with the geologic
framework, particularly with certain clays, soil-organic
matter, and selected hydroxides. Only conservative
substances, such as the chloride ion, will move
unaffected by retardation (see Chapter 5).
In addition, it is not only the water below the water table
that is moving, but also fluids within the capillary fringe.
Here the.velocity diminishes rapidly upward from the
water table. Movement in the capillary fringe .is
important where the contaminant is gasoline or other
substances less dense than water.
Transmlsslvlty and Storatlvlty
Hydrogeologists commonly use the term transmissivity
(T) to describe an aquifer's capacity to transmit water.
Transmissivity is equal to the product of the aquifer
thickness (m) and hydraulic conductivity (K) and it is
described in units of gpd/ft (gallons per day per foot of
aquifer thickness).
T = Km (17)
Another important term is storativity(S), which describes
the quantity of water that an aquifer will release from or
take into storage per unit surface area of the aquifer per
unit change in head. In unconfined aquifers the
storativity is, for all practical purposes, equal to the
specific yield and, therefore, it should range between
0.1 and 0.3. The storativity of confined aquifers is
substantially smaller because the water that is
released from storage when the head declines comes
from the expansion of water and compaction of the
aquifer, both of which are exceedingly small. For
confined aquifers the storativity generally ranges
between 0.0001 and 0.00001, and for leaky confined
aquifers it is in the range of 0.001. On method to
estimate storativity for confined aquifers is to multiply
theaquiferthicknessby 0.000001 The small storativity
for confined aquifers means that to obtain a sufficient
supply from a well there must be a large pressure
change throughout a wide area. This is not the case
with unconfined aquifers because the water derived is
not related to expansion and compression but comes
instead from gravity drainage and dewatering of the
aquifer.
89
-------
Hydrogeologists have found it necessary to use
transmissivity and storativity coefficients to calculate
the response of an aquifer to stresses and to predict
future water-level trends. These terms also are
required as input for most flow and transport computer
models.
Water-Level Fluctuations
Ground-water levels fluctuate throughout the year in
response to natural changes in recharge and
discharge, to changes in pressure, and to artificial
stresses, such as pumping. Fluctuations brought about
by changes in pressure are limited to confined aquifers.
Most of these changes, which are short term, are
caused by loading, such as a passing train compressing
the aquiferor an increase in discharge from an overlying
stream. Other water-level fluctuations are related to
changes in barometric pressure, tides, eartntides, and
earthquakes. None of these fluctuations reflect a
change in the volume of water in storage.
An examination of the rise and fall of the water level in
a well tapping flood plain deposits may lead to erroneous
conclusions. If the aquifer is unconfined, a water-level
rise implies ground-water recharge. On the other hand,
a similar rise in a confined aquifer may be the result of
loading brought about by the additional weight as the
discharge of the stream increases. Generally ground-
water recharge would lag behind an increase in stream
discharge, while pressure loading would be concomitant
(fig. 4-24).
Fluctuations that involve changes in storage are
generally more long lived (fig. 4-25). Most ground-
water recharge takes place during the spring and fall.
Following these periods, which are a month or two long.
• I*
4H
. ILLINOIS RIVER STAGE _
AT BEAROSTOWN
"V.
*~t*
• •
—
OMBM
"Si
— —
p
V
—
—
^
—
3-
J 414
•( 4M
410
411
- W
•— ^•i
ELL
=»
CS
~~^
1
S I8NI2
*• •
""•«»
M-l!
1.+ 9
^
^
-
^
— «v
I*M
JUNE
Figure 4-24. Effect of Increasing River Stage on
the Water Level In a Well Tapping a Confined
Aquifer (From Walton, 1970)
I
879
870
876
J 875
I 873}
872
871-
f-4.00
rs.co
s
2.00 g
0
TV
2
..oo £
I S
? ?
1984
?r
.--
0.00
1985
Figure 4-25. Relationship Between Precipitation
and Water Level In a Well Tapping a Fine-
Grained, Unconfined Aquifer
the water level declines in response to natural discharge,
which is largely to streams. Although the major period
of recharge occurs in the spring, minor events can
happen any time there is a rain.
The volume of water added or removed from ground-
waterstorage can be estimated by the following equation:
VrS (17)
where
Vw = the volume of water, in cubic feet
Vr = the volume of rock through which the
water level has changed
S = storativity
For example, following a rain the water table rises a half
a foot throughout an area of 10,000 square feet. If the
aquifer has a storativity of 0 .2, then 1000 cubic feet or
or nearly 7,500 gallons of water were added to storage.
In this regard, Figure 4-25 shows an interesting
relationship. Notice in April 1985 that the water table
rose about 1.3 feet following 1.5 inches of rain, but in
May the water table rose only about 1 foot after two
storms provided more than twice the amount of rain (3.1
inches). This phenomenon suggests that the storativity
changed, but actually the effect is related to soil moisture.
When the unsaturated zone has a high soil-moisture
content (April), the tillable porosity is less than it is when
the moisture content is low (May); therefore, the greater
the moisture content, the higher the water table rise.
Evapotranspiration effects on a surficial or shallow
aquifer are both seasonal and daily. Trees, each
serving as a minute pump, remove water from the
90
-------
capillary fringe or even from beneath the water table
during hours of daylight in the growing season (fig. 4-
26). In turn, this results in a diurnal fluctuation in the
water table and it might influence streamflow as well.
•11.0
Figure 4-26. Hydrograph of a Well, 14 Feet Deep,
that Is Influenced by Transpiration
Cone of Depression
When a well is pumped, the water level in its vicinity
declines to provide a gradient to drive water toward the
discharge point. The gradient becomes steeper as the
well is approached because the flow is converging from
all direction, and the area through which the flow is
occurring gets smaller. This results in a cone of
depression around the well. Relatively speaking, the
cone of depression around a well tapping an
unconf ined aquifer is small if compared to that around
a well in a confined system. The former may be a few
tens to a few hundred of feet in diameter, while the
latter may extend outward for miles (fig. 4-27). By
means of aquifer tests, which analyze the cone of
depression, coefficients of transmissivity and storativity,
as well asotherhydraulic parameters can be determined.
Cones of depression from several pumping wells may
overlap and, since their drawdown effects are
additive, the water-level decline throughout the area
of influence is greater than from a single cone (fig. 4-
28). In ground-water studies, and particularly
contamination problems, evaluation of the cone or
cones of depression can be critical because they
represent an increase in the hydraulic gradient, which
in turn controls ground-water velocity and direction of
flow. In fact, properly spaced and pumped wells
provide a mechanism to control the migration of
leachate plumes. Discharging and recharging well
schemes are commonly used in attempts to restore
contaminated aquifers (see Chapter 7).
Specific Capacity
The decline of the water level in a pumping well, or any
well for that matter, is called the drawdown and the pre-
Land surface
Limits of cone
of depression
Land surface •
Potenllon|ietrjc_surface~
Q """"'
x'~~~
Drawdown \ | | x'V^Cone of
depression
ff /S/SSSSSSSSSS S
Confined aquifer"
Confining unit
Figure 4-27. Cones of Depression In Unconfined and Confined Aquifers (From Heath, 1983)
91
-------
Well A
WeJiB
Cone of depression with Well A pumping
Static potentiometric surface
Cone of depression if Well B was
pumping and Well A was idle
Confined aquifer
Well A
WellB
.* — -A
Cone of depression with
-both Well A and Well B
pumping
Confined aquifer
Figure 4-28. Overlapping Cones of Depression Result In More Drawdown Than Would Be the Case for
a Single Well (Modified from Heath, 1983)
Land surface
_Pot&nt\om»tr\c_3ur1ac9_ (nonpumpingj_
Cone of depression
XXXXXXXXXXXXXXX/XXXX
Producing zone
Length
of
screen
A Drawdown In
aquifer --
A Well loss
XXXXXXXXXXXXXXXXXXXXX
— 'Nominal' radius
Confined
aquifer
° Effective radius
X / / S XX XX/XXX/^X/X/XXXXXXXXXX
X X// / ' Ss sss
Confining unit
xxxxxx
Figure 4-29. Values of Transmlsslvlty Based on Specific Capacity Commonly are Too Low Because of
Well Construction Details that Increase Well Loss (Modified from Heath, 1983)
92
-------
pumping level is the static water level (fig. 4-29). The
discharge rate of the well divided by the difference
between the static and the pumping level is the specific
capacity. The specific capacity indicates how much
water the well will produce per foot of drawdown.
where
Q
s
Specific capacity = Q/s (18)
the discharge rate, in gpm
the drawdown, in ft
If a well produces 100 gpm and the drawdown is 8 feet,
the well will produce 12.5 gallons per minute for each
foot of available drawdown. One can rather crudely
estimate transmissivity of confined aquifers by
multiplying specific capacity by 2,000 and by 1,550 in
the case of unconfined systems.
The material presented in this chapter is both brief
and generalized, but it should provide sufficient
information and general principles to allow one to
develop some understanding of hydrogeology. Greater
detail can be obtained from the literature mentioned in
the references.
References
Center For Environmental Research Information, 1985,
Protection of public water supplies from ground-water
contamination: U.S. Environmental Protection Agency,
EPA/625/4-85/016.
Freeze. R.A. and J.A. Cherry, 1979, Groundwater:
Prentice-Hall Publ. Co.. Englewood Cliffs. NJ.
Heath. R.C., 1980. Basic elements of ground-water
hydrology with reference to conditions in North Carolina:
U.S. Geol. Survey Water Resources Invest, Open-File
Rept. 80-44.
Heath, R.C. and F.W. Trainer, 1981, Introduction to
ground water hydrology: Water Well Jour. Publ. Co.,
Worthington, OH.
Heath. R.C., 1983. Basic ground-water hydrology:
U.S. Geol. Survey Water-Supply Paper 2220.
Heath, R.C.. 1984, Ground-water regions of the
United States: U.S. Geol. Survey Water-Supply Paper
2242.
Johnson, E.E. 1966, Ground water and wells: Edward
E. Johnson, Inc., Saint Paul, MM.
Pettyjohn, W.A., 1982, Cause and effect of cyclic
changes in ground-water quality: Ground-Water
Monitoring Review, vol. 2, no. 1.
Pettyjohn, W.A., Hal White, and Shari Dunn, 1983,
Water atlas of Oklahoma: Univ. Center for Water
Research, OK State Univ.
Pettyjohn, W.A., 1985, Regional approach to
ground-water investigations: in Ward, C.H., W. Giger,
and P.L McCarty, Ground Water Quality, John Wiley &
Sons. New York, NY.
Seaber, P.R., 1965, Variations in chemical character of
water in the Englishtown Formation of New Jersey:
U.S. Geol. Survey Prof. Paper 498-B.
Stefferud, Alfred. 1955, Water, the yearbook of
agriculture: U.S. Dept of Agriculture.
Todd, O.K., 1980, Groundwater hydrology: John Wiley
& Sons. New York, NY.
U.S. Geological Survey, 1985. Water resources data.
Oklahoma, water year 1983: U .S. Geol. Survey Water-
Data Rept OK-83-1.
Walton, W.C., 1970, Groundwater resource evaluation:
McGraw-Hill Book Co., New York. NY.
93
-------
Chapter 5
GROUND-WATER CONTAMINATION
Introduction
For millennia, hu mans have disposed of waste products
in a variety of ways. The method might reflect
convenience, expedience, expense, or best available
technology, but in many instances, leachates from
these wastes have come back to haunt latergenerations.
Ground-water contamination may lead to problems of
inconvenience, such as taste, odor, color, hardness, or
foaming, but the problems are far more serious when
pathogenic organisms, flammable or explosive
substances, or toxic chemicals and their by-products
are present.
Presently, most regulatory agencies are concerned
with ground-water contamination cases that involve
organic compounds, and this is the result of the rapid
growth of the synthetic organic chemical industry in the
United States during the last 50 years. At least 63,000
synthetic organic chemicals are in common industrial
and commercial use in the United States, and the
number increases by 500 to 1,000 each year.
Furthermore, health effects brought about by long term.
low level exposures are not well known.
More than 200 chemical constituents in ground water
have been documented, including approximately 175
organic compounds and more than 50 inorganic
chemicals and radionuclides (OTA, 1984). The sources
of these chemicals are both natural and human-induced.
In a survey conducted by the U.S. EPA, volatile organic
compounds (VOCs) were detected in 466 randomly
selected public ground-water supply systems. One or
more VOCs were detected in 16.8 percent of small
systems and 28.0 percent of the larger systems sampled.
Those occurring most often were trichloroethylene
(TCE) and tetrachloroethylene (PCE).
In the lesser developed countries, contamination of
water supplies by organic compounds is of minor
concern, or of no concern at all. In such places the
major hearth problems are the result of poor sanitary
conditions and illness brought about by pathogenic
organisms. In Mexico, for example, 10 percent of the
the individuals who perish each year die from diarrhea,
which is caused by the ingestion of contaminated food,
water, and air. The primary health-related goal of water
treatment is disinfection, and the emphasis over the
past several years on synthetic organic compounds in
drinking water in the United States has overshadowed
this goal.
Individual contaminated sites generally are not large,
but once degraded, ground water may remain in an
unusable or even hazardous condition for decades or
even centuries (Pettyjohn, 1979). The typically low
velocity of ground water prevents a great deal of mixing
and dilution; consequently, a contaminant plume may
maintain a high concentration as it slowly moves from
points of recharge to zones of discharge.
Sources of Ground-Water Contamination
As water moves through the hydrologic cycle, its quality
changes in response to differences in the environments
through which it passes. The changes may be either
natural or human-influenced; in some cases they can
be controlled, in other cases they cannot, but in most
instances they can be managed in order to limit adverse
water-quality changes.
The physical, chemical, and biological quality of water
may range within wide limits. In fact, it is often impossible
or at least difficult to distinguish the origin (human-made
or natural) of many water-quality problems. Natural
quality reflects the types and amounts of soluble and
insoluble substances with which the water has come in
contact. Surface water generally contains less dissolved
solids than ground water, although at certain times
where ground-water runoff is the major source of
streamflow, the quality of both surface water and ground
water is similar. During periods of surface runoff, streams
may contain large quantities of suspended materials
and, under some circumstances, a large amount of
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dissolved solids. Most commonly, however, during high
rates of flow streams have a low dissolved-mineral
concentration.
Although the chemical quality of water in surficial or
shallow aquifers may range within fairly broad limits
from one time to the next, deeper ground water is
characterized by nearly constant chemical and physical
properties, at least on a local scale where the aquifer is
unstressed by pumping. As a general rule, dissolved
solids increase with depth and with the time and distance
the water has traveled in the ground. A few uncommon
water-quality situations exist throughout the country,
reflecting peculiar geologic and hydrologic conditions.
These include, among others, thermal areas and regions
characterized by high concentrations of certain elements,
some of which may be health hazards.
For centuries humans have been disposing of waste
products by burning, placing them in streams, storing
them on the ground, or putting them in the ground.
Human-induced influences on surface-water quality
reflect not only waste discharge directly into a stream,
but also include contaminated surface runoff. Another
major influence on surf ace-waterquality is related to the
discharge of ground water into a stream. If the adjacent
ground water is contaminated, stream quality tends to
deteriorate. Fortunately in the latter case because of
dilution, the effect in the stream generally will not be as
severe as it is in the ground.
The quality of ground water most commonly is affected
by waste disposal and land use. One major source of
contamination is the storage of waste materials in
. excavations, such as pits or mines. Water-soluble
substances that are dumped, spilled, spread, or stored
on the land surface eventually may infiltrate. Ground
water also can become contaminated by the disposal of
fluids through wells and, in limestone terrains, through
sinkholes directly into aquifers. Likewise, infiltration of
contaminated surface water has caused ground-water
contamination in several places. Irrigation tends to
increase the mineral content of both surface and ground
water. The degree of severity in cases such as these is
related to the hydrologic properties of the aquifers, the
type and amount of waste, disposal techniques, and
climate.
Another cause of ground-water quality deterioration is
pumping, which may precipitate the migration of more
mineralized water from surrounding strata to the well. In
coastal areas pumping has caused seawaterto invade
fresh-water aquifers. In parts of coasta! west Florida,
wild-flowing, abandoned artesian wells have salted,
and consequently ruined, large areas of formerly fresh
or slightly brackish aquifers.
Ground-Water Quality Problems that Originate
on the Land Surface
1. Infiltration of contaminated surface water
2. Land disposal of solid and liquid waste materials
3. Stockpiles, tailings, and spoil
4. Dumps
5. Disposal of sewage and water-treatment plant sludge
6. Sah spreading on roads
7. Animal feedlots
8. Fertilizers and pesticides
9. Accidental spills
10. Paniculate matter from airborne sources
Ground-Water Quality Problems that Originate
Above the Water Table
1. Septic tanks, cesspools, and privies
2. Surface impoundments
3. Landfills
4. Waste disposal in excavations
5. Leakage from underground storage tanks
6. Leakage from underground pipelines
7. Artificial recharge
8. Sumps and dry wells
9. Graveyards
Ground-Water Quality Problems that Originate
Below the Water Table
1. Waste disposal in wet excavations
2. Agricultural drainage wells and canals
3. Well disposal of wastes
4. Underground storage
5. Secondary recovery
6. Mines
7. Exploratory wells and test holes
8. Abandoned wells
9. Water supply wells
10. Ground-water development
Table 5-1. Sources of Ground-Water Quality
Deterioration
Table 5-1 shows that ground-water quality problems
are most commonly related to: (I) water-soluble products
that are stored or spread on the land surface, (2)
substances that are deposited or stored in the ground
above the water table, and (3) material that is stored,
disposed of, or extracted from below the water table.
Many of the contamination problems related to these
activities are highly complex, and some are not well
understood.
Ground-Water Quality Problems thaf Originate on
the Land Surface
Infiltration of Contaminated Surface Water. The yield of
many wells tapping streamside aquifers is sustained by
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Contaminated
stream
PSP> ^
.•U5lsKX&V>
Figure 5-1. Induced Infiltration from a
Contaminated Stream Will Degrade Ground-Water
Quality
infiltration of surface water (fig. 5-1). In fact, more than
half of the well yield may be derived directly by induced
recharge from an adjacent surface-water source, which
may be contaminated. As the induced water migrates
through the subsurface, a few substances are diluted or
removed, particularly where the water flows through
filtering materials, such as sand and gravel, or organic
matter. Filtration is not likely to occur if the water flows
through large openings, such as those in some carbonate
aquifers. Chloride, nitrate, and several organic
compounds, are highly mobile, move freely with the
water, and are not removed by filtration.
Examples' of the degradation of ground-water supplies
by induced infiltration of contaminated surface water
are both numerous and widespread. In the greatest
number of cases, the contamination originated from the
disposal of municipal or industrial waste directly into a
stream, which was then induced by pumping into
adjacent aquifers. In hydrologic situations such as
these, months or even years may be required for the
contaminant to reach a well, but once there, all of the
intervening area may be completely degraded.
Land Disposal of Solid and Liquid Waste Materials. One
cause of ground-water contamination is the disposal of
waste materials directly onto the land surface. Examples
include manure, sludges, garbage, and industrial wastes.
The waste may occur as individual mounds or it may be
spread over the land. If the waste material contains
soluble substances, they may infiltrate. Similar problems
occur in the vicinity of various types of stockpiles.
Stockpiles. Tailings, and Spoil. Perhaps the prime
example of ground-water contamination caused by
stockpiles is unprotected storage of de-icing salt (sodium
and calcium chloride), commonly mixed with sand, at
highway maintenance lots. The salt readily dissolves to
either infiltrate or run off. An average sized stockpile
may contain 150 to 250 tons of salt, with anticaking
additives, such as ferric ferrocyanide and sodium
ferrocyanide, and perhaps phosphate and chromate to
reduce corrosivity (Williams, 1984).
Otherstockpiles include coal, metallic ores, phosphates,
and gypsum. Both coal and metal sulfide ores, when
weathered, may cause acid drainage, and the resulting
low pH water may dissolve additional constituents from
the ore or from other earth materials that it contacts.
Tailings, which consist of ore of a grade too low for
furthertreatment, also may generate acid waters. They
are commonly associated with ponds used for the
disposal of mining wastes from cleaning and ore
concentration. As a general rule, tailings ponds are
unlined and, when eventually filled with slurry, are
abandoned; they may serve as sources of acid, metals,
dissolved solids, and radioactivity.
The debris or waste material produced during mining is
called spoil. For over a century, iron-sulfide-rich spoil
has served as a major source of acid-mine drainage in
the eastern coal fields and at metal sulfide mines
throughout the country.
Dumps. During the past two decades, investigators
have taken a serious look at the environmental effects
of dumps. As rainwater infiltrates through trash in a
dump, it accumulates an ample assortment of chemical
and biological substances. The resulting fluid, or
leachate, may be highly mineralized, and as it infiltrates,
some of the substances it contains may not be removed
or degraded.
Disposal of Sewage and WaterTreatment Plant Sludge.
Sludge is the residue of chemical, biological, and physical
treatment of municipal and industrial wastes. They
include lime-rich material from water treatment plants,
as well as sewage sludge from wastewaler treatment
plants. Sludges typically contain partly decomposed
organic matter, inorganic salts, heavy metals, bacteria,
and perhaps viruses. Nitrogen in municipal sludge may
vary from 1 to 7 percent. Land application of wastewater
and sewage sludge is an alternative to conventional
treatment and disposal, and is in common usage by the
canning and vegetable industry, petroleum refining,
pulp and paper, and the power industry. Contamination
results from the infiltration of partly treated wastewaters
that have not undergone sufficient attenuation.
Infiltration from wastewater stabilization ponds also can
cause ground-water contamination. Ponds of this type
primarily are used for settlement of suspended solids
and biological treatment of primary and secondary
effluent.
Salt Spreading on Roads. Especially since the
construction of the interstate highway system, water
contamination due to wintertime road salting has become
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an increasing problem. From a quality viewpoint, the
salting may bring about deterioration of streams due to
surface runoff, and infiltration causes ground-water
contamination. Numerous instances of contamination
have been reported in the New England states and
Michigan.
On the outskirts of Muskegon, Ml, which lies on a sandy
plain adjacent to Lake Michigan, a class action suit was
filed against a county wastewater treatment operation
that uses large upground lagoons, alleging
contamination of several domestic wells. Evidence
presented at a pretrial hearing clearly showed, however,
that a few of the domestic wells had been contaminated
from time to time, but the source was de-icing salt
spread on high crowned roads that were bordered by
wide, deep ditches cut into the sand of an unconfined
aquifer. All of the domestic wells were adjacent to the
road ditches and, when pumping, induced salty water to
the well.
Accidental Spills. A large volume of toxic materials are
transported throughout the country by truck, rail, and
aircraft, transferred at handling facilities, and stored in
tanks; accidental spills of these materials are
commonplace. It has been estimated that about 16,000
spills, ranging from a few to several million gallons,
occur each year, and these include hydrocarbons, paint
products, flammable materials, acids and anhydrous
ammonia, among many others (National Academy of
Sciences, 1983). Virtually no .methods are available to
quickly and adequately clean up an accidental spill or
those caused by explosions or fires! Furthermore,
immediately following an accident, the usual procedure
is to spray the area with water. The resulting fluid may
either flow into a stream or infiltrate. In a few cases, the
fluids have been impounded by dikes, causing even
more infiltration.
Fertilizers and Pesticides. Increasing amounts of both
fertilizers and pesticides are being used in the United
States each year. Reportedly, there are more than
32,000 different compounds consisting of an excess of
1,800 active ingredients used in agricultural applications
(Houzim and others, 1986). Many are highly toxic and,
in countless cases, quite mobile in the subsurface.
Numerous compounds, however, become quickly
attached to fine-grained sediment, such as organic
matter and clay and silt particles. A part of this attached
material is removed by erosion and surface runoff. In
many heavily fertilized areas, the infiltration of nitrate, a
decomposition product of ammonia fertilizer, has
adversely affected ground water. The consumption of
nitrate-rich water leads to a disease in infants known as
"blue babies" (methemoglobinemia).
In some irrigated regions, automatic fertilizer feeders
are attached to irrigation sprinkler systems. When the
pump is shut off, water flows back through the pipe into
the well bore, creating a partial vacuum that may cause
fertilizer to flow from the feeder into the well. It is
possible that some individuals even dump fertilizers
(and perhaps pesticides) directly into the well to be
picked up by the pump and distributed to the sprinkler
system.
Aurelius (1989) described an investigation in Texas
where 188 wells were sampled for nitrate and pesticides
in 10 counties where aquifer vulnerability studies and
field characteristics indicated the potential for ground-
water contamination from the normal use of agricultural
chemicals. Nine pesticides (2,4,5-T, 2,4-DB,
metolachlor, dicamba, atrazine, prometon, bromacil,
picloram, and triclopyr) were found present in 10 wells,
nine of which were used for domestic supply. Also, 182
wells were tested for nitrate and ofthese. 101 contained
more than the recommended limit. Of the high nitrate
wells, 87 percent were used for household purposes. In
addition, 28 wells contained arsenic at or above the limit
of 0.05 mg/L, and 23 of these were domestic wells.
Animal Feedlots. Feedlots, usedforcattle, hogs, sheep,
and poultry, cover relatively small areas but provide a
huge volume of wastes. These wastes and seepage
from lagoons have contaminated both surface and
ground water with large concentrations of nitrate,
phosphate, chloride, and bacteria.
Paniculate Matter from Airborne Sources. A relatively
minor source of ground-water contamination is caused
by acid rain and the fallout of paniculate matteroriginating
from smoke, flue dust, or aerosols, and from automobile
emissions. Some of the paniculate matter is water-
soluble and toxic. Deutsch (1963) described an example
of ground-water contamination by chromium-rich dust
discharged through roof ventilators at a factory in
Michigan. Accumulating on the downwind side of the
plant, the highly soluble hexavalent chromium infiltrated,
contaminating a local municipal water supply. Along the
Ohio River in the vicinity of Ormet, Ohio, the airborne
discharge of fluoride from an aluminum processing
plant seriously affected dairy operations, and fluoride
concentrations in ground water at the plant exceeded
1,000 mg/L in the mid 1970s.
Ground-Water Quality Problems that Originate
Above the Water Table
Many different types of materials are stored, extracted,
or disposed of in the ground above the water table.
Table 5-1 shows that contamination can originate from
many of these operations.
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Septic Tanks. Cesspools, and Privies. Probably the major
cause of ground-water contamination in the United
States is effluent from septic tanks, cesspools, and
privies. Individually of little significance, these devices
are important in the aggregate because they are so
abundant and occur in every area not served by municipal
or privately owned sewage treatment systems. Onsite
sewage disposal systems number approximately 22
million and discharge an estimated one trillion gallons of
effluent. Conventional septic tanks and their associated
leach fields account for 85 percent of the systems in
use.
The area that each point source affects is generally
small, since the quantity of effluent is small, but in some
limestone areas effluent may travel long distances in
subterranean cavern systems. Biological contamination
of ground water is widely recognized. In areas where
the density of septic tanks is unusually high and the soils
are permeable, this form of waste disposal has caused
regional ground-water contamination (Nassau and
Suffolk counties, NY, and Dade County, FL).
Surface Impoundments. Surface impoundments,
including ponds and lagoons, generally consist of
relatively shallow excavations that range in area from a
few square feet to many acres. They are used in
agricultural, municipal, and industrial operations for the
treatment, retention, and disposal of both hazardous
and nonhazardous wastes. During the Surface
Impoundment Assessment (EPA. 1983), more than
180,000 impoundments were located at approximately
80,000 sites. Nearly half of the sites were located over
zones that are either very thin or very permeable, and
more than half of these contained industrial waste. In
addition, 98 percent of the sites on thick, permeable
aquifers were located within a mile of potential drinking
water supplies.
Special problems develop with surface impoundments
in limestone terrain with extensive near-surface solution
openings. In Florida, Alabama, Missouri, and elsewhere,
municipal sewage lagoons have collapsed into sinkholes
draining raw effluent into widespread underground
openings. In some cases the sewage has reappeared
in springs and streams several miles away. Wells
producing from the caverns could easily become
contaminated and cause epidemics of waterbome
diseases.
Oil-field brines, which are highly mineralized salt
solutions, are particularly noxious and without doubt
they have contaminated both surface and ground water
in every state that produces oil. The brine, an unwanted
by-product, is produced with the oil, as well as during
drilling. In the latter case, drilling fluids and brines are
stored in reserve pits, which are filled some time after
completion or abandonment of the well. Customarily,
produced oil-field brines are temporarily stored in holding
tanks or placed in an injection well. Owing to the
corrosive nature of the brine, leaky tanks and pipelines
are not uncommon.
Landfills. Lehman (1986) reported that there are
approximately 18.500 municipal and 75,700 industrial
landfills that are subject to RCRA Subtitle D regulations.
Of the 94,000 known landfills recorded during a 1979
inventory, only about 5,600 facilities were licensed, and
the remainder were open dumps (Peterson, 1983).
Sanitary landfills generally are constructed by placing
wastes in excavations and covering the material daily
with soil—thus the term "sanitary" to indicate that garbage
and other materials are not left exposed to produce
odors or smoke or attract vermin and insects. Even
though a landfill is covered, leachate may be generated
by the infiltration of precipitation and surface runoff.
Fortunately many substances are removed from the
leachate as it filters through the unsaturated zone, but
leachate may contaminate ground water and even
streams if it discharges at the surface as springs and
seeps.
Waste Disposal in Excavations. Following the removal
of clay, limestone, sand, and gravel, or other material.
the remaining excavations are traditionally left
unattended and often used as unregulated dumps. The
quantity and variety of materials placed in excavations
are almost limitless. They have been used for the
disposal of liquid wastes, such as oil-field brines and
spent acids from steel mill operations, and for snow
removed from surrounding streets and roads—snow
that commonly contains a large amount of salt.
Leakage from Underground Storage Tanks. A growing
problem of substantial potential consequence is leakage
from underground storage tanks and from pipelines
leading to them. These facilities store billions of gallons
of liquids that are used for municipal, industrial, and
agricultural purposes. Corrosion is the most frequent
cause for leakage. It has been estimated that at least
35 percent of all underground storage tanks are now
leaking (EPA, 1986). Gasoline leakage has caused
severe hazardous difficulties throughout the nation.
Since gasoline will float on the water table, it tends to
leak into basements, sewers, wells, and springs, causing
noxious odors, explosions, and fires.
Leakage from Underground Pipelines. Literally
thousands of miles of buried pipelines cross the U.S.
Leaks, of course, do occur, but they may be exceedingly
difficult to detect. Leaks are most likely to develop in
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lines carrying corrosive fluids. An example occurred in
central Ohio where a buried pipeline carried oil-field
brine from a producing well to a disposal well. The
corrosive brine soon weakened the metal pipe, which
then began to leak over a length of several tens of yards.
The brine infiltrated, contaminating the adjacent ground
water, then flowed down the hydraulic gradient to a
stream. During the ensuring months, nearly all of the
vegetation between the leaking pipeline and the stream
was killed. The leaking area of the pipe was detected
only because of the dead vegetation and salty springs.
A pipeline that cut through a municipal unconf ined well
field in south-central Kansas ruptured, spilling a
substantial amount of hydrocarbons. Restoration has
been both expensive and time consuming.
Sewers are used to transport wastes to a treatment
plant. Rarely watertight, fluids leak out of sewers if they
are above the water table, and into them il they are in the
saturated zone. In many places the water table fluctuates
to such a degree that the sewer is gaining in discharge
pan* of the time and losing at other times. Figure 5-2
shows the chloride content in wells 10.5 (D-3) and 14
feet (D-4) deep that are a few feet from a sewer and,
upgradient, a 14 feet deep control well (A-4). While the
shallower well reached a peak of nearly 175 mg/L, the
concentration is much reduced in the deeper well. Even
the lowest concentrations near the sewerare 50 percent
or more higher than the average background
concentration, which is less than 25 mg/L.
in
190
100
T5
\MMD-3pOJn)
Figure 5-2. Leakage from a Sewer Increases the
Chloride Concentration of the Ground Water
Artificial Recharge. Artificial recharge includes an
assortment of techniques used to increase the amount
of water infiltrating an aquifer. Methods consists of
spreading the water over the land or placing it in pits or
ponds, or injecting water through wells directly into the
aquifer. Waters used for artificial recharge consist of
storm runoff, excess irrigation water, streamf low, cooling
water, and treated sewage effluent, among others. The
quality of water artificially recharged can effect the
quality of that in the ground. In several places this has
led to increased concentrations of nitrates, metals,
detergents, synthetic organic compounds, bacteria,
and viruses.
Sumps and Drv Wells. Sumps and dry wells are used for
drainage, to control storm runoff, for the collection of
spilled liquids, and disposal. They are usually of small
diameter and may be filled with pea gravel, coarse
sand, or large rocks.
Orr (1990) described several .storm water drainage
wells in Ohio that receive a variety of contaminants
through intentional dumping, illegal disposal, and
inadvertent collection of leaks and spills. At Fairfield
and Fairborn, dry wells serve as runoff collection wells
(an estimated 2,900 in Fairfield) and discharge into very
permeable deposits that serve as the major source of
domestic, municipal, and industrial water supply. In
addition to typical storm water, other contaminants
have included used oil and filters, antifreeze, and, in one
well, a considerable number of dead catfish. At Fairfield
an accidental release of 21,000 gallons of fuel oil from
a surface tank flowed into two storm drainage wells in
March 1989. Although approximately 16,000 gallons
were recovered, by September 1989 .product thickness
in monitoring wells was as much as eight feet.
Graveyards. Leachate from graveyards may cause
ground-water contamination, although cases are not
well documented. In some of the lightly populated
glaciated regions in the north-central part of the U.S.,
graveyards are commonly found on deposits of sand
and gravel, because these materials are easier to
excavate than the adjacent glacial till and are better
drained so that burials are not below the water table.
Unfortunately, these same sand and gravel deposits
also may serve as a source of water supply. Graveyards
also are possible sources of contamination in many
hard rock terrains where there are sinkholes or a thin
soil cover.
Ground-Water Quality Problems that Originate
Below the Water Table
Table 5-1 lists a number of causes of ground-water
contamination produced by the use and misuse of
space in the ground below the water table.
Waste Disposal In Wet Excavations. Following the
cessation of various mining activities, the excavations
usually are abandoned; eventually they may fill with
water. These wet excavations have been used as
dumps for both solid and liquid wastes. The wastes,
being directly connected to an aquifer, may cause
extensive contamination. Furthermore, highly
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concentrated leachates may be generated from the
wastes due to seasonal fluctuations of the water table.
In the late 1960s at a lead-zinc mine in northwestern
Illinois, processing wastes were discharged into an
abandoned mine working. The wastes, moving slowly in
the ground water, contaminated several farm wells.
Analyses of waterfrom several of the wells showed high
concentrations of dissolved solids, iron, sulfate, and,
more importantly, heavy metals and cyanide.
Agricultural Drainage Wells and Canals. Where surf icial
materials consist of heavy clay, flat-lying land may be
poorly drained and contain an abundance of marshes
and ponds. Drainage of this type of land generally is
accomplished with field tiles and drainage wells. A
drainage well is merely a vertical, cased hole in the
ground or in the bottom of a pond that allows the water
to drain into deeper, more permeable materials. The
drainage water may contain agricultural chemicals and
bacteria.
Deepening of stream channels may lower the water
table. Where the fresh-saltwater interface lies at shallow
depths, lowering of the water table (whether by
channelization, pumping, or other causes) may induce
upward migration of the saline water; it may even flow
into the deepened channel. Underthese circumstances,
reduction of the depth to fresh water can result in a rise
in the level of saline water several times greaterthan the
distance the freshwater level is lowered.
In some coastal areas, the construction of extensive
channel networks has permitted tidal waters to flow
considerable distances inland. The salty tidal waters
infiltrate, increasing the salt content of the ground water
in the vicinity of the canal. Some canals are used for the
disposal of urban runoff and sewage effluent
Well Disposal of Wastes. For decades, humans have
disposed of liquid wastes by pumping them into wells.
Since World War II, a considerable number of deep
well-injection projects (Class I wells) have come into
existence, usually at industrial sites. Industrial disposal
wells range in depth from a few tens of feet to several
thousand feet. The injection of highly toxic wastes into
some of these wells has led to ground-water
contamination. The problems are caused by direct
injection into an aquifer, by leakage of contaminants
from the well head, through the casing, or via fractures
in confining beds.
Exclusive of oil-field brine, most deep well-injection
operations are tied to the chemical industry. Well depths
range from 1,000 to 9,000 feet and average 4,000 feet.
The deepest wells are found in Texas and Mississippi.
As of October'1983, EPA reported the existence of at
least 188 active hazardous waste injection wells in the
United States. There were an additional 24,000 wells
used to inject oil-field brine (Class II wells).
Properly managed and designed underground injection
systems can be effectively used for storage of wastes
deep underground and may permit recovery of the
wastes in the future. Before deep well disposal of
wastes is permitted by state regulatory agencies and
the EPA, however, there must be an extensive evaluation
of the well system design and installation, the waste
fluids, and the rocks in the vicinity of the disposal well.
Underground Storage. The storage of material
underground is attractive from both economic and
technical viewpoints. Natural gas is one of the most
common substances stored in underground reservoirs.
However, the hydrology and geology of underground
storage areas must be well understood in order to'
insure that the materials do not leak from the reservoir
and degrade adjacent water supplies.
Secondary Recovery. With increased demands for
energy resources, secondary recovery, particularly of
petroleum products, is becoming even more important.
Methods of secondary recovery of petroleum products
commonly consist of injection of steam or water into the
producing zone, which either lowers the viscosity of the
hydrocarbon or flushes it from the rocks, enabling
increased production. Unless the injection well is carefully
monitored and constructed, fluids can migrate from a
leaky casing or through fractures in confining units.
Mines. Mining has instigated a variety of water
contamination problems. These have been caused by
pumping of mine waters to the surface, by leaching of
the spoil material, by waters naturally discharging
through the mine, and by milling wastes, among others.
Literally thousands of miles of stream and hundreds of
acres of aquifers have been contaminated by highly
corrosive mineralized waters originating in coal mines
and dumps in Appalachia. In many western states, mill
wastes and leachates from metal sulfide operations
have seriously affected both surface water and ground
water.
Many mines are deeper than the water table, and in
order to keep them dry, large quantities of water are
pumped to waste. If salty or mineralized water lies at
relatively shallow depths, the pumping of f reshwaterfor
dewatering purposes may cause an upward migration,
which may be intercepted by the well. The mineralized
water most commonly is discharged into a surface
stream.
Many abandoned underground mine workings serve as
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a source of water supply for homes, cities, and industry.
They also are used as waste receptacles. Orr (1990)
described an Ohio situation where combined sanitary
sewers and storm water are discharged through wells
tapping an abandoned mine. An additional 200 drainage
wells were drilled through septic tank leach fields to the
underground workings. In the same town, a nationally
recognized food processing plant uses the mine for
water supply and the city installed their standby well
field in it. Water samples from borings into the mine
were opaque with sewage, and strong raw sewage and
diesel fuel odors were present, along with a strong flow
of methane..
Exploratory Wells and Test Holes. Literally hundreds of
thousands of abandoned exploratory wells dot the
countryside. Many of these holes were drilled to
determine the presence of underground mineral
resources (seismic shot holes, coal, salt, oil, gas, etc.).
The open holes permit water to migrate freely from one
aquifer to another. A freshwater aquifer could thus be
joined with a contaminated aquifer or a deeper saline
aquifer, or contaminated surface water could drain into
freshwater zones.
Abandoned Wells. Another cause of ground-water
contamination is the migration of mineralized fluids
through abandoned wells, and dumping wastes directly
into them. In many cases when a well is abandoned the
casing is pulled (if there is one) or the casing may
become so corroded that holes develop. This permits
ready access for fluids under higher pressure to migrate
either upward ordownward through the abandoned well
and contaminate adjacent aquifers. In other cases,
improperly cased wells allow high-pressure artesian
saline water to spread from an uncased or partly cased
hole into shallower, lower-pressure aquifers or aquifer
zones.
Although confined aquifers, to some extent, are protected
by overlying confining units, abandoned wells make the
seal ineffective. In addition, some individuals, probably
through a lack of awareness, use abandoned wells to
dispose of used motor oil and other liquid wastes,
permitting direct access to a drinking water supply.
Water Supply Wells. Improperly constructed water-
supply wells may either contaminate an aquifer or
produce contaminated water. Dug wells, generally of
large diameter, shallow depth, and poorly protected,
commonly are contaminated by surface runoff flowing
into the well. Other contamination has been caused by
infiltration of water through contaminated fill around a
well orthrough the gravel pack. Still other contamination
has been caused by barnyard, feedlot, septic tank, or
cesspool effluent draining directly into the well. Many
contamination and health problems can arise because
of poor well construction.
Although well construction standards institute rigid
guidelines, they may not be strictly adhered to during
the installation of domestic and livestock wells.
Furthermore, a great number of water supply wells were
constructed long before well standards were established.
Ground Water Development. In certain situations
pumping of ground water can induce significant water-
quality problems. The principal causes include
interaquifer leakage, induced infiltration, and landward
migration of sea water in coastal areas. In these
situations, the lowering of the hydrostatic head in a
freshwater zone leads to migration of more highly
mineralized water toward the well site. Undeveloped
coastal aquifers are commonly full, the hydraulic gradient
slopes towards the sea, and freshwater discharges'
from them through springs and seeps into the ocean.
Extensive pumping lowers the freshwaterpotentiometric
surface permitting sea water to migrate toward the
pumping center. A similar predicament which occurs in
inland areas where saline water is induced to flow
upward, downward, or laterally into a fresh water aquifer
due to the decreased head in the vicinity of a pumping
well. Wells drilled adjacent to streams induce water to
flow from the streams to the wells. If the stream is
contaminated, induced infiltration will lead to
deterioration of the water quality in the aquifer.
Natural Controls on Ground-Water Contamination
As Deutsch (1965) clearly pointed out, there are tour
major natural controls involved in shallow ground-water
contamination. The first includes the physical and
chemical characteristics of the earth materials through
which the liquid wastes flow. A major attenuating effect
for many compounds is the unsaturated zone. Many
chemical and biological reactions in the unsaturated
zone lead to contaminant degradation, precipitation,
sorption, and oxidation. The greater the thickness of the
unsaturated zone, the more attenuation there is likely to
take place. Below the water table, the mineral content
of the medium probably becomes more important
because assorted clays, hydroxides, and organic matter
take up some of the contaminants by exchange or
sorption. Many of the other minerals have no effect on
the contaminants with which they come into contact.
The second major control includes the natural processes
that tend to remove or degrade a contaminant as it flows
through the subsurface from areas or points of recharge
to zones or points of discharge. These processes
include filtration, sorption, ion-exchange, dispersion,
101
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oxidation, and microbial degradation, as well as dilution.
The third control relates to the hydraulics of the flow
system through which the waste migrates, beginning
with infiltration and ending with discharge. The
contaminant may enter an aquifer directly, by flowing
through the unsaturated zone, by interaquifer leakage,
by migration in the zone of saturation, or by flow through
open holes.
The final control is the nature of the contaminant. This
includes its physical, chemical, and biological
characteristics and, particularfy, its stability under varying
conditions. The stability of the more common constituents
and the heavy metals are fairly well known. On the other
hand, the stability of organic compounds, particularly
synthetic organic compounds, has only recently come
under close inspection and actually little is known of
their degradation and mobility in the subsurface. This
fact has been brought clearly to the attention of the
general public by the abundance of reported incidences
of contamination by EDB, TCE, and DBCP.
To a large extent, it is the aquifer framework that
controls the movement of ground water and
contaminants. Of prime importance, of course, is the
hydraulic conductivity, both primary and secondary. In
the case of consolidated sedimentary rocks, primary
permeability, in many respects, is more predictable
than secondary permeability. In sedimentary rocks,
similar units of permeability tend to follow bedding
planes orformational boundaries, even if the strata are
inclined. Permeable zones most often are separated by
layers of fine-grained material, such as clay, shale, or
silt, which serve as confining beds. Although leakage
through confining beds is the rule rather than the
exception, both water and contaminants are more likely
to remain in a permeable zone than to migrate through
units of low permeability. The movement of ground
water and contaminants through larger openings, such
as fractures, complicates the assumed picture. Not
only can the velocity change dramatically, but in fracture
flow, much of the attenuation capacity is lost, and it is
difficult to predict local directions of flow.
The geologic framework, in conjunction with surface
topography, also exerts a major control on the
configuration of the water table and the thickness of the
unsaturated zone. Generally speaking, the water table
would be relatively flat in a deposit of permeable surf icial
sand and gravel. In contrast, the water table in glacial
till, which is typically fine-gained, would more closely
conform to the surface topography. The position of the
water table is important not only because it is the
boundary between the saturated and unsaturated zones,
but also because it marks the bottom and, therefore, the
thickness of the unsaturated material.
In many, if not most, contaminated areas, the water
table has been or is intermittently affected by pumping.
The resulting cone of depression on the water table
changes both the hydraulic gradient and ground-water
velocity. A change in gradient and velocity also occurs
in the vicinity of recharge basins (lagoons, pits, shafts,
etc.), because the infiltrating water forms a mound in the
water table. As Figure 5-3 shows, the mound causes
radial flow and, therefore, contaminants can move in
directions that are different than the regional hydraulic
gradient, at least until the mounding effects are overcome
by the regional flow.
Ground-water or interstitial velocity is controled by the
hydraulic conductivity, gradient, and effective porosity.
Water movement through a permeable gravel with a
gradient of 10 feet per mile averages about 60 feet per
day, but in a clay with the same gradient and no
secondary permeability the water movement would be
only about 1 foot in 30,000 years. In most aquifers,
ground-water velocity ranges from a few feet per day to
a few feet per year.
Carlston (1964) determined that the mean residence
time of ground water in a basin in Wisconsin was about
45 days and in New Jersey about 30 days. This study
shows that ground water may discharge into closely
spaced streams in humid areas within a few days to a
few months. On the other hand, in less permeable
terrains ground water and contaminants may remain in
the subsurface for years or even decades.
Leachate
The causes of ground-water contamination are many,
but it is the source that needs special consideration. For
example, an accidental spill from a ruptured tank may
provide a considerable volume of liquid with an extremely
high concentration that is present only during a short
time, but leachate continuously generated from a landfill
may consist of a large volume of low concentration that
spans many years. Once it reaches the water table, a
spill might move largely as a conservative contaminant
because of its high concentration, despite the fact that
it might be degradable in smaller concentrations.
Leachate is more likely to be attenuated by microbial
degradation, sorption, dilution, and dispersion.
In the case of landfills and similar sources, leachate is
a liquid that has formed as infiltrating water migrates
through the waste material extracting water-soluble
compounds and paniculate matter. The mass of leachate
generated is directly related to precipitation, assuming
the waste lies above the water table. Much of the annual
precipitation, including snowmelt, is removed by surface
runoff and evapotranspiration; it is only the remainder
102
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Plan View
Original water table '::.\\\Z?.
Figure 5r3. Infiltration from a Surface Impoundment Will Create a Mound on the Water Table
103
-------
that is available to form leachate. Since the landfill
cover, to a large extent, controls leachate generation, it
is exceedingly important that a cover be properly
designed, maintained, and monitored.
The physical, chemical, and biological characteristics
of leachate are influenced by: (1) the composition of the
waste, (2) the stage of decomposition, (3) microbial
activity, (4) the chemical and physical characteristics of
the soil cover and of the landfill, and (5) the time rate of
release (recharge). Since all of the above can range
within remarkably wide limits, it is possible to provide
only a general range in concentration of leachate
constituents, as Table 5-2 shows.
Constituents Operating Landfill Abandoned Landfill
COD, mg/L
Ammonia-N, mg/L
Hardness, mg/L as
CaCOa
Total iron,
Sulfate.
Specific Conductance.
nmhos
1,800
3.850
160
900
40.4
225
3,000
18
246
100
290
2.2
100
2,500
Table 5-2. Comparison of Chemical
Characteristics of Leachate from an Operating
Landfill and a 20-Year-Old Abandoned Landfill In
Southeastern Pennsylvania (From Wu and Ahlert,
1976)
It also is important to account for the fact that materials
placed in landfills may vary seasonally. For example,
many municipal landfills are used to dispose of snow
and ice, which may contain calcium, sodium, and chloride
from de-icing salts. This could lead to the generation of
leachate that changes throughout the year, particularly
in regard to the chloride concentration. In addition,
leachate collected from a seep at the base of a landfill
should be more highly mineralized than that present in
the underlying ground water, which is diluted.
Changes In Ground-Water Quality
It is often assumed that natural ground-water quality is
nearly constant at any particular site. Field data
substantiate this assumption, and logic leads to the
same conclusion, if the aquifer is confined and not
subjected to a stress. Multiple samples from a single
well, however, are likely to show slight changes in
concentrations of specific constituents owing to
differences in sample collection, storage, and analytical
technique.
Deeper or confined aquifers in which ground-water flow
is lethargic, generally have a nearly constant chemical
quality that, at any particular place, reflects the
geochemical reactions that occurred as the water
migrated through confining layers and aquifers from
recharge area to points of collection or discharge.
The quality of deeper water can change, but generally
not abruptly, in response to stresses on the aquifer
system. Changes in hydrostatic head brought about by
pumping, for example, may cause migration of other
types of waters from adjacent units into the producing
zone. As shown in Figure 5-4. the sulfate content of a
municipal well in north-central North Dakota increased
fivefold, from 200 to 1,000 mg/L, over a period of a few
years.
1200
§
§
o
£
1
•»
1000
800
600
400 -
200
1000 2000 3000
Tlme.daya
4000
Figure 5-4. The Increase In Sulfate Concentration
Was Related to Natural Causes Brought About by
Pumping
In this instance, the first sample was collected in 1974
when the well was first pumped for an acceptance test.
The well, one of six in a new field, tapped a previously
unused and confined ground-water system. By 1978
the entire well field was in operation, overlapping cones
of depression had spread out several miles along the
trend of the buried glacial valley, and the sulfate
concentration had increased to nearly 700 mg/L. From
1982 through 1985 sulfate fluctuated between about
850 and 1,000 mg/L, and the slow change, either an
increase or a decrease, was in response to the pumping
durations and rates of all of the wells in the field. The
source of the naturally occurring sulfate was several
hundred feet from the nearest production well and,
fortunately, only one other well was affected, and then
to a far smaller degree. Consequently, it was possible
to blend the water from all of the production wells, and
104
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the concentration of sulfate in the mixed water
consistently was less than 250 mg/L.
Changes in water quality in confined aquifers also may
be due to fluid migration along the well casing or gravel
pack, or by leakage through confining beds, abandoned
wells, or exploration holes, and by well injection of
waste fluids.
Incontrasttoconfined aquifers, ground-waterquality, in
shallow and surficial aquifers, can change considerably
within a few hours or days. These aquifers are not well
protected from changes brought about by natural events
occurring at the land surface or from human-induced
contamination. Surficial aquifers, in fact, are highly
susceptible to rapid and sometimes dramatic changes
in quality.
In the majority of cases, neither water levels nor water
samples are measured or collected at regular intervals.
Annual or quarterly measurements or samples may be
satisfactory for most purposes, but they are likely to be
far too infrequent in ground-water studies if an
investigator is attempting to develop an understanding
of the manner in which a system functions. Figure 5-5
shows the March through December 1988 fluctuation of
the water table in a well 14 feet deep. Quarterly
measurements provide a good indication of the annual
082
880.
•6
g 876 -
i
874 .
872
100
200
Tim*, diyi
300
400
Figure 5-5. Weekly Water-Table Measurements
More Accurately Show the Aquifer Response
than do Quarterly Measurements
change, but they do not display the complexity of the
hydrograph, as shown by weekly measurements. It is
the short-term rise in water level that is most likely to
indicate changes in ground-waterquality.
Figure 5-6 shows the annual range in electrical
conductivity in the same well described above. Again,
quarterly measurements provide a general impression
of the change throughout the year, which in this example
is from about 925 to 1,070 umhos. On the other hand,
an average of five measurements per month reveal that
the electrical conductivity changed considerably from
one time to the next, and that the annual range is from
800 to 1,l75u,mhos.
1200
11100
I 1000 -
I
I »oo -
800
100
300
400
200
Tim*, days
Figure 5-6. Weekly Measurements of Electrical
Conductivity are more useful for Determining
Changes In Chemical Quality than are Quarterly
Determinations
The Concept of Cyclic Fluctuations
Several years ago, Pettyjohn (1971.1976. 1982)
described cyclic fluctuations of ground-water quality.
The mechanisms that lead to cyclic fluctuations will be
discussed in greaterdetail here because both the cause
and effect can have a significant impact on: (1) ground-
water quality monitoring and determination of
background quality; (2) transport and fate of organic
and inorganic compounds, as well as bacteria and
viruses; and (3) monitoring well design and installation.
The contaminated site that Pettyjohn used to develop
the concept of cyclic fluctuation lies on the flood plain of
the Olentangy River in central Ohio where precipitation
averages about 38 inches per year (fig. 5-7). Underlain
by shale, the alluvial deposits consist of 15 to 35 feet of
sand, gravel, silt, and clay. The water table, 1.5 to 5 feet
below land surface, oscillates a foot or so annually.
Oil production began at this site in mid-.l 964, but by July
1965, all wells had been plugged' Ground-water
contamination occurred because of leakage of oil-field
brine, containing about 35,000 mg/L of chloride, from
105
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three holding ponds. When samples were first collected
from 23 monitoring wells in July 1965, the aquifer locally
contained more than 35,000 mg/L of chloride.
Of particular importance in the monitoring of this site is
a cluster of three wells, one screened at a depth of 7 to
9 feet and another from 21 to 23 feet, while a third,
gravel-packed through much of its length (23 feet),
receives water from the entire aquifer (fig. 5-8). It is
assumed that the third well provides a composite sample
of the reservoir and that when it had a higher
concentration than both the deep and shallow wells, the
most highly mineralized water was between 9 and 23
feet, and vice versa.
EXPLANATION
Figure 5-7. Water-Table Map of the Contalminated
Olentangy River Site
Figure 5-9 shows the chloride fluctuations in the three
ns occurred at the shallowest depths, at other times at
the greatest depth, and at still other times the greatest
concentration was somewhere in the middle of the
aquifer. Figures 5-10 and 5-11 show the vertical
distribution of chloride in the aquifer. The only means for
accounting for the variable distribution, both in space
and time, is intermittent recontamination, which is
puzzling in view of the fact that oil-field activities ceased
in June 1965 before any of the samples were collected.
The chloride fluctuations that occurred during 1965 to
Figure 5-8. Construction Details of Three Wells in
a Cluster
1966 and 1969 are shown schematically in Figure 5-12.
The October 1965 samples apparently were collected
shortly after a recharge event, which leached salt from
the unsaturated zone. This slowly sinking mass (1) was
subsequently replaced with less mineralized water. A
month later, the first mass had reached and was migrating
along the bottom of the aquifer when another recharge
event occurred (2). By December, the second mass had
reached the bottom of the aquifer and was moving
toward the river. Recharge events also occurred in
January 1966 (3), and in February 1966 (4). Figure 5-
12 shows that the aquifer was recontaminated several
times during 1969, particularly during January, February,
and March. On the average, it appears that the chloride
concentration in the ground water at the Olentangy
River site was reduced by half every 250 or so days.
This was not a linear decline, but rather intermittent
flushing of the source.
Findings similar to those in the Olentangy River study.
have been reported by Hagen (1986). Hoyle (1987),
Ross(1988), Pettyjohn (1987a, I987b. 1988). Pettyjohn
and others (1986). Nelson (1989), and Froneberger
Tout
1*00
1100
200
0
Figure 5-9. Variations In Chloride Concentration in
Cluster Wells at the Olentangy River Site
106
-------
CMorUi ramnmtlcn
8 I II i 51 | 81 8
* ~ K ?? W ? R £
I
I"
I"
OK.
uw. **••
1«BS
Figure 5-10. Vertical Distribution of Chloride
During 1965 and 1966
CMorM* eonunlritlan ki mfl/l
* 1. 5 I 51 55 l§ 88 88
^ *V O ^ MO «» MO w MO ^ M O w MO f M
Fit).
April M*, »*g ta» CO.
Figure 5-11. Vertical Distribution of Chloride
During 1969
(1989). The investigations were conducted in an urban
area in north-central Oklahoma at a small but intensely
monitored field site. The site lies on the flood plain of a
small stream, and the alluvium consists of a fine-
grained silt loam that contains soil structures throughout
the entire thickness of 43 feet.
At the Oklahoma site, fertilizer application, followed by
rain, has a short lived but significant effect on the
concentration of nitrogen in ground water, regardless of
the soil-moisture content. As Figure 5-13 shows, nitrate
concentrations increased in one well (14 feet deep)
from about 4 to to 16 mg/L within a two-day period
following 1.3 inches of rain. The concentration then
decreased to about 2 mg/L during the next three days.
|At this time (September 1985) the soil-moisture content
'was very low. The change in nitrate concentration over
Nov '65
Dec'65
Jan '66
Feb '66
Mar '66
Jan '69 5
Feb '69
Mar '69
Apr '69
May '69
Aug '69
Figure 5-12. Conceptual Model Showing Leaching
and Recontamlnatlon of Ground Water at the
Olentangy River Site
the five-day interval appears to suggest the infiltration of
a relatively small volume of highly concentrated water
that is followed about two days later with a large volume
of water with a very low nitrate content. Flow through
the unsaturated zone was greater than 5 feet per day.
suggesting early flow through macropores that is followed
by piston-type flow.
A similar phenomenon occurred in April 1986 when the
soil-moisture content was twice as great as it had been
in September, 1985. Shown in Figure 5-14 is the nitrate
concentration in three wells at a cluster; the wells are
8.5 (A-1), 9.5 (A-2). and !4(A-4)feetdeep. The change
in concentration in all of the wells follows the same
pattern, but the concentration decrease with depth.
Following a rain, the concentration at a depthof 8.5 feet,
for example, increased only about 3 mg/L and this was
followed during the next two days by a decrease of
15 mg/L and then nitrate again slowly increased.
During the fall and spring events, nitrate accounted for
only a small percentage of the dissolved solids content,
and the concentration of the other major constituents in
the water followed a different pattern. As Figure 5-15
shows, there was a small decrease in electrical
107
-------
20
-. io -
i
0-
-10-
•20
1300
10 . 20
Time, In days (September, 1965]
30
Figure 5-13. During Dry Weather the Water-Table
Aquifer Responded Quickly to Rain and Flushed
Nitrate into the Ground
conductivity at the peak nitrate concentration, and as
the nitrate decreased, electrical conductivity began to
increase, reaching a maximum about 11 days later.
At the same site, another well cluster, adjacent to a
building, receives runoff from the roof that infiltrates in
the vicinity of the wells. The runoff has a low dissolved
mineral content and, when it infiltrates during a prolonged
wet period, the electrical conductivity of the ground
waterdecreasesf rom around 1,000 to about 400 umhos
(fig-5-16).
30
2O-
I
s
10
Nlral«lnWelA-1 (8.5(1)
10
Time, todays
20
Figure 5-14. Although Decreasing with Depth,
Nitrate Concentrations In all of the Wells
Followed a Similar Pattern after a Rain
1200-
1100 -
1000
Sp. Cond
Nitrate
30
20 £
10
10
Time, ki days
20
Figure 5-15. The Increase In Nitrate Was Caused
by a Small Volume of Rapidly Infiltrating Water,
While the Later Increase In Electrical
Conductivity Was Caused by a Large Mass of
Slowing Moving Water
The Ohio and Oklahoma studies indicate that water
soluble substances on the land surface or in the
unsaturated zone may be intermittently introduced into
a shallow aquifer, changing its quality, for many years.
The rate of introduction or leaching is dependent upon
the chemical and physical properties of the waste and
the soil, and the frequency of the recharge events.
Throughout most of the year in humid and semiarid
regions, the quantity of water that infiltrates and the
amount of contaminants that are flushed into an aquifer
are relatively small. During summer months, ground-
water quality changes would be expected to occur more
rapidly, perhaps in a matter of hours, because of the
large size and abundance of the macropores and
fractures. These changes, however, may occur only
over a relatively small area because of the local nature
of convective storms.
On the other hand, during the spring recharge period
and, in many places, during the fall as well, noteworthy
quantities of contaminants may infiltrate over wide
areas. Although the quantity of leached substances is
largerthan at any othertime during the year, the change
may occur more slowly and the resulting concentration
in ground water may not be at a maximum because of
the diluting effect brought about by the major influx of
water. Therefore, the major infusion of contaminants,
which is strongly influenced by climate, occurs twice a
year, although minor recharge events may occur at any
time.
This phenomenon has important implications in
108
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1MO
1400
1000
600
200
WdlE-4
(1411)
War April Miy Jim*
Oa Nov o*c
Figure 5-16. Runoff from a Roof Tends to Reduce
the Electrical Conductivity of the Underlying
Ground Water
monitoring and sampling. Since the natural quality of
shallow ground water ranges fairly widely, background
concentration is not a finite number but, rather, a range
that may encompass an order of magnitude for major
constituents, such as dissolved solids, and two or three
orders of magnitude for minor forms, such as nitrate. In
addition, the concentration might increase several fold
a day or two after a rain, or decrease even more three
to five or so days later. The question then arises as to
the most appropriate time to sample. Available data
suggest that the least biased sample could be obtained
at least two weeks after a recharge event, but the
interval is strongly influenced by the physical and
chemical characteristics of the unsaturated zone and
the depth to the water table.
In order to account for cyclic fluctuations in ground-
water quality it is assumed that: (1) the unsaturated
zone may store a considerable volume of water-soluble
substances for long periods of time, and (2) the main
paths along which contaminants rapidly move through
the unsaturated zone to the water table consist largely
of fractures and macropores.
Most macropores may be barely detectable without a
close examination. Ritchie and others (1972) suggested
that the interfaces between adjacent soil peds also
serve as macropores. Moreover, these openings need
not extend to the land surface in order for flow to occur
in them (Quisenberry and Phillips, 1976). Nonetheless,
water can flow below the root zone in a matter of
minutes. Thomas and Phillips (1973) suggested that
this type of flow does not appear to last more than a few
minutes or perhaps, in unusual cases, more than a few
hours after "cessation of irrigation or rain additions."
Even though there may be a considerable influx of
contaminants through macropores and fractures to the
water table following a rain, the concentration of solutes
in the main soil matrix may change little, if at all. This is
clearly indicated in studies by Shuford and others
(1977) and again shows the major role of large openings.
On the other hand, in the spring, when the soil-moisture
content is high, some of the relatively immobile or
stagnant soil water may percolate to the water table
transporting salts with it. A similar widespread event
may occur during the fall as a result of decreasing
temperature and evapotranspiration, and of wet periods
that might raise the soil-moisture content.
Ecologicconditions infractures and macropores should
be quite different from those in the main soil matrix,
largely because of the greater abundance of oxygen
and smaller moisture content. As a result, one might
expect different microbial populations and densities, as
well as chemical conditions in macropores and fractures
than in the bulk soil matrix. Coupled with theirfargreater
fracture permeability, this may help to explain why
some biodegradable organic compounds or those that
should be strongly sorbed actually reach the watertable
and move with the ground water.
Prediction of Contaminant Migration
In any ground-water contamination investigation it is
essential to obtain the background concentration of the
chemical constituents of concern, particularly those
that might be common both to the local ground water
and a contaminant. As mentioned previously, the water
in shallow or surficial aquifers can undergo substantial
fluctuations in chemical quality. Therefore, it is not
always a simple task to determine background
concentrations, particularly of the more conservative
constituents, such as chloride or nitrate.
The severity of ground-water contamination is partly
dependent on the characteristics of the waste or leachate,
that is, its volume, composition, concentration of the
various constituents, time rate of release of the
contaminant, the size of the area from which the
contaminants are derived, and the density of the
leachate, among others. Data describing these
parameters are difficult to obtain and commonly are
lumped together into the term "mass flow rate," which
is the product of the contaminant concentration and its
volume and recharge rate, or leakage rate.
Once a leachate is formed it begins to migrate downward
through the unsaturated zone where several physical,
chemical, and biological forces act upon it. Eventually,
however, the leachate may reach saturated strata where
it will then flow primarily in a horizontal direction as
defined by the hydraulic gradient. From this point on, the
109
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leachate will become diluted due to a number of
phenomena, including filtration, sorption, chemical
processes, microbial degradation, dispersion, time, and
distance of travel.
Filtration removes suspended particles from the water
mass, including particles of iron and manganese or
other precipitates that may have been formed by
chemical reaction. Dilution by sorption of chemical
compounds is caused largely by clays, metal oxides
and hydroxides, and organic matter, all of which function
as sorptive material. The amount of sorption depends
on the type of contaminant and the physical and chemical
properties of the solution and the subsurface material.
Chemical processes are important when precipitation
occurs as a result of excess quantities of ions in solution.
Chemical processes also include volatization as well as
radioactive decay. In many situations, particularly in the
case of organic compounds, microbiological degradation
effects are not well known, but it does appear, however,
that a great deal of degradation can occur if the system
is not overloaded and appropriate nutrients are available
(see Chapter 7).
Dispersion of a leachate in an aquifer causes the
concentration of the contaminants to decrease with
increasing length of flow. It is caused by a combination
of molecular diffusion, which is important only at very
low velocities, and dispersion or hydrodynamic mixing,
which occurs at higher velocities in laminar flow through
porous media. In porous media, different macroscopic
•velocities and flow paths that have various lengths are
to be expected. Leachate moving along a shorter flow
path or at a higher velocity would arrive at an end point
sooner than that part following a longer path or a lower
velocity; this results in hydrodynamic dispersion.
Dispersion can be both longitudinal and transverse and
the net result is a conic form downstream from a
continuous contamination source. As Figure 5-17 shows,
the concentration of the leachate is less at the margins
of the cone and increases toward the source. Because
dispersion is directly related to ground-water velocity,
the size of a plume of contamination tends to increase
with more rapid flow.
Since dispersion is affected by velocity and the
configuration of the aquifer's pore spaces, coefficients
must be determined experimentally or empirically for a
given aquifer. There is considerable confusion regarding
the quantification of the dispersion coefficient. Selection
of dispersion coefficients that adequately reflect
conditions that exist in an aquifer is a problem that can
o
«
500
i
1000
I
1500
Seal* In FM!
Figure 5-17. The Size and Concentration Distribution In a Contaminant Plume is Related to Ground-
Water Velocity. Upper Plume Velocity Is 1.5 feet/day; In tower Plume Velocity Is 0.5 feet/day
110
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not be readily solved and herein lies one of the major
stumbling blocks of chemical transport models.
Often confused with the term dispersion (Dx =
longitudinal dispersion and Oy = transverse dispersion)
is dispersivity. Dispersion includes velocity: to transform
from one to another requires either division or
multiplication by velocity.
The rate of advance of a contaminant plume can be
retarded if there is a reaction between its components
and ground-water constituents or if sorption occurs.
This is called retardation (R(j). The plume in which
sorption and chemical reactions occur generally will
expand more slowly and the concentration will be lower
than the plume of an equivalent nonreactive leachate.
Hydrodynamic dispersion affects all solutes equally
while sorption, chemical reactions, and microbial
degradation affects specific constituents at different
rates. As Figure 5-18 shows, a leachate source that
contains a number of different solutes can have several
solutes moving at different rates due to the attenuation
processes.
The area! extent of plumes may range within rather wide
extremes depending on the local geologic conditions,
influences on the hydraulic gradient, such as pumping,
ground-water velocity, and changes in the time rate of
release of contaminants.
The many complex factors that control the movement of
leachate and the overall behavior of contaminant plumes
are difficult to assess because the final effect represents
several factors integrated collectively. Likewise,
concentrations for each constituent in a complex waste
are difficult to obtain. Therefore, predictions of
concentration and plume geometry, at best, can only be
used as estimates, principally to identify whether or not
a plume might develop at a site and, if so, to what extent.
Models can be used to study plume migration, and as
an aid in determining potential locations for monitoring
wells, and to test various renovation or restoration
schemes.
References
Aurelius, L. A., 1989, Testing for pesticide residues in
Texas well water: Texas Department of Agriculture,
Austin.
Carlston.C.W., 1964, Trrtium-hydrologic research, some
results of the U.S. Geological Survey Research Program:
Science, v. 143, no. 3608.
900-
450-
0 —
450-
900-
1
0
1. Chlorob«nzen« RH - 35
2. Unknown - 15
3. Chloroform - 3
4. Chloride - 1
Plumea tiler 2800 days
900
1800
—I
2700
3600
4500
Figure 5-18. Constituents Move at Different Rates Because of Retardation
111
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Oeutsch, M., 1963, Ground-water contamination and
legal controls in Michigan: U.S. Geological Survey
Water-Supply Paper 1691.
Deutsch, M., 1965, Natural controls involved in shallow
aquifer contamination: Ground Water, v. 3, no. 3.
Froneberger, D.F., 1989, Influence of prevailing
hydrogeologic conditions on variations in shallow
ground-water quality: unpubl. M.S. thesis, School of
Geology, Oklahoma State University.
Hagen, D.J., 1986, Spatial and temporal variability of
ground-water quality in a shallow aquifer in north-
central Oklahoma: unpubl. M.S. thesis, School of
Geology, Oklahoma State Universit.
Houzim, V.. J. Vavra. J. Fuksa, V. Pekney, J. Vrba, and
J. Stribral, 1986, Impact of fertilizers and pesticides on
ground water quality: Impact of Agricultural Activities on
Ground Water, v. 5.
Lehman, J.P., 1986, An outline of EPA's subtitle D
program: Waste Age, v. 17, no. 2.
National Academy of Sciences, 1983, Transportation of
hazardous materials-toward a national strategy:
Transportation Research Board Special Report No.
197.
Nelson, M.J., 1989, Cause and effect of water-table
fluctuations in a shallow aquifer system, Payne County,
Oklahoma: unpubl. M.S. thesis, School of Geology,
Oklahoma State University.
Off ice of Technology Assessment, 1984, Protecting the
Nation's groundwater from contamination, v.ll, U.S.
Congress, OTA-0-276.
Orr, V.J., 1990, Wellhead protection-lessons learned:
Proc. Underground Injection Practices Council 1990
Summer Meeting.
Peterson, N.M., 1983, 1983 survey of landfills: Waste
Age, March, 1983.
Pettyjohn, W.A.,1971, Waterpollution by oil-field brines
and related industrial wastes in Ohio. Ohio Jour. Sci., v.
71, no. 5.
Pettyjohn, W.A., 1976, Monitoring cyclic fluctuations in
ground-water quality. Ground Water, v. 14, no. 6.
Pettyjohn, W.A., 1979, Ground-water pollution—an
imminent disaster: Ground Water, v. 17, no. 1.
Pettyjohn. W.A., 1982, Cause and effect of cyclic
fluctuations in ground-water quality: Ground Water
Monitoring Review, v. 2, no. 1.
Pettyjohn, W.A., David Hagen, Randall Ross, and A. W.
Hounslow, 1986, Expecting the unexpected: Proc. 6th
Nat. Symp. on Aquifer Restoration, and Ground
Water Monitoring, Nat. Water Well Assn.
Pettyjohn,W.A.,l987a, Where's the return key?: Proc.
Conf. Solving Ground-Water Problems with Models,
Nat. Water Well Assoc., v. 2.
Pettyjohn,W.A.,1987b, Hydrogeology of fine-grained
sediments: Proc. 3rd Nat. Water Conf., Philadelphia
Academy of Natural Sciences.
Pettyjohn, W.A., 1988, Hydrogeology of fine-grained
materials: NATO/CCMS 2nd Internat. Conf.,
Demonstration of Remedial Action Technologies for
Contaminated Land and Groundwater, Bilthoven,
Netherlands.
Quisenberry, V.I. and R.E. Phillips, 1976, Percolation of
surface applied water in the field: Soil Sci. Soc. A. Jour.,
v. 40.
Ritchie, J.T., D.E. Kissel, and E. Burnett, 1972, Water
movement in undisturbed swelling clay soil: Soil Sci.
Soc. Am. Proc., v. 36.
Ross. R.R., 1988, Temporal and vertical variability of
the soil- and ground-water geochemistry of the Ashport
silt loam, Payne County, Oklahoma: unpubl. M.S. thesis,
School of Geology, Oklahoma State University.
Shuford, J.W., D.D. Fritton, and D.E. Baker, 1977,
Nitrate-nitrogen and chloride movement through
undisturbed field soil: Jour. Environ. Qua!., v. 6.
Thomas, G.W., R.L. Blevins, R.E. Phillips, and M.A.
McMahon, 1973. Effect of killed sod mulch on nitrate
movement and corn yield: Jour. Agron., v. 65.
U.S. Environmental Protection Agency, 1978, Surface
impoundments and their effects on ground water quality
in the United States-a preliminary survey: Office of
Drinking Water, EPA 570/9-78-004.
U.S. Environmental Protection Agency, 1983. Surface
impoundment assessment national report: Office of
Drinking Water, EPA 570/9-84-002.
U.S. Environmental Protection Agency, 1986, Summary
of state reports on releases from underground storage
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tanks: Office of Underground Storage Tanks, EPA 600/
M-86/020.
Williams. J.S., 1984, Road salt—silent threat to ground
water: Maine Environmental News, v. 11, no. 3.
Wu, J.S. and R.C. Ahlert. 1976, State of the art review—
non-point source pollution: Water Resources
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Chapter 6
GROUND-WATER INVESTIGATIONS
Introduction
Within the last decade, a substantial numberof ground-
water investigations have been conducted. Many of
these have been centered at specific contaminated
sites in response to federal legislation concerned with
sources of drinking water, threats to human health and
the environment posed by toxic and hazardous waste,
and the restoration of contaminated aquifers. In general,
most of the sites consist of only several acres or a few
square miles, but a numberof reconnaissance studies
have focused on thousands of square miles.
In most cases the cost of these investigations has been
excessively high, due in large measure to the expense
of analytical services. The most disconcerting feature
of many of these investigations is that their results were
found to be inadequate, and additional work and
expense were required. It must be understood that a
data base will almost always be inadequate to some
and its resolution will eventually be dictated by time,
common sense, and budgetary constraints. Although
these constraints will always be present to one degree
or another, it is imperative that the most reliable and
applicable information be collected commensurate with
the available resources.
The reason many field investigations are both
inadequate and costly is that a comprehensive work
plan was not developed before the project was initiated,
or that it was not followed. Any type of an investigation
must be carefully planned, keeping in mind the overall
purpose, time limitations, and available resources.
Moreover, the plan must use a practical approach
based on sound, fundamental principles. As far as
ground-water quality investigations are concerned,
the basic questions are (1) is there a problem, (2)
where is it, and (3) how severe is it? A subsequent
question may relate to what can be done to reduce the
severity, that is, aquifer restoration.
Ground-water quality investigations can be divided
into three general types: regional, local, and site
evaluation. The first, which may encompass several
hundred or even thousands of square miles, is
reconnaissance in nature, and is used to obtain an
overall evaluation of the ground-water situation. A
local investigation is conducted in the vicinity of a
contaminated site, may cover a few tens or hundreds
of square miles, and is used to determine local ground-
water conditions. The purpose of the site evaluation is
to ascertain, with a considerable degree of certainty,
the extent of contamination, its source or sources,
hydraulic properties, and velocity, as well as all of the
other related controls on contaminant migration.
Ground-water investigations can be quite varied in
terms of purpose as well as scale and duration. Although
a few of these variations will be discussed briefly, the
main topic of this chapter will be site specific ground-
water investigations involving contamination with toxic
and hazardous wastes.
Purposes of Ground-Water Investigations
Ground-waterinvestigations are conducted fora variety
of purposes. One is for reconnaissance or the
establishment of background quality, such as those
done by the U. S. Geological Survey for many years,
which resulted in a historical documentation of the
quality and quantity of both surface and subsurface
waters. Usually these investigations are made using
existing private, municipal, industrial, and irrigation
wells. The data are useful for determining fluctuations,
trends, and cycles in water levels and chemical quality.
Another purpose may be to monitor a variety of ground-
water parameters in order to establish cause and effect
relationships, as for example, an assessment of the
design, construction and operation of a hazardous
waste disposal facility on area! ground-water quality.
Monitoring may be done to assure the integrity of lagoon
liners or, in general, prove compliance with any of the
regulatory standards dealing with waste disposal,
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storage, or treatment facilities. Ground-water quality
monitoring also is increasing with respect to possible
contaminant sources, such as underground storage
tanks, the application of agricultural chemicals, and
mining, just to mention a few.
Ground-water investigations traditionally have played
an important role in litigation. Under the civil laws of
trespasser negligence, the informal ion obtained during
a study may be used to determine the source of ground-
water contamination in order to establish liability, or be
used in response to federal legislation, such as RCRA
and Superiund, In these cases particular attention must
be given to the handling of samples as well as the
documentation of fie Id and laboratory procedures. During
the beginning of the study, legal counsel should be
obtained to assure that proper procedures are built into
the work plan.
Perhaps the most specialized types of ground-water
investigations are those driven by research objectives.
The goals of these studies are as varied as the nature
of research itself, and may range from model validation
to determining the rates and daughter products of
contaminant degradation. Specialized field equipment
and technologies often are required to obtain
representative samples of subsurface materials for use
in column and microcosm studies. Usually more
observation points are required for research studies
than for other types of ground-water investigations, as
are the demands for more stringent quality control.
Types of Ground-Water Investigations
Regional Investigations
Ground-water investigations can be carried out on a
regional, local, or site-specific scale. The first, which
may encompass hundreds or even thousands of square
miles, is reconnaissance in nature, and is used to obtain
an overall evaluation of a ground-water situation.
This broad-brush reconnaissance study can be the
starting point for two general types of investigations.
First, it can be carried out with the purpose of locating
potential sources of contamination, or it may provide an
understanding of the occurrence and availability of
ground water on a regional scale. The underlying
objectives are first, to determine if a problem exists, and
second, if necessary, to ascertain prevalent hydrologic
propertiesof earth materials, generalizedflowdirections
of both major and minor aquifers, the primary sources
and rates of recharge and discharge, the chemical
quality of the aquifers and surface water, and the
locations and yields of wells. These data can be useful
an more detailed studies because they provide
information on the geology and flow direction, both of
which affect studies of smaller scale.
Local Investigations
A local investigation, which is conducted in the vicinity
of a contaminated site, may covera f ewtens or hundreds
of square miles, and is used to determine local ground-
water conditions. The purpose is to define, in greater
detail, the geology, hydrology, and water quality in the
area surrounding a specific site or sites of concern. This
information is important in designing and carrying out
more detailed site investigations.
Site Investigations
The goals of an investigation at a contaminated site are
to ascertain, with considerable certainty, the nature and
extent of contamination, its source or sources, and the
relative movement of different contaminants and their
degradation products. The end result is to provide
information leading to an effective and cost-efficient
remediation plan.
The site investigation is usually the most detailed,
complex, costly, and, from the legal and restoration
viewpoint, the most critical of the three types of ground-
water studies. A site investigation must address a
myriad of pertinent parameters affecting contaminant
transport and transformation, including geology and
hydrogeology, geochemical interactions, biotic and
abiotic degradation processes, and the rate of movement
of contaminants through the unsaturated and saturated
zones. It also is important, when appropriate, to locate
and determine the effect of phenomena influencing the
movement of contaminant plumes such as nearby
pumping wells, multiaquifer interactions, and local
streams.
At the same time ground-water studies are being carried
out there are usually auxiliary investigations. These
may include tank inventories, toxicologies! evaluations.
air pollution monitoring, manifest scrutiny, and
manufacturing procedures, as well as other information
gathering, all of which eventually combine in the
development of a comprehensive report.
Organization and Development of the
Investigation
Regardless of the complexity ordetail of the investigation,
a logical series of steps should be followed. Although
each investigation is unique, these general rules are:
1. Establish objectives.
2. Prepare work plan
3. Data collection.
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4. Data interpretation.
5. Develop conclusions.
6. Present results.
Establish Objectives
Establishing the major goal or goals of an investigation
is paramount to a successful and cost-effective project.
The exact goals should be clearly defined and agreed
upon by all interested parties. They should be clearly
expressed in writing and referred to often during the life
of the study. Otherwise as the work progresses, there
may be a tendency for the study to drift from the stated
objectives, resulting in the collection of cost ly superfluous
information, perhaps at the expense of required
information.
The approach, time requirements, and funding can be
vastly different between a regional reconnaissance
evaluation and a site-investigation. The former, which
deals with gross features, may only require a few days,
while the latter, which necessitates minute detail, may
demand years. In either case the time and resource
requirements are dictated by the goals, and the success
of the work is measured by howdirectly the investigation
pursues those goals.
In one case the objective statement may be to "measure
the water levels in a given township using existing
wells." Another might be "evaluate the degradation rate
of tetrachloroethylene at a specific spill site, define the
plumes of the parent and degradation contaminants,
and predict the location and concentrations of these
contaminants after 10 years." In both of these examples
.the objective is clearly stated and the complexity is
evident. In the first case, the caveat "using existing
wells" states that the study, for whatever purpose, is
very limited. Clearly the second set of goals is vastly
more complicated and will undoubtedly require many
observation points; a detailed knowledge of the site's
soils, geochemistry, geology, and hydrogeology,
sophisticated analytical capabilities; predictive models
and the information necessary to drive them.
Once the general objective of the ground-water study
is established, a number of secondary purposes must
be considered. These involve the physical system
and the chemical aspects. Secondary objectives
include the following:
1. Determination of the thickness, soil
characteristics, infiltration rate, and water-bearing
properties of the unsaturated zone.
2. Determination of the geologic and
hydrologic properties and dimensions of each geologic
unit that potentially could be affected by ground-water
contamination. This includes rock type, thickness of
aquifers and confining units, their area! distribution,
structural configuration, transmissivity, hydraulic
conductivity, storativity, water levels, infiltration or
leakage rate, and rate of evapotranspiration, if
appropriate.
3. Determination of recharge and discharge
areas, if appropriate.
4. Determination of the direction and rate of
ground-water movement in potentially affected units.
5. Determination of the ground water and
surface water relationships.
6. Determination of the background water-
quality characteristics of potentially affected units.
7. Determination of potential sources of
contamination and types of contaminants. -
Prepare Work Plan
The preparation of the work plan or method of approach
should be made in direct response to the stated goals,
using existing data and information to the fullest extent
possible. The investigative plan needs to be flexible in
a practical way. For example, the position of all test
wells, borings, and monitoring points cannot be
determined in the office at the start of an investigation.
Rather, these locations should be adjusted on the basis
of information obtained as each hole is completed. In
this way, one can maximize the data acquired from each
drill site and more appropriately locate futures holes in
order to develop a better understanding of the ground
water and contamination situation at the site under
study.
Similarly, the exact contaminants of target, appropriate
analytical methods, detailed sampling techniques, and
the required number of samples cannot be accurately
estimated at the beginning of a project. These must be
refined as data are collected and the statistics of those
data interpreted.
The early development of a flexible plan of investigation
occasionally may be required to include, at least in part,
guidelines established by the Environmental Protection
Agency, such as the Ground Water Technical
Enforcement Guidance Document. State regulatory
agencies may have even more stringent requirements.
Also, in the case of Superfund and RCRA sites, the
investigator probably will be required to work with or at
least use data collected by consultants for the defendant.
In almost all cases, as the work progresses, it is
necessary to adjust the work plan to one degree or
another. In the event changes must be made, it is
important that they do not cause the work to drift from
the original objectives.
Even fairly simple ground-water investigations can result
in large amounts of data, adjustment of the project
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approach, statistical evaluations, interpretations and
conclusions, the preparation of graphics for
presentations, and the final report. The work plan should
contain provisions for dealing with data either by
developing an automatic data processing program or
selecting one from the software market. Also, if the
project requires the use of mathematical models, data•'
storage and retrieval systems should be developed in
concert with these needs.
One section of the work plan should be dedicated to the
health and safety of those actively participating in the
investigation, as well as the general public. Health
monitoring tests, performed before, during, and after
the field work is completed, are necessarily predicated
on estimates of the toxicity and concentration of
contaminants at the site. Protective clothing and other
safety considerations also must be based on these
estimates until collected information becomes available.
Access to the site should be limited to project personnel,
particularly when drilling or other heavy equipment is in
use.
Another section of the work plan should deal with chain-
of-custody requirements when working at Superfund,
RCRA, or other sites where litigation is involved. As
discussed in EPA's Technical Enforcement Guidance
Document, this section should include instructions
concerning:
1. Sample labels to prevent misidentif ication
of samples.
2. Sample seals to preserve the integrity of
samples from the time they are collected until opened in
the laboratory.
3. Field logbook to record information about
each sample collected during the ground-water
monitoring program.
4. Chain-of-custody record to document
sample possession from time of collection to analysis.
5. Sample analysis request sheets, which
serve as official communicationsto the laboratory of the
particular analyses required for each sample and provide
further evidence that the chain of custody is complete.
6. Laboratory logbook and analysis
notebooks, which are maintained at the laboratory and
record all pertinent information about the sample.
Data Collection
Existing Information. Data collection forms the basis for
the entire investigation, consequently, time must be
allocated and care exercised in addressing this part of
the project. As mentioned above, all existing information
should be collected, analyzed, and used to prepare a
work plan before field activities are begun. The amount
and types of data to be collected are dictated by the
objectives of the study. Materials that should be
collected, when available, include soil, geologic,
topographic, county and state maps, geologic cross-
sections, aerial photographs, satellite imagery, the
location of all types of wells with discharge rates, well
togs, climatological and stream discharge records,
chemical data, and the location of potential sources of
ground-water contamination.
Many of these data are readily available in the files and
reports of local, state, and federal agencies. Personnel
with these agencies also can be of great help because
of their knowledge withthe area and available literature.
Examples include the U.S. Geological Survey, which
has at least one office in each state, the state geological
survey, and several state agencies that deal with water,
such as the state water survey, water resources board,
or a water commission. Other sources of information
include the state or federal depanments of agriculture,
soil conservation, and the weather service, among
others.
It often is useful to talk with long-term local residents,
realizing that their information may be biased because
of prejudices involving the cause of the investigation.
Their historical knowledge often can assist in defining
possible sources of contamination. For example, "there
used to be a service station on that corner about 30
years ago," or "that company buried trash out in that
field until after World War II." Often their memory is of
events that are not available in the literature.
Climatological data are important because they indicate
precipitation events and patterns, which influence
surface runoff and ground-water recharge. Additionally,
these data include temperature measurements that can
be used for an evaluation of evapotranspiration, which,
for shallow ground water can produce a significant
effect on the water-table gradient, causing it to change
in slope and direction, both seasonally and diurnally.
Soil types are related to the original rock from which
they were derived. Consequently, soils maps can be
used as an aid in geologic mapping, and they are
valuable for estimating infiltration. Soil information also
is necessary to evaluate the potential for movement of
organic and inorganic compounds through the
unsaturated zone.
Exceedingly useful tools, both in office and field study,
are aerial photographs and satellite imagery. The latter
should be examined first in an attempt to detect trends
of lineaments, which may indicate the presence of faults
and major joints or joint systerns. These may reflect
zones of high permeability that exert a strong influence
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on fluid movement from the land surface or through the
subsurface. Satellite imagery also can be used to detect
the presence of shallow ground water owing to the
subtle tonal changes and differences in vegetation
brought about by a higher moisture content. Rock types
also may be evident on imagery.
Aerial photographs, particularly stereoscopic pairs,
should be an essential ingredient of any hydrotogic
investigation. They are necessary to further refine the
trends of lineaments, map rock units, determine the
location of cultural features and land use, locate springs
and seeps, as well as potential drilling sites, and detect
possible sources of contamination. Topographic and
state and county road maps also are useful for many of
these purposes.
Geologic reports, maps, and cross sections provide
details of the surface and subsurface, including the
area! extent, thickness, composition, and structure of
rock units. These sources of information should be
supplemented, if possible, by an examination of the
logs of wells and test holes. Depending on the detail of
the logs, they may provide a clear insight into the
complexities of the subsurface.
Logs of wells and test holes are essential in ground-
water investigations. They provide first-hand information
on subsurface strata, their thickness, and area! extent.
They also may allow inferences as to relative
permeability, well-construction details, and water-level
depths.
Inorganic chemical data may be available from reports,
but the most recent information is probably stored in
local, state, and federal files. Concentrations of selected
constituents, such as dissolved solids, specific
conductance, chloride, and sulfate, may be plotted on
base maps and used to estimate background quality
and, perhaps, indicate areas of contamination.
Sources of information that report concentrations of
organic compounds usually are scarce and should be
questioned, particularly if they are old. It only has been
within the last decade or so that organic compounds
have become of concern in ground water. The cost of
analysis is high, and much remains to be learned about
appropriate sampling methods, storage, and
interpretation. Consequently, when using existing data,
investigators normally will need to rely on inorganic
substances to detect contaminated ground-water sites.
In some cases both organic and inorganic substances
are present in a leachate. On the other hand, reliance on
concentrations of inorganic constituents to evaluate
contamination by organic compounds may not be
appropriate, possible, or desirable.
Field Investigations. Several generalized methods have
been available for a number of years to evaluate a
possible or existing site relative to the potential for
ground-water contamination. These rating techniques
are valuable, in a qualitative sense, for the formulation
of a detailed investigation. One of the most noted is the
LeGrand (1983) system, which takes into account the
hydraulic conductivity, sorption, thickness of the water-
table aquifer, position and gradient of the water table,
topography, and distance between a source of
contamination and a well or receiving stream. The
LeGrand system was modified by the U.S. Environmental
Agency (1983) for the Surface Impoundment
Assessment study.
Fenn and others (1975) formulated a water balance
method to predict leachate generation at solid waste
disposal sites. Gibb and others (1983) devised a
technique to set priorities for existing sites relative to
their threat to health. An environmental contamination
ranking system was developed by the Michigan
Department of Natural Resources (1983). On a larger
scale DRASTIC, prepared by the National Water Well
Association for EPA (1987), is a method to evaluate the
potential for ground-water contamination based on the
hydrogeologic setting. A methodology for the
development of a ground-water management and aquif er
protection plan was described by Pettyjohn (1989).
The field phase of a ground-water investigation is the
most intensive and important part of the project. The
data collected during this phase will determine its
success. Some of the main factors affecting the quality
of the field data include an understanding of the
hydrogeology at the site, a knowledge of the types of
contaminants involved and their behavior in the
subsurface, the location and construction of monitoring
wells, and how they are sampled and analyzed.
In order to detect and outline areas of contamination in
the subsurface, an understanding of the movement of
ground water is necessary. In soils the important
parameters to quantify by field investigations include
soil-moisture characteristic curves, soil texture,
unsaturated hydraulic conductivity curves and
preferential flow paths, such as fractures and
macropores, and the spatial and temporal variability of
these factors.
In the saturated zone it is important to determine the
hydraulic properties of the aquifer including gradient,
direction of flow and velocity, storativity, transmissivity,
and hydraulic conductivity.
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With respect to water levels and flow patterns, the
factors that affect seasonal and temporal variations
should be identified. Such factors include onsile and
offsite pumping and recharge, tidal and stream stage
fluctuations, construction, changes in land use, and
waste disposal practices.
In addition to determining the gross hydraulic
conductivity, its distribution also should be determined.
Variations in hydraulic conductivity, both within and
between strata, affect ground-water flow paths, including
magnitude and direction, and must be identified in order
to isolate major zones of contaminant migration.
If the aquifer is composed of a fractured media, the
nature of the fractures is required for use in flow models.
Although this information can be very difficult to obtain,
it can be helpful to collect information describing fracture
density, location, orientation, roughness, and the degree
to which they are connected.
A number of techniques are available to measure the
hydraulic properties of aquifers. A few of the techniques
include the following:
1. Aquifer tests are performed by pumping
from one well and observing the resulting drawdown in
nearby wells. These analyses can be used to determine
an aquifers coefficients of storativity and transmissivity.
2. Slug tests are conducted by suddenly
removing or adding a known volume of water from a well
and observing the return of the water level to its original
location. Slug tests are used to determine hydraulic
conductivity.
3. Flow-net analyses, both horizontal and
vertical, also permit an evaluation of hydraulic
parameters and flow directions, and aid in the
understanding of the role played by strata of different
permeability.
4. Tracer tests can be used to determine if
two locations are hydraulically connected, measure
flow velocities, and determine the variability of hydraulic
conductivity within an aquifer system.
5. Borehole dilution tests can be used to
determine the hydraulic conductivity in a single well by
introducing a tracer and measuring the dilution with time
caused by the inflow of wate r into the well that is brought
about by the natural hydraulic gradient in the vicinity of
the well.
6. Rock cores taken during the drilling of
wells and test holes can be analyzed in the laboratory
to determine a number of physical and chemical
properties, including porosity, hydraulic conductivity,
and mineralogy. Care must be used in an evaluation of
hydraulic parameters determined by laboratory analyses
of unconsolidated materials.
7. Surface and borehole geophysics, aerial
photography, and imagery are particularly helpful in
working with fractured media.
After the site has been described in terms of ground-
water movement, the work plan can be adjusted to
sample for contaminants in the unsaturated and
saturated zones. Nested lysimeters can be used to
detect contaminants in the unsaturated zone; however,
great care must be taken to assure that the collected
samples are representative and not affected by sorption
and volatilization. The placement of nested piezometers
in closely spaced, separate boreholes of different depths
generally is preferred to determine vertical head
differences and the vertical movement of contaminants.
while monitoring wells with appropriately located screens
are used to determine the lateral movement of
contaminants in the saturated zone.
As discussed above, an understanding of the variability
or distribution of hydraulic conductivity, in both the
vertical and horizontal dimension, allows one to isolate
the major zones of watertransmission and, therefore, to
select the proper lengths and depths for well screens.
This follows for offsite, upgradient, and downgradient
observation points.
The length and position of well screens also must be
predicated on the nature of the contaminant. For
example, it the contaminants are miscible with the liquid
phase, it may be possible to use only one well per
sampling point. It also may be possible to use only one
well if the transmissive zone is very thin. If the
contaminants are immiscible with the liquid phase
(sinkers or floaters), the well screens must be located
appropriately.
In carrying out a ground-water investigation it is not
uncommon for at least part of the chemical species of
concern to be dictated by state or federal regulations,
such as the RCR A list of priority pollutants. Beyond this
one must be aware of contaminant transformation
phenomena in the design and implementation of a
ground-water sampling program. For example, when
selecting proper contaminant targets it is imperative to
realize that the original species may have been reduced
in concentration, altered, or eliminated by chemical,
physical, or biological processes taking place in the
subsurface environment. Aerobic and anaerobic
biological degradation, and hydrolysis and redox
reactions are among these processes. The sampling
protocol also should be influenced by alterations in the
transport of contaminants caused by immiscible
compounds, sorption-desorption phenomena, and the
facilitated transport of hydrophobic compounds.
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Data Interpretation
Data interpretation should begin with the development
of the work plan. Today's widespread availability and
use of computers allow the application of data processing
to the results of almost all investigations and software
exists fora wide variety of data handling requirements.
To the extent possible, the amounts and types of data
should be anticipated early in the project and provisions
made for the continuous input of collected information
as the work progresses. Also, the quality assurance and
quality control program should be built into the data
handling system so that the quality of the data can be
continuously monitored.
If predictive models are required at some point in the
investigation, or later in the development of an aquifer
remediation project, they should be formulated or
selected from existing models as early as possible so
that requirements for acquisition of the appropriate data
can be built into the work plan. Steps also should be
taken for model calibration and validation as the
investigation proceeds.
Even a.t a moderately sized site, a ground-water
investigation of limited scope can result in the collection
of a great deal of information. The amount of time saved
and the amount of frustration avoided during data
interpretation is directly proportional to the skill with
which one anticipates (1) the types and amounts of data
collected; (2) the calculations required to determine
contaminant transformation process rates, support
conclusions, and make projections; (3) the correlations
required to prove cause and effect, define relationships,
and determine reaction coefficients; and (4) prepare
the graphic displays needed for reports and
presentations.
Develop Conclusions
In a very real sense the development of conclusions.
like preparing for the interpretation of data, should be
done in the early stages of the project by establishing
hypotheses. These hypotheses must be proposed in
direct response to the objectives of the investigation,
then, as in hypothesis testing in statistics, the project
designed around their acceptance or rejection. If done
correctly, this approach can play a significant role in
assuring that the project design is an efficient response
to the project goals, and that the collection of extraneous
information is kept to a minimum.
To carry this point further, assume that gasoline fumes
are detected in the basement of a small house. A
service station is located immediately to the east at a
slightly higher elevation. There is another service station
about 200 feet south of the first, across a street. The
goal of an investigation would be to determine the
source of gasoline so that negligence could be proven.
If one assumed that the shallow water table followed the
surface topography, the first hypothesis would be that
the gasoline originated from the closest, upgradient
buried tanks. After drilling only three shallow wells,
water-level data might prove that the ground water was
moving due west from the closest station and the first
hypothesis could be accepted.
On the other hand, the water-level data could show that
the gradient did not follow the lay of the land but was
about 30 degrees west of north. In this case the first
hypothesis must be rejected with the conclusion now
being that the second service station is at fault. At this
point the investigation might be ended or additional
proof provided by drilling wells to delineate the plume
and show that no other sources existed.
A more complicated example might involve the need to
define the plume of contamination at a Superfund site
so that a remediation plan could be developed. The goal
would be to locate the plume horizontally, as well as
vertically, and provide concentration isograms. If the
parent contaminant were trichloroethylene, the
hypothesis must be made that biodegradation is taking
place and that the well placements, sampling, and
analytical procedures must be designed to also locate
dichloroethylene and vinyl chloride.
Many of today's ground-water contamination problems
are extremely complex, particularly those associated
with hazardous waste sites. It is very important, therefore,
that conclusions be based on the collective wisdom and
experience of interdisciplinary teams to the fullest extent
possible.
Present Results
All investigations usually result in a report and commonly
other types of presentations as well. Their style and
content are determined by the type of study and can
vary in as many ways as the investigations themselves.
However, some general traits can be suggested.
Those studies designed to "Establish Background" and
those for "Monitoring" cause-and-effect relationships
should consist predominantly of field data appropriately
grouped and tabulated for easy access. Reports
prepared for use in litigation are usually brief with only
the essentials of the study highlighted along with the
essence of the findings—most often in proof or disproof
of a legal argument. "Site Characterization" reports
generally are more complete and detailed than other
reports because they generally serve as the basis of
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other activities, such as the design and implementation
of a remedial action plan or a complex and costly
compliance monitoring system.
Examples of Ground-Water Investigations
Regional Examples
Regional investigations are conducted for many
different purposes. One type is to detect potential
sources and locations of ground-water contamination.
Another type of exceedingly broad scope includes
library searches. Examples entail an early EPA effort
to evaluate ground-water contamination throughout
the United States (van der Leeden and others, 1975,
Miller and Hackenberry. 1977, Scalf and others, 1973,
Millerand others, 1974, and Fuhrimanand Barton. 1971).
The reports are useful for obtaining a general
appreciation of the major sources and magnitude of
contamination over a regionally extensive area.
In 1980, individuals in EPA Region VII became aware
of what appeared to be a large number of wells that
contained excessive concentrations of nitrate.
Suspecting a widespread problem, a regional
reconnaissance investigation was initiated. The general
approach consisted of a literature search, a meeting
in each state with regulatory and health personnel, an
evaluation of existing data, and an interpretation of all
of the input values.
The fundamental principle guiding this study was the
fact that abnormal concentrations of nitrate can arise in
a variety of ways, both from natural and human-made
sources or activities. The degradation may encompass
a large area if it results from the over-application of
fertilizer and irrigation water on a coarse textured soil,
from land treatment of waste waters, or from a change
in land use, such as converting grasslands to irrigated
plots. On the other hand, it may be a local problem
affecting only a single well if the contamination is the
result of animal feedlots, municipal and industrial waste
treatment facilities, or improper well construction or
maintenance.
Most of the data base for this study was obtained from
STORET. First, nitrate concentrations in well waters
were placed in a separate computer file. Two maps
were generated from the file, the first showing the
density of wells that had been sampled for nitrate, and
the second showing the density of wells that exceeded
10 mg/L of nitrate (fig. 6-1). These maps indicated the
areas of the most significant nitrate problems. Inturn,
the nitrate distribution maps were comparedto geologic
taps, which allowed some general identification of the
ihysical system that was or appeared to be impacted
fig. 6-2).
• >10 mg/l MO,
Figure 6-1. Location of Wells with Nitrate
Exceeding lOmg/L In Region VII
Figure 6-2. Generalized Rock Types with High
Nitrate Concentrations In Region VII
Iowa, eastern Nebraska, northeastern Kansas, and
the northern third of Missouri are characterized by
glacial till interbedded with local deposits of outwash.
Throughout the area are extensive deposits of alluvium.
Many of the aquifers are shallow and wells are
commonly dug, bored, or jetted. This area contained
the greatest number of domestic wells with high nitrate
concentrations. It also contained the greatest
number of municipal wells that exceeded the nitrate
Maximum Contaminant Level (MCL). The cause of
contamination in the shallow domestic wells was
suspected to be poor well construction and
maintenance, but this was possibly not the case for
many of the generally deeper municipal wells, where
the origin appeared to be from naturally occurring
sources in the glacial till.
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Most of Nebraska and western Kansas are mantled by
sand, gravel, and silt, which allow rapid infiltration. The
watertable is relatively shallow. The irrigated part of this
region, particularly adjacent to the Platte River and in
areas of Holt County, NB, contained the greatest regional
nitrate concentrations in the four state area. This was
brought about by the excessive application of fertilizers
and irrigation waters in this very permeable area.
The remaining area in Kansas and an adjacent part of
Missouri is underlain by sedimentary rocks across
which flow many streams and rivers with extensive
flood plains. Most of the contaminated wells tapped
alluvial deposits. The primary cause of high nitrate in
domestic wells was suspected to be poor well
construction and maintenance, orpoorsiting with respect
to feedlots, barnyards, and septic tanks.
The southern part of Missouri is represented by
carbonate rocks containing solution openings. Aquifers
in these rocks are especially susceptible to contamination
and the contaminants can be transmitted great distances
with practically no change in chemistry other than
dilution. The carbonate terrain is not easily managable
nor is monitoring a simple technique because of the vast
number of possible entry sites whereby contaminants
can enter the subsurface.
The STORET file also was used to generate a number
of graphs of nitrate concentration versus time for all of
the wells that were represented by multiple samples.
The graphs clearly showed that the nitrate concentration
in the majority of wells ranged within wide limits from
one sampling period to the next, suggesting leaching of
nitrate during rainy periods from the unsaturated zone.
The state seminars were exceedingly useful because
the personnel representing a number of both state and
federal agencies had a good working knowledge of the
geology, water quality, and land-use activities of their
respective states. .
Although the study extended over several months, the
actual time expended amounted to only a few days. The
conclusions, for the most part, were straightforward
and, in some cases, pointed out avenues for
improvement in sample collection and data storage/
access. The major conclusions are as follows:
1. High levels of nitrate in ground water
appeared to be randomly distributed throughout the
region.
2. The most common cause of high nitrate
concentration in wells was the result of inadequate well
construction, maintenance, and siting. Adequate well
construction codes could solve this problem. Dug wells,
those improperly sealed, and wells that lie within an
obvious source of contamination, such as a pig lot,
should probably be abandoned and plugged.
3. In areas of extensive irrigation where excess
water was applied to coarse textured soils, the nitrate
concentration in ground water appeared to be increasing.
4. In the western part of the region, changes
in land use, particularly the cultivation or irrigation of
grasslands, had resulted in leaching of substantial
amounts of naturally occurring nitrate from the
unsaturated zone.
5. The population that was consuming high-
nitrate water supplies was small, accounting for less
than 2 per cent of the population.
6. There had been no more than two reported
cases of methemoglobinemia in the entire Region within
the preceeding 15 years despite the apparent increase
in nitrate concentration in ground-water supplies. This
implied a limited health hazard.
7. State agency personnel were convinced
that they did not have significant nitrate-related health
problems
8. Many of the wells used in state and federal
monitoring networks are of questionable value because
little or nothing is known about their construction.
9. The volume of chemical data presently in
the files of most of the state agencies within the region
is not adequately represented in the STORET data
system.
This cursory examination provided only a general
impression of the occurrence, source, and cause of
abnormal nitrate concentrations in ground water in the
Region. Nonetheless, it furnished a base for planning
local or site investigations, was prepared quickly, and
did not require field work or extensive data collection.
As mentioned previously, the source of excessive nitrate
in many municipal wells could not be readily explained.
There could be multiple sources related to naturally
occurring high nitrate concentrations in the unsaturated
zone or the glacial till, to contamination, or to poor well
constructioin. Definitive answers would require more
detailed local or site studies.
The overall effect of changing from grazing land to
irrigated agriculture, in view of the great mass of nitrate
in the unsaturated zone, warrants additional local
investigation. Although the concentration of nitrate in
underlying ground water would increase following
irrigation, it is likely that some control on the rate of
leaching could be implemented by limiting the amount
of water applied to the fields.
The obvious relationship between the application of
excessive amounts of fertilizer and water on a coarse
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textured, as was the case in Nebraska, shows the need
for experimental work on irrigation techniques in order
to reduce the loading. Also implied is the necessity for
the development of educational materials and seminars
to offer means whereby irrigators can reduce water,
pesticide, and fertilizer applications, and yet maintain a
high yield.
Local Example
Local investigations can be as varied in scope and area!
extent as regional evaluations and the difference
between the two is relative. For example, one might
desire to obtain some knowledge on the hydrogeology
of an area encompassing a few tens or several hundred
square miles in order to evaluate the effect of oil-field
brine production and disposal. Examples of this scope
include Kaufmann (1978) and Oklahoma Water
Resources Board (1975). The'other extreme may
center around a single contaminated well. In this case
the local investigation would most likely focus on the
area influenced by the cone of depression, the size of
which depends of the geology, hydraulic properties,
and well discharge.
Consider an area in the Great Plains where a number of
small municipalities have reported that some of their
wells tend to increase in chloride content over a period
of months to years. The increase in a few wells has been
sufficient to cause abandonment of one or more wells in
the field. Additionally, a number of wells when drilled
yielded brackish or salty water necessitating additional
drilling elsewhere. This is an expensive process that
strains the operating budget of a small community.
In this case, a Iocs! investigation covered an area of
about 576 mi^. A review of files and reports and
discussions with municipal officials and state and federal
regulatory agencies indicated that the entire area had
produced oil and gas for more than 30 years. Inadequate
brine disposal appeared to be the most likely cause of
the chloride problem.
During the initial stage of the investigation, all files
dealing with the quality of municipal well water were
examined. This task was followed by a review of the
geology, which included a assessment of all existing
maps, cross sections, and well logs, both lithologic and
geophysical.
The chemical data clearly showed that the chloride
content in some wells increased with time, although not
linearly. The geologic phase of the study showed thai
the rocks consist largely of interbedded layers of shale
and sandstone and that the sandstone deposits, which
serve as the major aquifers, are lenticular and range
from 12 to about 100 feet in thickness. The sandstones
are fine-grained and cemented to some degree and, as
a result, each unit will not yield a large supply. Resultingly,
all sandstone strata are screened.
Trending north-south through the east-central part of
the area is an anticline (fig. 6-3) that causes the rocks
to dip about 50 feet per miles either to the east or west
of the strike of the structure (fig. 6-4). This means that
a particular sandstone will lie at greater depths with
increasing distances from the axis of the anticline.
In this example, the subsurface geology was examined
by an evaluation of geophysical and geologists logs of
wells and test holes, including oil and gas wells and
tests. As shown in Figure 6-4, interpretation of the logs,
in the form of a geologic cross section, brings to light an
abundance of interesting facts. The municipal wells
range in depth from 400 to 900 feet, but greater depth
does not necessarily indicate a larger yield nor does
depth imply a particularchemical quality. The difference
in well depth and yield is related to the thickness and
permeability of the sandstone units encountered within
the well bore. Secondly, the volume of the sandstone
components ranges widely, but the thinnest and most
discontinuous units increase in abundance westward.
More importantly, the mineral content of the ground
Scale (miles)
•••^ Sandstone outcrop area
M Aquifer thickness exceeds 125 ft
Figure 6-3. Generalized Geologic Map of a Local
Investigation
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1100 -
I » 5
-100
-300 -
Figure 6-4. Geologic Cross-Section Showing Downdlp Change In Water Quality
water, which can be determined from geophysical logs,
increases down the dip of the sandstone, from fresh in
the outcrop area, to brackish, and finally to salt water
(fig. 6-4). Notice also that brackish and saline water lie
at increasingly shallower depths to the west of the
outcrop area.
The position and depth of a few municipal wells and test
holes are also shown on the cross section. Well 1 would
be expected to have a small yield of brackish water.
Well 2 is an abandoned test hole that penetrated a thick
saline zone and a thick brackish water zone. In the case
of Well 3, the freshwater derived from the thin, shallower
sandstones is sufficient to dilute water derived from the
more mineralized zones. On the other hand, as the
artesian pressure in the shallow sandstones decreases
with pumping and time, an increasing amount of the well
yield might be derived from the deeper brackish layer.
causing the quality to deteriorate.
The major conclusion derived from this study is that the
most readily apparent source of high chloride content in
municipal wells, that is, inadequate oil-field brine
disposal, is not the culprit. Ratherall of the problems are
related to natural conditions in the subsurface, brought
about by the downdip increase in the dissolved solids
content as freshwater grades into brackish and
eventually into saline water. Deterioration of municipal
well water quality is related to the different zones
penetrated by the well and to a decrease in artesian
pressure in freshwater zones brought about by pumping.
The latter allows updip migration of brackish or saline
water. Furthermore, the migration of mineralized water
could occur through the well bore or by lateral or vertical
leakage from one aquifer to another, which again is the
result of a pressure decline in the freshwater zones. The
problem could be diminished by constructing future
wells eastward toward the axis of the anticline, limiting
them to those areas either within the outcrop or where
the thickness of the freshwater aquifers comprise a total
thickness that exceeds 125 feet (fig. 6-3).
Site Example
Site investigations are ordinarily complex, detailed, and
expensive. Furthermore, the results and interpretations
are likely to be thoroughly questioned in meetings,
interrogatories, and in court, because the expenditure
of large sumsof money may be at stake. The investigator
must exercise extreme care in data collection and
interpretation. The early development of a flexible plan
of investigation is essential and it must be based, at
least in part, on guidelines established by the EPA,
such as the Ground-Water Monitoring Technical
Enforcement Guidance Document. State regulatory
agencies may have even more stringent requirements.
In the case of Superfund and RCR A sites, the regulatory
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investigator probably will be required to work with or at
least use data collected by consultants for the defendant.
In some cases, the defendant conducts and pays forthe
entire investigation; regulatory personnel only modify
the work plan so that it meets established guidelines.
There are two points to consider in these situations.
First, the consultant is hired by the defendant and
should act in his best interest. This means that his
interpretations may be biased toward his client and
concepts detrimental to the client are not likely to be
freely given. Second, even though the regulatory
investigator and the consultant, to some degree, are
adversaries, this does not mean that the consultant is
dishonest, ignorant, or that his ideas are incorrect. It
must always be remembered that the entire purpose of
the investigation is to determine, insofar as possible,
what has or is occurring so that effective and efficient
corrective action can be undertaken. In the long run
cooperation leads to success.
As an example of a ground-water quality site
investigation, consider a rather small refinery that has
been in existence for several decades. For some
regulatory reason an examination of the site is required.
The facility, which has not been in operation for several
years, includes an area of about 245 acres. The geology
consists of alternating layers ot sandstone and shale
that dip slightly to the west; the upper 20 to 30 feet of the
rocks are weathered.
Potential sources of ground-water contamination include
wastewater treatment ponds, a land treatment unit, a
surface runoff collection pond, and a considerable
number of crude and product storage tanks. Line sources
of potential contaminants include unimproved roads,
railroad lines, and a small ephemeral stream that carries
surface runoff from the plant property to a holding pond.
After considering the topography and potential sources
of contamination, the location of 11 test borings was
established. The purpose of the holes was to determine
the subsurface geologic conditions underlying the site.
Following completion, the holes were geophysically
logged and then plugged to the surface with a bentonite
and cement slurry. The borehole data were used to
determine drilling sites lor 20 observation wells, in order
to ascertain the quality of the ground water, to establish
the depth to water, and to determine the hydraulic
gradient. Eight of the observation wells were constructed
so that they could be used later as a part of the
monitoring system. Two of the wells tapped the
weathered shaie, their purpose being to monitor the
watertable, evaluate the relation between precipitation
and recharge, and ascertain the potential fluctuation of
water quality in the weathered material in order to
determine if it might serve as a pathway for contaminant
migration from the surface to the shallowest aquifer.
(From a technical perspective. the weathered shale and
sandstone is not an aquifer, but from a regulatory point
of view it could be considered a medium into which a
release could occur and, therefore, would fall under
RCRA guidelines.)
Regulations required that the uppermost aquifer be
monitored, which in this case was a relatively thin,
saturated sandstone. After the initial investigative
information was available, all of the findings were used
to design a ground-water monitoring system. This plan
called for an additional 12 monitoring wells.
Graphics based on all of the drilling information (geologic
and geophysical logs) included several geologic cross
sections (fig. 6-5) and maps showing the thickness of
shale overlying the aquifer (fig. 6-6), thickness of the
aquifer, and the hydraulic gradient (fig. 6-7). The major
purpose of the first map was to show the degree of
natural protection that the shale provided to the aquifer
relative to infiltration from the surface. The aquifer
thickness map was needed for the design of monitoring
wells. The water-level gradient map was necessary to
estimate ground-water velocity and flow direction. During
the drilling phases, cores of the aquifer and the overlying
shale were obtained for laboratory analyses of hydraulic
conductivity, porosity, specific yield, grain size,
mineralogy, and general description. Aquifer tests were
conducted on two of the wells.
The cross sections and maps indicate that the sandstone
dips gently eastward and nearly crops out in a narrow
band alongthewestemmarginof the facility. Elsewhere,
owing to the change in topography and the dip of the
aquifer, the sandstone is overlain by 25 feet or more of
shale; throughout nearly all of the site the shale exceeds
50 feet in thickness. Consequently, only one small part
of the aquifer, its outcrop and recharge area, is readily
subject to contamination.
A'
.S*nd*ton«'.
S«nd*tone
ra SK«I«
Potantiometric
Surlecs
Uppotntost
Aquifer
Figure 6-5. Geologic Cross-Section for the Site
Investigation
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Figure 6-6. Map Showing Thickness of Shale
Overlying the Uppermost Aquifer
The water-level map indicates that the hydraulic gradient
is not downdip but rather about 55 degrees from it. It is
controlled by the topography off site. The average
gradient is about 0.004 ft/ft, but from one place to
another it differs to some extent, reflecting changes in
aquifer thickness and hydraulic conductivity.
The topographic map indicates that surface runoff from
the entire facility is funneled down to a detention pond.
The pond and the lower part of the drainage way lie in
the vicinity of the aquifer's recharge or outcrop area.
Logs of the drill holes list specific depths in six of the
holes inwhich highly viscous hydrocarbons were present.
All were reported in the unsaturated zone at depths of
2 to 9 feet with thicknesses ranging from a half inch to
nearly a foot. At these locations the shale overlying the
aquifer exceeded 55 feet in thickness.
Chemical analyses of water from the observation wells
indicated, with one exception, that the quality was within
Figure 6*7. Potentlometric Surface of the
Uppermost Aquifer
background concentrations and no organic compounds
were present. The exception was an observation well
near the surface runoff retention pond.
Evaluation of all of the data indicated two potential
problems—hydrocarbons in the unsaturated zone and
ground-watercontaminationinthe vicinity of the surf ace
runoff detention pond. Since the plant had been in
operation more than 50 years, the hydrocarbons had
migrated from the surface into the weathered shale no
more than 9 feet, and there was a minimum of at least
45 feet of tight, unfractured shale between the
hydrocarbons and the shallowest aquifer, it did not
appear that the soil contamination would present a
hazard to ground water.
The existence of contaminated ground water, however,
was a problem that needed to be addressed even
though the sandstone aquifer is untapped and is never
likely to serve as a source of supply. Four additional
monitoring wells were installed downgradient in order to
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determine the size of the plume and its concentration.
Corrective action called for removal of sediment and
sludge from the pond, backfilling with clean material, a
cap, and pumping to capture the plume. The
contaminated .water was treated on site with existing
facilities.
References
Fenn, D.G., K.J. Hanley, andT.V. DeGeare, 1975, Use
of the water balance method for predicting leachate
generation from solid waste disposal sites: U.S.
Environmental Protection Agency Solid Waste Rept.
No. 168, Cincinnati, OH.
Fuhriman, O.K. and J.R. Barton, 1971, Ground water
pollution in Arizona, California, Nevada and Utah; U.S.
Environmental Protection Agency 16060 EPA 12/71.
Gibb, J.P., M.J. Barcelona. S.C. Schock, and M.W.
Hampton, 1983, Hazardous waste in Ogle and
Winnebago Counties, potential risk via ground water
due to past and present activities: Illinois Dept. Energy
and Natural Resources, Doc. No. 83/26.
Kaufmann. R.F., 1978. Land and water use effects on
ground-water quality in Las Vegas valley: U.S.
Environmental Protection Agency, EPA-600/2-78-179.
LeGrand, H.E., 1983, A standardized system for
evaluating wastedisposal sites: Nat. Water Well Assn..
Worthington, OH.
Michigan Department of Natural Resources. 1983, Site
assessment system (SAS) for the Michigan priority
ranking system under the Michigan Environmental
Response Act: Michigan Dept. Nat. Resources.
Miller, D.W., F.A. DeLuca, and T.L. Tessier, 1974.
Ground water contamination in the northeast states:
U.S. Environmental Protection Agency, EPA-660/2-74-
056.
Miller, J.C. and P.S. Hackenberry, 1977, Ground-water
pollution problems in the southeastern United States:
U.S. Environmental Protection Agency, EPA-600/3-77-
012.
Oklahoma Water Resources Board, 1975, Salt water
detection in the Cimarron terrace, Oklahoma: U.S.
Environmental Protection Agency, EPA-660/3-74-033.
Pettyjohn, W.A., 1989, Development of a ground-water
management aquifer protection plan: Underground
Injection Practices Council and Texas Water Comm.
Scatf, M.R., J.W. Keeley and C.J. LaFevers, 1973,
Ground water pollution in the south central states: U.S.
Environmental Protection Agency, EPA-R2-73-268.
U.S. Environmental Protection Agency, 1983, Surface
impoundment assessment national report: U.S.
Environmental Protection Agency 570/9-84-002.
U.S. Environmental Protection Agency, 1986, RCRA
Ground Water Monitoring Technical Enforcement
Guidance Document: Nat. Water Well Assn.
Van der Leeden, Frits, L.A. Cerrilto, and D.W. Miller,
1975. Ground-water pollution problems in the
northwestern United States: U.S. Environmental
Protection Agency, EPA-660/3-75-018.
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Chapter 7
GROUND-WATER RESTORATION
Introduction
Prevention of ground-water contamination is far more
logical, simple, and cost-effective than attempting to
correct a problem—a problem that may have been in
existence for years. A great deal of time, effort, and
money are presently being expended to develop
remedial measures to counteract the effects of
contaminated aquifers and public water supplies. These
include traditional as well as innovative construction
techniques, water management, and research initiatives.
Several options or combinations of options are available
to restore a contaminated aquifer: (1) provide inground
treatment/containment, (2) provide aboveground
treatment, (3) remove or isolate the source of
contamination, (4) abandon the source of supply, or (5)
ignore the problem. Generally, several techniques are
coupled in order to achieve the desired results
Restoration of contaminated aquifers to former
background or near background conditions or to contain
contaminated ground water in certain locations is
generally accomplished through one of two overall
approaches. One approach involves natural or induced
in situ treatment, while the other approach uses
engineered systems to contain the contaminated ground
water. In the latter case pumping wells or engineered
structures are installed in order to develop hydraulic
gradients that cause the contaminated water to remain
in a specified, general location from which it may be
removed for later treatment.
Regardless of the restoration approach, any source or
sources that continue to contaminate the ground water
should be removed, isolated, or treated. Treatment or
removal of an existing contamination source eventually
may result in restoration of ground-water quality through
natural processes. In other situations, contaminated
ground water is removed from the aquifer by pumping
or is allowed to discharge to a stream in which the flow
is sufficient to dilute the contaminant to nondetectable
concentrations. Natural replacement of the ground
water is relied upon to eventually restore the quality of
the water in the aquifer. Typically the natural restoration
processes require many years orperhaps even decade.s
for completion. As a result, ground-water restoration
commonly requires a combination of approaches that
involve ground-water removal and treatment or, if
necessary, induced in situ treatment coupled with source
control (removal, isolation, treatment). Site-specific
conditions, properly defined and understood, provide
the ground-waterinvestigatorwtth the basic information
needed for the determination of a viable approach and
for selecting and designing a cost-effective restoration
scheme.
This chapterprovides an overview of aquifer restoration
technologies utilizing techniques derived from
interrelated disciplines of geology, hydrology,
geochemistry, engineering, construction, biology, and
agronomy. The major emphasis of the chapter is on
ground-water pumping systems and in situ biological
treatment for organic contaminants, which are found at
almost all hazardous waste sites. Many of the
technologies have been developed by demonstration
and research in conjunction with remedial activities in
the Superf und program. Detailed information on selected
techniques can be obtained from the references.
Contaminant Mobility
The design of a ground-water restoration program is
complicated by the fact that all contaminants do not
behave in the same manner. Although discussed
previously, it is important to briefly redescribe the
significance of contaminant mobility in developing and
designing a ground-water restoration program.
The movement of most ground-water contaminants is
controlled by gravity, the permeability and wetness of
the geological materials, and the rniscible character of
the contaminants in ground water. When a material,
particularly a hydrocarbon, is released to the soil, capillary
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attraction and gravity actively draw it into the soil. As the
main body of material moves downward into the more
moist regions of the soil, capillary forces become less
important as the contaminants move through the more
favorable channels by displacing air.
When the contaminants reach the water table, those
less dense than water tend to spread laterally along the
air-water interface or capillary fringe, while the heavier
ones continue to move downward in the saturated zone.
In both cases, the contaminants tend to migrate in the
direction of ground-water flow. In unusual circumstances
very dense contaminants may be more affected by
gravity than by advective flow and move in directions
other than that of the ground water.
The amount of a contaminant that reaches the water
table depends on the quantity involved, the
characteristics of the contaminant, the chemical and
biological properties of the unsaturated zone,
precipitation (ground-water recharge), and the physical
and chemical characteristics of the earth materials. In
general, the more permeable the earth material, the
greater the quantity of contaminant that is likely to reach
the ground water. The entire amount of a contaminant
may be temporarily immobilized in the unsaturated
zone so that it only migrates downward after rainfall
events, becoming a continual or long-term contamination
source. Material so immobilized in the unsaturated
zone may remain there unless physically, chemically, or
biologically removed.
A hydrocarbon liquid phase, for example, generally is
considered to be immiscible with both water and air.
Residual hydrocarbons can occupy from 15 to 40 percent
of the available pore space. However, it is important to
realize that various components of the hydrocarbon
may slowly volatilize into the vapor phase and then
dissolve into the liquid phase. A halo of dissolved
components of the hydrocarbon precedes the immiscible
phase, some of which becomes trapped in the pore
spaces and is left behind as isolated masses. Even
when the so-called residual phase is entirely immobile,
ground water coming into contact with the trapped
material leaches soluble components and continues to
contaminate ground water.
Interaction of the contaminant and the aquifer materials
is anotherconsideration in the evaluation of contaminant
mobility. Some contaminants tend to partition between
the liquid, solid, and vapor phases in amounts dictated
by the characteristics of each contaminant, the nature
of the aqurfermaterial, particularly the amount of organic
carbon, and other geochemical parameters. For many
contaminants, these associations are not fixed but can
be completely reversible. In addition .these compounds
may move freely from one phase to another, depending
upon their concentration in each phase. The processes
of ion-exchange and sorption. chemical precipitation,
and biotransformation all result in retardation or
transformation of the contaminants. Ground water can
become contaminated as freshwater moves through or
past the aquifer material where contaminants are
attached, or as infiltrating water moves through the
unsaturated zone, which contains contaminants in the
vapor phase. The subsurface transport of hydrophobic
compounds is an active field of research.
Highly soluble contaminants, such as salts, some metal
species, and nitrates, have little affinity for sorption to
the solid phase. For aquifer restoration purposes, these
contaminants can be considered to move essentially in
the same direction and velocity as the ground water and
are ideal candidates for pump-and-treat technology.
Site Characterization
In most restoration schemes, all too often the physical
features of the subsurface are largely ignored and little
understood, and most of the effort is involved with the
design and construction of engineering structures. The
important point to consider, however, is that the physical
features of the subsurface, that is. the distribution of
permeability and porosity, and the resulting
hydrogeologic characteristics control the movement
and storage of fluids in the subsurface.
Ground-water restoration activities require dedication
of sufficient resources to collect and understand site
conditions. An adequate amount of field data must be
collected to provide a detailed understanding of the
geology, hydrology, and geochemistry of the site, as
well as the types of contaminants to be removed, their
concentrations, and distribution. The literature should.
be reviewed to determine, to the fullest extent possible.
the contaminants characteristics of sorption,
volatilization, partitioning, and ability to be degraded.
Finally, laboratory investigations, including treatability
studies, development of sorption isotherms, and column
and microcosm examinations to determine contaminant
transport and transformation parameters, assist in
developing a full understanding of the site conditions,
and potential alternatives forground-water remediation.
Many ground-water texts and reports, particularly the
older ones, show ground-water flow nets to be
homogeneous in both the horizontal and vertical
dimensions—at least on a regional scale. In reality such
depictions are rare and the actual water movement is
much more complicated. Flow lines drawn on a water-
table map, for example, imply that the fluids are moving
directly downgradient when, in fact, the flow actually
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follows curvilinear paths (see Chapter 4). All too often
significant amounts of the flow may be through limited
parts of the aquifer, both horizontally and vertically. This
could result from the spatial variability of permeability
for water, or it could result from density or other
considerationsforcontamHiants. Inotherwords, neither
the bulk of the water flow nor the distribution of the
contaminants can be assumed as homogeneous.
Figure 7-1 is the map of a contaminated waste disposal
site that shows the location of a number of monitoring
wells and the altitude of the water surface in them.
Notice that there is as much as 100 feet of difference in
head in wells that are relatively close. The reason for
this difference is well depth, with the deeper wells
having the greatest depth to water. Figure 7-2 is a
water-level map of the same area; contours were based
on shallow wells of nearly the same depth and screen
length. Flow lines depict the general direction of ground-
water movement. Figure 7-3 is a hydrologic cross
section, that is, a vertical flow net, constructed along the
line A-A'. Notice in this example that in the upper50 feet
or so the ground water is flowing across different
geologic units with little loss in head. This indicates that
secondary permeability (fractures), rather than the
10t7
11 If
1070
Seal*, ft
A'
Figure 7-2. Map Showing Configuration of the
Water Table and Flow Lines
primary permeability of the various geologic units, is the
major control on ground-water flow. In the lower part of
the cross section the water-level contours or
equipotential lines are closely spaced and roughly
parallel land surface. This reflects the depth at which
the fractures tend to disappear. The hydrologic cross
section shows that fluid movement, both contaminants
and ground water, is largely limited to the upper 50 feet
of the strata.
Figure 7-1. Map of a Contaminated Area Showing Figure 7-3. Hydrologic Cross Section Showing
Location of Monitoring Wells and Elevation of Equipotential and Flow Lines. Numbers
Water Levels Represent Total Head
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Obviously, inert her pump-and-t real or in situ restoration
systems, or in ground-water monitoring, the location,
depth, and length of the screens of monitoring or
extraction wells are of paramount importance. If the
wells are improperly located, monitoring results would
not adequately represent the aquifer being studied, and
its restoration would be more costly and less effective'
than necessary. Therefore, in planning and carrying out
ground-water restoration activities, it is essential to
dedicate adequate resources to the collection of
background information. In designing remediation
activities, it is more important to describe the most
permeable zones so that it can be determined where the
water can go, under a remediation system, rather than
its natural state.
Source Control
The objective of source- control strategies is to reduce
or eliminate the volume of waste, thereby removing or
minimizing ongoing contamination of the ground-water
environment. Source-control techniques include removal
of the source(s), surface-water controls, ground-water
barriers, interceptors, and hydrodynamic controls.
Source Removal
Soil and water at a hazardous waste site may be
removed for treatment or relocation to a site that is more
acceptable from an engineering or environmental
viewpoint. While the removal and treatment or reburial
of contaminated materials at a more controlled site may
appear to solve a contamination problem .various factors
need to be evaluated before excavation commences.
These factors include:
(1). Problems associated with the excavation of bulky,
partially decomposed or hazardous waste.
(2). Distance to an acceptable treatment/reburial site.
(3). Road conditions between sites.
(4). Accessibility of both sites.
(5). Political, social, and economic factors associated
with locating a new site.
(6). Disposition of contaminated ground water.
(7). Control of nuisances and vectors during
excavation.
(8). Reclamation of excavated site.
(9). Costs.
These considerations suggest that excavation and
relocation may be a viable alternative only where costs
are not significant compared to the importance of the
resource being protected. In some cases, removal and
reburial in an approved facility transfers a problem from
one location to another, and possibly creates additional
problems.
Surface Runoff Controls
Surface runoff control measures are used to minimize
the infiltration and percolation of overland flow or
precipitation at a waste site. It is the infiltration of these
waters that serve as the moving or driving force that
leaches contaminants from the surface or unsaturated
zone to the water table. According to an EPA estimate
(Schuller and others, 1983), a disposal site consisting of
17 acres with 10 inches per year of infiltration could
produce 4.6 million gallons of leachate each year for 50
to 100 years. This estimate, of course, is site-specific.
Reduction of infiltration through a contaminated site can
be accomplished by contouring the site, providing a cap
or barrier to infiltration, and revegetating the site.
Several standard engineering techniques can be used
to change the topographic configuration of the land
surface in order to control the movement of overland
flow. Some of the more common techniques are dikes
and berms, ditches, diversion waterways, terraces,
benches, chutes, downpipes, levees, sedimentation
basins, and surface grading.
A mounded and maintained cover or cap of low
permeability material greatly reduces or even prevents
water from entering the source, thus reducing leachate
generation. Covers also can control vapors or gases
produced in a landfill. They may be constructed of
native soils, clays, synthetic membranes, soil cement,
bituminous concrete, or asphalt, a combination of these
materials.
Revegetation can be a cost-effective method of
stabilizing the surface of a waste site, especially when
preceded by capping and contouring. Vegetation
reduces raindrop impact and the velocity of overland
flow, and strengthens the soil mass, thereby reducing
erosion by wind and water. It also improves the site
aesthetically.
Schuller and others (1983) described the effect of
regrading, installation of a PVC topseal, and revegetation
of a landfill in Windham, Connecticut. As Figures 7-4
and 7-5 illustrate, field data clearly indicate that the
cover reduced infiltration and leachate generation, which
caused a reduction in the size and concentration of the
leachate plume.
Ground-Water Barriers
Subsurface barriers are designed to prevent or control
ground-waterf tow into, through, orfrom a certain location.
Barriers keep fresh ground water from coming into
contact with a contaminated aquifer zone or ground
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Pond3
• Control Point
SpKllle Conducttnct Contour
x 9PKIIKC
C (meromnc
mtioMcin)
Figure 7-4. Distribution of Specific Conductance,
May 19,1981
fond]
Pond 5,
t Control Mm
.x Specific Conductwic* Contour
Figure 7-5. Distribution of Specific Conductance,
November 12,1981
water from existing areas of contamination from moving
into areas of clean ground water. Usually it is necessary
to incorporate other technologies, such as pump-and-
treat systems, with ground-water barriers.
The types of barriers commonly used include:
1. Slurry trench walls
2. Grout curtains
3. Vibrating beam walls
4. Bottom sealing
5. Block displacement
Slurry trench walls are placed either upgradient from a
waste site to prevent flow of ground water into the site,
downgradient to prevent offsite flow of contaminated
water, or around a source to contain the contaminated
ground water. A slurry wall may extend through the
water-bearing zone of concern, or it may extend only
several feet below the water table to act as a barrier to
floating contaminants. Intheformercase, the foundation
should lie on, or preferably in, an underlying unit of low
permeability so that contaminants do not flow under the
wall. A slurry wall is constructed by excavating a trench
at the proper location and to the desired depth, while
keeping the trench filled with a clay slurry composed of
a 5 to 7 percent by weight suspension of bentonite in
water. The slurry maintains the vertical stability of the
trench walls and forms a low permeability filter cake on
the walls of the trench. As the slurry trench is excavated,
it is simultaneously backfilled with a material that forms
the final wall. The three major types of slurry backfill
mixtures are soil bentonite, cement bentonite, and
concrete. Slurry walls, under proper conditions, can be
constructed to depths of 100 feet or so.
Slurry trench walls are reported to have a long service
life and short construction time, cause minimal
environmental impact during construction, and be a
cost-effective method for enclosing large areas under
certain conditions (Nielsen, 1983). A concern regarding
the use of a slurry wall where contaminated materials
are in direct contact with the wall is the long-term
integrity of the wall (Wagner and others, 1986). In such
cases, the condition of the wall needs to be verified over
time by ground-water monitoring.
Two separate slurry walls were constructed along parts
of the margin of the Rocky Mountain Arsenal near
Denver in order to contain plumes that originate on the
plant property (Shukle, 1982, Pendrell and Zeltinger,
1983, and Hager and others. 1983). Along the north
boundary, where surficial, unconsolidated sand and
gravel occur with a thickness that averages about 30
feet, the slurry wall, about 2 feet thick, is 6,800 feet long.
On the upgradient side are a series of 35 12-inch-
diameter discharging wells on 200 foot centers that
pump contaminated ground water into a -treatment
facility. After flowing through a carbon filtration system
the water is reinjected into 50 6-inch diameter recharge
wells on 100 foot centers on the opposite side of the
barrier.
Along the northwest boundary of the Arsenal is another
bentonite slurry barrier, 1,425 feet long, that extends
southwestward from a bedrock high. The wall, excavated
into the sand and gravel with the bentonite slurry trench
method, is 30 inches wide and extends 3 feet into the
underlying bedrock. The barrier contains about 7,000
cubic yards of backfill that were obtained from a borrow
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pit and blended with the bentonile prior to emplacement.
The barrier was constructed where the saturated
thickness of the permeable material is less than 10 feet.
Paralleling the downgradient side of the barrier is a
series of 21 recharge wells, stretching nearly 2,100 feet
along the Arsenal boundary. Directly behind (upgradient)
the barrier and extending into the thicker part of the
surficial aquifer are 15 discharge wells. The
contaminated ground water is pumped to a treatment
plant and then reinjected into the recharge wells, thus
forming a hydraulic barrier. Farther southeast along the
boundary is another hydraulic barrier system, about
1,500 feet long, that consists of two parallel rows of
discharge wells with 15 wells perrow and, downgradient,
a row of 14 recharge wells. The contaminated water,
originating from a spill, is pumped, treated, and then
reinjected. This system and the one along the north
boundary was put into operation in late 1981 and the
system along the northwest boundary began operation
in 1984.
Grouting is the process of pressure-injecting stabilizing
materials into the subsurface to fill and, thereby, seal
voids, cracks, fissures, or otheropenings. Grout curtains
are underground physical barriers formed by injecting
grout through tubes. The amount of grout needed is a
function of the available void space, the density of the
grout, and the pressures used in setting the grout. Two
or more rows of grout are normally required to provide
a good seal. The grout used may be either paniculate
(i.e., Portland cement) or chemical (i.e., sodium silicate)
depending on the soil type and the contaminant present.
Grouting creates a fairly effective barrier to ground-
water movement, although the degree of completeness
of the grout curtain is difficult to ascertain (Nielsen,
1983). Incomplete penetration of the grout into the voids
of the earth material permits leakage through the curtain.
A variation of the grout curtain is the vibrating beam
technique for placing thin (approximately 4 inches)
curtains or walls. Although this type of barrier is
sometimes called a slurry wall, it is more closely related
to a grout curtain since the slurry is injected through a
pipe in a manner similar to grouting. A suspended
I-beam connected to a vibrating driver-extractor is
vibrated through the ground to the desired depth. As the
beam is raised at a controlled rate, slurry is injected
through a set of nozzles at the base of the beam, filling
the void left by the beam's withdrawal. The vibrating
beamtechnique is most efficient in loose, unconsolidated
deposits, such as sand and gravel.
Another method that uses grouting is bottom sealing,
where grout is injected through drill holes to form a
horizontal or curved barrier below the site to prevent
downward migration of contaminants.
Block displacement is a relatively new plume
management method, in which a slurry is injected so
that it forms a subsurface barrier around and below a
specific mass or "block" of material. Continued pressure
injection of the slurry produces an uplift force on the
bottom of the block, resulting in a vertical displacement
proportional to the slurry volume pumped. Brunsing and
Clean/ (1983) described an example of slurry-induced
block displacement. Demonstrated in Whitehouse,
Florida, a slurry wall was constructed around a small
area, 60 feet in diameter, to a depth of 23 feet in
unconsolidated material. Injection wells were then used
to force a soil bentonite slurry outward along the bof.om
of the cell. Subsequent test holes indicated that the new
floor of the cell contained 5 to 12 inches of slurry.
Sheet pile cutoff walls have been used for many years
forexcavatton bracing and dewatering. Where conditions
are favorable, depths of 100 feet or more can be
achieved. Sheet piling cutoff walls can be made of
wood, reinforced concrete, or steel, with steel being the
most effective material for constructing a ground-water
barrier. The construction of a sheet pile cutoff wall
involves driving interlocking sheet piles down through
unconsolidated materials to a unit of low permeability.
Individual sheet piles are connected along the edges
with various types of interlocking joints. Unfortunately,
sheet piling is seldom water-tight and individual plates
can move laterally several to several tens of feet while
being driven. Acidic or alkaline solutions, as well as
some organic compounds, can reduce the expected life
of the system.
Membrane and synthetic sheet curtains can be used in
applications similar to grout curtains and sheet piling.
With this method, the membrane is placed in a trench
surrounding or upgradient of the plume, thereby
enclosing the contaminated source or diverting ground-
water flow around it. Placing a membrane liner in a
slurry trench application also has been tried on a limited
basis. Attaching the membrane to an underlying confining
layer and forming perfect seals between the sheets is
difficult but necessary in order for membranes and other
synthetic sheet curtains to be effective. Ariotta and
others (1983) described a system triat consists of a
trench lined with 100 mil high density polyethylene and
backfilled with sand. It was installed by the slurry trench
construction method in New Brunswick, New Jersey, in
the fall of 1982.
Hydrodynamlc Controls
Hydrodynamic controls are used to isolate a plume of
contaminationf romthe normal ground-waterf low regime
to prevent the plume from moving into a well field,
another aquifer, or to surface water. Controlling the
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movement of ground water by means of recharge and
discharge wells has been practiced for several years.
The major disadvantages include the commonly long
pumping periods, well construction and maintenance
costs, and the fact that the subsurface geology dictates
system design.
The extent of the cone of depression around a pumping
well can be controlled by the discharge rate and thus the
cone, which is a change in the hydraulic gradient, can
be used to control ground-water flow directions and
velocity. Management of the cone or cones permits the
operator to capture contaminants, which can then be
diverted to a treatment plant. Well placement is
particularly important since proper spacing and pumping
rates are required to capture the contaminants.
Moreover, well placement should be optimized so that
as little uncontaminated water as possible is produced
in order to reduce treatment costs.
Recharge wells are used to develop a hydraulic barrier
(an inverted cone of depression) or pressure ridge. In
this way, recharge wells can be used to force the
contaminant plume to move in preferred directions,
such as toward a drain or discharging well.
The design of well systems is, in large part, based on
trial and error methods coupled with experience. Herein
also lies one of the more useful exercises of computer
simulations, because with this approach one can
quickly and easily evaluate different well location and
pumping schedules, and estimate costs.
Gradient-control techniques are used at a great number
of sites undergoing restoration and nearly always play
some role in containment methods, as is the case at the
Rocky Mountain Arsenal.
A well point system, which is a common technique used
fordewatering at construction sites, consists of several
closely spaced shallow wells connected to a main
header pipe. The header pipe is connected to a suction
lift pump. Well point systems are used only for shallow
aquifers and are designed so that the drawdown
produced by the system completely intercepts the plume
of contamination.
Deep wells are similarto well point systems except they
are generally deeper and normally are pumped
individually. This system commonly is used in places
where the ground-water surface is too deep for the use
of a suction lift system.
A thorough knowledge of the hydrogeotogical conditions
of a site is required for the development of a
hydrodynamic control system. The effect of the injection
wells on the drawdown and the radius of influence of the
pumping wells must be analyzed. Of particular
importance are the potential well yield or injection rate,
and the effect of hydrotogic flow boundaries. Monitoring
of the system is essential.
Ground-Water Collection and Treatment
The cleanup of a contaminated ground-water site
involves the collection and treatment of the contaminated
water. Some of the techniques used for source control
often are used as part of a ground-water cleanup
program, including pumping well systems, interceptor
systems, and some of the techniques used for source
control. In addition, in situ treatment, enhanced
desorption, encapsulation, and biodegradation may be
part of a cleanup plan.
Pumping Systems
A ground-water pumping scheme combined with a
treatment procedure, also called a pump-and-treat
system, is usually designed for a specific ground-water
contamination problem. The use of pump-and-treat
systems is probably more widespread and successful
than all other restoration techniques combined. Large
expenditures are made each year to prepare for and
operate pump-and-treat remediation of ground-water
contamination (Keely, 1989). The hydrogeology of the
site, the source of the contaminant, and the
characteristics of the contaminant must be understood
if an efficient and cost-effective program is to be
conducted.
The operation of a well field to remove ground water
causes the formation of stagnation zones downgradient
from the extraction wells, which must be considered in
the system design. For example, if remedial action wells
are located within the bounds of a contaminant plume,
the portion of the plume lying within the stagnation
zones will not be effectively remediated because the
contaminants are removed only from the zone of
advective ground-water flow. In this case, the only
remediation in the stagnation zone will result from the
process of chemical diffusion and degradation, which
may be very slow. Proper location of wells based on
pumping rates and drawdown tends to mitigate this
effect.
The tailing effect also can affect the removal and
renovation of ground water containing a low solubility
contaminant. Tailing is the slow, nearly asymptotic
decrease in contaminant concentration in ground water
moving through contaminated geologic material. The
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contaminants migrate into the finer pore structures of
the earth materials and are slowly exchanged with the
bulk water present in larger pores and this results in
"tailing."
Many human-made and natural organic compounds
found in ground watertend to adsorb to the organic and
mineral components of the aquifer material. When
water is removed by pumping, contaminants can remain
on the aquifer material, the amount depending on the
geologic materials and characteristics of the
contaminants. Once sorbed to the geologic material,
contaminants may desorb slowly into the ground water,
thus requiring extended periods of pumping and treating
to attain desired levels of restoration.
The removal of a water-insoluble liqu id, such as gasoline.
can be difficult since the product may become trapped
in the pores of earth materials and is not easily removed
by pumping. Pumping ground water to remove the
components of a residual phase initially may reduce the
concentration, but this reduction may only be the result
of dilution or lowering of the water table below the level
of contamination. A contaminant will not be removed
faster than it is released into the ground water, so if the
pumping stops for a period of time, water-soluble residual
phase components again will dissolve into the ground
water bringing the concentrations back to the previous
level.
An innovation in pump-and-treat technology is pulsed
pumping. This technique involves alternating the periods
of pumping, allowing contaminants time to come to
equilibrium with the ground water in each cycle.
Equilibrium is achieved by diffusion from stagnant zones
or zones of lower permeability, and by partitioning of
sorbed contaminants or those associated with residual
contaminant phases. Alternating pumping among wells
also can establish active flow paths in the stagnant
zones.
Another innovation is the use of pump-and-treat systems
in conjunction with other remediation technologies.
Examples are the use of extraction wells with barrier
walls to limit plume expansion while reducing the amount
of clean water pumped, and the use of surface ponds or
flooding to flush contaminants from the unsaturated
zone prior to collection by a pumping system.
Interceptor Systems
Interceptor systems may be an alternate to pumping
systems. The subsurface drains used in interceptor
systems essentially function as an infinite line of
extraction wells, and can perform many of the same
functions. Subsurface drains create a continuous zone
of influence in which ground water flows towards the
drain. Subsurface drains are installed perpendicular to
the direction of ground-water flow and collect ground
water from an upgradient source for treatment.
Interceptor systems prevent leachate or contaminated
ground water from moving downgradient toward wells
or surface water.
Two types of interceptor systems used for source
control are the passive system, which relies on gravity
flow, and the active system, which uses pumps. An
interceptor system consists of a trench excavated to a
specified depth below the water table in which a
perforated collection pipe is installed in the bottom.
Active interceptor systems have vertical removal wells
spaced along the interceptor trench or a horizontal
removal pipe in the bottom of the trench. Active systems
are usually backfilled with a coarse sand or gravel to
maintain the stability of the wall. These interceptor
systems can be used as preventive measures, such as
leachate collection systems, as abatement measures,
such as interceptor drains, or in product recovery from
ground water, such as the removal of gasoline or oil.
Interceptor drains generally are used to either lower the
water table beneath a contamination source or to collect
contaminated ground water!rom an upgradient source.
Interceptor systems are relatively inexpensive to install
and operate, but they are not well suited for soils with a
low permeability.
In stratified soils with variable hydraulic conductivities,
the drain is normally installed on a layer with a low
hydraulic conductivity to minimize leachate leakage
under the drain. An impermeable liner placed in the
bottom of a trench also can be used to control underflow.
The design, spacing, and location of drains for various
soil and ground water conditions are described further
in Wagner and others (1986).
A combined interceptor and ground-water dam
installation was described by Giddings (1982). In this
case, a landfill that began as abuming dump, was found
to be discharging leachate both to the surface and to the
ground water, much of which eventually flowed into an
adjacent river. A leachate interceptor trench was
constructed on the downgradient side of the disposal
area, as shown in Figure 7-6. In the trench on the
upgradient side was placed a perforated pipe in a gravel
envelope that was covered with permeable material.
The remainder of the trench on the downgradient side
was then backfilled with fine-grained materials as
shown in Figure 7-7. Leachatef romthe landfillflows into
the filled trench, seeps into the perforated pipe, and
then is collected for treatment. In this case, the main
135
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A I Upgradlent Monitoring
Well
Figure 7-6. Site Layout
Perforated -
Pipe
Gray Till
Figure 7-7. Site Cross Section
136
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purpose of the ground-water dam was to prohibit water
originating in the adjacent river from flowing into the
trench, which would have substantially increased the
volume of wastewater.
Ground-Water Treatment after Removal
Of course the technology of pumping and treating of
ground water implies that a cadre of engineering
processes are available fortreating the extracted water
at the surface. A detailed discussion of these is beyond
the scope of this document. They will only be mentioned
to give the reader a familiarity with the processes so that
detailed searches can be made elsewhere.
Treatment technologies for pumped or intercepted
ground water can be grouped into three broad areas:
physical, chemical, and biological. Physical treatment
methods include adsorption, density separation,
filtration, reverse osmosis, air and steam stripping, and
incineration. Precipitation, oxidation/reduction, ion
exchange, and neutralization are commonly used
chemical treatment methods. Biological treatment
methods include activated sludge, aerated surface
impoundments, anaerobic digestion, trickling filters,
and rotating biological discs.
In Situ Treatment
In situ treatment is an alternative to the removal and
subsequent treatment of contaminated ground water.
This method requires minimal surface facilities and
reduces exposure to the contaminant. The success of
various treatment methods is highly dependent on
physical factors including aquifer permeability, the
characteristics of the contaminants involved, and the
geochemistry of the aquifer material.
I n situ treatment technology has not yet been developed
to the extent of other currently available technologies
for restoring contaminated aquifers. However, some in
situ treatment technologies have demonstrated success
in actual site remediations (Wagner and others, 1986).
Laboratory and pilot-scale testing generally must be
performed to evaluate the applicability of a particular
technology to a specific site.
In situ treatment may be grouped into two broad
categories: physical/chemical and biological. Brief
descriptions follow of the available technologies that
have potential for success at hazardous waste sites.
In Situ Physlcal/Chemlca! Treatment
Organic and inorganic contaminants may be treated
chemically to cause immobilization, mobilization for
extraction, ordetoxif ication. The application of oxidation
and reduction reactions to in situ treatment is largely
conceptual, but potentially may be used to accomplish
immobilization by precipitation, mobilization by
solubilizing metals ororganics, ordetoxif ication of metals
and organics (Wagner and others, 1986). The chemicals
used in these processes, however, have the potential to
degrade compounds other than those targeted and to
form degradation products that may be more toxic than
the original ones.
Precipitation, chelation, and polymerization are three
methods used to immobilize a contaminant. Precipitation
using caustic solutions is effective in immobilizing
dissolved metals in ground water. Chelation also may
be effective in immobilizing metals, although
considerable research is needed (Wagner and others,
1986). Polymerizat ion is effective in immobilizing organic
monomers. However, the chemicals added to the
contaminants in the ground water may react to form
toxic by-products. Solidification methods used for
treatment of soils also can immobilize contaminants.
Mobilization of contaminants is accomplished by soil
flushing or vacuum extraction. Neutralization, hydrolysis,
and permeable treatment bed technologies may be
used for detoxification. Precipitation and polymerization
will lower the hydraulic conductivities near the injection
wells making closely spaced wells necessary for effective
treatment.
One interesting example of polymerization, reported by
Williams (1982), involved a 4,200 gallon leak of acrylate
monomer from a corroded pipeline at a small plant in
Ohio. The contaminant migrated through a layer of fill,
consisting largely of cinders, and then downward through
a storm sewer trench into a thin sand and gravel aquifer.
A test boring and soil sampling program delineated the
plume and indicated that the contaminant was slowly
beginning to undergo polymerization and, therefore,
immobilization. To increase the rate of reaction, 2-inch-
diameter perforated PVC pipe was buried, about 2 feet
below land surface, in four narrow trenches that trended
across the plume. A riser and manifold header connected
each pipe to solution tanks containing a catalyst in one
and an activator in the other. Both solutions contained
a wetting agent. A total of 8,000 gallons of solution were
injected during the two treatment operations and 1,000
gallons had been injected previously during the
investigative phase. On the basis of pre- and post-
treatment soil borings, it was estimated that 85 to 90
percent of the liquid monomer contaminant was solidified,
and in some places it exceeded 99 percent
polymerization. It was assumed that the remaining
mate rial would polymerize naturally.
In situ physical/chemical treatment processes generally
entail the installation of a series of injection wells at the
head of or within the plume of contaminated ground
137
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water. An alternative technique that has been used in
shallow aquifers, is the installation of in situ permeable
treatment beds. Trenches are filled with a reactive
permeable medium and contaminated ground water
entering the trench reacts with the medium to produce
a nonhazardous soluble product or a solid precipitate.
Among the materials commonly used in permeable bed'
trenches are limestone to neutralize acidic ground
water and remove heavy metals, activated carbon to
remove nonpolar contaminants, such as carbon
tetrachforide, polychlorinated biphenyls, and benzene,
and zeolites and other ion exchange resins for removing
solubilized heavy metals.
Permeable treatment beds are applicable only in
relatively shallow aquifers because the trench must be
constructed down to a layer of tow permeability. They
also are often effective for only a short time because
they lose their reactive capacity or become plugged
with solids. An overdesign of the system or replacement
of the reactive medium can lengthen the time during
which permeable treatment is effective.
Mobilization for Extraction
Pump-and-treat remediation techniques often are
inefficient when a preponderance of the contaminants
are sorted to the solid phase of the aquifer. The same
can be said for in situ treatment if the reactive chemicals
are unable to come into contact with the contaminants.
In these cases, the enhanced desorption or mobilization
of contaminants would be of considerable interest in
aquifer restoration activities.
Soil flushing is the process of flooding a contaminated
area with water or a solvent to mobilize the contaminant,
followed by the collection of the elutriate The process
is basedonthe solvent solubilizingorchermcaiiy reacting
with the contaminants and mobilizing them into the
solvent phase. Water is used if the contaminant is
readily soluble. Acid solutions tend to flush metals and
basic organics.
The mobilization of contaminants by injecting surfactants
into the aquifer matrix is possible. Techniques used for
the secondary recovery of oil are being used
experimentally, with moderate success. Both surfactant
and alkaline floods have been attempted. Most oil-field
surfactants are expensive, while alkaline floods produce
lye: therefore, this approach promises little benefit to
aquifer restoration.
In the recovery of hydrocarbons, there are three possible
physical-chemical methods. At shallow depths, thermal
or steam flooding may be helpful while on a larger scale,
alcohol flooding may at some future date prove to be
helpful. Alcohol is easily produced and dissolves the
hydrocarbon, but tentative research results indicate
that the required alcohol-water ratio must be so high as
to make the technique questionable.
Another emerging technology, which is increasingly
being used, is alternately called in situ vacuum extraction
or in situ volatilization. It is used to extract volatile
organic contaminants from the unsaturated zone where
contaminants exist as a result of underlying contaminated
ground water, or free product riding on top of the ground
water, or from leaks or spills. The technology has
enjoyed considerable success in this and other
industrialized countries.
The plumbing associated with this type of remediation
is obviously dictated by site conditions, including the
thickness of the unsaturated zone, the volatility of the
contaminants involved as well as their source and
extent, and the porosity and permeability of the
unsaturated zone (Pacific Environmental Services,
1989).
Generally these vapor extraction projects consists of a
series of slotted PVC wells configured to span the area
of contamination. Air inlet wells located both inside and
outside of the plume increase the introduction of air from
the atmosphere (fig. 7-8).
Like pump-and-treat remediation techniques, vacuum
extraction projects usually require some type of surface
treatment facility to deal with the collected vapors.
When surface treatment is required, activated carbon
columns are widely in operation, however the use of
biologically active columns is being studied, which will
allow the introduction of oxygen or other gases needed
for biodegradation.
Vacuum extraction is best suited for areas of high,
relatively homogeneous, permeability. There should be
no underground structures, and great care must be
given to the explosive nature of the extracted vapors.
The unit cost, which appears to be very promising,
varies widely according to the size of the area under
remediation and the specific site characteristics.
Radio frequency heating has been under development
since the mid-1970s and the concept is being applied to
in situ decontamination of uncontrolled hazardous waste
landfills and sites (Rich and Cherry, 1987). In this
process, the ground is heated with radio frequency
waves that vaporize the hazardous contaminants. The
vapors emanating from the soil are then treated.
Detoxification
Neutralization of ground water may be accomplished by
injecting dilute acids or bases into the aquifer through
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Vapor Treatment
Air/Water Separator
Extraction Well
Inlet Well
'/rvXr-1'.?-'. Water Table
Figure 7-8. Schematic of a Vaccum Extraction System
injection wells to adjust the pH to the desired level.
Toiman and others (1978) recommended that
neutralization only be applied to ground water at
industrial waste disposal sites since municipal landfills,
which constantly generate anaerobic decomposition
products, would require neutralization over a long period
of time.
Hydrolysis may be used for detoxification, however, the
intermediate products formed during hydrolysis of a
particular compound must be known since they may be
more toxicthanthe targeted compound. Esters, amides,
carbamates, phosphoric and pnosphonic acid esters,
and pesticides are potentially degradable by hydrolysis
(Wagner and others, 1986).
Blodegradatlon
There are two basic approaches to in situ biodegradation.
The first relies on the natural biological activity in the
subsurface. The second approach, called enhanced
biorestoration, involves the stimulation of the existing
microorganisms by adding nutrients.
Natural Subsurface Biological Activity
Biological treatment in the subsurface involves the use
of microorganisms to break down hazardous organic
compounds into nonhazardous materials. The site
hydrology, environmental conditions, and the
biodegradability of the contaminants are factors that
determine the potential effectiveness of in situ biological
treatment. Most compounds are more rapidly degraded
aerobically, howeversome compounds will only degrade
under anaerobic conditions. Biodegradation in ground
water and solids can be a slow process and may take
several years for completion depending on the
compounds present. In situ biodegradation, however, is
a desirable method of treatment because the
contaminants are destroyed, thus, removal of ground
water for external treatment and residual handling
possibly can be avoided.
In situ biorestoration of the subsurface is a relatively
new technology that has recently gained considerable
attention. Scarcely more than a decade ago,
conventional wisdom assumed that the subsurface
below the root zone of plants was, for all practical
purposes, sterile. Research during the last decade has
indicated that the deeper subsurface is not sterile, but
in fact, harbors significant populations of
microorganisms. Bacterial densities of around a million
organisms per gram of dry soil have been found in
several unccntaminated aquifers. Water-table aquifers
examined so far exhibit considerable variation in the
rate of biodegradation of specific contaminants and
rates can vary two or three orders of magnitude from
one aquifer to another or over a vertical separation of
only a few feet in the same aquifer. Although extremely
variable, the rates of biodegradation are fast enough to
protect ground-water quality in many aquifers.
Although not clearly defined, several environmental
factors are known to influence the capacity of indigenous
microbial populations to degrade contaminants. These
139
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factors include dissolved oxygen, pH, temperature,
oxidation-reduction potential, availability of mineral
nutrients, salinity, soil moisture, the concentration of
specific contaminants, and the nutritional quality of
dissolved organic carbon in ground water.
Natural biorestoration does occur in the subsurface
environment. Contaminants in solution in ground water,
as well as vapors in the unsaturated zone, can be
completely degraded ortransf ormed to new compounds.
Undoubtedly, thousands of contamination events are
remediated naturally before the contamination reaches
a point of detection. On the other hand, methods are
needed to determine when natural biorestoration is
occurring, the stage the restoration is in, whether
enhancement of the process is possible or desirable,
and what will happen if natural processes are allowed to
run their course.
For information on in situ biorestoration of specific
compounds and conditions see Bower and McCarty
(1983), Jhaveri and Mazzacca (1983), Lee and Ward
(1984), Parsons and others (1985), Parsons and others
(1984), SuHlita and Gibson (1985), Sutflita and Miller
(1985), Wilson (1985), Wilson and Rees (1985). Wood
and others (1985). and Young (1984).
Enhanced Biorestoration
In the subsurface environment, populations of organisms
capable of degrading contaminants increase until limited
by metabolic requirements, such as mineral nutrients or
oxygen. Once this point is reached, the rate of
biodegradation or transformation of organic compounds
is controlled by the transport mechanisms that supply
the limiting nutrients.
The majority of microbes in the subsurface are firmly
attached to soil particles. As a result, nutrients must be
brought to the active sites by advection and diffusion of
water in the saturated zone, or by soil gas, in the
unsaturated zone. In the simplest and perhaps most
common case, the compounds to be degraded for
microbial energy and cell synthesis are transported in
the aqueous phase by infiltrating water or by advective
flow through the ground water. In the unsaturated zone,
volatile organic compounds can move readily as vapors
in the soil gas where oxygen is present. Below the water
table, aerobic metabolism is limited by the low solubility
of oxygen in water. Factors that control the rate of
biological activity are the stoichiometry of the metabolic
process, the concentration of the required nutrients in
the mobile phases, the flow of the mobile phases, the
opportunity for colonization in the subsurface by
metabolically capable organisms, and the toxicity of the
waste.
Much of the development work in the area of ground-
water and soil remediation by biodegradation has been
performed using petroleum products. The number of
gasoline stations, underground tanks, and gasoline
pipelines throughout the country and the potential for
ground-water contamination have prompted
considerable laboratory and field studies on in situ
biodegradation of hydrocarbons.
Many of the enhanced biorestoration techniques now
in use are variations on those developed by Raymond
and his coworkers (Raymond, 1974; Raymond and
others, 1986). This process reduces hydrocarbon
contaminants in aquifers by enhancing the indigenous
hydrocarbon-utilizing microflora. Nutrients and oxygen
are introduced through injection wells and circulated
through the contaminated zone by pumping one or
more producing wells. The increased supply of nutrients
and oxygen stimulates biodegradation of the
hydrocarbons.
Raymond's process has been used with reasonable
success to restore aquifers contaminated with gasoline.
The overall removal of total hydrocarbons using this
technology usually ranges from 70 to 80 percent. Some
of the sites treated by this technique have been restored
to the point where no dissolved gasoline was present in
the ground water, and state regulatory standards were
satisfied. State agencies charged with restoring other
sites, however, have required that the operation continue
until no trace of liquid gasoline could be detected. Most
of the sites restored in this manner have had appropriate
monitoring programs installed following remediation.
Usually the first step in the process is to use physical
methods to recover as much of the gasoline as possible
and then a detailed investigation of the hydrogeology is
undertaken to determine the extent of the contamination.
Laboratory studies are conducted to determine if the
native microbes can degrade the contaminants and to
determine the combination of minerals required to
promote maximum cell growth at the ambient ground-
water temperature and under aerobic conditions.
Considerable variations in nutrient requirements among
aquifers have been noted. One aquifer required only the
addition of nitrogen and phosphorus, while another was
best stimulated by the addition of ammonium sulfate,
mono- and disodium phosphate, magnesium sulfate,
sodium carbonate, calcium chloride, and manganese
and ferrous sulfate. It was found that a chemical analysis
of the ground water was not helpful in estimating the
nutrient requirements of the system.
Field investigations and laboratory studies guide the
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design and installation of a system of wells for injecting
the nutrients and oxygen, and for the control of ground-
water flow. Controlling the ground-water flow is critical
to moving oxygen and nutrients to the contaminated
zone and optimizing the degradation process.
The technique developed by Raymond does not provide
fortreatment above the water table. Soils contaminated
by leaking underground storage tanks may be physically
removed during the process of removing the tank,
however, this may not be practical with deep water
tables or large areas of contamination. An alternative to
soil removal is the construction of one or more infiltration
galleries, which are used to recirculate the treated water
back through the contaminated unsaturated zone.
Oxygen may be added to the infiltrated water during an
in-line stripping process for volatile organic contaminants
or through aeration devices placed in the infiltration
galleries.
The rate of biorestoration of hydrocarbons, either above
or below the water table, is effectively the rate of supply
of oxygen. Table 7-1 compares the number of times the
water in the aquifer, orthe air above it, must be replaced
to restore subsurface materials of various textures. The
calculations assume typical values for the volume
occupied by air, water and hydrocarbons (De Pastrovich
and others.1979, Clapp and Horberger, 1978). The
calculations further assume that the oxygen content of
the water is 10 mg/L, that of the air is 200 mg/L and that
the hydrocarbons are completely metabolized to carbon
dioxide. These values are provided only to exemplify
the processes involved and would differ at an actual
site. The oxygen concentration in the water can be
increased by using oxygen rather than air. which also
would reduce the volumes of recirculated water required.
Hydrogen peroxide is an alternative source of oxygen in
biorestoration and Raymond and others (1986) have
patented a process of treatment with hydrogen peroxide.
Iron or an organic catalyst may be used to decompose
the hydrogen peroxide to oxygen. The rate at which
hydrogen peroxide decomposes to oxygen must be
controlled to limit the formation of bubbles that could
lead to gas blockage and the loss of permeability.
Hydrogen peroxide may mobilize metals, such as lead
and antimony, and, if the water is hard, magnesium and
calcium phosphates can precipitate and plug the injection
well or infiltration gallery. To determine the
microorganism's hydrogen peroxide tolerance level
laboratory studies are performed.
Treatment Trains
In most contaminated hydrogeologic systems, the
remediation process may be so complex, in terms of
contaminant behavior and site characteristics, that no
single system or unit is capable of meeting all
requirements. Consequently, several unit operations
may be combined in series or in parallel to effectively
restore ground-water quality to the required level.
Barriers and hydrodynamic controls may serve as
temporary plume control measures, however,
hydrodynamic processes are integral parts of any
withdrawal and treatment or in situ treatment process.
Most remediation projects typically are started by
removing the source. The next step may be the
installation of pumping systems to remove free product
floating on the water surface or the removal of soluble
contaminants for treatment at the surface. Barriers also
might be constructed to slow an advancing plume or to
reduce the amount of water requiring treatment.
Proportion of Total Subsurface Volume
Occupied by:
Texture
Stone to Coarte Gravel
Gravel to Cotna Sand
Coam to Medium Sand
Medium to Fne Sand
Rne Sand to SHt
Hydrocarbons
(when drained)
0.006
o.ooe
0.015
0.025
0.040
Air
(whan drained)
0.4
0.3
0.2
0.2
0.2
Water
(when flooded)
0.4
0.4
0.4
0.4
0.6
Volume* Requked
to meet
Hydrocarbons
Oxygen Demand
Ar
250
530
1.500
2,500
4.000
Water
5.000
8.000
15.000
25,000
32.000
Table 7-1. Estimated Volumes of Water or Air Required to Completely Renovate Subsurface Material
that Contained Hydrocarbons at Residual Saturation
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Enhanced biorestoration techniques may be feasible in
some of the more diluted areas of the plume. In some
circumstances, a site may reach final restoration goals
using natural chemical and biological processes. An
adequate monitoring program would be required to
establish data on the progress of the restoration program.
Steps in treatment of contaminated ground water include
the removal, collection, and delivery of the contaminated
water to the treatment units, and in the case of in situ
processes, delivery of the treatment materials to the
contaminated areas in the aquifer. A thorough knowledge
and understanding of the hydrogeologic and
geochemical characteristics of the site are required to
design a system that will optimize the remediation
techniques selected, maximize the predictability of
restoration effectiveness, and allowforthe development
of a cost-effective and lasting remediation program.
Institutional Limitations on Controllng Ground-
Water Contamination
The principal criteria for selecting remediation
procedures are the water-quality level to which to restore
an aquifer, and the most economical technology available
to reach that level. Institutional limitations, however,
sometimes override these criteria in determining if,
when, and how remediation will be selected and carried
out.
Response to a ground-water contamination problem is
likely to require compliance with several local, state,
and federal pollution control laws and regulations. If the
response involves handling hazardous wastes,
discharging substances into the air or surface waters, or
injecting wastes underground, federal and state pollution
control laws will apply. These laws do not exempt the
activities of federal, state, or local officials or other
parties attempting to remediate contamination problems.
They apply to both generators and responding parties,
and it is not unusual for these pollution control laws to
conflict. A hazardous waste remediation project must
meet RCRA permit requirements governing the transport
and disposal of hazardous wastes, which can influence
the selection of the remediation plan and the scheduling
of cleanup activities.
In situ remediation procedures may be subject to
permitting or other requirements under federal or state
underground injection control programs. Withdrawal
and treatment approaches may be subject to regulation
under federal or state air pollution control programs or
to pretreatment requirements if contaminated ground
water is to be discharged to a surface water or to a
municipal wastewater treatment system. A remediation
plan involving pumping from an aquifer may be subject
to state ground-water regulations on well construction
and well spacing, and may need to consider various
competing legal rights to extract ground water.
Other factors influencing selection and design of a
ground-water remediation program include the
availability of alternative sources of watersupply, political
and judicial constraints, and the availability of funds.
Where alternate water supplies are plentiful and
economical, there may not be a demand for total
remediation; adequate remediation to protect human
health and the environment may be sufficient. In the
final analysis, responsible agencies can pursue
remediation measures to the extent that resources are
made available.
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on remedial actions for contaminated ground water at
Superfund sites: EPA-540/G-88/003.
U. S. Environmental Protection Agency, 1989, Seminar
on site characterization for subsurface remediations:
CERI-89-224.
U.S. Environmental Protection Agency, 1989, Transport
and fate of contaminants in the subsurface: EPA/625/
4-89/019.
U.S. Environmental Protection Agency. 1990. Handbook
on in situ treatment of hazardous waste-contaminated
soils: EPA/540/2-90/002.
U.S. Environmental Protection Agency, 1990, Basics of
pump-and-treat ground-water remediation technology:
EPA/600/8-90/003.
Wagner. K.. K. Boyer. R. Claff. M. Evans. S. Henry, V.
Hodge, S. Mahmud, D. Sarno, E. Scopina, and P
Spooner. 1986, Remedial action technology for waste
disposal sites: 2nd ed. Noyes Data Corporation, Park
Ridge, NJ.
143
-------
Williams, E.B., 1982, Contaminant containment by in
situ polymerization: 2nd Nat. Symp. on Aquifer
Restoration and Ground-Water Monitoring, Nat. Water
Well Assoc.
Wilson, B., 1985, Behavior of trichloroethylene, 1,1-
dichloroethylene in anoxic subsurface environments:
unpubl. M.S. thesis, Univ. of Oklahoma.
Wilson, B.H. and J.F. Rees, 1985, Btotransformation of
gasoline hydrocarbons inmethanogenicaquifermaterial:
Proc. of NWWA/API Conf. on Petroleum Hydrocarbons
and Organic Chemicals in Ground Water, Houston, TX.
Wood, P.R., R.F. Lang, and I.L Payan, 1985, Anarobic
transformation, transport, and removal of volatile
chlorinated organics in ground water: Ground Water
Quality, John Wiley & Sons, New York.
Young, L.Y.. 1984, Anaerobic degradation of aromatic
compounds: Microbial Degradation of Aromatic
Compounds.
• U.S. GOVERNMENT PRINTING OFTICE: 1994-550-001/80346
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VOL. II
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United Slater Office of
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Agency Washington. DC 20460
EPA'625'6-90'016D
July 1991
Handbook
Ground Water
Volume II: Methodology
i
-------
EP A/625/6-90/016b
July 1991
Handbook
Ground Water
Volume II: Methodology
i
U.S. Environmental Protection Agency . 1
Office of Research and Development '
Center for Environmental Research Information
Cincinnati, OH 45268
(££ Printed on Recycled Paper
-------
NOTICE
This document has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
This document is not intended to be a guidance or support document for a specific regulatory program.
Guidance documents are available from EPA and must be consulted to address specific regulatory issues.
-------
Contents
Page
Chapter 1. Monitoring Well Design and Construction 1
Chapter 2. Ground-Water Sampling 22
Chapter 3. Transport and Fate of Contaminants in the Subsurface 41
Chapter 4. Ground-Water Tracers 67
Chapter 5. Introduction to Aquifer Test Analysis 96
Chapter 6. Models and Computers in Ground-Water Investigations 117
ill
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Acknowledgment
Many individuals contributed to the preparation and review of this handbook. The document was prepared by
Eastern Research Group, Inc., for EPA's Center for Environmental Research Information, Cincinnati, OH.
Contract administration was provided by the Center for Environmental Research Information.
Volume I, Ground Water and Contamination, was published in September 1990 (EPA/625/6-90/016a). Volume
II, Methodology describes various investigative approaches and techniques. Although extensively revised, part
of Volume II was obtained from previous publications, "Handbook: Ground Water (EPA/625/6-87/016) and
"Protection of Public Water Supplies from Ground-Water Contamination" (EPA/625/4-85/016).
Authors and Reviewers
Michael J. Barcelona - Western Michigan University, Kalamazoo, Ml
Russell Boulding - Eastern Research Group, Inc., Arlington, MA
Ralph C. Heath - Private Consultant, Raleigh, NC
Wayne A. Pettyjohn - Oklahoma State University, Stillwater, OK
Ron Sims - Utah State University, Logan, UT
Judy Sims - Utah State University, Logan, UT
Paul van der Heijde - IGWMC, Holcomb Research Institute
H. Allen Wehrman - Illinois State Water Survey, Champaign, IL
Project Officer
Carol Grove - EPA-CERI, Cincinnati, OH
iv
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Preface
The subsurface environment of ground water is characterized by a complex interplay of physical, geochemical
and biological forces that govern the release, transport and fate of a variety of chemical substances. There are
literally as many varied hydrogeologic settings as there are types and numbers of contaminant sources. In
situations where ground-water investigations are most necessary, there are frequently many variables of land
and ground-water use and contaminant source characteristics which cannot be fully characterized.
The impact of natural ground-water recharge and discharge processes on distributions of chemical constituents
is understood for only a few types of chemical species. Also, these processes may be modified by both natural
phenomena and man's activities so as to further complicate apparent spatial or temporal trends in water quality.
Since so many climatic, demographic and hydrogeologic factors may vary from place to place, or even small areas
within specific sites, there can be no single "standard" approach for assessing and protecting the quality of grourvl
water that will be applicable in all cases.
Despite these uncertainties, investigations are underway and they are used as a basis for making decisions about
the need for, and usefulness of, alternative corrective and preventive actions. Decision makers, therefore, need
some assurance that elements of uncertainty are minimized and that hydrogeologic investigations provide reliable
results.
A purpose of this document is to discuss measures that can be taken to ensure that uncertainties do not undermine
our ability to make reliable predictions about the response of contamination to various corrective or preventive
measures.
EPA conducts considerable research in ground water to support its regulatory needs. In recent years, scientific
knowledge about ground-water systems has been increasing rapidly. Researchers in the Office of Research and
Development have made improvements in technology for assessing the subsurface, in adapting techniques from
other disciplines to successfully identify specific contaminants in ground water, in assessing the behavior of certain
chemicals in some geologic materials and in advancing the state-of-the-art of remedial technologies.
An important part of EPA's ground-water research program is to transmit research information to decision makers,
field managers and the scientific community. This publication has been developed to assist that effort and,
additionally, to help satisfy an immediate Agency need to promote the transfer of technology that is applicable to
ground-water contamination control and prevention.
The need exists for a resource document that brings together available technical information in a form convenient
for ground-water personnel within EPA and state and local governments on whom EPA ultimately depends for
proper ground-water management. The information contained in this handbook is intended to meet that need. It
is applicable to many programs that deal with the ground-water resource. However, it is not intended as a guidance
or support document for a specific regulatory program.
GUIDANCE DOCUMENTS ARE AVAILABLE FROM EPA AND MUST BE CONSULTED TO ADDRESS
SPECIFIC REGULATORY ISSUES.
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Chapter 1
MONITORING WELL DESIGN AND CONSTRUCTION
The principal objective of constructing monitoring wells
is to provide access to an otherwise inaccessible
environment. Monitoring wells are used to evaluate
topics within various disciplines, including geology,
hydrology, chemistry, and biology. In ground-water
quality monitoring, wells are used (orcollecting ground-
water samples, which upon analysis may allow
description of a contaminant plume, or the movement of
a particular chemical (or biological) constituent, or ensure
that potential contaminants are not moving past a
particular point.
Ground-Water Monitoring Program Goals
Each purpose for ground-water monitoring-ambient
monitoring, source monitoring, case preparation
monitoring, and research monitoring-must satisfy
somewhat different requirements, and may necessitate
different strategies for well design and construction
(Barcelona and others, 1984). At the outset.the goals
of the intended monitoring program must be clearly
understood and thought should be given to the potential
future use of the wells in other, possibly different,
monitoring programs.
Regional investigations of ground-water quality involve
ambient monitoring. Such investigations seek to
establish an overall picture of the quality of water within
all or parts of an aquifer. Generally, sample collection is
conducted routinely over a period of many years to
determine changes in quality overtime. Often, changes
in quality are related to long-term changes in land use
(e.g., the effects of urbanization). Monitoring conducted
for Safe Drinking Water Act compliance generally falls
in this category.
Samples commonly are collected from a variety of
public and private water supply wells for ambient quality
investigations. Because of the diversity of sources, the
data obtained through some ambient monitoring
programs may not meet the strict well design and
construction requirements imposed by the three other
types of monitoring. However, such programs are
important for detecting significant changes in aquifer
water quality overtime and space and protecting public
health.
Regulatory monitoring at potential contaminant sources
is considered source monitoring. Under this type of
program, monitoring wells are located and designed to
detect the movement of specific pollutants outside of
the boundaries of a particular facility (e.g., treatment,
storage, or disposal). Ground-water sampling to define
contaminant plume extent and geometry would fall into
this monitoring classification. Monitoring well design
and construction are tailored to the site geology and
contaminant chemistry. With source monitoring,
quantitative aspects of analytical results become most
important because the level of contaminant
concentration may require specific regulatory action.
Monitoring for case preparation, such as legal
proceedings in environmental enforcement, requires a
level of detail similar to source monitoring. Source
monitoring, in fact, often becomes a part of legal
proceedings to establish whether or not environmental
damage has occurred and to identify the responsible
party. This is a prime example of one type of monitoring
program evolving into another. The appropriateness
and integrity of monitoring well design and construction
methods will come under close scrutiny in legal
proceedings. In such cases, the course of action taken
during the monitoring investigation, the decisions made
concerning well design and construction, and the reasons
for those decisions must be clearly established and
documented.
Monitoring for research generally requires a level of
sophistication beyond that required of any other type of
monitoring (this, of course, depends upon the types and
concentrations of constituents being sought and the
overall objectives of the research). Detailed information
is often needed to support the basic concepts and
expand understanding of the complex mechanisms of
-------
ground-water movement and solute/contaminant
transport.
The goals of any proposed ground-water monitoring
program should be clearly stated and understood before
making any decisions on the types and numbers of
wells needed, their locations, depth, constituents of
interest, and methods of collection, storage,
transportation, and analysis.
As each of these decisions is made, consideration must
be given to the costs involved in each step of the
monitoring program and how compromises in one step
may affect the integrity and outcome of the other steps.
For example, cost savings in well construction materials
may so severely limit the usefulness of a well that
another well may need to be constructed at the same
location for the reliable addition of a single chemical
parameter.
Monitoring Well Design Components
Monitoring well design and construction methods follow
production well design and construction techniques; a
monitoring well .however, is built specifically to give
access to the ground water so that a "representative"
sample of water can be withdrawn and analyzed. While
well efficiency and yield is important, the ability to
produce large amounts of water for supply purposes is
not the primary objective.
Emphasis is placed instead on constructing a well that
will provide easily obtainable ground-water samples
that will give reliable, meaningful information. Therefore,
materials and techniques used for constructing a
monitoring well must not materially alter the quality of
the water being sampled. An understanding of the
chemistry of suspected pollutants and the geologic
setting in which the monitoring well is to be constructed
play a major role in the drilling technique and well
construction materials used.
Several components needto be considered in monitoring
well design: location and number of wells, diameter,
casing and screen material, screen length and depth of
placement, sealing material, well development, and
well security. Often, discussion of one component will
impinge upon other components.
Location and Number
Locating monitoring wells spatially and vertically to
ensure that the ground-water flow regime of concern is
being monitored is obviously one of the most important
components in ground-water quality monitoring design.
Monitoring well locations (sites) and the numberof wells
inthe monitoring program are closely linked. The number
of wells and their location are principally determined by
the purpose of the monitoring program. In most
monitoring situations, the goal is to determine the effect
that some surface or near-surface activity has had on
nearby ground-water quality. Most dissolved constituents
will descend vertically through the unsaturated zone
beneath the area of activity and then, upon reaching the
saturated zone, move horizontally in the direction of
ground-water flow. Therefore, monitoring wells are
normally completed downgradient in the first permeable
water-bearing unit encountered. Consideration should
be given to natural (seasonal) fluctuations, which can
amount to several feet throughout the year and from
one year to the next, and artificial fluctuations brought
about largely by pumping, which can amount to several
tens of feet in only a few hours. Artificial fluctuations
also are caused by lagoon operation, which can cause
a rise or "mound" in the water table.
Preliminary boreholes and monitoring wells can be
constructed to collect and analyze geologic material
samples, to measure ground-water levels, and to collect
water-quality samples, all of which provide a guide to
thefuture placement of additionalwells. Accurate water-
level information must be obtained to determine if local
ground-waterf low paths and gradients differ significantly
from the regional appraisal.
The analysis of water-quality samples from the
preliminary wells can direct the placement of additional
wells. Such data are helpful in the vertical arrangement
of sampling points, especially for a contaminant that is
denser than water. Without some preliminary chemical
data, it is usually very difficult to determine the location
of the most contaminated zone.
Site geology, site hydrology, source characteristics,
contaminant characteristics, and the size of the area
under investigation all help determine where and how
many wells should be constructed. Certainly, the more
complicated the geology and hydrology, the more
complex the contaminant and source, and the largerthe
area being investigated, the greater the number of
monitoring wells that will be required.
Diameter
In the past, the diameter of a monitoring well was based
primarily on the size of the device (bailer, pump, etc.)
used to withdraw the water samples. This practice was
similar to that followed for water supply well design. For
example, a domestic water well is commonly 4 to 6
inches in diameter, which is of sufficient size to
accommodate a submersible pump capable of delivering
from 5 to 20 gallons per minute. Municipal, industrial,
-------
and irrigation wells have greater diameters to handle
larger pumps, and to increase the available screen
open area so the well can produce water efficiently.
This practice worked well in very permeable formations,
where an aquifer capable of furnishing large volumes of
water was present. However, unlike most water-supply
wells, monitoring wells are quite often completed in very
marginal water-producing zones. Pumping one or more
well volumes of water (the amount of water stored in the
well casing under nonpumping conditions) from a well
built in low-yielding materials (Gibb and others, 1981)
may present a serious problem if the well has a large
diameter.
Figure 1 -1 illustrates the amount of water in storage per
foot of casing for different well casing diameters. Well
casings with diameters of 2 and 6 inches will contain
0.16 and 1.47 gallons of water per foot of casing,
respectively. Purging four well volumes from a well
containing 10 feet of water would require removal of 6.4
gallons of water from a 2-inch well and 58.8 gallons of
waterf rom a 6-inch well. Under low-yielding conditions,
it can take considerable time to recover enough water
from the well to collect a sample (see Figure 1-2).
40
30
20
10
Will Diameter (Inch**)
Assumption*: K «1 x 10'* cm/Me, well screen * 10', 10' of water
above screen, 6' of water Instantaneously
removed
Figure 1-2. Time Required for Recovery After
Slug of Water Removed (from Rlnaldo-Lee, 1983)
2.6
2.0
I
!„
1.0
0.5
3456
Well Diameter (Inches)
Figure 1-1. Volume of Water Stored Per Foot of
Well Casing for Different Diameter Casings (from
Rlnaldo-Lee, 1983)
In addition, when hazardous constituents are present in
the ground water, the purged water must be properly
disposed. Therefore, the quantity of water pumped
from the well should be minimized for reasons of safety,
as well as disposal cost. Cost of well construction also
is a consideration. Wells less than 4 inches in diameter
are much less expensive than large diameter wells in
terms of both cost of materials and cost of drilling.
For these reasons and with the advent of a variety of
commercially available small-diameter pumps (less than
2 inches OD) capable of lifting water over 100 feet, 2-
inch ID wells have become the standard in monitoring
well technology.
Large diameter wells can be useful in situations where
monitoring may be followed by remedial actions involving
reclamation and treatment of the contaminated ground
water. In some instances, the "monitoring" well may
become a "supply" well to remove contaminated water
for treatment. Larger diameter wells also merit
consideration when monitoring is required at depths of
hundreds of feet and in situations where the additional
strength of large diametercasing is needed. Forsampling
at several depths beneath one location, several
monitoring wells have been nested in a single borehole
(Johnson, 1983). This type of technique will require
drilling a larger diameter hole to accommodate the
multiple well casings. Again, the use of smaller diameter
-------
casing provides advantages by allowing more wells to
be nested in the borehole, thus easing construction and
reducing drilling expenses.
Casing and Screen Material
The type of material used for a monitoring well can have
a distinct effect on the quality of the water sample to be
collected (Barcelona and others, 1985; Gillham and
others, 1983; and Miller, 1982). The materials of choice
should retain their structural integrity for the duration of
the monitoring program under actual subsurface
conditions. They should neither adsorb nor leach
chemical constituents that would bias the
representativeness of the samples collected.
Galvanized steel casing can impart iron, manganese,
zinc, and cadmium to many waters, and steel casing
may contribute iron and manganese to a sample. PVC
pipe has been shown to release and adsorb trace
amounts of various organic constituents to water after
prolonged exposure (Miller, 1982). PVC solvent cements
used to attach sections of PVC pipe also have been
shown to release significant quantities of organic
compounds.
Teflon0 and glass are among the most inert materials
considered for monitoring well construction. Glass,
however, is difficult and expensive to use under most
field conditions. Teflon0 also is very expensive; with
technological advances, TeflonR-coated casings and
screens may become available. Stainless steel also
offers desirable properties for monitoring, but it too is
expensive.
A reasoned strategy for ground-water monitoring must
consider the effects of contaminated water on well
construction materials as well. Unfortunately, there is
limited published information on the performance of
specific materials in varied hydrogeologic settings
(Pettyjohn and others, 1981). The following is a
preliminary ranking of commonly used materials exposed
to different solutions representing the principal soluble
species present in hazardous waste site investigations
(Barcelona and others, 1984). They are listed in order
of best to worst in terms of chemical resistance:
Teflon0
Stainless Steel 316
Stainless Steel 304
PVC Type 1
Lo-Carbon Steel
Galvanized Steel
Carbon Steel
Polyvinyl chloride (PVC Type I) is very chemically
resistant except to low molecular weight ketones,
aldehydes, and chlorinated solvents. Generally, as the
organic content of a solution increases, direct attack on
the polymer matrix or solvent absorption, adsorption, or
leaching may occur. This reaction, however, has not
been observed with Teflon0. Provided that sound
construction practices are followed, Teflon0 can be
expected to outperform all other casing and sampling
materials (Barcelona and others, 1984).
Stainless steels are the most chemically resistant of the
ferrous materials. Stainless steel, however, may be
sensitive to the chloride ion, which can cause pitting
corrosion, especially over long-term exposures under
acidic conditions. Given the similarity in price, workability,
and performance, the remaining ferrous materials (lo-
carbon, galvanized steel, and carbon) provide little
advantage over one another for casing/screen
construction.
Significant levels of organic components found in PVC
primers and adhesives (such as tetrahydrofuran,
methylethylketone, cyclohexanone, and
methylisobutylketone) were detected in well water
several months after well installation (Sosebee and
others, 1982). The presence of compounds such as
these can mask the presence of other similar volatile
compounds (Miller, 1982). Therefore, when using PVC
and other similar materials, such as ABS, polypropylene,
or polyethylene, for well construction, threaded joints
are the preferred means for connecting sections together.
In many situations, it may be possible to compromise
accuracy or precision for initial cost, depending on the
objectives of the monitoring program. For example, if
the contaminants of interest are already defined and
they do not include substances that might bleed or sorb,
it may be reasonable to use wells cased with a less
expensive material.
Wells constructed of less than optimum materials might
be used for sampling if identically fabricated wells are
constructed in uncontaminated parts of the monitored
aquifer to provide ground-water samples for use as
"blanks" (Pettyjohn and others, 1981). Such blanks,
however, may not adequately address problems of
adsorption on, or leaching from, the casing material
induced by contaminants in the ground water. It may be
feasible to use two or more kinds of casing materials in
the saturated zone and above the seasonal high water
table, such as Teflon0 or stainless steel, and use a
more appropriate material, such as PVC or galvanized
steel casing, above static water level.
Trying to save money by compromising on material
quality or suitability, however, may eventually increase
program cost by creating the need for reanalysis, or
-------
worse, monitoring well reconstruction. Each case
requires careful consideration and the analytical
laboratory should be fully aware of the construction
materials used.
Care also must be taken in preparing the casing and
well screen materials prior to installation. At a minimum,
materials should be washed with detergent and rinsed
thoroughly with clean water. Steam-cleaning and high
pressure, hot water cleaners provide excellent cleaning
of cutting oils and lubricants left on casings and screens
after their manufacture (particularly for metal casing
and screen materials). To ensure that these and other
sampling materials are protected from contamination
prior to placement down-hole, materials should be
covered (with plastic sheeting or other material), and
kept off the ground.
All wells should allow free entry of water. The water
produced should be as clear and silt-free as possible.
For drinking water supplies, sediment in the raw water
can create additional pumping and treatment costs and
lead to the general unpalatability of the water. With
monitoring wells, sediment-laden water can greatly
lengthen filtering time and create chemical interference
in sample analyses.
Commercially manufactured well screens are preferred
for monitoring wells so long as the screen slot size is
appropriate. Sawed or torch-cut casing may be
satisfactory in deposits where medium to coarse sand
or gravel predominate. In formations where fine sand,
silt, and clay predominate, sawed or torch-cut slots will
be too large to retain the aquifer materials, and the well
may clog or fill with sediment. The practice of sawing
slots in PVC pipe should be avoided in monitoring
situations where organic chemicals are of concern,
because this procedure exposesf resh surfaces of PVC,
increasing the possibility of releasing compound
ingredients or reaction products.
It may be helpful to have several slot-sized well screens
on site so that the proper manufactured screen and
slots can be placed in the hole after the aquifer materials
have been inspected. Gravel pack of a size compatible
with the selected screen slot size will further help retain
the finer fractions of material and allow free entry of
water into the well by creating a zone of higher
permeability around the well screen.
For natural-packed wells, where relatively
homogeneous, coarse materials predominate, a slot
size should be selected based on the effective size and
uniformity coefficient of the formation materials. The
effective size is equivalent to the sieve size that will
retain 90 percent (or passes 10 percent) of the formation
material; the uniformity coefficient is the ratio of the
sieve size that will retain 40 percent (or pass 60 percent)
of the formation material to the effective size (Alter and
others 1989). If an artificial pack is used, a uniform
gravel-pack size that is from three to five times the 50
percent retained size of the formation and a screen size
that will retain at least 90 percent of the pack material
should be selected (Walker, 1974). The gravel-pack
should be composed of clean, uniform quartz sand.
The gravel-pack should be placed carefully to avoid
bridging in the hole and to allow uniform settling around
the screen. A tremie pipe can be used to guide the sand
to the bottom of the hole and around the screen. The
pipe should be lifted slowly as the annulus between the
screen and borehole as the borehole fills. If the depth of
water standing in the annulus is not great, the sand can
be simply poured from the surface. The volume of sand
required to fill the annulus to the desired depth (usually
about 1 foot above the top of the screen) should be
calculated. Field measurements should be taken to
confirm that the pack has reached this level before
backfilling or sealing procedures start.
Screen Length and Depth of Placement
The length of screen and the depth at which it is placed
depend, to a large degree, on the behavior of the
contaminant as it moves through the unsaturated and
saturated zones, and on the goal of the monitoring
program. When monitoring an aquifer used as a water
supply, the entire thickness of the water-bearing
formation could be screened (just as a production well
might be). In regional aquifer studies, production wells
commonly are used for sampling. Such samples would
provide water integrated overthe entire thickness of the
water-bearing zone(s), and would be similar in quality to
what would be found in a drinking-water supply.
When specific depth intervals must be sampled at one
location, vertical nesting of wells is common. This
technique is often necessary when the saturated zone
istoothickto adequately monitor with one long-screened
section (which would dilute the collected sample). Since
contaminants tend to stratify within the saturated zone,
collecting a sample integrated over a thick zone will
provide little or no information on the depth and
concentration that a contaminant may have reached.
Furthermore, nested wells provide information on the
water level or potential that exists at each well screen.
These data are essential to an understanding of the
vertical component of flow.
Screen lengths of 1 to 2 feet are common in detailed
plume geometry investigations. Thick aquifers would
require that several wells be completed at different
depth intervals. In such situations (and depending on
-------
the magnitude of the aquifer saturated thickness), screen
lengths of no more than 5 to 10 feet should be used.
Monitoring wells can be constructed in separate holes
placed closely together or in one larger diameter hole,
as shown in Figure 1-3. Vertical movement of
contaminants in the well bore before and after well
completion may be difficult to prevent since it is difficult
to seal several wells in one hole.Thus, multiple holes
may need to be drilled to ensure well integrity. Specially
constructed installations have been developed to sample
a large number of points vertically over short intervals
(Morrison and Brewer, 1981; Pickens, 1981; and
Torstensson, 1984; Figures 1-4 and 1-5).
-nu>s«i(Tvp.i-
I
Figure 1-3. Typical Multlwell Installations (from
Johnson, 1983)
In other situations, only the first water-bearing zone
encountered will require monitoring (for example, when
monitoring near a potential contaminant source in a
relatively impermeable glacial till). The "aquifer" or zone
of interest in such an instance may be only several
inches to a few feet thick. Screen length should be
limited to 1 to 2 feet in these cases to minimize siltation
problems from surrounding fine-grained materials and
possible dilution effects from water contributed by
uncontaminated zones.
Because of the chemical reactions that occur when
ground water contacts the atmosphere, particularly
when dealing with volatile compounds, the screened
section should not be aerated. Generally, well depth
should assure that the screened section is always fully
submerged. The design should consider fluctuations in
the elevation of the top of the saturated zone caused by
seasonal variations or human-induced changes.
Onuni Surt**
-rvcf*.
Figure 1-4. Schematic Diagram of a Multilevel
Sampling Device (from Pickens and others, 1981)
Figure 1-5. Single (a) and Multiple (b) Installation
Configurations for an Alr-Llft Sampler (from
Morrison and Brewer, 1981)
Monitoring for contaminants with densities different
than water demands special attention. In particular, low
density organic compounds, such as gasoline, will float
on the ground-water surf ace (Gillham and others. 1983).
Monitoring wells constructed to detect floating
contaminants should contain screens that extend above
the zone of saturation so that these lighter substances
can enter the well. The screen length and position must
-------
accommodate the magnitude and depth of variations in
water-table elevation. However, the thickness of floating
products in the well does not necessarily indicate the
thickness of the product in the aquifer.
Sealing Materials and Procedures
It is critical that the screened part of each monitoring
well access ground water from a specific depth interval.
Vertical movement of ground water in the vicinity of the
well can greatly influence sample quality (Keith and
others, 1982). Rainwatercan infiltrate backfill, potentially
diluting or contaminating samples; vertical seepage of
leachate along the well casing will also produce
unrepresentative samples (particularly important in
multilevel installations such as in Figures 1-3,1-4, and
1 -5). Even more importantly, the creation of a conduit in
the annulus of the monitoring well that could contribute
to or hasten the spread of contamination is to be strictly
avoided. Several methods have been employed
successfully to isolate contaminated zones during the
drilling process (Burkland and Raber, 1983; Perry and
Hart. 1985).
Monitoring wells are usually sealed with neat cement
grout, dry bentonite (powdered, granulated, or
pelletized). or bentonite slurry. Well seals usually are
installed at two places within the annulus: one just
above the screened interval and the other at the ground
surface to inhibit downward leakage of surface
contaminants.
Bentonite traditionally has been considered to provide
a much better seal than cement. However, recent
investigations on the use of clay liners for hazardous
waste disposal have shown that some organic
compounds migrate through bentonite with little or no
attenuation (Brown, and others, 1983). Therefore,
cement may offer some benefits over bentonite.
Bentonite most often is used as a down-hole seal to
prevent vertical migration withinthewell annulus. When
bentonite must be placed below the water table (or
where water has risen in the borehole), it is recommended
that a bentonite slurry be tremied down the annulus to
fill the hole from the bottom upward. In collapsible
material conditions, where the borehole has collapsed
to a point just above the water table, dry bentonite
(granulated or pelletized works best) can be poured
down the hole.
Bentonite clay has appreciable ion-exchange capacity,
which may interfere with the chemistry of collected
samples when the seal is adjacent to the screen or well
intake. When improperly placed, cement grout has
been known to seriously affect the pH of sampled water.
Therefore, special attention and care should be exercised
during placement of a down-hole seal. Approximately 1
foot (at a minimum) of gravel-pack or naturally collapsed
material should extend above the top of the well screen
to ensure that the sealing materials do not migrate
downward into the well screen. If the sealing material is
too watery before being placed down the hole, sealing
materials may settle or migrate into the gravel-pack or
screened area, and the fine materials in the seal may
penetrate the natural or artificial pack.
While a neat cement (sand and cement, no gravel) grout
is often recommended, especially for surface sealing,
shrinkage and cracking of the cement upon curing and
weathering can create an improperseal. Shrink-resistant
cement (such as Type K Expansive Cement) and
mixturesof small amounts of bentonite with neat cement
have been used successfully to help prevent cracking.
Development
Development is a facet of monitoring well installation
that often is overlooked. During the drilling process,
fine-grained materials smearonthe sides of the borehole,
forming a mud "cake" that reduces the hydraulic
conductivity of the materials opposite the screened part
of the well. To facilitate entry of water into the monitoring
well (a particularly important factor for low-yielding
geologic materials), this mud cake must be broken
down and the fine-grained materials removed from the
well or well bore. Development also removes fluids,
primarily water, which are introduced to the water-
bearing formations during the drilling process.
Additionally, monitoring wells must be developed to
provide water free of suspended solids for sampling.
When sampling for metal ions and other inorganic
constituents, water samples must be filtered and
preserved at the well site at the time of sample collection.
Improperly developed monitoring wells will produce
samples containing suspended sediments that will both
bias the chemical analysis of the collected samples and
frequently cause clogging of the field filtering
mechanisms.
The time and money spent for this important procedure
will expedite sample filtration and result in samples
more representative of water contained in the formation
being monitored. The time saved in field filtration alone
will more than offset the cost of development.
Successful development methods include bailing,
surging, and flushing with airorwater. The basic principle
behind each method is to create reversals of flow in and
out of the well (and/or borehole), which tend to break
down the mud cake and draw the finer materials into the
hole for removal. This process also aids in removing the
finer fraction of materials in proximity to the borehole,
-------
leaving behind a
materials.
"natural" pack of coarser-grained
ComprMMd AJr
Years ago, small-diameter well development most
commonly was achieved through use of a bailer. The
bailer was about the only "instrument" that had been
developed for use in such wells. Rapidly dropping and
retrieving the bailer in and out of the water caused a
back-and-forth action of water in the well, moving some
of the more loosely bound fine-grained materials into
the well where they could be removed.
Depending on the depth of water in the well, the length
of the well screen, and the volume of water the bailer
could displace, this method was not always very efficient.
"Surge blocks," which could fit inside 2-inch diameter
wells, provided some improvement over bailing
techniques. Such devices are simply plungers that,
when moved vigorously up and down, transfer that
energy to an in-and-out action on the water nearthe well
screen. Surge blocks have the potential to move larger
quantities of water with higher velocities, but they pose
some risk to the well casing and screen if the surge
block fits too tightly or if the up-and-down action becomes
too vigorous. Improved surge block design has been
the subject of some recent investigation (Schalla and
Landick, 1985).
In more productive aquifers, "overpumping" has been a
popular method for well development. With this method,
a pump is alternately turned on (usually at a slightly
higher rate than the well can sustain) and off to simulate
a surging action in the well. A problem with this method
is that overpumping does not create as pronounced an
outward movement of water as does surging.
Overpumping may tend to bridge the fine and coarse
materials, limiting the movement of the fine materials
into the well and thereby limiting the effectiveness of the
method.
Pumping with air also has been used effectively (Figure
1-6). Better development has been accomplished by
attaching differently shaped devices to the end of an
airline to force the air out into the formation. Figure 1 -7
shows an example. Such a device causes a much more
vigorous action on the movement of material in proximity
to the well screen while also pushing water to the
ground surface.
Air development techniques may expose field crews to
hazardous constituents when highly contaminated
ground water is present. The technique also may cause
chemical reactions with species present in the ground
water, especially volatile organic compounds. Care
also must be taken to filter the injected air to prevent
contamination of the well environment with oil and other
lubricants present in the compressor and airlines.
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Figure 1-6. Well Developments with Compressed
Air
Continuous Slot Well Screen
Figure 1-7. The Effects of High-Velocity Jetting
Used for Well Development through Openings In
a Continuous-Slot Well Screen
8
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Development procedures for monitoring wells in
relatively unproductive geologic materials are somewhat
limited. Due to the low hydraulic conductivity ot the
materials, surging of water in and out of the well casing
is extremely difficult. Also, when the well is pumped, the
entry rate of the water is inadequate to effectively
remove fines from the well bore and the gravel-pack
material outside the well screen.
Where an open borehole can be sustained in this type
of geologic setting, clean water can be circulated down
the well casing, out through the screen, and back up the
borehole (Figure 1-8). Relatively high water velocities
can be maintained and the mud cake from the borehole
wall can be broken down effectively and removed.
Because of the low hydraulic conductivity of the geologic
materials outside the well, only a small amount of water
will penetrate the formation being monitored. This
development procedure can be done before and after
placement of a gravel-pack but must be conducted
before a well has been sealed. Afterthe gravel-pack has
been installed, water should not be circulated too quickly
or the gravel-pack will be lifted out of the borehole.
Immediately following development, the well should be
sealed, backfilled, and pumped for a short period to
stabilize the formation around the outside of the screen
and to ensure that the well will produce fairly clear
water.
Figure 1-8. Well Development by Back-Flushing
with Water
Security
For most monitoring well installations, some precautions
must be exercised to protect the surface portions of the
well from damage. In many instances, inadvertent
vehicular accidents do occur; also, monitoring well
installations seem particularly vulnerable to grass
mowers. Vandalism is often a major concern, from
spontaneous "hunters" looking for a likely target to
premeditated destruction of property associated with
an unpopular operation. Several simple solutions can
be employed to help minimize the damage due to
accidental collisions. However, outwitting the determined
vandal may be an impossible undertaking and certainly
an expensive one.
The basic problem in maintaining the physical condition
of any monitoring well is anticipating the hazards that
might befall that particular installation. Some situations
may call for making the well highly visible whereas
others may require keeping the well inconspicuous.
Where the most likely problem is one of vehicular
contact, be it mowers, construction traffic, or othertypes
of two-, three-, or four-wheeled traffic, the first thing that
can be done is to make the top of the well easy to see.
It should extend far enough above ground to be visible
above grass, weeds, or small shrubs. If that is not
practical, use a "flag" that extends above the well
casing. A flag is also helpful for periods when leaves or
snow have buried low-lying objects.
The well casing should also be painted a bright color
(orange and yellow are the most visible). This not only
makes the well more visible but also protects metal
casing material from rusting. Care should be taken to
prevent paint from getting inside the well casing or in
threaded fittings that may contact sampling equipment.
The owners/operators of the site being monitored should
also know the location of each installation. They should
receive maps clearly and exactly indicating the position
of the wells, and their employees should be informed of
the importance of those installations, the cost associated
with them, and the difficulty involved in replacement.
The segment of the well that extends above the ground
also can be reinforced, particularly if the well is
constructed of PVC or TeflonR. The well could be
constructed such that only the portion of the well above
the water table is metal. In this manner, the integrity of
the sample is maintained as ground water contacts only
inert material, and the physical condition of the well is
maintained as the upper metal portion is better able to
withstand impact.
There are two arguments to consider when constructing
-------
a well in this manner. The arguments focus on the weak
point in the well construction: at or near the juncture of
the metal and nonmetal casings. One argument suggests
that a longer section of metal casing is superior because
its additional length in the ground provides more strength.
Thus, a break is less likely to occur (althoughthe casing
is likely to be bent). The other argument suggests that
should a break in the casing occur, a shorter length of
metal casing is superior because a break nearer to
ground surface is easier to repair. Each argument has
its merits; only experience with site conditions is likely
to produce the best solution.
The use of "well protectors" is another popular solution
that involves the use of a larger diameter steel casing
placed around the monitoring well at the ground surface
and extending several feet below ground (Figure 1-9).
The protectors are usually seated in the cement surface
seal to a depth below the frost line.
Commonly, well protectors are equipped with a locking
cap, which ensures against tampering with the inside of
the well. Dropping objects down the well may clog the
well screen orprohibit the sampling device from reaching
water, and the quality of the ground water may be
altered, particularly where small quantities (perhaps
drops) of an organic liquid may be sufficient to completely
contaminate the well.
Monitoring Well
_ Well Protector with
LockaWe Cao
Figure 1-9. Typical Well Protector Installation
Problems associated with vandalism run from simple
curiosity to outright wanton destruction. Obviously, sites
within secured, fenced areas are less likely to be
vandalized. However, there is probably no sure way to
deter the determined vandal, short of posting a 24-hour
guard. In such situations, well protectors are a must.
The wells should be kept as inconspicuous as possible.
However, the benefits of "hiding" monitoring wells must
be weighed against the costs of delays in finding them
for sampling and the potential costs for repairs or
maintenance on untried security designs.
In some situations, it might be a good policy to notify the
public of the need for the monitoring wells. Properly
asserting that each well serves an environmental
monitoring purpose and that the wells have been
constructed to ensure public well-being may create a
civic conscience that would help to minimize vandalism.
As with all the previously mentioned monitoring well
components, no single solution will best meet every
monitoring situation. Knowledge of the social, political,
and economic conditions of the geographic area and
circumstances surrounding the need for ground-water
monitoring will dictate, to a large degree, the type of well
protection needed.
Monitoring Well Drilling Methods
As might be expected, different drilling techniques can
influence the quality of a ground-water sample. This
applies to the drilling method employed (e.g., augered,
driven, or rotary), as well as the driller. There is no
substitute for a conscientious driller willing to take the
extra time and care necessary to complete a good
monitoring well installation.
Among the criteria used to select an appropriate drilling
method are the following factors, listed in order of
importance:
1. Hydrologic information
a. type of formation
b. depth of drilling
c. depth of desired screen setting below the top of
the zone of saturation
2. Types of pollutants expected
3. Location of drilling site, i.e., accessibility
4. Design of monitoring well
5. Availability of drilling equipment
Table 1-1 summarizes several different drilling methods,
and their advantages and disadvantages when used for
monitoring well construction. Several excellent
publications are referenced for detailed discussions
(Campbell and Lehr, 1973; Fenn and others. 1977;
10
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Method
Drillina Principle
Advantages
Disadvantages
Drive Point
1.25 ID 2 inch 10 cuing with
pointed screen mechanically
driven to depth.
Auger. Hollow-
and Solid-stem
Jetting
Cable-tool
(Percussion)
Successive 5-toot flights of spiral-
shaped drDI stem are rotated into
the ground to create a hole.
Cuttings are brought to the suriace
by the turning action of the auger.
Washing action ol water forced out
of the bottom of the drill rod clears
hole to allow penetration. Cuttings
brought to surface by water flowing
up the outside of the drill rod.
Hole created by dropping a heavy
'string* of dril tools into well bore.
crushing materials at bottom.
Cuttings are removed occasionally
by bailer. Generally, casing is
driven jutt ahead of the the bottom
of the hole: a hole greater than 6
inches in dlamerter is usually
made.
Inexpensive.
Easy 10 install, by hand If
necessary.
Water sample* can be collected as
driving proceeds.
Depending on overburden, a good
seal between casing and formation
can be achieved.
Inexpensive.
Fairly simple operation. Small rigs
can get to ditficult-to-reach areas.
Quick set-up time.
Can quickly construct shallow wells
In firm, noncavey materials.
No drilling fluid required.
Die of hollow-stem augers greatly
facilitate* collection of split-spoon
samples.
Small-diameter wells can be built
inside hollow-stem flights when
geologic materis! are cavey.
Inexpensive. Driller often not
needed lor shallow holes.
In firm, noncavey deposits where
hole will stand open, weR
construction fairly simple.
Can be used in rock formations as
well as unconaoCdated formations.
Fairly accurate logs can be
prepared from cuttings if collected
often enough.
Driving a casing ahead of hole •
minimizes cross-contamination by
vertical leakage of formation
waters.
Core samples can be obtained
easily.
Difficult to sample from smaller diameter
drive points if water level Is below suction
lift. Bailing possible.
No formation samples can be collected.
Limited to fairly soft materials. Hard to
penetrate compact, gravelly materials.
Hard to develop. Screen may become
clogged if thick days are penetrated.
PVC and Teflon casing and screen are not
strong enough to be driven. Must use
metal construction materials which may
Influence some water quality determina-
tions.
Depth of penetration limited, especially in
cavedy materials. Maximum depths 150
feet.
Cannot be used in rock or well-cemented
formations. Difficult to drill in cobbles/
boulders.
Log of well is difficult to interpret without
collection of split spoons due to the lag
time for cuttrings to reach ground suriace.
Vertical leakage of water through borehole
during drifing Is likely ID occur.
Solid-tiem limited to fine grained,
unconsolidated materials that will not
collapse when unsupported.
With hollow-stem flights, heaving materials
can present a problem. May need to add
water down auger to control heaving or
wash materials from auger before
completing well.
Somewhat slow, especially with increasing
depth.
Extremely difficult to use in very coarse
materials, i.e.. cobbles/boulders.
A water supply Is needed that is under
enough pressure to penetrate the geologic
materials present
Difficult to interpret sequence of geologic
materiil from cuttings.
Maximum depth 1 SO feet, depending on
geology and water pressure capabilities.
Requires an experienced driller.
Heavy steel drive pipe used to keep hole
open and drilling *toots' can Emit
accessibility.
Cannot run some geophysical logs due to
presence of drive pipe.
Relatively slow drilling method.
Table 1-1. Advantages and Disadvantages of Selected Drilling Methods for Monitoring Well
Construction
11
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Method
Drilling Principle
Advantaaes
Disadvantage*
Hydraulic Rotary
Reverse Rotary
Air Rotary
Rotating bit breads formation;
cutting* are brought to the
surface by * circulating fluid
(mud). Mud It forced down trw bit.
and up in* annulu* between the dril
stem and note wal. Cuttings arc
removed by settling In a "mud pit" at
the ground surface and the mud is
circulated back down the dril stem.
Similar to Hydraulic Rotary method
except the drilling fluid is circulated
down the borehole outside the drill
stem and Is pumped up the inside. Just
the reverse ol the normal rotary
method. Water is used as the drilling
fluid, rather than a mud, and the hole
Is kept open by the hydrostatic
pressure of the water standing In the
borehole.
Very similar to Hydraulic Roatary. the
main difference being that air is used
as (he primary drilling fluid as opposed
to mud or water.
Air-Percussion
Rotary or Downhole-
Hammer
Air Rotary with a reciprocating hammer
connected to the bit to fracture rock.
Drilling is fairly quick in all types of
geologic materials.
Borehole will stay open from
formation of a mud wall on sides of
borehole by the circulating drilling
mud. Eases geophysical logging
and well construction.
Geologic core* can be collected.
Virtually unlimited depths possible.
Creates a very "clean* hole, not
dirtied with drilling mud.
Can be used in all geolotc
formations.
Very deep penetrations possible.
Split-spoon sampling possible.
Can be used in all geologic
formations; most successful In
highly fractured environments.
Useful at any depth.
Fairly quick.
Drilling mud or water not required.
Very fast penetrations.
Useful in an geolbogic
formations.
Only small amounts of water
needed for dust and bit
temperature control.
Cross contamination potential
can be reduced by driving
casing.
Expensive, requires experienced drifter
and fair amount of peripheral equipment
Completed well may be difficult to
develop, especially small-diameter wells.
because of mud wal on borehole.
Geologic logging by visual Inspection of
cuttings Is lair due to personce ol drling
mud. Thin beds of sand, gravel, or day
may be missed.
Presence of drilling mud can contaminate
water samples, •specially the organic,
biodegradable muds.
Circulation of drilling fluid through a
contaminated zone can create a hazard at
the ground surface with the mud pit and
cross-contaminate dean zones during
circulation.
A large water supply Is needed to
maintain hydrostatic pressure In deep
holes and when highly conductive
formations are encountered.
Expensive-experienced driller and much
peripheral equipment required.
Hole diameters are usually large.
commonly IB inches or greater.
Cross-contamination from circulating
water likely.
Geologic samples brought to surface are
generally poor, circulating water will
•wash* finer materials from sample.
Relatively expensive.
Cross-contamination from vertical
communication possible.
Air will be mixed with water In the hole
and that which Is blown from the hole.
potentially creating unwanted reaction*
with contaminants; may affect 'represen-
tative* samples.
Cutting* and water blown from the hole
can pose a hazard to crew and
surrounding environment If toxic
compounds encountered.
Organic foam additives to aid cutting*
removal may contaminate sample*.
Relatively expensive.
As with most hydraulic rotary methods.
the rig is fairfy heavy, limiting accessibil-
ity.
Vertical mixing of water and air create*
cross-contamination potential.
Hazard posed to surface environment if
toxic compound* encountered.
Organic foam additive* for cutting*'
removal may contaminate samples.
Table 1-1. Continued
12
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Johnson, Inc.. 1972; and Scalf and others, 1981). The
table also gives a concept of the advantages and
disadvantages that need to be considered when
choosing a drilling technique for different site and
monitoring situations (see, also, Lewis, 1982; Luhdorff
and Scalmanini, 1982; Minning, 1982; and Voytek,
1983).
Hollow-stem augering is one of the most desirable
drilling methods for constructing monitoring wells. No
drilling fluids are used and disturbance of the geologic
materials penetrated is minimal. Depths are usually
limited to no more than 150 feet. Typically, auger rigs
are not used when consolidated rock must be penetrated.
In formations where the borehole will not stand open,
the monitoring well can be constructed inside the hollow-
stem auger prior to its removal from the hole. Generally,
this limits the diameter of the well that can be built to 4
inches. The hollow-stem has an added advantage in
offering the ability to collect continuous in situ geologic
samples without removal of the auger sections.
The solid-stem auger is most useful in fine-grained,
unconsolidated materials that will not collapse when
unsupported. The method is similar to the hollow-stem
except that the auger flights must be removed from the
hole to allow the insertion of the well casing and screen.
Cores cannot be collected when using a solid-stem.
Therefore, geologic sampling must rely on cuttings that
come to the surface, which is an unreliable method
because the depth from which the cuttings are derived
is not precisely known.
Cable-tool drilling is one of the oldest methods used in
the water well industry. Even though the rate of
penetration is rather slow, this method offers many
advantages for monitoring well construction. With the
cable-tool, excellent formation samples can be collected
and the presence of thin permeable zones can be
detected. As drilling progresses, a casing is normally
driven and this provides an ideal temporary casing
within which to construct the monitoring well.
In air-rotary drilling, air is forced down the drill stem and
back up the borehole to remove the cuttings. This
technique has been found to be particularly well suited
to drilling in fractured rock. If the monitoring is intended
for organic compounds, the air must be filtered to
ensure that oil from the air compressor is not introduced
into the formation to be monitored. Air-rotary should not
be used in highly contaminated environments because
the water and cuttings blown out of the hole are difficult
to control and can pose a hazard to the drill crew and
observers. Where volatile compounds are of interest,
air-rotary can volatilize them and cause water samples
to be unrepresentative of in situ conditions. The use of
foam additives to aid cuttings' removal presents the
opportunity for organic contamination of the monitoring
well.
Air-rotary with percussion hammer increases the
effectiveness of air-rotary for materials likely to cave
and highly creviced formations. Addition of the
percussion hammer gives air-rotary the ability to drive
casing, which reduces the loss of air circulation in
fractured rock and aids in maintaining an open hole in
soft formations. The capability to construct monitoring
wells inside the driven casing, prior to its being pulled,
adds to the appeal of air-percussion. However, the
problems with contamination and crew safety must be
considered.
Reverse circulation rotary drilling has limited application
for monitoring well construction. Reverse circulation
rotary requires that large quantities of water be circulated
down the borehole and up the drill stem to remove
cuttings. If permeable formations are encountered,
significant quantities of watercan move into the formation
to be monitored, thus altering the quality of the water to
be sampled.
Hydraulic or "mud" rotary is probably the most popular
method used in the water well industry. Hydraulic rotary,
however, presents some disadvantages for monitoring
well construction. In hydraulic rotary technique, a drilling
mud (usually bentonite) is circulated down the drill stem
and up the borehole to remove cuttings. The mud
creates a wall on the side of the borehole that must be
removed from the screened area by development
procedures. With small diameterwells, the drilling mud
is not always completely removed. The ion-exchange
potential of most drilling muds is high and may effectively
reduce the concentration of trace metals in waterentering
the well. In addition, the use of biodegradable, organic
drilling muds, rather than bentonite, can introduce
organic components to water sampled from the well.
Most ground-water monitoring wells will be completed
in glacial or unconsolidated materials, and generally will
be less than 75 feet in depth. In these applications,
hollow-stem augering usually will be the method of
choice. Solid-stem auger, cable-tool, and air-percussion
also offer advantages depending on the geology and
contaminant of interest.
Geologic Samples
Permit applications for disposal of waste materials often
require that geologic samples be collected at the disposal
site. Investigations of ground-water movement and
contaminant transport also should include the collection
of geologic samples for physical inspection and testing.
13
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Stratigraphic samples are best collected during
monitoring well drilling.
Samples can be collected continuously, at each change
in Stratigraphic unit, or, in homogeneous materials, at
regular intervals. These samples may later be classified,
tested, and analyzed for physical properties, such as
particle-size distribution, textural classification, and
hydraulic conductivity, and for chemical analyses, such
as ion-exchange capacity, chemical composition, and
specific parameter teachability.
Probably the most common method of material sampling
is a "split-spoon" sampler. This device consists of a
hollow cylinder, 2 inches in diameter, that is 12 or 18
inches long, and split in half lengthwise. The halves are
held together with threaded couplings at each end; the
top end attaches to the drill rod, and the bottom end is
a drive shoe (Figure 1-1 Oa). The sampler is lowered to
the bottom of the hole and driven ahead of the hole with
a weighted "hammer" striking an anvil at the upper end
of the drill rod. The sample is forced up the inside of the
hollow tube and is held in place with a basket trap or flap
valve. The trap or valve allows the sample to enter the
tube but not exit, although retention of noncohesive,
sandy material in the tube is often difficult. After the
sampler is withdrawn from the hole, the sample is
removed by unscrewing the couplings and separating
the collection tube.
Another common sampler is the thin wall tube or "Shelby"
tube. These tubes are usually 2 to 5-1/2 inches in
diameter and about 24 inches long. The cutting edge of
the tube is sharpened and the upper end is attached to
a coupling head by means of cap screws or a retaining
pin (Figure 1-1 Ob). A Shelby tube has a minimum ratio
of wall area to sample area and creates the least
disturbance to the sample of any drive-type sampler in
current use (for hydraulic conductivity tests, minimal
disturbance is critical). After retraction, the tube is
disconnected from the head and the sample is forced
from the tube with a jack or press. If sample preservation
is a major concern, the tube can be sealed and shipped
to the laboratory.
Apart from permit requirements, material samples are
very helpful for deciding at what depth to complete a
monitoring well. Unexpected changes encountered
during drilling can alter preconceived ideas concerning
the local ground-waterflow regime. In many instances,
the driller will be able to detect a variation in the
formation by a change in penetration rate, sound, or
leel" of the drilling rig. However, due to the lag time for
cuttings to come to the surface and the amount of
mixing the cuttings may undergo as they come up the
a.
b.
Figure 1-10. Cross-Sectional Views of (a) Split
Spoon and (b) Shelby Tube Samplers (from
Mobile Drilling Co., 1972)
borehole, the only way to exactly determine the character
of the subsurface is to stop drilling and collect a sample.
Case History
Several different types of monitoring wells were
constructed during the investigation of a volatile organic
contaminant plume in northern Illinois (Wehrmann,
1984). A brief summary of the types of wells employed
and the reasons for their use helps illustrate how an
actual ground-water quality monitoring problem was
approached.
During the final weeks of a 1 -year study of nitrate in
ground water in north-central Illinois, the presence of
several organic compounds was detected in the drinking
water of all five homes sampled within a large rural
residential subdivision. The principal compound.
trichloroethylene (TCE), was present in concentrations
ranging from 50 to 1,000 micrograms per liter (u.g/L). All
the homes in the subdivision used private wells, 65 to 75
feet deep, that tapped a surficial sand and gravel
deposit. Figure 1-11 shows a geologic cross section of
the study area.
Two immediate concerns needed to be addressed.
First, how many other water wells were affected and.
second, what was the contaminant source? Early
thoughts connected the TCE to the contamination
potential of the large number of septic systems in the
subdivision. Earlier work (Wehrmann, 1983) had
established a south-southwest direction of ground-
14
-------
BOD-
'S
I
I I
« 700-1
,ww
•900
Roscoe
Upland*
High Terrace
Low Rock River Floodplain tow Terrace
Terrace '
High Terrace T
WW
WW
-800
600-
500
Cateita-Pianeville Dolomite
Glen wood St. Peter Sandstone
WW • Water Well
T - Tollway boring
U 700
Scale (miles)
K":'.': A Outwash sand and gravel ^;..'•.
i- • • i V*'*
i |':V-'XI Lacustrine sands, silt and clay^'.' ~'~":..'. ~."~.~~.\'. •'j*-
[Jl Ht] Organic materials (or) buried soil >Ol'Y::-"f '.•'^r^
-600
ianner Formation ••;'
-500
Figure 1-11. East-West Cross Section Across Rock River Valley at Roscoe (from Berg and others,
1981)
water flow beneath the subdivision. Because the area
upgradient of the subdivision was primarily farmland,
several monitoring wells were placed in that area to help
confirm or deny the possibility that the septic systems
were the source of the TCE.
Five "temporary" monitoring wells were constructed
upgradient of the affected subdivision. Original plans
called for driving a 2-inch diameter sandpoint to depths
from 40 to 70 feet. Water samples would be collected at
10-foot intervals as the point was driven. Once 70 feet
was reached, the sandpoint would be pulled, the hole
properly abandoned, and the point driven at a new
sampling location. The first hole was to be placed north
(upgradient) of a domestic well found to be highly
contaminated, and additional holes were to be placed
successively in an upgradient direction across the field.
In this manner, ground-water samples could be collected
quickly at many depths and locations, the we!! materials
recovered, and the field left relatively undisturbed.
Once drilling commenced, however, it became clear
that driving sandpoints into the coarse sand and gravel
was not possible. Consequently, an air-percussion rig
was brought on site and a new approach was established.
A 4-inch diameter screen, 2 feet long and with a drive
shoe, was welded to a 4-inch diameter steel casing.
This assembly was driven by air hammer to the desired
sampling depth. The bottom of the drive shoe, being
open, forced the penetrated geologic materials into the
casing and screen. These materials were then removed
by air rotary once the desired depth was reached. All
well construction materials were steam-cleaned priorto
use to avoid cross-contamination. Figure 1-12 shows
the locations of the temporary well sites and the analytical
results for TCE from samples bailed at depths of 40 and
50 feet.
The temporary sampling program revealed that the
contaminant source was outside of the subdivision. Due
to the construction and sampling methods employed for
these wells, emphasis was not placed on the quantitative
aspects of the sampling results; however, important
qualitative conclusions were made. The temporary wells
confirmed the presence of VOCs directly upgradient of
the subdivision and provided information for the
15
-------
HOUSE WELL
SAMPLED
SAME WEEK.
2178 ppb TCE *> 65'
MOOSE HAVENl
SUBDIVISION I
Figure 1-12. Locations and TCE Concentrations for Temporary Monitoring Wells at Roscoe, Illinois
(from Wehrmann, 1984)
subsequent location and depth of nine permanent
monitoring wells.
Due to the problems associated with organic compound
teachability and adsorptionfrom PVC casing and screen.
flush-threaded stainless steel casing and screen, 2
inches in diameter, were used for the permanent
monitoring wells. The screens were 2 feet long with
0.01-inch wire-wound slot openings. All materials
associated with the monitoring well construction,
including the drill rig, were steam-cleaned prior to the
commencement of drilling to avoid organic contamination
fromcutting oils and grease. Priorto use, the casing and
screen materials were kept off site in a covered, protected
area. To ensure that the sandy materials would not
collapse after drilling, casing lengths and the screen
were joined aboveground and placed inside of the
augers before the auger flights were pulled out of the
16
-------
hole. The sand and gravel below the water table
collapsed around the screen and casing as the augers
were removed. To prevent vertical movement of water
down along the casing, about 3 feet of a wet bentonile/
cement mixture was placed in the annulus just above
the water table. Cuttings (principally clean, fine to
medium sand) were backfilled above the bentonite/
cement seal to within 4 feet of land surface. Another
bentonite/cement mixture was placed to form a seal at
ground surface, further preventing movement of water
down along the well casing. A 4-inch diameter steel
protective cover with locking cap was placed over the
casing and into the surface seal to protect against
vandalism.
The nine wells were drilled at four locations with paired
wells at three sites and a nest of three wells at one site
(Figure 1 -13). The locations were basedonthe analytical
results of the samples taken from the temporary wells
and basic knowledge of the ground-water flow direction.
-f<\ MONONEGAM
<-\\ VCQUNTBY ESTA:
Figure 1-13. Location of Monitoring Well Nests and Cross Section A-A' at Roscoe, Illinois (from
Wehrmann, 1984)
17
-------
Locations were numbered as nests 1 through 4 in order
of their construction. Nest 1, located immediately north
of the affected subdivision, consists of three wells
completed at depths of approximately 60, 70, and 80
feet below ground surface. Nest 2 consists of two wells,
one 50 feet deep, and the other 60 feet deep. Nest 3
consists of two wells 40 and 55 feet deep, while nest 4
consists of two wells 50 and 60 feet deep.
Prior to completing the monitoring wells, it was felt an
additional well, 100 feet deep and adjacent to nest 1,
was needed to further define the vertical extent of the
contaminant plume. Because the hollow-stem auger rig
was no longer available, arrangements were made to
use a cable-tool rig. The well was constructed over a
period of 2 days, which was somewhat slower than any
of the other methods previously used (but typical of
cable-tool speeds). A 6-inch casing was driven several
feet, a bit was used to break up the materials inside the
casing, and then the materials were removed from the
casing with a dart-valve bailer. This procedure was
repeated until 100 feet was reached; then the well
casing and screen were screwed together and lowered
down the hole. The 6-inch casing was then pulled back,
which allowed the hole to collapse about the well, which
was constructed of stainless steel exactly like the other
nine monitoring wells. All drilling equipment and well
construction materials were steam-cleaned prior to
use.
Appraisal of the sampling results of the monitoring wells
and the domestic wells in the area produced the pictorial
representations shown in Figures 1-14 and 1-15. Figure
1-14 conceptually illustrates a downgradient cross
section of the TCE plume in the vicinity of monitoring
nests 2,3, and 4. Figure 1-15 shows the likely extent of
the VOC contaminant plume. This map includes a
limited amount of data from privately owned monitoring
wells located on industrial property just upgradient of
monitoring nests 2 and 4. The dashed lines indicate the
probable extent of the contaminant plume based on the
dimensions of the plume where it passes beneath the
developed area along the Rock River.
This monitoring situation clearly indicates the role
different drilling and construction techniques can play in
a ground-water sampling strategy. In each instance,
much consideration was given to the effect the methods
used for construction and sampling would have on the
resultant chemical data. Where quantitative results for
a fairly "quick" preliminary investigation were not
necessary and driving sandpoints was too difficult, air-
percussion rotary methods were deemed acceptable.
For the placement of the permanent monitoring wells,
wells that may become crucial for contaminant source
identification and possibly for litigation, the hollow-stem
auger was the technique of choice. Finally, when the
hollow-stem auger was not available, a cable-tool rig
was chosen. Since only one hole was to be drilled, the
relative slowness of the method became less important.
Also, the depth of completion (100 feet) in the cavey
sand and gravel made the cable-tool preferable over
the hollow-stem. In addition, each method chosen was
capable of maintaining an open hole without the use of
drilling mud, which could have affected the results of the
chemical analyses of the ground water.
A' A
Ne
'4
\ *
V
st 3 Nest 4 s Land Surface -^ Nes
* X
X ('.
^^
^ Top of Saturated Zone (Water Table) ^ y
( >1000«/L ~^ 500-1000«/L ) \\
~— — • '~~~
t2
\
\
I
250
.100-260MB"-
— <100Mfl"-
Figure 1-14. Cross Section A-A' through Monitoring nests 2,3, and 4, Looking in the Direction of
Ground-Water Flow (from Wehrmann, 1984)
18
-------
OLOE FARMnfiESEMEH SUBDIVISION
31 DOMESTIC WELLS
SAMPLED ton ~4/i3
in/Lie 4 14/L
Figure 1-15. General Area of Known TCE Contamination (from Wehrmann, 1984)
Summary
Critical considerations for the design of ground-water
quality monitoring networks include alternatives for well
design and drilling techniques. With a knowledge of the
principal chemical constituents of interest and the local
hydrogeology, and appreciation of subsurface
geochemistry, appropriate materials for we!! design and
drilling techniques can be selected. Whenever possible,
physical disturbance and the amount of foreign material
introduced into the subsurface should be minimized.
The choices of drilling methods and well construction
materials are very important in every type of ground-
water monitoring program. Details of network
construction can introduce significant bias into monitoring
data, which frequently may be corrected only by
repeating the process of well siting, installation,
completion, and development. This can be quite costly
in time, effort, money, and loss of information. Undue
expense is avoidable if planning decisions are made
cautiously with an eye to the future.
The expanding scientific literature on effective ground-
water monitoring techniques should be read and
evaluated on a continuing basis. This information will
help supplement guidelines, such as this, for applications
to specific monitoring efforts.
19
-------
References
Aller, and others, 1989, Handbook of Suggested
Practices f orthe design and installation of ground-water
monitoring wells: National Water Well Association,
Dublin, OH EPA 600/4-89/034.
Barcelona, M.J., J.P. Gibb, J.A. Helfrich, and E.E.
Garske, 1985, Practical guide forground-watersampling:
Illinois State Water Survey, U.S. Environmental
Protection Agency, Robert S. Kerr Environmental
Research Laboratory, Ada, OK, and Environmental
Monitoring and Support Laboratory, Las Vegas, NV.
Barcelona, M.J., 1984, TOC determinations in ground
water: Ground Water, v.22, no.1, pp.18-24.
Barcelona, M.J., J.A. Helfrich, E.E. Garske, and J.P.
Gibb, 1984, A laboratory evaluation of ground water
sampling mechanisms: Ground Water Monitoring
Review, v.4, no. 2, pp. 32-41.
Barcelona, J.J.. J.P. Gibb, and R.A. Miller, 1984. A
guide to the selection of materials for monitoring well
construction and ground-water sampling. Illinois State
Water Survey Contract Report 327. Illinois State Water
Survey, Champaign, IL.
Berg, R.C., J.P. Kempton, and A.N. Stecyk. 1981,
Geology for planning in Boone and Winnebago Counties,
Illinois: Illinois State Geological Survey Circular 531,
Illinois State Geological Survey, Urbana,
Brown, K.W., J. Green, and J.C. Thomas. 1983, The
influence of selected organic liquids on the permeability
of clay liners: Proc. of the Ninth Annual Research
Symposium: Land Disposal, Incineration, and Treatment
of Hazardous Wastes. U.S. Environmental Protection
Agency SHWRD/EPCS, May 2-4, 1983, Ft. Mitchell,
KY.
Burkland, P.W., and E. Raber, 1983, Method to avoid
ground-water mixing between two aquifers during drilling
and well completion procedures: Ground Water
Monitoring Review, v. 3, no. 4, pp. 48-55.
Campbell, M.D. and J.H. Lehr, 1973, Water well
technology: McGraw-Hill Book Company, New York,
NY.
Fenn, D., E. Cocozza, J. Isbister, 0. Braids, B. Yare, and
P. Roux, 1977, Procedures manual for ground water
monitoring at solid waste disposal facilities (SW-611).
U.S. Environmental Protection Agency, Cincinnati, OH.
Gibb, J.P., R.M. Schuller, and R.A. Griffin, 1981,
Procedures for the collection of representative water
quality data from monitoring wells: Cooperative
Groundwater Report. Illinois State Water and Geological
Surveys, Champaign, IL.
Gillham, R.W., M.J.L. Robin. J.F. Barker, and J.A.
Cherry, 1983, Groundwater monitoring and sample
bias: American Petroleum Institute Publication 4367,
Environmental Affairs Department.
Illinois State Water Survey and Illinois State Geological
Survey. 1984. Proceedings of the 1984 ISWS/ISGS
Groundwater Monitoring Workshop. February 27-28.
Champaign, IL.
Illinois State Water Survey and Illinois State Geological
Survey. 1982. Proceedings of the 1982 ISWS/ISGS
Groundwater Monitoring Workshop. Illinois Section of
American Water Works Association. February 22-23,
1982, Champaign, IL.
Johnson, T.L., 1983, A comparison of wellnests vs.
single-well completions: Ground Water Monitoring
Review, v. 3, no. 1, pp. 76-78.
Johnson, E.E., Inc., 1972, Ground water and wells.
Johnson Division, Universal Oil Products Co., St. Paul,
MN.
Keith, S.J.. L.G. Wilson, H.R. Fitch, and D.M. Esposito,
1982, Sources of spatial-temporal variability in Ground-
Water Quality Data and Methods of Control: Case
Study of the Cortaro Monitoring Program, Arizona:
Proc. of the Second National Symposium on Aquifer
Restoration and Ground Water Monitoring. National
Water Well Association. May 26-28. Columbus, OH.
Lewis, R.W., 1982, Custom designing of monitoring
wells for specif ic pollutants and hydrogeologic conditions:
Proc. of the Second National Symposium on Aquifer
Restoration and Ground Water Monitoring. National
Water Well Association, May 26-28, Columbus, OH.
Luhdorff, E.E., Jr., and J.C. Scalmanini, 1982, Selection
of drilling method, well design and sampling equipment
for wells for monitoring organic contamination: Proc. of
the Second National Symposium on Aquifer Restoration
and Ground Water Monitoring, National Water Well
Association, May 26-28, Columbus, OH.
Mackay, D.M., P.V. Roberts, and J.A. Cherry, 1985,
Transport of organic contaminants in groundwater:
Environmental Science & Technology, v. 19, no. 5, pp.
384-392.
Miller, G.D., 1982, Uptake and release of lead, chromium
and trace level volatile organics exposed to synthetic
well casings: Proc. of Second National Symposium on
20
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Aquifer Restoration and Ground Water Monitoring.
National Water Well Association, May 26-28, Columbus,
OH.
Minning, R.C., 1982, Monitoring well design and
installation: Proc. of Second National Symposium on
Aquifer Restoration and Ground Water Monitoring.
National WaterWell Association, May 26-28, Columbus,
OH.
Mobile Drilling Company, 1972, Soil sampling
equipment-accessories: Catalog 650. Mobile Drilling
Company, Indianapolis, IN.
Morrison, R.D. and P.E. Brewer, 1981, Air-lift samplers
for zone of saturation monitoring: Ground Water
Monitoring Review, v. 1, no. 1, pp. 52-55.
Naymik, T.G. and M.E. Sievers, 1983, Groundwater
tracer experiment (II) at Sand Ridge State Forest,
Illinois: Illinois State Water Survey Contract Report 334,
Illinois State Water Survey, Champaign, IL.
Naymik, T.G. and J.J. Barcelona, 1981 .Characterization
of a contaminant plume in groundwater, Meredosia,
Illinois: Ground Water, v. 16, no. 3, pp. 149-157.
O'Hearn, M., 1982, Groundwater monitoring at the
Havana Power Station's ash disposal ponds and
treatment lagoon: Confidential Contract Report, Illinois
State Water Survey, Champaign, IL.
Perry, C.A. and R.J. Hart, 1985, Installation of
observation wells on hazardous waste sites in Kansas
using a hollow-stem auger: Ground Monitoring Review,
v. 5. no. 4. pp. 70-73.
Pettyjohn, W.A. and A.W. Hounslow, 1982, Organic
compounds andground-waterpollution: Proc. of Second
National Symposium on Aquifer Restoration and Ground
Water Monitoring. National Water Well Association,
May 26-28, Columbus, OH.
Pettyjohn, W.A.. W.J. Dunlap. R. Cosby, and J.W.
Keeley, 1981, Sampling ground water for organic
contaminants: Ground Water v. 19, no. 2, pp. 180-189.
Pfannkuch, H.O., 1981, Problems of monitoring network
design to detect unanticipated contamination: Proc. of
First National Ground Water Quality Monitoring
Symposium and Exposition, National Water Well
Association, May 22-30, Columbus, OH.
Pickens, J.F.. J.A. Cherry, R.M. Coupland, G.E. Grisak,
W.F. Merritt, and B.A. Risto. 1981, A multi-level device
for ground-water sampling: Ground Water Monitoring
Review, v. 1, no. 1. pp. 48-51.
Rinaldo-Lee, M . B ., 1983. Small- vs. large-diameter
monitoring wells: Ground Water Monitoring Review, v.
3, no. 1,pp. 72-75.
Scalf, M.R.. J.F. McNabb. W.J. Dunlap. R.L. Cosby,
and J. Fryberger, 1981, Manual of ground-water
sampling procedures: NWWA/EPA Series, National
Water Well Association, Worthington, OH. EPA-600/2-
81-160.
Schalla, R., and R.W. Landick, 1985, A new valved and
air-vented surge plunger for developing small-diameter
monitor wells: Proc. of Third National Symposium and
Exposition on Ground-Water Instrumentation, National
Water Well Association. October 2-4, San Diego, CA.
Sosebee, J.B., Jr. and others 1982. Contamination of
groundwater samples with PVC adhesives and PVC
primer from monitor wells: Environmental Science and
Engineering, Inc., Gainesville, FL.
Torstensson, B.A., 1984, A new system for ground
water monitoring: Ground Water Monitoring Review, v.
4, no. 4, pp. 131-138.
Voytek, J.E., Jr., 1983, Considerations in the design
and installation of monitoring wells: Ground Water
Monitoring Review, v. 3, no. 1, pp. 70-71.
Walker, W.H., 1974, Tube wells, open wells, and
optimum ground-water resource development: Ground
Water, v. 12, no. 1,pp. 10-15.
Wehrmann, H.A., 1984, An investigation of a volatile
organic chemical plume in northern Winnebago County,
Illinois: Illinois State Water Survey Contract Report 346.
Illinois State Water Survey. Champaign,
Wehrmann, H.A., 1983, Monitoring well design and
construction: Ground Water Age, v. 17, no.8, pp. 35-38.
Wehrmann, H.A., 1983, Potential nitrate contamination
of groundwater in the Roscoe Area, Winnebago County,
Illinois: Illinois State Water Survey Contract Report 325,
Illinois State Water Survey, Champaign, IL.
Wehrmann. H.A., 1982, Groundwater monitoring for fly
ash leachate, Baldwin Power Station, Illinois Power
Company. Confidential Contract Report, Illinois State
Water Survey. Champaign. IL.
21
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Chapter 2
GROUND-WATER SAMPLING
Introduction
Background
Ground-water sampling is conducted to provide
information on the condition of subsurface water
resources. Whether the goal of the monitoring effort is
detection or assessment of contamination, the
information gathered during sampling efforts must be of
known quality and be well documented. The most
efficient way to accomplish these goals is by developing
a sampling protocol, which is tailored to the information
needs of the program and the hydrogeology of the site
or region under investigation. This sampling protocol
incorporates detailed descriptions of sampling
procedures and other techniques that, of themselves,
are not sufficient to document data quality or reliability.
Sampling protocols are central parts of networks or
investigatory strategies.
The needforreliableground-watersampling procedures
has been recognized for years by a variety of
professional, regulatory, public, and private groups.
The technical basis for the use of selected sampling
procedures for environmental chemistry studies has
been developed for surface-water applications over the
last four decades. Ground-water quality monitoring
programs, however, have unique needs and goals that
are fundamentally different from previous investigative
activities. The reliable detection and assessment of
subsurface contamination require minimal disturbance
of geochemical and hydrogeologic conditions during
sampling.
At this time, proven well construction, sampling, and
analytical protocols for ground-water sampling have
been developed for many of the more problematic
chemical constituents of interest. However, the
acceptance of these procedures and protocols must
await more careful documentation and firm regulatory
guidelines for monitoring program execution. The time
and expense of characterizing actual subsurface
conditions place severe restraints on the methods that
can be employed. Since the technical basis for
documented, reliable drilling, sample collection, and
handling procedures is in the early stages of
development, conscientious efforts to document method
performance under real conditions should be a part of
any ground-water investigation (Barcelona and others,
1985; Scalf and others, 1981).
Information Sources
Much of the literature on routine ground-water monitoring
methodology has been published in the last 10 years.
The bulk of this work has emphasized ambient resource
or contaminant resource monitoring (detection and
assessment), rather than case .preparation or
enforcement efforts. General references that are useful
to the design and execution of sampling efforts are the
U.S. Geological Survey (1977), Wood (1976), the U.S.
Environmental Protection Agency (Brass and others,
1977; Dunlap and others, 1977; Fenn and others, 1977;
Sisk, 1981) and others (National Council of the Paper
Industry, 1982;Tinlin, 1976). In large part, these works
treat sampling in the context of overall monitoring
programs, providing descriptions of available sampling
mechanisms, sample collection techniques, and sample
handling procedures. The impact of specific
methodologies on the usefulness or reliability of the
resulting data has received little discussion (Gibb and
others, 1981).
High-quality chemical data collection is essential in
ground-water monitoring programs. The technical
difficulties involved in "representative" samplings have
been recognized only recently (Gibb and others, 1981;
Grisak and others, 1978). The long-term collection of
high-quality ground-water chemistry data is more
involved than merely selecting a sampling mechanism
and agreeing on sample handling procedures. Efforts to
detect and assess contamination can be unrewarding
without accurate (i.e.. unbiased) and precise (i.e.,
comparable and complete) concentration data on
22
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ground-water chemical constituents. Also, the expense
of data collection and management argue for
documentation of data quality.
'Gillham and others (1983) published a very useful
reference onthe principal sources of bias and imprecision
in ground-water monitoring results. Their treatment is
extensive and stresses the minimization of random
error, which can enter into well construction, sample
collection, and sample handling operations. They further
stress the importance of collecting precise data over
time to maximize the effectiveness of trend analysis,
particularly for regulatory purposes. Accuracy also is
very important, since the ultimate reliability of statistical
comparisons of results from different wells (e.g.,
upgradieni versus downgradient samples) may depend
on differences between mean values for selected
constituents from relatively small replicate sample sets.
Therefore, systematic error must be controlled by
selecting proven methods for establishing sampling
points and sample collection to ensure known levels of
accuracy.
The Subsurface Environment
The subsurface environment may be categorized broadly
into two zones, the unsaturated or vadose zone and the
saturated zone. The use of the term "vadose" is more
accurate because isolated saturated areas may exist in
the unsaturated zone above the water table of
unconf ined aquifers.
Investigators have discovered recently that the
subsurface is neither devoid of oxygen (Winograd and
Robertson, 1982) nor sterile (Wilson and McNabb,
1983; Wilson and others, 1983). These facts may
significantly influence the mobility and persistence of
chemical species, as well as the transformations of the
original components of contaminant mixtures
(Schwarzenbach and others, 1985) that have been
released to the subsurface.
The subsurface environment also is quite different from
surface water systems in that vertical gradients in
pressure and dissolved gas content have been observed
within the usual depth ranges of monitoring interest
(i.e., 1 to 150 m [3 to 500 ft]). In some cases, these
gradients can be linked to well-defined hydrologic or
geochemical processes. However, reports of apparently
anomalous geochemical processes have increased in
recent years, particularly at contaminated sites
(Barcelona and Garske, 1983; Heaton and Vogel, 1981;
Schwarzenbach and others. 1985; Winograd and
Robertson, 1982; Wood and Petraitis, 1984).
The subsurface environment is not as readily accessible
as surface water systems, and some disturbance is
necessary to collect samples of earth materials or
ground water. Therefore, "representative" (i.e., artifact
or error free) sampling is really a function of the degree
of detail needed to characterize subsurface hydrologic
and geochemical conditions and the care taken to
minimize disturbance of these conditions in the process
(Claasen, 1982). Each well or boring represents a
potential conduit for short-circuited contaminant
migration or ground-water flow, which must be
considered a potential liability to investigative activities.
The subsurface environment is dynamic over extended
time frames and the processes of recharge and ground-
water flow are very important to a thorough
understanding of the system. Detailed descriptions of
contaminant distribution, transport, and transformation
necessarily rely on the understanding of basic flow and
fluid transport processes. Short-term investigations may
only provide a snapshot of contaminant levels or
distributions. Since water-quality monitoring data are
normally collected on discrete dates, it is very important
that reliable collection methods are used to assure high
data quality over the course of the investigation. The
reliability of the methods should be investigated
thoroughly during the preliminary phase of monitoring
network implementation.
Although the scope of this discussion is on sampling
ground water for chemical analysis, the same data
quality requirements apply to water-level measurements
and to hydraulic conductivity testing. These hydrologic
determinations form the basis for interpreting chemical
constituent data and may well limit the validity of fluid or
solute transport model applications. Hydrologic
measurements must be included in the development of
the quality assurance/quality control (QA/QC) program
for ground-water quality monitoring networks.
The Sampling Problem and Parameter Selection
Cost-effective water-quality sampling is difficult in
ground-water systems because proven field procedures
have not been extensively documented. Regulations
that call for "representative sampling" alone are not
sufficient to ensure high-quality data collection. The
most appropriate monitoring and sampling procedures
for a ground-water quality network will depend on the
specific purpose of the program. Resource evaluation,
contaminant detection, remedial action assessments,
and litigation studies are purposes for which effective
networks can be designed once the information needs
have been identified. Due to the time, personnel needs,
and cost of most water-quality monitoring programs, the
optimal network design should be phased so as to make
the most of the available information as it is collected;"
This approach allows for the gradual refinement of
program goals as the network is implemented.
23
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Two fundamental considerations are common to most
ground-waterquality monitoring programs: establishing
individual sampling points (i.e., in space and time) and
determining the elements of the water sampling protocol
that will be sufficient to meet the information needs of
the overall program. The placement and number of
sampling points can be phased to gradually increase
the scale of the monitoring program. Similarly, the
chemical constituents of initial interest should provide
background ground-water quality data from which a list
of likely contaminants may be prepared as the program
progresses. Table 2-1 shows candidate chemical and
hydrologic parameters for both detective and
assessment monitoring activities. Special care should
be taken to account for possible subsurface
transformation of the principal pollutant species. Ground-
water transport of contaminants can produce chemical
distributions that vary substantially overtime and space.
In particular, transformation of organic compounds can
change substantially the identity of the original
contaminant mixture (Mackay and others, 1985;
Schwarzenbach and others, 1985).
Detective Monitoring
Chemical Parameters'
pH. O", TOC. TOX. Alkalinity, TDS. Eh, CI". NO,', SO.', PO.«,
SiO,. Na'. 1C', Ce~, M8". NH.'. ft. Mn
Hydrologic Parameters
Watar Level, Hydraulic Conductivity
Assessment Monitoring
Chemical Parameters'
pH, B-. TOC. TOX. Alkalinity. TDS, Eh. CI'. NOT. SO.'. PO.*.
SiOj, 8, Na', K', Ce", Mg", NH/. Fe. Mn, Zn. Cd, Cu, Pb, Cr.
Ni. AB, Hg. As. Sb, Se. Be
Hydrologic Parameters
Water Level, Hydraulic Conductivity
•Q"1 - specific conductance, a measure of the charged species in
solution.
In this respect, monitoring in the vadose zone is attractive
because it should provide an element of "early" detection
capability. The methodologies available for this type of
monitoring have been under development for some
time. There are distinct limitations, however, to many of
the available monitoring devices (Everett and McMillion,
1985; Everett and others. 1982; Wilson, 1981; Wilson,
1983), and it is frequently difficult to relate observed
vadose zone concentrations quantitatively to actual
contaminant distributions in ground water (Everett and
others, 1984; Lindau and Spalding, 1984). Soil gas
sampling techniques and underground storage tank
monitors have been commercially developed that can
be extremely useful for source scouting. Given the
complexity of vadose zone monitoring procedures and
the need for additional investigation (Bobbins and
Gemmell, 1985), implementing these techniques in
routine ground-water monitoring networks may be
difficult.
This chapter addresses water-quality sampling in the
saturated zcne, reflecting the advanced state of
monitoring technology appropriate forthis compartment
of the subsurface. There are a numberof useful reference
materials forthe development of effective ground-water
sampling protocols, which include information on the
types of drilling methods, well construction materials.
sampling mechanisms, and sample handling methods
currently available (Barcelona and others, 1985;
Barcelona and others, 1983;Gillham and others, 1983;
Scalf and others. 1981; Todd and others. 1976). To
collect sensitive, high-quality contaminant concentration
data, investigators must identify the type and magnitude
of errors that may arise in ground-water sampling.
Figure 2-1 presents a generalized diagram of the steps
involved in sampling and the principal sources of error.
Table 2-1. Suggested Measurements for Ground-
Water Monitoring Programs
Contaminant detection is generally the most important
aspect of a water-quality program, and must be assured
in network design. False negative contaminant readings
due to the loss of chemical constituents orthe introduction
of interfering substances that mask the presence of the
contaminants in water samples can be very serious.
Such errors may delay needed remedial action and
expose either the public or the environment to an
unreasonably high risk. False positive observations of
contaminants may call for costly remedial actions or
more intensive study, which are not warranted by the
actual situation. Thus, reliable sample collection and
data interpretation procedures are central to an optimized
network design.
Step
In-SHu Condition
EsubUshing a Sampling Point
Raid Measurements
-t
Sample Collection
Sample DeUvsry/Transfer
Held Blank*. Standard*
FiaU Determinations
Preservation / Storage
Transportation
Sources of Error
Improper wed construction/
placement: inappropriate
material* selection
Instrument malfunction;
operator error
Sampling mechanism bias;
operator error
Sampling mechanism bias;
sample exposure, degassing,
oxygenation; field condition*
Operator error; matrix
interferences
Instrument malfunction;
operator error; field conditions
Matrix interferences; handling/
labeling errors
Delay; sample Iocs
Figure 2-1. Steps and Sources of Error In Ground-
Water Sampling
24
-------
Strict error control at each step is necessary for the
collection of high-quality data representative of in situ
conditions.
There are two major obstacles to controlling ground-
water sampling errors. First, field blanks, standards,
and split samples used in data quality assurance
programs cannot account for changes that may occur in
the integrity of samples prior to sample delivery to the
land surface. Second, most of the sources of error that
may affect sample integrity prior to delivery are not well
documented in the literature for many of the contaminants
of current interest. Among these sources of error are the
contamination of the subsurface by drilling fluids, grouts,
or sealing materials; the sorptive or leaching effects on
water samples due to well casing; pump or sampling
tubing materials' exposures; and the effects on the
solutionchemistryduetooxygenation.depressurization,
or gas exchange caused by the sampling mechanism.
These sources of error have been investigated to some
extentforvolatile organic co ntaminants under laboratory
conditions. However, to achieve confidence in field
monitoring and sampling instrumentation for routine
applications, common sense and a "research" approach
to regulatory monitoring may be needed. Two of the
most critical elements of a monitoring program are
establishing both reliable sampling points and simple,
efficient sampling protocols that will yield data of known
quality.
Establishing a Sampling Point
Taking adequate care in selecting drilling methods, well
construction materials, and well development techniques
should altowthe approximation of representative ground-
water sampling from a monitoring well. The
representative nature of the water samples can be
maintained consistently with a trained sampling staff
and jgood field-laboratory communication. Also,
important hydro logic measurements, such as water
level and hydraulic conductivity, can be made from the
same sampling point. A representative water sample
may then be defined as a minimally disturbed sample
taken after proper well purging, which will allow the
determination of the chemical constituents of interest at
predetermined levels of accuracy and precision.
Sophisticated monitoring technology and sampling
instrumentation are poor substitutes for an experienced
sampling team that can follow a proven sampling
protocol.
This section details some of the considerations in
establishing a reliable sampling point. There are a
number of alternative approaches for selecting a
sampling point in monitoring network design, including
deploying arrays of either nested monitoring wells or
multilevel devices (Barvenik and Cadwgan, 1983;
Pickens and others, 1978) at various sites within the
area of interest. Different approaches have their
individual merits, based onthe ease of verifying sampling
point isolation, durability, cost, ease of installation, and
site-specific factors.
The most effective option for specific programs should
be chosen with representative sampling criteria in mind.
The sampling points must be durable, inert towards the
chemical constituents of interest, allow for purging of
stagnant water, provide sufficient water for analytical
work with minimum disturbance, and permit the
evaluation of the hydrologic characteristics of the
formation of interest. Monitoring wells can be constructed
to meet these criteria because a variety of drilling
methods, materials, sampling mechanisms, and
pumping regimes for sampling and hydrologic
measurements can be selected to meet the current
needs of most monitoring programs.
The placement and number of wells will depend on the
complexity of the hydrologic setting and the degree of
spatial and temporal detail needed to meet the goals of
the program. Both the directions and approximate rates
of ground-water movement must be known in order to
satisfactorily interpret the chemical data. With this
knowledge, it also may be possible to estimate the
nature and location of pollutant sources (Gorelick and
others, 1983). Subsurface geophysical techniques can
be very helpful in determining the optimum placement
of monitoring wells under appropriate conditions and
when sufficient hydrogeologic information is available
(Evans and Schweitzer, 1984). Well placement should
be viewed as an evolutionary activity that may expand
or contract as the needs of the program dictate.
Well Design and Construction
Effective monitoring well design and construction require
considerable care and at least some understanding of
the hydrogeology and subsurface geochemistry of the
site. Preliminary borings, well drilling experience, and
the details of the operational history of a site can be very
helpful. Monitoring well design criteria include depth,
screen size, gravel-pack specifications, and yield
potential. These considerations diflersubstantially from
those applied to production wells. The simplest, small
diameterwell completions that will permit development,
accommodate the sampling gear, and minimize the
need to purge large volumes of potentially contaminated
water are preferred for effective routine monitoring
activities. Helpful references include Barcelona and
others (1983), Scalf and others (1981), and Wehrmann
(1983).
25
-------
Well Drilling
The selection of a particular drilling technique should
depend on the geology of the site, the expected depths
of the wells, and the suitability of drilling equipment for
the contaminants of interest (see Chapter 1). Regardless
of the technique used, every effort should be made to
minimize subsurface disturbance. For critical
applications, the drilling rig and tools should be steam-
cleaned to minimize the potential forcross-conomination
between formations or successive borings. The use of
drilling muds can be a liability for trace chemical
constituent investigations because foreign organic
matterwill be introduced into the penetrated formations.
Even "clay" muds without polymeric additives contain
some organic matter, which is added to stabilize the
clay suspension and may interfere with some analytical
determinations. Table 2-2 contains information on the
total and soluble organic carbon contents of some
common drilling and grouting materials (Wood. 1976).
The effects of drilling muds on ground-water solution
chemistry have not been investigated in detail.
However, existing reports indicate that the organic
carbon introduced during drilling can cause false water-
quality observations for long periods of time (Barcelona,
1984; Brobst, 1984). The fact that these interferences
are observable for gross indicators of levels of organic
carbon compounds (i.e., TOO) and reduced substances
(i.e., COD) strongly suggests that drilling aids are a
potential source of serious error. Special situations may
call for innovative drilling techniques (Yare, 1975).
Well Development, Hydraulic Performance, and
Purging Strategy
Once a well is completed, it is necessary to prepare the
sampling point for water sampling and begin to evaluate
the hydraulic characteristics of the producing zone.
These steps provide a basis for maintaining reliable
sampling points over the duration of a ground-water
monitoring program.
Well Development. The proper development of
monitoring wells is essential to the collection of
"representative" water samples. During the drilling
process, fine panicles are forced through the sides of
the borehole into the formation, forming a mud cake that
reduces the hydraulic conductivity of the materials in
the immediate area of the well bore. To allow water from
the formation being monitored to freely enter the
monitoring well, this mud cake must be broken down
opposite the well screen and the fine material removed
from the well. This process also enhances the yield
potential of the well, which is a critical factor when
constructing monitoring wells in low-yielding geologic
materials.
More importantly, monitoring wells must be developed
to provide water free of suspended solids. When
sampling for metal ions and other dissolved inorganic
constituents, water samples must be filtered and
preserved at the well site at the time of sample collection.
Improperly developed monitoring wells will produce
samples containing suspended sediments that may
both bias the chemical analysis of the collected samples
and cause frequent clogging of field filtering mechanisms.
The additional time and money spent for well
development will expedite sample filtration and result in
samples that are more representative of water chemistry
in the formation being monitored.
Development procedures used for monitoring wells are
similar to those used for production wells. The first step
in development involves the movement of water at
alternately high and low velocity into and out of the well
Ash
(%bywt)
Organic Content
1% by wt)
Soluble Carbon
«%bywt> '
Soluble
Carbon In Total
Organic Content
<%bywtl
"Bentonite" muda/grouts
Volclay* (~90% montmorlllonlte)
Benseaf
"Organic" muds/drilling aidt
Ez-Mud* (acrylamlde-sodium acrylate coporymer
dispersed in food-grade oil
I normally used In 0.25% dilution])
Revert* (guar bean starch-based mixturel
98.2
B8.5
11.6
1.6
1.8
11.6
21.5
98.4
<0.001
<0.001
17.9
33.B
94.4
3.7
2.1
85.6
•AD percentages determined on a moisture-free basis.
•Trademark of American Colloid Co.
Trademark of ML Baroid/NL Industries Inc.
Trademark of Johnson Division, UOP Inc.
Source: Wood. 1976.
Table 2-2. Composition of Selected Sealing and Drilling Muds
26
-------
screen and gravel-pack to break down the mud cake on
the well bore and loosen fine particles in the borehole.
This step is followed by pumping to remove these
terials from the well and the immediate area outside
e well screen. This procedure should be continued
until the water pumped from the well is visually free of
suspended materials or sediments.
Hydraulic Performance of Monitoring Wells. The
importance of understanding the hydraulics of the
geologic materials at a site cannot be overemphasized.
Collection of accurate water-level data from properly
located and constructed wells provides information on
the direction of ground-water flow. The success of a
monitoring program also depends on knowledge of the
rates of travel of both the ground water and solutes. The
response of a monitoring well to pumping also must be
known to determine the proper rate and length of time
of pumping prior to collecting a water sample.
Hydraulic conductivity measurements provide a basis
for judging the hydraulic connection of the monitoring
well and adjacent screened formation to the
hydrogeologic setting. These measurements also allow
an experienced hydrologist to estimate an optimal
sampling frequency for the monitoring program
(Barcelona and others, 1985).
Traditionally, hydraulic conductivity testing has been
hieved by collecting drill samples, which were then
ken to the laboratory for testing. Several techniques
involving laboratory permeameters are routinely used.
Falling head or constant head permeameter tests on
recompacted samples in fixed wall or triaxial test cells
are among the most common. The relative applicability
of these techniques depends on both operator skill and
methodology since calibration standards are not
available. The major problem with laboratory test
procedures is that the determined values are based on
recompacted geologic samples ratherthan undisturbed
geologic materials. Only limited work has been done to
date on performing laboratory tests on "undisturbed"
samples to improve the field applicability of laboratory
hydraulic conductivity results. Melby (1989) reported
that laboratory-determined values of hydraulic
conductivity for cores of unconsolidated, fine-grained
material from Oklahoma were three to six orders of
magnitude smaller than values determined by aquifer
testing. Considerable care must be exercised when
evaluating laboratory-derived hydraulic conductivity
coefficients.
Hydraulic conductivity is most effectively determined
under field conditions by aquifer testing methods, such
,s pumping or slug testing (see Chapter 4). The water-
'vel drawdown can be measured during pumping.
Alternatively, water levels can be measured after the
static water level is depressed by application of gas
pressure or elevated by the introduction of a slug of
water. These procedures are rather straightforward for
wells that have been properly developed.
Well Purging Strategies. The number of well volumes
to be removed from a monitoring well prior to collecting
a water sample must be tailored to the hydraulic
properties of the geologic materials being monitored,
the well construction parameters, the desired pumping
rate, and the sampling methodology to be employed.
No single number of well volumes to be pumped fits all
situations. The goal in establishing a well purging strategy
is to obtain water from the geologic materials being
monitored while minimizing the disturbance of the
regional flow system and the collected sample. To
accomplish this goal, a basic understanding of well
hydraulics and the effects of pumping on the quality of
water samples is essential. Water that has remained in
the well casing more than about 2 hours has had the
opportunity to exchange gases with the atmosphere
and to interact with the well casing material. Therefore,
the chemistry of water stored in the well casing is not
representative of that in the aquifer and should not be
collected for analysis. Purge volumes and pumping
rates should be evaluated on a case-by-case basis.
Gibb (1981) has shown how the measurements of
hydraulic conductivity can be used to estimate the well-
purging requirement. Figures 2-2a and 2-2b show an
example of this procedure. In practice, it may be
necessary to test the hydraulic conductivity of several
wells within a network. The calculated purging
requirement should then be verified by measurements
of pH and specific conductance during pumping to
signal equilibration of the water being collected.
The selection of purging rates and volumes of water to
be pumped prior to sample collection also can be
influenced by the anticipated waterquality. In hazardous
environments where purged water must be contained
and disposed of in a permitted facility, it is desirable to
minimize this amount. This can be accomplished by
pumping the wells at very low pumping rates (100 mL/
min) to minimize the drawdown in the well and maximize
the percentage of aquifer water delivered to the surface
in the shortest period of time. Pumping at low rates, in
effect, isolates the column of stagnant water in the well
bore and negates the need for its removal. This approach
is only valid in cases where the pump intake is placed
at the top of, or in, the well screen.
In summary, well purging strategies should be
established by (1) determining the hydraulic performance
of the well; (2) calculating reasonable purging
27
-------
requirements, pumping rates, and volumes based on
hydraulic conductivity data, well construction data, site
hydrologic conditions, and anticipated waterquality; (3)
measuring the well purging parameters to verify chemical
"equilibrated" conditions; and (4) documenting the entire
effort (actual pumping rate, volumes pumped, and
purging parameter measurements before and after
sample collection).
Givan:
48-(oot dMp, 2-Inch diameter well
2-foot long acreen
3-toot thick aquifer
italic watar lava! about IE fael below land aurface
hydraufe conductivity « 10'* cm/aec
Awumptlona:
A desired purga rate of GOO mL/min and campling rate of 100
mL/mlr. will be uaed.
Calculation*:
One well volume - (48 ft - IB ft) x 613 mL/fi 12 inch diameter
wall)
= 20.2 tttera
Aquifer Traniminlvlty - hydraulic conductivity » aquifer thickness
= 10'* m/iec » 1 malar
-- 10-* m>/aec or 8.64 rn'/dey
From Figure 2-3b:
At 6 minute*: 95% aquilar watar and
(5 mln » 0.5 L/mln)/20.2 L
- 0.12 well volume*
At 10 minute*: 100% aquifer water and
(10 min x O.S Uminl/20.2 L
• 0.24 well volume*
It appear* that a high percentage of aquifer witar can be obtained
within a relatively ertort time of pumping at 500 mL-mln'1. Thl*
pumping rate i* below that ueed during well development to prevent
well damange or further development.
Figure 2-2a. Example of Well Purging
Requirement Estimating Procedure (Barcelona
and others, 1985)
1201-
100
60
40
20
620.0m'/day
Q o SOOmL/mhi
Diameter = 5.08 cm
10 IB 20
Time (minutes)
25
30
Figure 2-2b. Percentage of Aquifer Water Versus
Time for Different Transmlsslvltles
Sampling Materials and Mechanisms. In many
monitoring situations, it is not possible to predict the
requirements that either materials for well casings,
pumps, and tubing, or pumping mechanisms must meet
in order to provide error-free samples of ground water.
Ideally, these components of the system should be
durable and inert relative to the chemical properties of
samples orthe subsurface so asto neither contaminate
nor remove chemical constituents from the water
samples. Duetothe long duration of regulatory program
requirements, well casing materials, in particular, must
be sufficiently durable and nonreactive to last several
decades. It is generally much easier to substitute more
appropriate sampling pumps or pump/tubing materials
as knowledge of subsurface conditions improves than
to drill additional wells to replace inadequate well casing
or screen materials. Also, there is no simple way to
account for errors that occur prior to handling a sample
at the land surface. Therefore, it is good practice to
carefully choose the components of the sampling system
that make up the rigid materials in well casing/screens
or pumps, and the flexible materials used in sample
delivery tubing.
Rigid Materials. An experienced hydrologist can base
well construction details mainly on hydrogeologic criteria,
even in challenging situations where a separate
contaminant phase may be present (Villaume, 1985).
However, the best material for a specific monitoring
application must be selected by considering subsurface
geochemistry and the likely contaminants of interest.
Therefore, strength, durability, and inertness should be
balanced with cost considerations in the choice of rigid
materials for well casing, screens, pumps, etc. (see
Chapter 1).
Common well casing materials include TFE (TeflonR),
PVC (polyvinyl chloride), stainless steel, and other
ferrous materials. The strength, durability, and potential
for sorptive or leaching interferences with chemical
constituents have been reviewed in detail for these
materials (Barcelona and others, 1985; Barcelona and
others, 1983). Unfortunately, there is very little
documentation of the severity or magnitude of well
casing interferences from actual field investigations.
This is the point at which optimized monitoring network
design takes on an element of "research," as the
components of the monitoring installation will need to
be systematically evaluated.
Polymeric materials have the potential to absorb
dissolved chemical constituents and leach either
previously sorbed substances or components of the
polymer formulations. Similarly, ferrous materials may*
adsorb dissolved chemical constituents and leach metal
28
-------
^^co
•ut
^•^an
ions or corrosion products, which may introduce errors
into the results of chemical analysis. This potential in
both cases is real, yet not completely understood. The
mmendations in the references noted above can
summarized as follows:
Teflon0 is the well casing material least likely to cause
significant error in ground-water monitoring programs
focused on either organic or inorganic chemical
constituents. It has sufficient strength for most
applications at shallow depth (i.e., < 100 m) and is
among the most inert materials ever made. For deeper
installations, it can be linked to another material above
the highest seasonal water level.
Stainless steel (either 316 or 304 type) well casing,
under noncorrosive conditions, is the second least like ly
material to cause significant error for organic chemical
constituent monitoring investigations. Fe, Mn, or Cr
may be released, under corrosive conditions. Organic
constituent sorption effects also may provide significant
sources of error after corrosion processes have altered
the virgin surface.
Rigid PVC well casing material that has National
Sanitation Foundation approval should be used in
monitoring well applications when noncemented or
threaded joints are used, and organic chemical
constituents are not expected to be of either present or
future interest. Significant losses of strength, durability
and inertness (i.e., sorption orleaching) maybe expected
under conditions where organic contaminants are
present in high concentration. PVC should, however,
perform adequately in inorganic chemical constituent
studies when concentrations of organic constituents
are not high and tin or antimony species are not being
targeted.
Monitoring wells made of appropriate materials and
screened over discrete sections of the saturated
thickness of geologic formations can yield a wealth of
chemical and hydrologic information. Whether or not
this level of performance is achieved frequently may
depend on the care taken in evaluating the hydraulic
performance of the sampling point.
Flexible Materials. Pump components and sample
delivery tubing may contact a water sample more
intimately than other components of a sampling system,
including storage vessels and well casing. Similar
considerations of inertness and noncontarrtinating
properties apply to tubing, bladder, gasket and seal
materials. Experimental evidence (Barcelona and others,
1985) has supported earlier recommendations drawn
from manufacturers' specifications (Barcelona and
others, 1983). A summary is provided in Table 2-3.
Again, the care taken in materials' selection for the
specific needs of the sampling program can pay real
dividends and provides greater assurance of error-free
sampling.
Sample Mechanisms. It is important to remember that
sampling mechanisms themselves are not protocols.
The sampling protocolforaparticular monitoring network
Materiala
Recommendation*
Porytatrafluoroethyten*
(Teflon-)
Polypropylene
Polyethylene (linear)
PVC (flexbto)
Vrton-
SBcone (medic*) grid* only)
Neoprene
Recommended for moat monitoring work, particularly (or detailed
organic analytical achemea. The material lean likely to Introduce
lignificant eampfing blaa or Impreclalon. The eaalett material to clean
In order to prevent croea-contamination.
Strongly recommended for corroaive high diaaolved aolida aokjtiont.
Lax Ekaty to Introduce aignlficant blai Into analytical result* than
polymer lormuUUon* (PVC) or other flexible materiala whh the
exception ol Teflon*.
Not recommended for detailed organic analytical achemea. Plasticizen
and etabnzen make up a tliebU percentage of the material by weight
at long a* It remains flexible. Documented Interference* ere likely with
several priority pollutant claaaea.
Flexible eumomerfc material* for gasket*, O-rlngi, bladder, and tubing
application*. Performance expected to be a function of exposure type
and the order of chemical resistance aa shown. Recommended only
when • more aultabla material la not avalable for the apeclflc uae.
Actual controlled expoaure trials may be useful In aiaetalng the
potential for analytical biaa.
•Trademark of DuPont. Inc.
'able 2-3. Recommendations for Flexible Materials In Sampling Applications
29
-------
is basically a step-by-step written description of the
procedures used for well purging, delivering samples to
the surface, and handling samples inthe field. Once the
protocol has been developed and used in a particular
investigation, it provides a basis for modifying the
program, if the extent or type of contamination requires
more intensive work. An appropriate sampling
mechanism is, however, an important part of any
protocol. Ideally, the pumping mechanism should be
capable of purging the well of stagnant water at rates of
litersorgallons per minute and also of delivering ground
waterto the surface so that sample bottles may be filled
at low flow rates (i.e., about 100 mL/min'1) to minimize
turbulence and degassing of the sample. In this way the
criteria for representative sampling can be met while
keeping the purging and sample collection steps simple.
Nielsen and Yeates (1985) reviewed the types of sample
collection mechanisms commercially available
(Anonymous, 1985). This review supports the results,
of research studies of their performance (Barcelona
and others, 1984; Stoltzenburg and Nichols, 1985).
Figure 2-3 shows examples of types of pumps or other
samplers, which are fully described in a number of
references (Barcelona and others, 1985; Gillham and
others, 1983; Scalf and others, 1981). Given all of the
varied hydrogeologic settings and potential chemical
constituents of interest, several types of pumps or
sampling mechanisms may be suitable for specific
applications. Figure 2-4 contains some
recommendations for reliable sampling mechanisms
relative to the sensitivity of the sample to error. The
main criteria for sampling pumps are the capabilities to
purge stagnant water from the well and to deliver the
water samples to the surface with minimal loss of
sample integrity. Clearly, a mechanism that is shown to
provide accurate and precise samplesfor volatile organic
compound determinations should be suitable for most
chemical constituents of interest.
After establishing a sampling point and the means to
collect a sample, the next step is the development of the
detailed sampling protocol.
Elements of the Sampling Protocol
There are few aspects of this subject that generate
more controversy than the sampling steps, which make
up the sampling protocol. Efforts to develop reliable
protocols and optimize sampling procedures require
particular attention to sampling mechanism effects on
the integrity of ground-water samples (Barcelona and
others, 1984; Stolzenburg and Nichols. 1985), as well
as to the potential errors involved in well purging,
delivery tubing exposures (Barcelona and others, 1985;
Ho, 1983), sample handling, and the impact of sampling
frequency on both the sensitivity and reliability of
chemical constituent monitoring results. Quality
assurance measures, including field blanks, standards,
and split control samples, cannot account for errors in
these steps of the sampling protocol. Actually, the
sampling protocol is the focus of the overall study
network design (Nacht. 1983), and it should be prepared
flexibly so that it can be refined as information on site
improves.
Each step within the protocol has a bearing on the
quality and completeness of the information being
collected. This is perhaps best shown by the progression
of steps depicted in Figure 2-5. Corresponding to each
step is a goal and recommendation for achieving that
goal. The principal utility of this description is that it
provides an outlined agenda for high-quality chemical
and water-quality data.
To ensure maximum utility of the sampling effort and
resulting data, it is essential to document the sampling
protocol as performed in the field. In addition to noting
the obvious information (i.e., persons conducting the
sampling, equipment used, weather conditions,
adherence to the protocol, and unusual observations),
three basic elements of the sampling protocol should be
recorded: (1) water-level measurements made prior to
sampling, (2) the volume and rate at which water is
removed from the well prior to sample collection (well
purging), and (3) the actual sample collection, including
measurement of well-purging parameters, sample
preservation, sample handling, and chain of custody.
Water-Level Measurement
Priorto well purging or sample collection, it is extremely
important to measure and record the water level in the
well. These measurements are needed to estimate the
amount of waterto be purged prior to sample collection.
Likewise, this information can be useful when interpreting
monitoring results. Low water levels may reflect the
influence of the cone of depression surrounding a
nearby production well. High water levels, compared to
measurements made at other times of the year, may be
indicative of recent recharge events. In relatively shallow
settings, high water levels from recent natural recharge
events may result in the increase of certain constituents
leached from the unsaturated zone or in the dilution of
the dissolved solids content in the collected sample.
Documenting the nonpumping water levels for all wells
at a site will provide historical information on the hydraulic
conditions at the site. Analysis of this information may
reveal changes in flow paths and serve as a check on
the effectiveness of the wells to monitor changing
hydrologic conditions. It is very useful to develop an
30
-------
Simple In*
Lifting be*
Discharge Check
Verve Assembly
(Inside Body)
Perforated
Flow Tube
Bladder
Intake Check Verve
Assembly
(Inside Screen)
Air tin*
to Pressure
W.ur Row
BliktrUm
--- L
Anti-Clogging
Screen
.1-1/4-O.D.Kl-1.0.
Rigid Tubing.
Utuily 18 to 36' Long
—Wttw Row
Motor
- 3/4'DiarrMMr Ball
Helical Rotor Electric
Submcnlble Pump
kcsf
Cut-Aw«y Oiagom
ol • Gu-Operited Bliddw Pump
Bailer
1 * Diameter Threaded Seat
S/16* Diameter Hole
Gai Entry Tube
Sample Discharge Tube
JU 31 W riser tube
Jjy 1/2' Bas drive tuba
pLJJT* Compresaion tube fining
i
i-
1
'
r
-
r^_— »
^—__— >
*- -^
sS
^ — Sampler body
•* Teflon seal
4 — Porous filter
Well
Note*:
1. Sampler length can be Inc/eeied
for ipecbl application*
2. Fabrication materiali can be selected
to meet analysia requirement!
and in aftu chemical environment
3. Tubing aiiei can be modified lor
special application*
Teflon Connector
6 mm ID
Polypropylene Tubing
Threaded Access Cap
• PVC Pipe
Check Valve
Arrangement
- Stoned Well Screen
Simple Slotted WeD Point
Caa-Drive Sampling Device
Glass Tubing
6mm 00
Tubing
Gss-Drnre Sampler Designed
for Permanent Installation in a
Borehole {Barcad Systems!
Outlet
Peristaltic Pump
Sample Collection Bottle
Igure 2-3. Schematic Diagrams of Common Ground-Water Sampling Devices (Neilsen and Yeates,
85)
31
-------
Type ot
constituent
Votati*
Organic
Compound*
Orgenometalac*
Well-purging
Parameters
Trace Inorganic
Metal Species
Reduced
Species
M«jof Cation*
frAniona
Example of
oonatttuent
Chlorofonn
TOX
CH,Hg
O..CO,
PH. 0-'
Eh
Fe, Cu
NO,'. S-
Na'. 1C. Ca~
Mg~
ci-. so.-
PoeWve
Displacement
bladder pumpa
Thief, in shu or
dual check va»ve
beler*
Mechanical
poaitive
displacement
pump*
INCREASING RELIABILITY OF SAMPLING MECHANISMS
1 INCREASING SAMPLE SENSITIVITY »•
Superior
performenoe
for most
applications
Superior
performance
for moat
applications
Superior
perform* nc«
forrnott
applications
Superior
performance
for most
applicatlona
Maybe
adequate If wel
purging la
auured
Maybe
adequate If well
purging la
auured
Maybe
adequate If well
purging la
auured
Adequate
Maybe
adequate rf well
purging is
auured
May be ade-
quate X dealgn
and operation are
controaed
May be ade-
quate If deeign
and operation ire
controlled
Adequate
Adequate
Gee-drive
Owiciw
_
Not recom-
mended
Not recom-
mended
Maybe
adequate
Adequate
Suction
mechanisms
Not recom-
mended
Not recom-
mended
May be ade-
quate If material*
are appropriate
Adequate
Figure 2-4. Matrix of Sensitive Chemical Constituents and Various Sampling Mechanisms
Step
Hydroteglc Measurements
Well Purging
Sample Collection
Filtration/ Preservation
Field Determinations
Field Blanks/Standards
Sample Storage/Transport
Goal
Establish nonpumplng water level.
Remov* or liotat* stagnant H,0
which would otherwise biu repre-
sentative sample.
Collect ssmples at land surface
or In wall-bore with minimal distur-
bance of sample chemistry.
Filtration permits determination of
soluble constituents end is e form of
preservation. It should be don* In the
field at soon as possible after
collection.
Held analyses of samples will effec-
tively avoid bias ki determining
parametsrs/constfaients which do
not stor* weR; e.g.. gases, alkainity,
PH.
These blanks and etandardi wDI
permit tht correction of analytical
result* for changes which may occur
after sample collection: preservation,
storage, and transport.
Refrigerate end protect samples to
minimize their chemical alteration
prior to analysis.
Recommendations
Messure the water level to ±0.3 cm (±0.01 ft).
Pump water until well purging parameters (e.g., pH,
T, Q->, Eh) stabilize to ± 10% over at least two
successive well volumes pumped.
Pumping rate* should be limited to ~100 mL/min
lor volatile organic* end Dai-sensitive parameters.
Filer: Trace metals. Inorganic anions/cations,
alkalinity.
Do not niter: TOC. TOX, volatile organic com-
pound umples; other organic compound samples
only when required.
Samples for determining gases, alkalinity and
pH should be analyzed in the field if at ell possible.
At least one blank and one standard lor each
aensitive parameter should be made up in the field
on each day of sampling. Spiked sample* are also
recommended for good OA/QC.
Observe maximum sample holding or storage periods
recommended by the Agency. Documentation of
actual holding periods should be carefully performed.
Figure 2-5. Generalized Ground-Water Sampling Protocol
understanding of the seasonal changes in water levels
and associated chemical concentration variability at the
monitored site.
Purging
The volume of stagnant water that should be removed
from the monitoring well should be calculated from the
analysis of field hydraulic conductivity measurements.
Rule-of-thumb guidelines for the volume of water to be
purged can cause time delays and unnecessary pumping
of excess contaminated water. These rules (i.e., 3-, 5-
or 10-well volumes) largely ignore the hydraulic
characteristics of individual wells and geologic settings.
One advantage of using the same pump to both purge
stagnant water and collect samples is the ability to
measure pH and specific conductance in an in-line flow
32
-------
cell. These parameters aid in verifying the purging
efficiency and also provide a consistent basis for
mparing samples from a single well or wells at a
rttcular site. Since pH is a standard variable for
iqueous solutions that is affected by degassing and
depressurization (i.e., toss of C02), in-line measurements
provide more accurate and precise determinations than
discrete samples collected by grab sampling
mechanisms.
The following example illustrates some of the other
advantages of verifying the purge requirement for
monitoring wells.
Documentation of the actual well purging process
employed should be a part of a standard field sampling
protocol. The calculated well purging requirement (e.g.,
>90 percent aquifer water) calls for the removal of five
well volumes prior to sample collection. Field
measurements of the well purging parameters have
historically confirmed this recommended procedure.
During a subsequent sampling effort, 12 well volumes
were pumped before stabilized well purging parameter
readings were obtained. Several possible causes could
be explored: (1) a limited plume of contaminants may
have been present at the well at the beginning of
sampling and inadvertently discarded while pumping in
an attempt to obtain stabilized indicator parameter
Jadings; (2) the hydraulic properties of the well may
ve changed due to silting or encrustation of the
screen, indicating the need for well rehabilitation or
maintenance; (3) the flow-through device used for
measuring the indicator parameters may have been
malfunctioning; or (4) the well may have been tampered
with by the introduction of a contaminant or relatively
clean water in an attempt to bias the sample results.
Sample Collection and Handling
Water samples should be collected when the solution
chemistry of the ground water being pumped has
stabilized as indicated by pH, Eh, specific conductance,
and temperature readings.
In practice, stable sample chemistry is indicated when
the purging parameter measurements have stabilized
over two successive well volumes. First, samples for
volatile constituents, TOC, TOX, and those constituents
that require field filtration or field determination should
be collected. Then large-volume samples for extractable
organic compounds, total metals, or nutrient anion
determinations should be collected.
All samples should be collected as close as possible to
je well head. A lee" fitting placed ahead of the in-line
Hormeasuring the well purging parameters makes
this more convenient. Regardless of the sample
mechanism in use or the components of the sampling
train, wells that are located upgradient of a site, and
therefore are expected to be representative of
background quality, should be sampled first to minimize
the potential for cross-contamination. Laboratory
detergent solutions and distilled water should be used
to clean the sampling train between samples. An acid
rinse (0.1 N HCI) or solvent rinse (i.e.. hexane or
methanol) may be used to supplement these cleaning
steps, if necessary. Cleaning procedures should be
followed by distilled water rinses, which may be saved
to check cleaning efficiency.
The order in which samples are taken for specific types
of chemical analyses should be decided by the sensitivity
of the samples to handling (i.e., most sensitive first) and
the need for specific information. For example, the
flowchart shown in Figure 2-6 depicts a priority orderfor
a generalized sample collection effort. The samples for
organic chemical constituent determinations are taken
in decreasing order of sensitivity to handling errors,
while the inorganic chemical constituents, which may
require filtration, are taken afterwards.
Instances arise, even with properly developed monitoring
wells, that call for the filtration of water samples. It
should be evident, however, that adequate well
development procedures, which require 2 to 3 hours of
bailing, swabbing, pumping, or air purging at each well,
may save many hours in sample filtration. Well
development may have to be repeated at periodic
intervals to minimize the collection of turbid samples. In
this respect, it is important to minimize the disturbance
of fines that accumulate in the well bore. This can be
achieved by careful placement of the sampling pump
intake at the top of the screened interval, low pumping
rates, and avoiding the use of bailing techniques that
disturb sediment accumulations.
It is advisable to refrain from filtering TOC, TOX, or other
organic compound samples because the increased
handling required may result in the loss of chemical
constituents of interest. Allowing any fine material to
settle prior to analysis, followed by decanting the sample,
is preferable to filtration in these instances. If filtration is
necessary for the determination of extractable organic
compounds, it should be performed in the laboratory
using nitrogen pressure. When samples must be filtered,
it may be necessary to run parallel sets of filtered and
unfiltered samples with standards to establish the
recovery of hydrophobic compounds. All of the materials'
precautions used in the construction of the sampling
train should be observed forfiltration apparatus. Vacuum
filtration of ground-water samples is not recommended.
33
-------
PROCEDURE
ESSENTIAL ELEMENTS
VM Purging
Smplt Cotaction
Fujmovol or fcohbon of Stagnant Want
Oaujrmkiatton of WaB-Purghia Paramatan
tpH. Bi. T. B-l"
Maaauramanu
Rapraaanubw Walar
VorMcalian of
Sampla CoBaclhin by
Volatta Organlca. TOX
Larga Votum. Sanv
pfca to Organic
Compound Datarml-
UMmal SameM
Haad-Spaea
FraaSamplaa
Minimal Aaralten or
FlaW Blank.
Standard!
Aaaartad Sanaitn*
Inorganic SptCMa
NOT. NH.-. Mil)
Uanoadad lor good
QA/OCI
Akallnrry/Acldity"
I
Traca Matal SampM
Inorganica
I
Major Caliona and
Ankm
smga
Minimal Air Conuet.
FiaM Datarmlnarian
AdcquiW Nirvng «a*lr«
Contaminotion
MMnvl Air Conucl.
Pmorvotiort
MMnwl Lou of SmpU
Integrity Prior lo Anotyvli
• Oonoui Hmpta ~rJch ihai^d t< (Uund In onto to dMorrnkw diMOlmd canilkuwiu. Rm.iton ihouk) bo KoompWMd prilorobly with In-
DM flura ond pump ptouura or by N, proHur* nwthoO.. S*mplM tot djnolrad gaM or voKrt. orgMiica >hoiM not b* liRonKt. In
hounoH whm IM« llovolopinont procodurM do not >low lor turbidity-lro* umploi «nd moy bin tnolytiul rMuki. ipU HmplM tfwuld
b> ipkod •*« lurntonh bilori ntrllion. Both v&od Hmpl« md rogulv umplo> ihoukl to antfyfod lo d»tann>i« rocoworioi from both
lypwo In th« Hold.
Figure 2-6. Generalized Flow Diagram of Ground-Water Sampling Steps (Barcelona and others, 1985)
Water samples for dissolved inorganic chemical
constituents (e.g., metals, alkalinity, and anionicspecies)
should be filtered in the field. The preferred arrangement
is an in-line filtration module, which utilizes sampling
pump pressure for its operation. These modules have
tubing connectors on the inlet and outlet parts and
range in diameter from 2.5 to 15 cm. Large diameter
filter holders, which can be rapidly disassembled for
filter pad replacement, are the most convenient and
efficient designs (Kennedy and others, 1976; Skougstad
andScarbo, 1968).
Representative sampling results from the execution of
a carefully planned sampling protocol. An important
consideration for maintaining sample integrity after
collection is to minimize sample handling, which may
bias subsequent determinations of chemical
constituents. Since opportunities to collect high-quality
data for the characterization of site conditions may be
limited by time, it is prudent to conduct sample collection
as carefully as possible from the beginning of the
sampling period. It is preferable to risk error on the
conservative side when doubt exists as to the sensitivity
of specific chemical constituents to sampling or handling
errors. Repeat sampling or analysis cannot make up for
lost data collection opportunities.
For samples collected for specif ic chemical constituents,
recommended sample handling and analysis procedures
may need to be modified. Samples that contain several
chemicals and have undergone extended storage
periods can cause significant problems. It is frequently
more effective to perform a rapid field determination of
specific inorganic constituents (e.g., alkalinity, pH,
ferrous iron, sulfide, nitrite, or ammonium) than to
attempt sample preservation followed by laboratory
analysis of these samples.
Many samples can be held for the U.S. EPA
recommended maximum holding times after proper
preservation (Table 2-4).
Quality Assurance/Quality Control
Planning for valid water-quality data collection depends
upon both the knowledge of the system and continued
refinement of all sample handling/collection procedures.
34
-------
Parameter*
(Type)
Wen Purging
pH (grab)
0" (grab)
T (grab)
Eh (grab)
Contamination Indicator*
pH. O" (grab)
TOC
TOX
Water Quality
Dissolved ga*e*
(O* CH«, CO,)
Alkalinity /Actdity
(Fe, Mn. Na',
K'. Ca",
Mg")
(PO.-. CI-.
Silicate)
NOr
so«-
NH.'
Phenol*
Volume
Required (mU
1 Sample*
50
100
1000
1000
Ai above
40
500
10 mL minimum
100
Filtered under
pressure with
appropriate
media
All filtered
1000 ml
&50
100
SO
400
500
Container
(Material)
T.S.P.G
T.S.P.G
T.S.P.G
T.S.P.G
A* above
G.T
G.T
G.S
T.G.P
T.P
(T.P.G
glass only)
T.P.G
T.P.G
T.P.G
T.G
Preservation
Method
None; nek) det.
None: field dat.
None; field dat.
None; field det.
A* above
Dark. 4°C
Dark. 4°C
Dark. 4°C
4°C/None
Field acidified
to pH <2 with
HNO,
4°C
4°C
4°C
4°C/H,SO4to
pH<2
4°C/H,PO. to
pH<4
Maximum
Holding
DA»u>u4
~vnoo
<1 hr"
<1 hr"
None
None
As above
24 hr
5 day*
<24hr
<6hr"/
<24hr
6 month**"
24 hr/
7 days;
7 days
24 hr
7 days
24 hr/
7 days
24 hr
Drinking Witer Suitability
A*. Ba, Cd. Cr.
Pb, Hg. Se. Ag
F-
Remainlng Organic
Parameter*
Sams as above
for wat*r
quality cation*
(Fa, Mn, etc.)
Same as chloride
above
Samt a*
above
Sama a*
above
Same es above
Same a* above
A* for TOX/TOC, except where analytical method call* for acidification
of Mmple
6 month*
7 days
24 hr
*h is assumed that at each site, for each sampling date, replicates, a field blank end standards must be taken at equal volume to those of
the samples.
"Temperature correction must be made for reliable reporting. Variations greater than ± 10% may result from longer holding period.
"*ln the event that HNO, cannot be used because of shipping restrictions, the sample should be refrigerated to 4°C, shipped immediately,
and acidified on receipt at the laboratory. Container should be rinsed with 1:1 HNO, and included whh sample.
Note: T = Teflon; S = stainless steel; P = PVC. polypropylene, polyethylene; G •= borosJlicate glass.
From Scalf et al.. 1381.
Table 2-4. Recommended Sample Handling and Preservation Procedures for a Detective Monitoring
Program
As discussed earlier, the need to begin QA/QC planning
with the installation of the sampling point cannot be
overemphasized.
The use of field blanks, standards, and spiked samples
for field QA/QC performance is analogous to the use of
laboratory blanks, standards, and procedural or
validation standards. The fundamental goal of field QC
is to ensure that the sample protocol is being executed
faithfully and thai situations that might lead to error are
recognized before they seriously impact the data. The
use of field blanks, standards, and spiked samples can
account for changes in samples that occur during
sample collection.
35
-------
Field blanks and standards enable quantitative correction
for bias (i.e., systematic errors), which arise due to
handling, storage, transportation, and laboratory
procedures. Spiked samples and blind controls provide
the means to correct combined sampling and analytical
accuracy or recoveries for the actual conditions to
which the samples have been exposed.
All QC measures should be performed for at least the
most sensitive chemical constituents for each sampling
date. Examples of sensitive constituents would be
benzene or trichloroethylene as volatile organic
compounds and lead or iron as metals. It is difficult to
use laboratory blanks alone for determining the limits of
detection or quantitation. Laboratory distilled water may
contain apparently higher levels of volatile organic
compounds (e.g., methylene chloride) than would
uncontaminatedground-watersamples. The field blanks
and spiked samples should be used for this purpose,
conserving the results of lab blanks as checks on
elevated laboratory background levels.
Whether or not the ground water is contaminated with
interfering compounds, spiked samples provide a basis
for both identifying the constituents of interest and
correcting their recovery (or accuracy) based on the
recovery of the spiked standard compounds. For
example, if trichloroethylene in a spiked sample is
recovered at a mean level of 80 percent (-20 percent
bias), theconcentrationsof trichloroethylene determined
in the samples for this sampling date may be corrected
by a f actorof 1.2 for low recovery. Similarly, if 50 percent
recovery (-50 percent bias) is reported for the spiked
standard, it is likely that sample handling or analytical
procedures are out of control and corrective measures
should be taken at once. It is important to know if the
laboratory has performed these corrections or taken
corrective action when it reports the results of analyses.
It should be further noted that many regulatory agencies
require evidence of QC and analytical performance but
do not generally accept data that have been corrected.
Field blanks, standards, and blind control samples
provide independent checks on handling and storage,
as well as the performance of the analytical laboratory.
Ground-water analytical data are incomplete unless the
analytical performance data (e.g., accuracy, precision,
detection, and quantitation limits) are reported, with
each set of results. Discussions of whether ground-
waterquality has changed significantly must be tempered
by the accuracy and precision performance for specific
chemical constituents.
Table 2-5 is a useful guide to the preparation of field
standards and spiking solutions for split samples. It is
important that the field blanks and standards are made
on the day of sampling and are subjected to all conditions
to which the samples are exposed. Field spiked samples
or blind controls should be prepared by spiking with
concentrated stock standards in an appropriate
Stock Solution lor field Spike of Split Sample*
Simple Type
Alkalinity
Aniont
Cations
Trace Metal*
Volume
SO ml
1 L
1 L
1 L
Composition
Na'. HCO,"
k-. N»; a-, so.-
f. NO,', PO.B. SI
Na'. K-
Ca~. Mg". O-. NO,'
Cd". Cu". Pb"
Cr~, Ni". Ag'
Fe"', Mn"
TOC
TOX
Volatile*
Eitractablea A
Extractablai B
40 ml
50 ml
40 mL
1 L
1 L
Field Standard
(Concentration)
10.0: 25 (ppm) H,0
25, 50 Ippml H.O
Solvent
Concentration of
Components
10.000; 25.000 (ppml
25.000; 50.000 (ppml
5.0; 10.0 (ppm) H.O. H' (add) 6.000: 10.000 I ppm)
10.0; 25.0 (ppml H,0. H* (acid) 10.000; 25.000 (ppm)
Acetone
KHP
Chloroform
2.4,6 TricMorophenol
Okhlorobutane, Toluene
Dibromopropane, Xylan*
Phenol Standard*
Polynuclaar Aromatic
0.2; 0.5 (ppm-CI
1.8; 4.5
-------
background solution priorto the collection of any actual protocol. Due care must be taken in sample collection,
samples Additional precautions should be taken against field determinations, and handling. Transport should be
the depressurization of samples during air transport planned so as not to exceed sample holding time before
and the effects of undue exposure to light during sample laboratory analysis. Every effort should be made to-
handling and storage. All of the QC measures noted inform the laboratory staff of the approximate time of
above will provide both a basis for high-quality data arrival so that the most critical analytical determinations
reporting and a known degree of confidence in data can be made within recommended storage periods.
interpretation. Well-planned overall quality control This may require that sampling schedules be adjusted
programs also will minimize the uncertainty in long-term so that the samples arrive at the laboratory during
trends when different personnel have been involved in working hours.
collection and analysis.
The documentation of actual sample storage and
Sample Storage and Transport treatment may be handled by chain of custody
The storage and transport of ground-water samples procedures. Figure 2-7 shows an example of a chain of
often are the most neglected elements of the sampling custody form. Briefly, the chain of custody record should
CHAIN OF CUSTODY RECORD
Stapling Date .^_______ Sit* Nane
Hell or Stapling Polntai
Sample S«ta for E«ch; Inorganic, Organic, Both-
Inclusive Sampl* Numbersi
Company's Name Telephone ( }
Address •
nun Mr streetcity atatezip
Collector'a Han* Telephone ( )
Date Sampled Time Started Tl«e Completed
Field Information (Precautions, Umber of Samples, Umber of Sample
Boxes, Etc.)t
1.
name organization location
2.
name organization location
Chain of Possession (After aaaplea are transported off-alt* or to
laboratory):
1. (IK)
signature title
__^ (OUT)
name (printed) date/tine of receipt
2. ' (IK)
algnatur* title
(OUT)
naoe (printed) date/tine of receipt
Analysis Information!
Analysis Begun Analysis Coapletnd
Aliquot (date/tine) Initials (date/tine) Initial*
1.
2.
3.
1. *».
3.
Figure 2-7. Sample Chain of Custody Form
37
-------
contain the dates and times of collection, receipt, and
completion of all the analyses on a particular set of
samples. Frequently, it is the only record that exists of
the actual storage period prior to the reporting of
analytical results. The sampling staff members who
initiate the chain of custody should require that a copy
of the form be returned to them with the analytical
report. Otherwise, verification of sample storage and
handling will be incomplete.
Shipping should be arranged to ensure that samples
are neither lost nor damaged enroute to the laboratory.
Several commercial suppliers of sampling kits permit
refrigeration by freezer packs and include proper
packing. It may be useful to include special labels or
distinctive storage vessels for acid-preserved samples
to accommodate shipping restrictions.
Summary
Ground-water sampling is conducted for a variety of
reasons, ranging from detection or assessment of the
extent of a contaminant release to evaluations of trends
in regional water quality. Reliable sampling of the
subsurface is inherently more difficult than either air or
surface water sampling because of the inevitable
disturbances that well drilling or pumping can cause
and the inaccessibility of the sampling zone. Therefore,
"representative" sampling generally requires minimal
disturbance of the subsurface environment and the
properties of a representative sample are scale
dependent. For any particular case, the applicable
criteria should be set at the beginning of the effort to
judge representativeness.
Reliable sampling protocols are based on the
hydrogeologic setting of the study site and the degree
of analytical detail required by the monitoring program.
Quality control begins with the evaluation of the hydraulic
performance of the sampling point or well and the
proper selection of mechanisms and materials for well
purging and sample collection. All other elements of the
program and variables that affect data validity may be
accounted for by field blanks, standards, and control
samples.
Although research is needed on a host of topics involved
in ground-water sampling, defensible sampling protocols
can be developed to ensure the collection of data of
known quality for many types of programs. If properly
planned and developed, long-term sampling efforts can
benefit from the refinements that research progress will
bring. Careful documentation will provide the key to this
opportunity.
References
Anonymous, 1985, Monitoring products, a buyers guide:
Ground Water Monitoring Review, v. 5, no. 3, pp. 33-45.
Barcelona, M.J.. J.P. Gibb, J.A. Helfrich, and E.E.
Garske, 1985, Practical guide forground-watersampling:
Illinois State Water Survey Contract Report 374, U.S.
Environmental Protection Agency, Robert S. Kerr
Environmental Research Laboratory, Ada. OK, and
U.S. Environmental Protection Agency, Environmental
Monitoring and Support Laboratory, Las Vegas, NV.
Barcelona, M.J.. J.A. Helfrich, and E.E. Garske, 1985,
Sampling tubing effects on ground water samples:
Analytical Chemistry, v. 47, no. 2, pp. 460-464.
Barcelona, M.J., J.A. Helfrich, E.E. Garske, and J.P.
Gibb, 1984, A laboratory evaluation of ground-water
sampling mechanisms: Ground Water Monitoring
Review, v. 4, no, 2, pp. 32-41.
Barcelona, M.J., 1984, TOC determinations in ground
water: Ground Water, v. 22, no. 1, pp. 18-24.
Barcelona, M.J., and E.E. Garske, 1983, Nitric oxide
interference in the determination of dissolved oxygen
by the azide-modified Winkler method: Analytical
Chemistry, v. 55, pp. 965-967.
Barcelona, M.J., J.P. Gibb, and R.A. Miller, 1983. A
guide to the selection of materials for monitoring well
construction and ground-water sampling: Illinois State
Water Survey Contract Report, U.S. EPA-RSKERL.
EPA-600/S2-84-024. 78 pp.
Barcelona, M.J., 1983, Chemical problems in ground-
water monitoring: Proc. of Third National Symposium
on Aquifer Rehabilitation and Ground Water Monitoring,
May 24-27, Columbus, OH.
Barvenik, MJ. and R.M. Cadwgan, 1983, Multilevel
gas-drive sampling of deep fractured rock aquifers in
Virginia: Ground Water Monitoring Review, v. 3, no. 4,
pp. 34-40.
Brass, H.J., M.A. Feige, T. Halloran, J.W. Mellow. D.
Munch, and R.F. Thomas. 1977, The national organic
monitoring survey, samplings and analyses forpurgeable
organic compounds:in Pojasek. R.B., ed. Drinking Water
Quality Enhancement through Source Protection, Ann
Arbor, Ml: Ann Arbor Science Publishers.
Brobst, R.B.. 1984, Effects of two selected drilling fluids
on ground water sample chemistry: in Monitoring Wells,
38
-------
Their Place in the Water Well Industry Educational
Session, NWWA National Meeting and Exposition, Las
Vegas. NV.
Claasen, H.C., 1982, Guidelines and techniques for
obtaining water samples that accurately represent the
water chemistry of an aquifer: U.S. Geological Survey
Open File Report, Lakeland, CO.
Dunlap, W J., J.F. McNabb, M.R. Scalf, and R.L. Cosby.
1977, Sampling for organic chemicals and
microorganisms in the subsurface: U.S. Environmental
Protection Agency, Robert S. Kerr Environmental
Research Laboratory, Ada, OK. EPA-600/2-77-176.
Evans. R.B., and G.E. Schweitzer, 1984, Assessing
hazardous waste problems: Environmental Science
and Technology, v. 18, no. 11 pp. 330A-339A.
Everett, L.G. and LG. McMillion, 1985, Operational
rangesfor suction lysimeters: Ground Water Monitoring
Review, v. 5, no! 3, pp. 51-60.
Everett, L.G.. L.G. Wilson, E.W. Haylman, and L.G.
McMillion, 1984, Constraints and categories of vadose
zone monitoring devices: Ground Water Monitoring
Review, v. 4, no. 4.
Everett, L.G., L.G. Wilson, and L.G. McMillion, 1982,
Vadose zone monitoring concepts for hazardous waste
sites: Ground Water, v. 20, no. 3, pp. 312-324.
Fenn, D., E. Cocozza, J. Isbister, 0. Braids, B. Yare, and
P. Roux, 1977, Procedures manual for ground water
monitoring at solid waste disposal facilities: U.S.
Environmental Protection Agency, Cincinnati, OH: EPA-
530/SW611.
Gibb, J.P., R.M. Schuller, and R.A. Griffin, 1981,
Procedures for the collection of representative water
quality data from monitoring wells: Illinois State Water
Survey Cooperative Report. 7, Champaign, IL: Illinois
State Water Survey and Illinois State Geological Survey.
Gillham, R.W., M.J.L. Robin, J.F. Barker, and J.A.
Cherry, 1983, Ground water monitoring and sample
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Grisak, G.E., R.E. Jackson, and J.F. Pickens, 1978.
Monitoring groundwaterquality, the technical difficulties:
Water Resources Bulletin, v. 6, pp. 210-232.
Gorelick, S.M., B. Evans, and I. Remsan, 1983,
Identifying sources of grouno*water pollution, an
optimization approach: Water Resources Research, v.
19, no. 3, pp. 779-790.
Heaton, T.H.E., and J.C. Vogel, 1981, "Excess air in
ground water: Journal Hydrology, v. 50, pp. 201-216.
Ho, J.S-Y., 1983, Effect of sampling variables on recovery
of volatile organics in water: Journal American Water
Works Association, v. 12, pp. 583-586.
Kennedy, V.C., E.A. Jenne, and J.M. Burchard, 1976,
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126, U.S. Geological Survey Water- Resources
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Lindau, C.W.. and R. F. Spalding, 1984, Majorprocedural
discrepancies in soil extracted nitrate levels and nitrogen
isotopic values: Ground Water, v. 22, no. 3, pp. 273-
278.
Mackay, D.M.. P.V. Roberts, and J.A. Cherry, 1985.
Transport of organic contaminants in ground water:
Environmental Science and Technology, v. 19, no. 5,
pp. 384-392.
Melby, J.T., 1989, A comparative study of hydraulic
conductivity determinations for a fine-grained aquifer:
unpubl. M.S. theses, School of Geology, Oklahoma
State University, 171 p.
Nacht, S.J., 1983, Monitoring sampling protocol
considerations: Ground Water Monitoring Review
Summer, pp. 23-29.
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sampling mechanisms available for small diameter
ground water monitoring wells: Ground Water Monitoring
Review , v. 5, no. 2, pp. 83-99.
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water sampling and piezometric monitoring: Ground
Water, v. 16, no. 5, pp. 322-327.
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OHEPA-600/2-81-160..
39
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Schwarzenbach, R.P. and others, 1985, Ground-water
contamination by volatile halogenated alkanes, abiotic
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40
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Chapter 3
TRANSPORT AND FATE OF CONTAMINANTS IN THE SUBSURFACE
Introduction
Protection and remediation of ground-water resources
require an understanding of processes that affect fate
and transport of contaminants in the subsurface
environment. This understanding allows: (1) prediction
of the time of arrival and concentration of contaminants
at a receptor, such as a monitoring well, a water supply
well, or a body of surface water; (2) design of cost-
effective and safe waste management facilities; (3)
installation of effective monitoring systems; and (4)
development of efficient and cost-effective strategies
for remediation of contaminated aquifers (Palmer and
Johnson, 1989a).
Contaminants in ground water will move primarily in a
horizontal direction that is determined by the hydraulic
gradient. The contaminants will decrease in
* concentration because of such processes as dispersion
(molecular and hydrodynamic), filtration, sorption,
various chemical processes, microbial degradation,
time rate release of contaminants, and distance of
travel (U.S. Environmental Protection Agency, 1985).
Processes such as hydrodynamic dispersion affect all
contaminants equally, while sorption, chemical
processes, and degradation may affect various
contaminants at different rates. The complex factors
that control the movement of contaminants in ground
water andthe resulting behaviorof contaminant plumes
are commonly difficult to assess because of the
interaction of the many factors that affect the extent and
rate of contaminant movement. Predictions of movement
and behavior can be used only as estimates, and
modeling is often a useful tool to integrate the various
factors.
The U.S. Environmental Protection Agency (EPA)
sponsored a series of technology transfer seminars
beff/een October 1987 and February 1988 that provided
an overview of the physical, chemical, and biological
processes that govern the transport and fate of
contaminants in the subsurface. The following discussion
is a summary of the workshops, and is based on the
seminar publication, Transport and Fate of Contaminants
in the Subsurface (U.S. Environmental Protection
Agency, 1989).
Physical Processes Controlling the Transport of
Contaminants In the Aqueous Phase In the
Subsurface
Advectlon-Dlsperslon Theory
The study of advection and dispersion processes is
useful for predicting the time when an action limit, i.e.,
a concentration limit used in regulations such as drinking
water standards, will be reached. Knowledge of
advection-dispersion also can be used to select
technically accurate and cost-effective remedial
technologies for contaminated aquifers.
If concentrations of a contaminant were measured in a
monitoring well that was located between a contaminant
source and a receptor such as a water supply well, a
graph of concentrations versus time would show a
breakthrough curve, i.e., the concentrations do not
increase in a step-function (i.e., plug flow), but rather in
an S-shaped curve (Figure 3-1). In a one-dimensional,
homogeneous system, the arrival of the center of the
mass is due to advection, while the spread of the
breakthrough curve is the result of dispersion (Palmer
and Johnson, 1989a).
Advection
Advection is defined by the transport of a non-reactive,
conservative tracerat an average ground-watervelocity
(Palmer and Johnson, I989a). The average linear
velocity is dependent on (1) the hydraulic conductivity of
the subsurface geologic formation in the direction of
ground-water flow, (2) the porosity of the formation and
(3) the hydraulic gradient in the direction of ground-
water flow. For waste contaminants that react through
precipitation/dissolution, adsorption, and/or partitioning
reactions within the subsurface formation, the velocity
can be different from the average ground-water velocity.
41
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BREAKTHROUGH CURVE
1.0
O
F
cc
Ul
"I
O
0.5
0.0
PLUQ r
FLOW!
ACTION LIMIT
ADVECT10N
I
t
0.1 0.6
TIME
Figure 3-1. Breakthrough Curve for a Contaminant, as Measured In a Monitoring Well (Palmer and
Johnson,1989a)
Dispersion
Dispersion of waste contaminants in an aquifer causes
the concentration of contaminants to decrease with
increasing length of flow (U.S. Environmental Protection
Agency, 1985). Dispersion is caused by: (1) molecular
diffusion (important only at very low velocities) and (2)
hydrodynamic mixing (occurring at higher velocities in
laminar flow through porous media). Contaminants
traveling through porous media have different velocities
and flow paths with different lengths. Contaminants
moving along a shorter flow path or at a higher velocity.
therefore, arrive at a specific point sooner than
contaminants following a longer path or traveling at a
lower velocity, resulting in hydrodynamic dispersion.
Figure 3-2 shows that dispersion can occur in both
longitudinal (in the direction of ground-water flow) and
transverse (perpendicular to ground-water flow)
directions, resulting in the formation of a conic waste
plume downstream from a continuous pollution source
(U.S. Environmental Protection Agency, 1985). The
concentration of waste contaminants is less at the
margins of the plume and increases towards the source.
A plume will increase in size with more rapid flow within
a time period, because dispersion is directly related to
ground-water velocity.
SCO
1000
Figure 3-2. The Effects of Ground-Water Velocity on Plume Shape. Upper Plume Velocity: 1.5 ft/day
and Lower Plume Velocity: 0.5 ft/day (U.S. Environmental Protection Agency, 1985).
42
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The dispersion coefficient varies with ground-water
velocity. At low velocity, the dispersion coefficient is
relatively constant, but increases linearly with velocity
as ground-water velocity increases. Based on these
observations, investigators proposedthatthe dispersion
coefficient can be expressed as a sum of an effective
molecular diffusion coefficient and a mechanical
dispersion coefficient (Palmer and Johnson, 1989a).
Theeffective molecular diffusion coefficient is a function
of the solution diffusion coefficient and the tortuosity of
the medium. Tortuosity accounts for the increased
distance a diffusing ion must travel around sand grains.
The mechanical dispersion coefficient is proportional to
velocity. Specifically, mechanical dispersion is a result
of: (1) velocity variations within a pore, (2) different pore
geometries, and (3) divergence of flow lines around
sand grains present in a porous medium (Gillham and
Cherry, 1982).
The term dispersivity is often confused with dispersion.
Dispersivity does not include velocity, so to convert
dispersivity to dispersion requires multiplication by
velocity. Since dispersion is dependent on site-specific
velocity parameters and configuration of pore spaces
within an aquifer, a dispersion coefficient should be
determined experimentally or empirically for a specific
aquifer. The selection of appropriate dispersion
coefficients that adequately reflect existing aquifer
conditions is critical to the success of chemical transport
modeling(U.S. Environmental Protection Agency, 1985).
Advectlon-Disperslon Equation
An advection-dispersion equation is used to express
the mass balance of a waste contaminant within an
aquifer as a result of dispersion, advection, and change
in storage. The mass balance is a function of the
dispersion coefficient, the ground-water velocity,
concentration of the contaminant, distance, and time
(Palmer and Johnson, 1989a). An advection-dispersion
equation can be applied to the description of three-
dimensional transport of waste contaminants in an
aquifer, using three dispersion coefficients (one
longitudinal and two transverse). Mathematically detailed
descriptions of the advection-dispersion equation are
presented in Bear (1969,1979).
Discrepancies between results generated from
advection-dispersion equations and laboratory and field
experiments have been found. These discrepancies
have been attributed to: (1) immobile zones of water
within the aquifer, (2) solution-solid interface processes,
(3) anion exclusion, and (4) diffusion in and out of
aggregates (Palmer and Johnson, 1989a).
Field observations using field tracer studies also have
shown that longitudinal dispersivity values are usually
much largerthan transverse dispersivity measurements
(Palmer and Johnson, 1989a). Figure 3-3 shows three-
dimensional field monitoring that has corroborated these
observations by identifying long, thin contaminant
plumes rather than plumes spread over the thickness of
an aquifer. (Kimmel and Braids, 1980; MacFarlane and
others, 1983). The large longitudinal dispersion
coefficients are thought to result from aquifer
heterogeneity. In an ideally stratified aquifer with layers
of sediment of different hydraulic conductivities,
contaminants move rapidly along layers with higher
permeabilities and more slowly along the lower
permeability layers (Figure 3-4) (Palmer and Johnson;
1989a). Sample concentration of a contaminant is an
integration of the concentrations of each layer, if water
is sampled from monitoring wells that are screened
A. HYPOTHETICAL CONTAMINANT PLUMt
WITH A LARGE TRANSVIMf OUPEJWIVITY
V//////////////////^^^^
B. HYPOTHETICAL CONTAMINANT PLUME
WITH A (MALL TRANSVERSE DHPEKSIVITY
W»«Tt
Figure 3-3. Hypothetical Contaminant Plumes for
Large (A) and Small (B) Dlsperslvitles (Palmer
and Johnson, 1989a)
through the various layers. Results from plotting
concentration versus distance show a curve with large
differences in concentrations, even though only
advection is considered. This dispersion is the result of
aquifer heterogeneity and not pore-scale processes.
However, defining hydraulic conductivities in the
subsurface is difficult, since not all geologic formations
are perfectly stratified, but may contain cross-
stratification or graded bedding (talmer and Johnson,
1989a). To quantify heterogeneity in an aquifer, hydraulic
conductivity is considered to be random, and statistical
characteristics, such as mean, variance, and
autocorrelation function, are determined.
43
-------
iiiiuiiiiiiiiiiiiiniiiuiiiiiiiiiiifiiiiiiiiiii
DISTANCE
Figure 3-4. Contaminant Distributions and
Concentrations In an Ideally Stratified Aquifer
(after Glllham and Cherry, 1982, by Palmer and
Johnson,I989a)
In addition to aquifer heterogeneity, other processes
contributing to the spread of contaminants include: (1)
diverging flow lines resulting in the spread of
contaminants by advection over a larger cross section
of the aquifer. (2) temporal variations in the water table
resulting in change of direction of ground-water flow
and lateral spread of contamination, and (3) variations
in concentration of contaminants at the source resulting
in apparent dispersion in the longitudinal direction (Frind
and Hokkanen, 1987; Palmer and Johnson, 1989a).
Ground-water sampling methods also may result in
detection of apparent spreading of contaminant plumes
(Palmer and Johnson, 1989a). An underestimation of
contaminant concentrations at specific locations in an
aquifer may be due to insufficient well-purging.
Monitoring wells with different screen lengths that
integrate ground water from different sections of the
aquifer may yield dissimilar contaminant concentrations.
Diffusive Transport through Low Permeability
Materials
In materials with low hydraulic conductivities (e.g.,
unfractured clays and rocks with conductivities less
than 10 to 9 m/s), diffusive transport of waste
contaminants is large compared to advective transport
(Neuzil, 1986; Palmer and Johnson, 1989a).
Contaminants can diffuse across natural aquitards or
clay liners with low hydraulic conductivities, resulting in
aquifer contamination. The extent of movement is
dependent on diffusive flux, rate of ground-waterf low in
the aquifer, and the length of the source area in the
direction of ground-water flow.
Effects of Density on Transport of Contaminants
The density of a contaminant plume may contribute to
the direction of solute transport if dissolved
concentrations of contaminants are large enough
(Palmer and Johnson, 1989a). For example, assume
that the density of ground water within an aquifer is 1.00,
the natural horizontal gradient is 0.005, and the natural
vertical gradient is 0.000. If the density of the contaminant
plume is equal to the density of the ground water, the
plume moves horizontally with the naturally existing
hydraulic gradient. If the density of the contaminated
water is 1.005 (a concentration of approximately 7,000
mg/L total dissolved solids), then the driving force in the
vertical direction is the same as the driving force in the
horizontal direction. If the aquifer is isotropic, then the
resulting vector of these two forces descends at 45
degrees into the aquifer. The contaminant plume
moves deeply into the aquifer and may not be detected
with shallow monitoring systems installed under the
assumption of horizontal flow.
Retardation of Contaminants
If contaminants undergo chemical reactions while being
transported through an aquifer, their movement rate
may be less than the average ground-water flow rate
(Palmer and Johnson, 1989a). Such chemical reactions
that slow movement of contaminants in an aquifer
include precipitation, adsorption, ion exchange, and
partitioning into organic matter or organic solvents.
Chemical reactions affect contaminant breakthrough,
as shown in Figure 3-5. If the retardation factor, R
(calculated from equations for contaminant transport
that include retardation), is equal to 1.0, the solute is
o
o
o
o
1.0
0.5
0.0
*, *2
TIME
Figure 3-5. Time Required for Movement of
Contaminants at Different Retardation Factors
(Palmer and Johnson, 1989a)
44
-------
nonreactive and moves with the ground water. If R is
greater than 1.0, the average velocity of the solute is
less than the velocity of the ground water, and the
dispersion of the solute is reduced. If a monitoring well
is located a distance from a contaminant source such
that a nonreactive solute requirestime, t1, to travel from
the source to the well, a contaminant with a retardation
factor of 2 will require 2t1 to reach the well, and 4tl will
be required for a contaminant with a retardation factor
of 4.
Contaminants with lower retardation factors are
transported greater distances over a given time than
contaminants with larger retardation factors (Figure 3-
6) (Palmer and Johnson, I989a). A monitoring well
network has a greater chance of detecting contaminants
with lower retardation factors because they are found in
a greater volume of the aquifer. Estimates of the total
mass of a contaminant with a retardation factor of 1.0 in
an aquifer may be more accurate than estimates for
contaminants with greater amounts of retardation.
Therefore, estimates of time required to remove
nonreactive contaminants may be more accurate than
time estimates for retarded contaminants. The slow
movement of retarded contaminants may control the
time and costs required to remediate a contaminated
aquifer.
Transport through Fractured Media
Because fractured rock has both primary and secondary
porosity, models used to describe solute transport in
porous media, such as aquifers in recent alluvial deposits
or glacial sediments, may not be appropriate for use at
sites on fractured rock (Palmer and Johnson, 1989a).
Primary porosity is the pore space formed at the time of
deposition and formation of the rock mass, and
secondary porosity is the pore space formed as the
result of fracture of the rock.
Transport mechanisms inf ractured media are advection
and dispersion, the same as in porous media (Figure 3-
7) (Palmer and Johnson, 1989a). In fractured media,
however, contaminants are transported by advection
only along fractures. Dispersion in fractured media is
due to: (1) mixing atfracture intersections, (2) variations
in opening widths across the width of the fracture, (3)
variations in opening widths along stream lines, (4)
molecular diffusion into microfractures penetrating the
interfracture blocks and (5) molecular diffusion into
interfracture porous matrix blocks (more important in
fractured porous rock than in fractured crystalline rock).
Transport of contaminants through fractured media is
described by one of four general models: continuum,
RETARDATION AND MONITORING
1,2,3 1 & a
WASTE DETECTED DETECTED
1 ONLY
DETECTED
R-5
R-3
AQUIFER
R"2
R- 1
Figure 3-6. Transport of Contaminants with Varying Retardation Factors at a Waste Site (Palmer and
Johnson, 1989a)
45
-------
FRACTURED POROUS ROCK
Diffusion:
into Rock
Matrix
Diffusion:
Into Rock
Matrix
Figure 3-7. Transport In Fractured Porous Rock
(Palmer and Johnson, I989a)
discrete fracture, hybrid, and channel (Palmer and
Johnson, 1989a).
In continuum models, individual fractures are ignored
and the entire medium is considered to act as an
equivalent porous medium. Single porosity continuum
models are applicable where the only porosity of the
rock mass is the fracture porosity, such as in fractured
granite or basalt. Double porosity models are applicable
to media in which there is both primary and secondary
porosity such as sandstones and shales.
Discrete fracture models try to describe flow andtransport
in individual fractures.Becqause it can be difficult to
obtain information about each fracture in the rock mass,
stochastic models usually are required. These models
use statistical information about distribution of fracture
properties such as orientation and aperture widths to
describe flow and transport.
Hybrid models are combinations of discrete fracture
and continuum models, while channel models describe
solutetransportassmallfingersorchannelsratherthan
as a uniform front along the width of a fracture.
Particle Transport through Porous Media
In addition to solute transport through porous media,
the transport of particles (including bacteria, viruses,
inorganic precipitates, natural organic matter, asbestos
fibers, orclays) also may be important in investigations
of contaminant transport. Particles can be removed
from solution by surface filtration, straining, and
physical-chemical processes (Figure 3-8) (Palmer and
Johnson, 1989a).
The effectiveness of each process is dependent on the
size of the specific particles present (Palmer and
Johnson, 1989a). If particles are largerthanthe largest
SURFACE
FILTRATION
gogogogo
ogogogog
gogogogo
STRAINING
ogogqgqg
gbgogogo
PHYSICAL-
CHEMICAL
Figure 3-8. Mechanisms of Filtration (Palmer and
Johnson,1989a)
pore diameters, they cannot penetrate into the porous
medium and are filtered at the surface of the medium.
If particles are smaller than the largest pores but larger
than the smallest, the particles are transported through
the larger pore channels, but eventually encounter a
pore channel with a smaller diameter and are removed
by straining. If particles are smaller than the smallest
pore openings, the particles can be transported long
distances through the porous medium.
The rate at which particles move through the porous
medium depends on several physical-chemical
processes (Palmer and Johnson, 1989a). Particles
may undergo random collisions with sand grains, and in
a percentage of those collisions particles will adhere to
the solid matrix by interception. Chemical conditions
may affect particle transport; e.g., such processes as
aggregation formation due to pH changes may change
particle surface properties. These larger aggregates
46
-------
can then be strained or filtered from the water.
Microorganism movement through geologic materials
1 is limited by many processes (Palmer and Johnson,
1989a). Some bacteria are large enough to be strained
from the water. Although viruses, which are smaller
than bacteria, can pass through the pores, they may
adsorb to geologic materials because their surfaces are
charged. Microorganisms, like chemical constituents,
can be transported by diffusion, or if they are motile, can
move in response to changes in environmental
conditions and chemical concentrations. Since
microorganism live and die, the rates of these processes
should be included in the description of their transport
in the subsurface.
Physical Processes Controlling the Transport of
Non-Aqueous Phase Liquids (NAPLs) In the
Subsurface
Transport and Dissolution of NAPLs
Non-aqueous phase liquids (NAPLs) are those liquids
that do not readily dissolve in water and can exist as a
separate fluid phase. (Palmer and Johnson, 1989b).
NAPLs are divided into two classes: those that are
lighter than water (LNAPLs) and those with a density
greater than water (DNAPLs). LNAPLs include
hydrocarbon fuels, such as gasoline, heating oil,
kerosene, jet fuel, and aviation gas. DNAPLs include
the chlorinated hydrocarbons, such as 1,1,1-
trichloroethane, carbon tetrachloride. chlorophenols.
chlorobenzenes. tetrachloroethylene, and
polychlorinated biphenyls (PCBs).
100% NAPL SATURATION
CL
UJ
UJ
cc
0.0
Irr*duc0al*
Watar
Saturation
Snw
WATER SATURATION
100%
Figure 3-9. Relative Permeability as a Function of
Saturation (Palmer and Johnson, I989b)
in the fraction of water within the pore space. As the
water fraction decreases, the relative permeability with
respect to the water phase decreases to zero. Zero
relative permeability is not obtained whenthe fraction of
water within the pore space equals zero, but at the
irreducible water saturation (Sw), i.e., the level of water
saturation at which the water phase is effectively
immobile and there is no significant flow of water. The
relative permeability of NAPL is similar. At 100 percent
NAPL saturation, the relative permeability for the NAPL
is equal to 1.0, but as the NAPL saturation decreases,
. ..._. . the relative permeability of the NAPL decreases. At the
As NAPLs move through geologic media, they displace residual NAPL saturation (Sm), the relative permeability
water and air (Palmer and Johnson. 1989b). Water is for the NAPL is effectively zero, and the NAPL is
the wetting phase relative to both air and NAPLs and
tends to line edges of pores and cover sand grains.
NAPLs are the non-wetting phase and tend to move
through the center of pore spaces. Neitherthe water nor
the NAPL phase occupies the entire pore, so the
considered immobile. These immobile fractions of NAPL
cannot be easily removed from pores except by
dissolution by flowing water.
Transport of Light NAPLs
• r-- — — -• ••- » *• r*r* vi ww *i »w • • aiio|*%/i | wi t_iy III Wnr l.5>
permeability of the medium with respect to these fluids If small volumesof a spilled LNAPLentertheunsaturated
isdifferentthanwhenthe pore space is entirely occupied zone (i.e., vadose zone), the LNAPL will flow through
by a single phase. This reduction in permeability depends the central portion of the unsaturated pores until residual
UPOn the SDecHiC medium and can he rlpsrrihori in eatnrotinn ie ra~.nw.nsi /c:_..— i
-------
surface is relatively impermeable, vapors will not diffuse
across the surface boundary and concentrations of
contaminants in the soil atmosphere may build up to
equilibrium conditions. However, if the surface is not
covered with an impermeable material, vapors may
diffuse into the atmosphere.
If large volumes of LNAPL are spilled (Figure 3-1 Ob),
the LNAPL flows through the pore space to the top of
the capillary fringe of the water table. Dissolved
components of the LNAPL precede the less soluble
components and may change the wetting properties of
the water, causing a reduction in the residual water
content and a decrease in the height of the capillary
fringe.
Since LNAPLs are lighter than water, they will float on
top of the capillary fringe. As the head formed by the
infiltrating LNAPLs increases, the water table is
depressed and the LNAPLs accumulate in the
depression. If the source of the spilled LNAPLs is
removed or contained, LNAPLs within the vadose zone
continue to flow underthe force of gravity until reaching
residual saturation. As the LNAPLs continue to enter
the water table depression, they spread laterally on top
of the capillary fringe (Figure 3-1 Oc). The draining of the
upper portions of the vadose zone reduces the total
head at the interface between the LNAPLs and the
ground water, causing the water table to rebound
slightly. The rebounding water displaces only a portion
of the LNAPLs because the LNAPLs remain at residual
saturation. Ground water passing through the area of
residual saturation dissolves constituents of the residual
LNAPLs, forming a contaminant plume. Water infiltrating
from the surface also can dissolve the residual LNAPLs
and add to the contaminant load of the aquifer.
Decrease in the water table level from seasonal variations
or ground-water pumping also causes dropping of the
pool of LNAPLs. If the water table rises again, part of the
LNAPLs may be pushed up, but a portion remains at
residual saturation below the new water table. Variations
in the water table height, therefore, can spread LNAPLs
over a greater thickness of the aquifer, causing larger
volumes of aquifer materials to be contaminated.
Selection of a remedial technology for LNAPLs in the
ground water should not include techniques that move
LNAPLs into uncontaminated areas where more LNAPLs
can be held at residual saturation.
Transport of Dense NAPLs
DNAPLs are very mobile in the subsurface because of
their relatively low solubility, high density, and low
viscosity (Palmer and Johnson, I989b). The low
solubility means that DNAPLs do not readily mix with
water and remain as separate phases. Their high density
TTITT
PRODUCT COUMCC
i t i * i
Figure 3-10. Movement of LNAPLs into the
Subsurface: (A) Distribution of LNAPLs after
Small Volume has Been Spilled; (B) Depression
of the Capillary Fringe and Water Table; (C)
Rebounding of the Water Table as LNAPLs Drain
From Overlying Pore Space (Palmer and
Johnson,1989b)
provides a driving force that can carry them deep into
aquifers. The combination of high density and low
viscosity results inthe displacement of the lowerdensity,
higher viscosity fluid, i.e., water, by DNAPLs, causing
"unstable" flow and viscous fingering (Saffman and
Taylor, 1958; Chouke and others, 1959; Homsy, 1987;
Kueper and Frind, 1988).
If a small amount of DNAPL is spilled (Figure 3-11 a),
the DNAPL will flow through the unsaturated zone
under the influence of gravity toward the water table,
flowing until reaching residual saturation in the
unsaturated zone (Palmer and Johnson, 1989b). If
water is present in the vadose zone, viscous fingering
48
-------
of the DNAPLs will be observed during infiltration. No
kiscous fingering will be exhibited if the unsaturated
rone is dry. The DNAPLs can partition into the vapor
phase, with the dense vapors sinking to the capillary
fringe. Residual DNAPLs or vapors can be dissolved by
infiltrating water and be transported to the water table,
resulting in a contaminant plume within the aquifer.
If a greater amount of DNAPL is spilled (Figure 3-11 b),
the DNAPLsf low until they reach the capillaryfringe and
begin to penetrate the aquifer. To move through the
capillaryfringe, the DNAPLs must overcome the capillary
forces between the water and the medium. A critical
height of DNAPLs is required to overcome these forces.
Larger critical heights are required for DNAPLs to move
through unfractured, saturated clays and silts; thus
these types of materials may be effective barriers to the
movement of DNAPLs if the critical heights are not
exceeded.
After penetrating the aquifer, DNAPLs continue to move
through the saturated zone until they reach residual
saturation. DNAPLs are then dissolved by ground water
passing through the contaminated area, resulting in a
contaminant plume that can extend over a large thickness
of the aquifer. If finer-grained strata arecontainedwithin
the aquifer, infiltrating DNAPLs accumulate on top of
pie strata, creating a pool. At the interface between the
ground water and the DNAPL pool, the solvent dissolves
into the water and spreads vertically by molecular
diffusion. As water flows by the DNAPL pool, the
concentration of the contaminants in the ground water
increases until saturation is achievedorthe downgradient
edge of the pool is reached. DNAPLs, therefore, often
exist in fingers or pools in the subsurface, rather than in
continuous distributions.The density of pools and fingers
of DNAPLs within an aquifer are important for controlling
the concentrations of dissolved contaminants originating
from DNAPLs.
If even larger amounts of DNAPLs are spilled (Figure 3-
11c), DNAPLs can penetrate to the bottom of the
aquifer, forming pools in depressions. If the impermeable
lower boundary is sloping, DNAPLs flow down the dip of
the boundary. This direction can beupgradientfrom the
original spill area if the impermeable boundary slopes in
that direction. DNAPLs also can flow along bedrock
troughs, which may be oriented differently from the
direction of ground-water flow. Flow along impermeable
boundaries can spread contamination in directions that
would not be predicted based on hydraulics.
Chemical Processes Controlling the Transport of
contaminants In the Subsurface
Introduction
Subsurface transport of contaminants often is controlled
MUPlUIMCI
Figure 3-11. Movement of DNAPLs Into the
Subsurface (A) Distribution of DNAPLs after
Small Volume has Been Spilled; (B) Distribution
of DNAPLs after Moderate Volume has Been
Spilled; (C) Distribution of DNAPLs after Large
Volume has Been Spilled (after Feenstra and
Cherry, 1988, by Palmer and Johnson, 1989b)
by complex interactions between physical, chemical,
and biological processes. The advection-dispersion
equation used to quantitatively describe and predict
contaminant movement in the subsurface also must
contain reaction terms added to the basic equation to
account for chemical and biological processes important
in controlling contaminant transport and fate (Johnson
and others, 1989).
49
-------
Chemical Reactions of Organic Compounds
Chemical reactions may transform one compound into
another, change the state of the compound, or cause a
compound to combine with other organic or inorganic
chemicals (Johnson and others, 1989). For use in the
advection-dispersion equation, these reactions
represent changes in the distribution of mass within the
specified volume through which the movement of the
chemicals is modeled.
Chemical reactions in the subsurface often are
characterized kinetically as equilibrium, zero, or first
order, depending on how the rate is affected by the
concentrations of the reactants. A zero-order reaction is
one that proceeds at a rate independent of the
concentration of the reactant(s). In a first-order process,
the rate of the reactions is directly dependent on the
concentration of one of the reactants. The use of zero
or first-order rate expressions may oversimplify the
description of a process, but higher order expressions,
which may be more realistic, are often difficult to measure
and/or model in complex environmental systems. Also
first-order reactions are easy to incorporate into transport
models (Johnson and others, 1989).
Sorptlon. Sorption is probably the most important
chemical process affecting the transport of organic
contaminants in the subsurface environment. Sorption
of non-polar organics is usually considered an
equilibrium-partitioning process between the aqueous
phase and the porous medium (Chiou and others,
1979). When solute concentrations are low (i.e., either
< 10~5 Molar, or less than half the solubility, whichever
is lower), partitioning often is described using a linear
Freundlich isotherm, where the sorted concentration is
a function of the aqueous concentration and the partition
coefficient (Kp) (Karickhoff and others, 1979; Karickhoff,
1984). Kp usually is measured in laboratory batch
equilibrium tests, and the data are plotted as the
concentration in the aqueous phase versus the amount
sorbed onto the solid phase (Figure 3-12) (Chiou and
others, 1979).
Under conditions of linear equilibrium partitioning, the
sorption process is represented in the advection-
dispersion equation as a "retardation factor," R (Johnson
and others, 1989). The retardation factor is dependent
on the partition coefficient Kp, bulk density of aquifer
materials, and porosity.
The primary mechanism of organic sorption is the
formation of hydrophobia bonding between a
contaminant and the natural organic matter associated
with aquifers (Tanford, 1973; Karickhoff and others,
1979; Karickhoff, 1984; Chiou and others, 1985; MacKay
and Powers, 1987). Therefore, the extent of sorption of
1200
800
400
1.1.1-TRICHLOROETHANE
'l.1,2£-TETRACHUOROETHANE
1 • 1.2-MCHLOROETHANE
jja 0 400 800 1200 1600 2000 2400
§ AQUEOUS CONCENTRATION (ug/U
Figure 3-12. Batch Equilibrium Data for 1,1,1-TCA,
1,1,2,2,-TeCA and 1,2-DCA (adapted from Chiou
and others, 1979, by Johnson and others, 1989)
a specific chemical can be estimated from the organic
carbon content of the aquifer materials (f^) and a
proportionality constant characteristic of the chemical
(Koc)' ft tne °r9anic content is sufficiently high (i.e.,
f raction organic carbon content (f^) > 0.001) (Karickhoff
andothers, 1979; Karickhoff, 1984). K^. values for many
compounds are not known, so correlation equations
relating K^. to more easily available chemical properties,
such as solubility or octanol-water partition coefficients
(Kenaga and Goring, 1980; Karickhoff, 1981;
Schwarzenbach and Westall, 1981; Chiou and others,
1982,1983), have been developed. Within a compound
class, KOC values derived from correlation expressions
often can provide reasonable estimates of sorption.
However, if correlations were developed covering a
broad range of compounds, errors associated with the
use of Koc estimates can be large (Johnson and others,
1989).
This method of estimation of sorption, using K^and f^
values, is less expensive than the use of batch equilibrium
tests. However, in soils with lower carbon content,
sorption of neutral organic compounds onto the mineral
phase can cause significant errors in the estimate of the
partition coefficient (Chiou and others, 1985).
Hydrolysis. Hydrolysis, an important abiotic
degradation process in ground waterforcertain classes
of compounds, is the direct reaction of dissolved
compounds with water molecules (Mabey and Mill,
1978). Hydrolysis of chlorinated compounds, which are
often resistant to biodegradation (Siegrist and McCarty,
1987), forms an alcohol or alkene (Figure 3-13).
Most information concerning rates hydrolysis is obtained
from laboratory studies, since competing reactions and
50
-------
RX + HOH
ROH * HX
HX
i i
c-c
H*er
OH'
Figure 3-13. Schematic of Hydrolysis Reactions
for Halogenated Organic Compounds (Johnson
and others, 1989)
slow degradation rates make hydrolysis difficult to
measure in the field. (Johnson and others, 1989).Often
data for hydrolysis are fitted as a first-order reaction,
and a hydrolysis rate constant, K, is obtained. The rate
constant multiplied by the concentration of the
contaminant is added to the advection-dispersion
equation to account for hydrolysis of the contaminant.
Cosolvatlon and lonlzatlon. Cosolvation and ionization
are processes that may decrease sorption and thereby
increase transport velocity (Johnson and others, 1989).
The presence of cosolvents decreases entropic forces
that favor sorption of hydrophobia organic contaminants
by increasing interactions between the solute and the
solvent (Nkedi-Kizza and others, 1985; Zachara and
fchers, 1988). If biologically derived or anthropogenic
lolvent compounds are present at levels of 20 percent
ormore by volume, the solubility of hydrophobicorganic
contaminants can be increased by an order of magnitude
or more (Nkedi-Kizza and others, 1985). In Figure 3-14,
decrease in sorption of anthracene in three soils, as
described by the sorption coefficient Kp, is illustrated,
with methanol as the cosolvent. Since cosolvent
concentration must be large for solute velocity to be
increased substantially, cosolvation is important primarily
near sources of ground-water contamination.
In the process of ionization, acidic compounds, such as
phenols or organic acids, can lose a proton in solution
to form anions that, because of their charge, tend to be
water-soluble (Zachara and others, 1986). For example,
the Kgc of 2,4,5-trichlorophenol can decrease from 2,330
for the phenol, to almost zero for the phenolate (Figures
3-15 and 3-16) (Johnson and others, 1989). Acidic
compounds tend to ionize more as the pH increases.
However, for many compounds, such as the
chlorophenols, substantial ionization can occur at neutral
pH values.
Volatilization and Dissolution. Two important
tethways for the movement of volatile organic
^npounds in the subsurface are volatilization into the
unsaturated zone and dissolution into the ground water
1000
100
10
0.1
ANTHRACENE
.1 .2 .3 .4 .5-
FRACTION CO-SOLVENT
(METHANOL)
Figure 3-14. Effect of Methanol as a Cosolvent on
Anthracene Sorption for Three Soils (Adapted
from Nkedl-Klzza and others, 1985, by Johnson
and others, 1989)
Figure 3-15. K^ values for 2,4,5-trlchlorophenol
and 2,4,5-trichlorophenolate (Johnson and
others, 1989)
2500
2000
1500
1000
500
0
2,4,5-
TRICHLOROPHENOL
6.0 6.5 7.0 7.5 8.0 8.5
Figure 3-16. K^ versus pH for 2,4,5-
trichlorophenol (Johnson and others, 1989)
51
-------
(Johnson and others, 1989). Contaminants in the
aqueous and vapor phases are also more amenable to
degradation.
The degree of volatilization of a contaminant is
determined by: (1) the area of contact between the
contaminated area and the unsaturated zone, which is
affected by the nature of the medium (e.g., grain size,
depth to water, water content) and the contaminant
(e.g., surface tension and liquid density); (2) the vapor
pressures of the contaminants; and (3) the rate at which
the compound diffuses in the subsurface (Johnson and
others, 1989).
The residual saturation remaining when immiscible
liquids move downward through unsaturated porous
media provides a large surface area for volatilization
(Johnson and others, 1989). Vapor concentrations in
the vicinity of the residual are often at saturation
concentrations. Movement of vapor away from the
residual saturation is usually controlled by molecular
diffusion, which is affected by the tortuosity of the path
through which the vapors move. Tortuosity also is
affected by the air-filled porosity of the medium, so
diffusion is reduced in porous media with a high water
content.
Diffusion also is reduced by the partitioning of the
vapors out of the gas phase and into the solid or
aqueous phases (Johnson and others, 1989). The
retardation factor developed for partitioning between
the aqueous and solid phases can be modified with a
term to describe partitioning between the vapor and
aqueous phases.
When immiscible fluids reach the capillary fringe, their
further movement is determined by the density of the
fluids relative to water (Scheigg, 1984; Schwille, 1988).
The LNAPLs pool on top of the water table while the
DNAPLs penetrate into the ground water. Floating
pools of LNAPL can provide substantial surface areafor
volatilization, with diffusion controlling the mass transfer
of organic contaminants into the vapor phase.
The transport and fate of DNAPLs that penetrate into
the ground water is controlled by dissolution.
Experiments have shown that saturation concentration
values can be maintained even with high ground-water
velocities (e.g., 1 m/day) through a zone of contamination
(Anderson and others, 1987). During remedial activities,
such as pump-and-treat, ground-water velocities may
be high, but the dissolution process should still be
effective.
Chemical Reactions of Inorganic Compounds
In studies of organic contamination, the most important
chracteristic is the total concentration of a contaminant
in a certain phase (e.g., in water versus aquifer solid
materials). However, studies of inorganic contamination
are often more difficult because inorganic materials can
occur in many chemical forms, and knowledge of these
forms (i.e, species) is required to predict their behavior
in ground water (Morel, 1983; Sposito, 1986).
In ground water, an inorganic contaminant may occur
as: (1) "free ions" (i.e., surrounded only by water
molecules); (2) insoluble species; (3) metal/ligand
complexes; (4) adsorbed species; (5) species held on a
surface by ion exchange; or (6) species differing by
oxidation state (e.g., manganese (II) and (IV) or
chromium (III) and (VI)) (Johnson and others, 1989).
The total concentration of an inorganic compound may
not provide sufficient information to describe the fate
and behaviorof that compound in ground water. Mobility,
reactivity, biological availability, and toxicity of metals
and other inorganic compounds depend upon their
speciation (Johnson and others, 1989). The primary
reactions affecting the speciation of inorganic
compounds are solubility and dissolution, complexation
reactions, adsorption and surface chemistry, ion
exchange, and redox chemistry.
Solubility, Dissolution, and Precipitation. Dissolution
and weathering of minerals determine the natural
composition of ground water (Johnson and others,
1989). Dissolution is the dissolving of all components
within a mineral, while weathering is a partial dissolution
process in which certain elements leach out of a mineral,
leaving others behind.
Mineral dissolution is the source of most inorganic ions
in ground water. In principle a mineral can dissolve up
to the limits of its solubility, but in many cases, reactions
occur at such a slow rate that true equilibrium is never
attained (Morgan, 1967).
The contribution of ions from one mineral may affect the
solubility of other minerals containing the same ion (i.e.,
the "common ion effect"). Computer programs such as
MINTED (Felmy and others, 1984). MINEQL (Westall
and others, 1976), and WATEQ2 (Ball and others,-
1980) may be used to predict the equilibrium distribution
of chemical species in ground water and indicate if the
waterisundersaturated,supersaturated,orat equilibrium
with various mineral phases. Some of these programs
also may be used to predict the ionic composition of
ground water in equilibrium with assumed mineral
phases (Jennings and others, 1982).
The weathering of silicate minerals contributes cations,
such as calcium, magnesium, sodium, potassium, and
52
-------
silica, to water andforms secondary weathering products
such as kaolinite and montmorillonite clays (Johnson
.and others, 1989). This weathering increases the
'alkalinity of ground water to a level greater than its
rainwater origins.
Weathering and dissolution also can be a source of
contaminants. Leachates from mine tailings can yield
arsenate, toxic metals, and strong mineral acids (Hem,
1 970), while leachates from fly-ash piles can contribute
selenium, arsenate, lithium, and toxic metals (Stumm
and Morgan, 1981; Honeyman and others, 1982;
Murarkaand Macintosh, 1987).
The opposite of dissolution reactions is precipitation of
minerals or contaminants from an aqueous solution
(Johnson and others, 1989). During precipitation, the
least-soluble mineral at a given pH level is removed
from solution. An element is removed by precipitation
when its solution concentration saturates the solubility
of one of its solid compounds. If the solution concentration
later drops below the solubility limit, the solid will begin
to dissolve until the solubility level is attained again.
Contaminants may initially precipitate, then slowly
dissolve later after a remedial effort has reduced the
solution concentration; thus complete remediation of
the aquifer may require years.
A contaminant initially may be soluble but later precipitate
after mixing with other waters or after contact with other
minerals (Drever, 1982; Williams, 1985; Palmer, 1989).
For example, pumping water from an aquifer may
mobilize lead until it converges and mixes with waters
high in carbonates from a different formation and
precipitates as a lead carbonate solid.
Complexatlon Reactions. In complexation reactions,
a metal ion reacts with an anion that functions as a
ligand (Johnson and others, 1989). The metal and the
ligand bind together to form a new soluble species
called a complex. Transition metals form the strongest
complexes (Stumm and Morgan, 1981); alkaline earth
metals form only weak complexes, while alkali metals
do not form complexes (Dempsey and O'Melia, 1983).
The approximate order of complexing strength of
metals is:
Hg> Cu> Pb> Ni> Zn> Cd> Fe(ll)> Mn> Ca> Mg
Common inorganic ligands that bind with metals include :
OH", Cr, SO4-, CO3-, S-, F-, NH3, PO4 CN-, and
polyphosphates. Their binding strength depends
primarilyonthe metal ion withwhich they are complexing
(Johnson and others, 1989). Inorganic ligands are
usually in excess compared to the trace" metals with
which they bind, and, therefore, they affect the fate of
the metals in the environmental system, ratherthan vice
versa (Morel, 1983).
Organic ligands generally form stronger complexes
with metals than inorganic ligands (Johnson and others,
1989). Organic ligands include: (1) synthetic compounds
from wastes, such as amines, pyridines, phenols, and
other organic bases and weak acids; and (2) natural
organic materials, primarily humic materials (Schnitzer,
1969; Hayes and Swift, 1978; Stevenson, 1982,1985;
Johnson andothers, 1989). Humic mate rials are complex
structures, and their complexation behavior is difficult to
predict (Perdue and Lytle, 1983; Sposito, 1984; Perdue,
1985; Dzombak and others, 1986; Fish and others,
1986). Generally, humic materials are found in significant
concentrations only in shallow aquifers. Inthese aquifers,
however, they may be the primary influence on the
behavior of metals (Thurman, 1985).
Equilibrium among reactants and complexes for a given
reaction is predicted by an equilibrium (or "stability")
constant, K, which defines a mass-law relationship
among the species (Johnson and others, 1989). For
given total ion concentrations (measured analytically),
stability constants can be used to predict the
concentration of all possible species (Martell and Smith,
1974,1977; Smith and Martell, 1975).
Because complexes decrease the amount of free ions
in solution, less metal may sorb onto aquifer solid
materials or participate in precipitation reactions
(Johnson and others, 1989). The metal is more soluble
because it is primarily bound up in the soluble complex.
Research has demonstrated that a metal undergoing
complexation may be less toxic to aquifer
microorganisms (Reuterand others, 1979).
Sorptlon and Surface Chemistry. Surface sorption,
in many cases, is the most important process affecting
toxic metal transport in the subsurface (Johnson and
others, 1989). Changes in metal concentration, as well
as pH, can have a significant effect on the extent of
sorption (Figure 3-17).
Approaches to predicting behavior of metal ions based
on sorption processes include using isotherms
(indicating that data were collected at a fixed
temperature) to graphically and mathematically
represent sorption data (Johnson and others, 1989).
Two types of isotherms are commonly used: the
Freundlich isotherm and the Langmuir isotherm (Figure
3-18). The Freundlich isotherm is empirical, and sorbed
(S) and aqueous (C) concentration data are fitted by
adjusting two parameters (K and a). The Langmuir
53
-------
F«OU)
Pb
4
PH
8
Figure 3-17. Adsorption of Metal Ions on
Amorphous Silica as a Function of pH (adapted
from Schlndler and others, 1976, by Johnson and
others, 1989)
logS
a=1
logC
Figure 3-18. Schematic Representation of
Freundlich and Langmulr Isotherm Shapes for
Batch Equilibrium Tests (Johnson and others,
1989)
isotherm is based on the theory of surface complexation,
using a parameter corresponding to the maximum
amount that can be sorbed and the partition coefficient,
K (Morel, 1983).
Another method to describe sorption is to use surface
complexation models that represent sorption as ions
binding to specific chemical functional groups on a
reactive surface (Johnson and others, 1989). All surface
sites may be identical or may be grouped into different
classes of sites (Benjamin and Leckie, 1981). Each type
of site has a set of specific sorbing constants, one for
each sorbing compound. Electrostatic forces at the
surface also contribute to the overall sorption constant
(Davis and others, 1978). Binding of ions to the surf ace
is calculated from constants using mass-law equations
similar to those used to calculate complex formation
(Schindler and others, 1976; Stumm and others, 1976;
Dzombak and Morel, 1986). However, the parameters
used in surface complexation models are data-fitting
parameters, which fit a specified set of data to a particular
model, but have no thermodynamic meaning and no
generality beyond the calibrating data set (Westall and
others, 1980).
Ion-Exchange Reactions. Ion-exchange reactions
are similarto sorption. However, sorption is coordination
bonding of metals (or anions) to specific surface sites
and is considered to be two-dimensional, while an ion-
exchanger is a three-dimensional, porous matrix
containing fixed charges (Helfferich, 1962; Johnson
and others, 1989). Ions are held by electrostatic forces
rather than by coordination bonding. Ion-exchange
"selectivity coefficients" are empirical and vary with the
amount of ion present (Reichenburg, 1966). Ion
exchange is used to describe the binding of alkali
metals, alkaline earths, and some anions to clays and
humic materials (Helflerich, 1962; Sposito, 1984).
Knowledge of ion exchange is used to understand the
behavior of major natural ions in aquifers and also is
useful for understanding behavior of contaminant ions
at low levels. In addition, ion exchange models are used
to represent competition among metals for surface
binding (Sposito, 1984).
Redox Chemistry. Reduction-oxidation (redox)
reactions involve a change in the oxidation state of
elements (Johnson and others, 1989). The amount of
change is determined by the number of electrons
transferred during the reaction (Stumm and Morgan,
1981). The oxidation status of an element can be
important in determining the potential for transport of
that element. For example, in slightly acidic to alkaline.
environments, Fe(lll) precipitates as a highly sorptive
phase (ferric hydroxide), while Fe(ll) is soluble and
does not retain other metals. The reduction of Fe(lll) to
Fe(ll) releases not only Fe+2 to the water, but also other
contaminants sorbed to the ferric hydroxide surfaces
(Evans and others. 1983; Sholkovitz, 1985).
Chromium (Cr) (VI) is a toxic, relatively mobile anion,
while Cr (III) is immobile, relatively insoluble, and strongly
sorbs to surfaces. Selenate (Se) (VI) is mobile but less
toxic, while selenite Se(IV) is more toxic but less mobile
(Johnson and others, 1989).
54
-------
The redox state of an aquifer is usually closely related
§microbial activity and the type of substrates available
the microorganisms (Johnson and others, 1989). As
ganic contaminants are oxidized in an aquifer, oxygen
is depleted and chemically reducing (anaerobic)
conditions form. The redox reactions that occur depend
on the dominant electron potential, which is defined by
the primary redox-active species. The combination of
Fe(ll)/Fe(lll) defines a narrow range of electron
potentials, while (S)(sulfur)(+IV)/S(-ll) defines a broader
range. Pairs of chemical species are called redox
couples.
After oxygen is depleted from ground water, the most
easily reduced materials begin to react and, along with
the reduced product, determine the dominant potential.
After that material is reduced, the next most easily
reduced material begins to react. These series of
reactions continue, usually catalyzed by
microorganisms. An aquifer may be described as "mildly
reducing" or "strongly reducing," depending on where it
is in the chemical series (Stumm and Morgan, 1981).
The electron potential of water may be measured in
volts, as Eh, or expressed by the "pe," which is the
negative logarithm of the electron activity in the water
(Johnson and others, 1989). A set of redox reactions is
Iften summarized on a pH-pe (or pH-Eh) diagram,
piich shows the predominant redox species at any
specified pH and pe (or Eh). In this theoretical approach,
only one redox couple should define the redox potential
of the system at equilibrium. However, in an aquifer,
many redox couples not in equilibrium can be observed
simultaneously (Lindberg and Runnels, 1984).
Therefore, redox behavior of chemicals in aquifers is
difficult to predict. However, the redox status of an
aquifer is important because of its effects on the mobility
of elements and the potential effects on biodegradation
of organic contaminants. Anaerobic (reducing)
conditions are not favorable for hydrocarbon
degradation, but reducing conditions favor
dehalogenation of chlorinated and other halogenated
compounds (Johnson and others, 1989).
Biological Processes Controlling the Transport of
Contaminants In the Subsurface
Introduction
Historically, ground water was thought to be a safe
watersource because it was protected by a metabolically
diverse "living filter" of microorganisms in the soil root
zone that converted organic contaminants to innocuous
end-products (Suflita,1989a).Aquiferswere considered
•be abiotic environments, based on studies that showed
inat microbial numbers decreased with soil depth
(Waksman, 1916) and that indicated that most
microorganisms were attached to soil particles (Balkwill
and others, 1977). In addition, by estimating the time
required for surface water to vertically penetrate
subsurface formations, researchers felt that
microorganisms travelling with water would utilize
available nutrients and rapidly die off. Therefore, since
aquifers were considered to be sterile, they could not be
biologically remediated if contaminated with organic
contaminants. However, microscopic, cultivation,
metabolic, and biochemical investigations, using
aseptically obtained aquifer materials, have shown that
there are high numbers of metaboNcally diverse
procaryotic and eucaryotic organisms present in the
terrestrial subsurface environment (Suflita, 1989a).
Evidence of Subsurface Microorganisms
Microbiological investigations have detected high
numbers of microorganisms (up to 50 x 106 total cells/
ml) in both contaminated and uncontaminated aquifers
at various depths and geological composition (Suflita,
I989a). Even deep geological formations may be
suitable habitats for microorganisms (Kuznetsov and
others, 1963; Updegraff, 1982). The microorganisms
that have been detected in the subsurface are small,
capable of response to addition of nutrients, and are
primarily attached to solid surfaces. Eucaryotic
organisms are present in the subsurface but are few in
numbers and are probably of minor significance, existing
as inert resting structures (Suflita, 1989a).
Suitable sampling technology was developed to
demonstrate the existence of subsurface
microorganisms (Suflita, 1989a). Samples must not be
contaminated with nonindigenous microorganisms
originating from drilling machinery, surface soil layers,
drilling muds, and water used to make up drilling muds.
Since most subsurface microorganisms are associated
with aquifer solid materials, current sampling efforts use
core recovery and dissection to remove microbiologically
contaminated portions of the cores (McNabb and
Mallard, 1984). This dissection is performed in the field,
to prevent nonindigenous organisms from penetrating
to the inner portions of the core, or in the laboratory if it
is nearby. The outer few centimeters and the top and
bottom portions of the aquifer cores are removed
because of possible contamination by nonindigenous
bacteria, and the center portions of the cores are used
for microbiological analysis. An alcohol-sterilized paring
device is used in the dissection process. The paring
device has an inner diameter that is smaller than the
diameter of the core itself. As the aquifer material is
extruded out of the sampling core barrel and over the
paring device, the potentially contaminated material is
stripped away. For anaerobic aquifers, this field paring
dissection is performed inside plastic anaerobic glove
bags while the latter is purged with nitrogen to minimize
55
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exposure of the microorganisms to oxygen (Beeman
and Suflita, 1987). Samples obtained by this technique
are considered to be aseptically acquired and are
suitable for microbiological analyses.
Evidence of Activity of Subsurface Microorganisms
Although direct and conclusive evidence had been
obtained about the existence of microorganisms in the
subsurface,quest ions remained about their significance
in ground water. Such questions included: (1) whether
ornotthe indigenous microorganisms were metabolically
active, (2) what was the diversity of the metabolic
activities, (3) what factors served to limit and/or stimulate
the growth and metabolism of these organisms, and (4)
could the inherent metabolic versatility of aquifer
microorganisms be utilized to remediate contaminated
aquifers (Suflita, 1989a).
Microbial subsurface activity was studied, and the
following metabolic processes were identified in the
subsurface environment: (1) biodegradation of organic
pollutants, including petroleum hydrocarbons,
alkylpyridines, creosote chemicals, coal gasification
products, sewage effluent, halogenated organic
compounds, nitriloacetate (NTA), and pesticides; (2)
nitrification; (3) denitrification; (4) sulfur oxidation and
reduction; (5) iron oxidation and reduction; (6)
manganese oxidation; and (7) methanogenesis (Suflita,
1989a). These metabolic processes include aerobic
and anaerobic carbon transformations, many of which
are important in aquifer contaminant biodegradation.
The other processes are those required for the cycling
of nitrogen, sulfur, iron, and manganese in microbial
communities.
Biodegradation may referto complete mineralization of
organic contaminants (i.e., the parent compounds), to
carbon dioxide, water, inorganic compounds, and cell
protein (Sims and others, 1990). The ultimate products
of aerobic metabolism are carbon dioxide and water,
while under anaerobic conditions, metabolic activities
also result in the formation of incompletely oxidized
simple organic substances such as organic acids and
other products such as methane or hydrogen gas.
Since contaminant biodegradation in the natural
environment is frequently a stepwise process involving
many enzymes and many species of organisms, a
contaminant may not be completely degraded. Instead,
it may be transformed to intermediate product(s) that
may be less, equally, or more hazardous than the
parent compound, and more or less mobile in the
environment (Sims and others, 1990). The loss of a
chemical, therefore, may or may not be a desirable
consequence of the biodegradation process if
biodegradation results in the production of undesirable
metabolites with their own environmental impact and
persistence characteristics (Suflita, 1989b). For
example, the reductive removal of tetrachloroethylene
(TeCE) under anaerobic conditions results in a series of
dehalogenated intermediates. TeCE's halogens are
removed and replaced by protons in a series of sequential
steps. However, the rate of reductive dehalogenation
decreases as fewer and fewer halogens remain.
Consequently, highly toxic vinyl chloride accumulates
and, from a regulatory standpoint, causes greater
concern than the parent contaminant. Bioremedial
technologies should be selected with knowledge of
metabolic processes of the specific contaminants at the
site.
Biodegradation of most organic compounds in aquifer
systems may be evaluated by monitoring their
disappearance from the aquifer through time.
Disappearance, or rate of degradation, is often
expressed as a function of the concentration of one or
more of the contaminants being degraded (Sims and
others, 1990). Biodegradation in natural systems often
can be modeled as a first-order chemical reaction
(Johnson and others, 1989). Both laboratory and field
data suggest that this is true when none of the reactants
are in limited supply. A useful term to describe reaction
kinetics is the half-life, 11/2, which is the time required to
transform 50 percent of the initial constituent.
As decomposable organic matter enters an oxygenated
aquifer (Figure 3-19), microbial metabolism will likely
begin to degrade the contaminating substrate; i.e., the
indigenous microorganisms utilize the contaminant as
an electron donorforheterotrophic microbial respiration
(Suflita, 1989a). The aquifer microorganisms use oxygen
as a co-substrate and as an electron acceptorto support
their respiration. This oxygen demand may deplete
oxygen and establish anaerobic conditions. When
oxygen becomes limiting, aerobic respiration slows,
and other microorganisms become active and continue
to degrade the organic contaminants. Under conditions
of anoxia, anaerobic bacteria use organic chemicals or
certain inorganic anions as alternate electron acceptors.
Nitrate present in ground water is not rapidly depleted
until oxygen is utilized. Organic matter is still metabolized,
but, instead of oxygen, nitrate becomes the terminal
electron acceptor during denitrification. Sulfate becomes
a terminal electron acceptor when nitrate is limiting.
When this occurs, hydrogen sulfide, an odorous gas,
can often be detected in the ground water as a metabolic
end-product. When very highly reducing conditions are
present in an aquifer, carbon dioxide becomes an
electron acceptor and methane is formed. Sometimes
a spatial separation of dominant metabolic processes
can occur in an aquifer, depending on the availability of
56
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PLOW •
CO
>-
LJJ
O
ai
•MO
CHEMICAL SPECIES
NO;
ELECTRON ACCEPTORS
ACETATE—CO,
SOI
CO,
-10
BIOLOGICAL CONDITIONS
AEROBIC
HETEROTROPHtC
RESPIRATION
SULFATE
RESPIRATION
bOHUJDGEKCSS
Figure 3-19. Microblally Mediated Changes In Chemical Species, Redox Conditions, and Spatial
Regions Favoring Different Types of Metabolic Processes Along the Flow Path of a Contaminant
Plume (adapted from Bouwer and McCarty, 1984, by Suflita, 1989a)
electron acceptors, the presence of suitable
microorganisms, and the energy benefit of the metabolic
process to the specific microbial communities. Asorganic
mattefis transported in a contaminant plume, a series
of redox zones can be established that range from
'highly oxidized to highly reduced conditions. The
biodegradation potential and the expected rates of
metabolism will bedifferertt in each zone (Suflita, 1989a).
For many contaminants, aerobic decomposition is
relatively fast, especially compared to methanogenic
conditions. However, some contaminants, such as
certain halogenated aliphatic compounds and 2,4,5-T,
degradefasterwhen anaerobic conditions exist (Bouwer
and others, 1981; Bouwer and McCarty, 1984; Gibson
and Suflita, 1986).
57
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Environmental Factors Affecting Blodegradation
Microorganisms need a suitable physical and chemical
environment to grow and actively metabolize organic
contaminants, (SufPta, I989a). Extremes of temperature,
pH, salinity, osmotic or hydro static pressures, radiation,
free water limitations, contaminant concentration, and/
orthe presence of toxic metals or other toxicant materials
can limit the rate of microbial growth and/or substrate
utilization. Often, two or more environmental factors
interact to limit microbial decomposition processes.
Selected critical environmental factors are presented in
Table 3-1.
Limitations in the ability to alter environmental factors in
the subsurface environment are important in selecting
and implementing aquifer bioremedial technologies
(Suf lita, 1989a). For example, the temperature of aquifers
probably cannot be significantly altered to stimulate in
situ microbial growth and metabolism, but temperatures
could be changed in a surface biological treatment
reactor.
Physiological Factors Affecting Blodegradation
In addition to environmental conditions, microbial
physiotogicalf actors also influence organic contaminant
biodegradation (Suflita, 1989a). The supply of carbon
and energy contained in organic contaminants must be
sufficient for heterotrophic microbial growth. Too high
a substrate concentration can limit microbial metabolism
due to the toxicity of the substrate to microorganisms. If
concentrations are too low, microbial response may be
inhibited, or the substrates may not be suitable for
Environmental Factor
Optimum Levels
Available soil water
Oxygen
Redox potential
PH
Nutrients
Temperature
25-85% of water holding capacity;
-0.01 MPa
Aerobic metabolism: Greater than
0.2 mg/l dissolved oxygen,
minimum air-filled pore
space of 10% by volume;
Anaerobic metabolism: 02
concentrations less than 1%
by volume
Aerobes & facultative anaerobes:
greater than 50 millivolts;
Anaerobes: less than 50 millivolts
pH values of 5.5 - 8.5
Sufficient nitrogen, phosphorus,
and other nutrients so as to
not limit microbial growth
(Suggested C:N:P ratio of
120:10:1)
15 - 45° C (Mesophiles)
Table 3-1. Critical Environmental Factors for Microbial Activity (Sims and others, 1984; Huddleston
and others, 1986; Paul and Clark, 1989)
58
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growth. Growth and energy sources do not have to be
supplied by the same carbon substrate. Growth and
pnetabolism of microorganisms can be stimulated by
providing a non-toxic primary carbon substrate so that
the rate and extent of contaminant degradation can be
increased (McCarty and others,1981; McCarty, 1985;
McCarty and others, 1984).
A contaminant also will be poorly metabolized if it is
unable to enter microbial cells and gain access to
intracellular metabolic enzymes, which may occur with
larger molecular weight compounds (Suflita, I989a). A
substrate also will persist if it fails to de-repress the
enzymes required for its degradation. Appropriate
enzymes sometimes can be induced by an alternate
chemical compound. Sometimes initial biochemical
reactions result in metabolites that tend to inhibit
degradation of the parent molecule.
The absence of other necessary microorganisms can
limit contaminant degradation, since often several
microbial groups are required for complete degradation
(Suflita, 1989a). Microbial consortia are especially
important in anaerobic mineralization of contaminants
(Mclnemey and Bryant, 1981); if any individual members
of a consortium are absent, biodegradation of the
parent material effectively ceases.
Ibhemlcal Factors Affecting Biodegradation
One of the most important factors affecting contaminant
biodegradation in aquifers is the structure of the
contaminant, which determines its physical state (i.e.,
soluble, sorted) and its tendency to biodegrade (Suflita,
1989a). Aquifer contaminants may contain chemical
linkages that tend to favor or hinder microbial
degradation. The number, type, and position of
substrtuents on a contaminant molecule should be
considered when evaluating its metabolic fate in an
aquifer.
Usually the closer a contaminant structurally resembles
a naturally occurring compound, the betterthe possibility
that the contaminant will be able to enter a microbial
cell, de-repress the synthesis of metabolic enzymes,
and be converted by those enzymes to metabolic
intermediates (Suflita, 1989a). Biodegradation is less
likely (though not precluded) forthose molecules having
unusual structural features infrequently encountered in
the natural environment. Therefore, xenobiotic
compounds tend to persist in the natural environment
because microorganisms have not evolved necessary
metabolic pathways to degrade those compounds.
however, microorganisms are nutritionally versatile,
liave the potential to grow rapidly, and possess only a
single copy of ON A. Therefore, any genetic mutation or
recombination is immediately expressed. If the alteration
is of adaptive significance, new species of
microorganisms can be formed and grow. Contaminated
environments supply selection pressure forthe evolution
of organisms with new metabolic potential that can grow
utilizing the contaminating substance.
Aquifer Bloremediatlon
If an aquifer contaminant is determined to be susceptible
to biodegradation, the goal of bioremediation is to
utilize the metabolic capabilities of the indigenous
microorganisms to eliminate that contaminant (Suflita,
1989a). This practice generally does not include the
inoculation of the aquifer with foreign bacteria.
Bioremedial technologies attempt to impose particular
conditions in an aquifer to encourage microbial growth
and the presence of desirable microorganisms.
Bioremediation is based on knowledge of the chemical
and physical needs of the microorganisms and the
predominant metabolic pathways (Suflita, 1989a). Most
often, microbial activity is stimulated by supplying
nutrients necessary for microbial growth. Bioremediation
can take place either above ground or in situ. In situ
systems are especially appropriate for contaminants
that sorb to aquifer materials, since many decades of
pumping may be required to reduce the contaminants to
sufficiently low levels.
Successful implementation of aquifer bioremediation
depends on determining site-specific hydrogeological
variables, such as type and composition of'an aquifer,
permeability, thickness, interconnection to other
aquifers, location of discharge areas, magnitude of
water table fluctuations, and ground-water flow rates
(Suflita, 1989b). Generally, bioremediation is utilized in
more permeable aquifer systems where movement of
ground water can be more successfully controlled.
Removal of free product also is important forthe success
of bioremediation. Many substances that serve as
suitable nutrients for microbial growth when present at
low concentrations are inhibitory at high concentrations
(Suflita, 1989b).
Modeling Transport and Fate of Contaminants In an
Aquifer
Introduction
Models are simplified representations of real-world
processes and events, and their creation and use
require many judgments based on observation of
simulations of specific natural processes. Models may
be used to simulate the response of specific problems
to a variety of possible solutions (Keely, 1989b).
59
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Physical models, including sand-filled tanks used to
simulate aquifers and laboratory columns used to study
contaminant flow through aquifer materials, often are
used to obtain information on contaminant movement
(Keely, 1989b). Analog models also are physically
based, but are only similar to actual processes. An
example is the electric analog model, where capacitors
and resistors are used to replicate the effects of the rate
of water release from storage in aquifers. The main
disadvantage of physical models is the time and effort
required to generate a meaningful amount of data.
Mathematical models are non-physical and rely on
quantification of relationships between specific
parameters and variables to simulate the effects of
natural processes (Keely, 1989b, Weaver and others,
1989). Because mathematical models are abstract,
they often do not provide an intuitive knowledge of real-
world situations. However, mathematical models can
provide insights into the functional dependencies
between causes and effects in an actual aquifer. Large
amounts of data can be generated quickly, and
experimental modifications made easily, making
possible for many situations to be studied in detail for a
given problem.
Use and Categories of Mathematical Models
The application of mathematical models is subject to
error in real-world situations when appropriate field
determinations of natural process parameters are
lacking. This source of error is not addressed adequately
by sensitivity analyses or by the application of stochastic
techniques for estimating uncertainty. The high degree
of hydrogeological, chemical, and microbiological
complexity typically present in field situations requires
the use of site-specific characterization of the influences
of various natural processes by detailed field and
laboratory investigations (Keely, 1989b).
Mathematical models have been categorized by their
technical bases and capabilities as:
(1) parameter identification models; (2) prediction
models; (3) resource management models; and (4)
data manipulation codes. (Bachmat and others, 1978;
van der Heidje and others, 1985).
Parameter identification models are used to estimate
aquifer coefficients that determine fluid flow and
contaminant transport characteristics (e.g., annual
recharge, coefficients of permeability and storage, and
dispersivity (Shelton, 1982; Guven and others, 1984;
Puri, 1984; Khan, 1986a, b; Strecker and Chu, 1986)).
Prediction models are the most numerous type because
they are the primary tools used for testing hypotheses
(Mercer and Faust, 1981; Anderson and others, 1984;
Krabbenhoft and Anderson, 1986).
Resource management models are combinations of
predictive models, constraining functions (e.g., total
pumpage allowed), and optimization routines for
objective functions (e.g., scheduling wellfield operations
for minimum cost or minimum drawdown/pumping lift).
Few of these types of models are developed well
enough or supported to the degree that they are useful
(van der Heidje, 1984a and b; van derHeidje and others,
1985).
Data manipulations codes are used to simplify data
entry to other kinds of models and facilitate the
productions of graphic displays of model outputs (van
der Heidje and Srinivasan, 1983; Srinivasan, 1984;
Moses and Herman, 1986).
Quality Control Measures
Quality control measures are required to assess the
soundness and utility of a mathematical model and to
evaluate its application to a specific problem. Huyakorn
et al. (1984) and Keely (I989b) have suggested the
following quality control measures:
1. Validation of the model's mathematical basis by
comparing its output with known analytical solutions
to specific problems.
2. Verification of the model's application to various
problem categories by successful simulation of
observed field data.
3. Benchmarking the problem-solving efficiency of a
model by comparison with the performance of other
models.
4. Critical review of the problem conceptualization to
ensure that the modeling considers all physical,
chemical, and biological processes that may affect
the problem.
5. Evaluation of the specifics of the model's application,
e.g., appropriateness of the boundary conditions,
grid design, time steps.
6. Appraisal of the match between the mathematical
sophistication of the model and the temporal and
spatial resolution of the data.
Summary
Transport and fate assessments require interdisciplinary
analyses and interpretations because processes are
interdependent (Keely 1989a). Each transport process
should be studied from interdisciplinary viewpoints, and
interactions among processes identified and understood.
In addition to a sound conceptual understanding of
60
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transport processes, the integration of information on
geologic, hydrologic, chemical, and biological processes
into an effective contaminant transport evaluation
requires data that are accurate, precise, and appropriate
at the intended problem scale and that attempt to
account for spatial and temporal variations.
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66
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Chapter 4
GROUND-WATER TRACERS
In hydrogeology, "tracer" is a distinguishable matter or
energy in ground water that carries information on the
ground-water system. A tracer can be entirely natural,
such as the heat carried by hot-spring waters;
accidentally introduced, such as fuel oil from a ruptured
storage tank; or intentionally introduced, such as dyes
placed in water flowing within limestone caves.
Types and Uses of Tracer Tests
The variety of tracer tests is almost infinite, considering
the various combinations of tracertypes, local hydrologic
conditions, injection methods, sampling methods, and
geological settings. Tracer tests mainly are used (1)
to measure one or more hydrogeologic parameters of
an aquifer; and (2) to identify sources, velocity, and
direction of movement of contaminants. Tracer tests
also can be broadly classified according to whetherthey
rely on natural gradient flow or an induced flow from
pumping or some other means. Quinlan and others
(1988) discuss how to recognize falsely negative or
positive tracer results.
Measurement of Hydrogeologic Parameters
Tracers can be used to measure or estimate a wide
variety of hydrogeologic parameters, most commonly
direction and velocity of flow and dispersion. Depending
on the type of test and the hydrogeologic conditions,
other parameters such as hydraulic conductivity,
porosity, chemical distribution coefficients, source of
recharge, and age of ground water can be measured.
Figure 4-1 shows six examples of tracer measurement
of hydrogeologic characteristics by natural gradient
flow. Figure 4-1 a shows flow velocity in a cave system.
and Figure 4-1 b shows subsurface flow patterns in a
karst area with sinking and rising streams. Figure 4-1 c
shows the velocity of movement of dissolved material
between two wells. Both velocity and direction of flow
can be measured in a single well as shown in Figure 4-
1d and by using multiple downgradient sampling wells
as shown in Figure 4-1 e. Finally, hydrodynamic
dispersion can be measured by multiwell, multilevel
sampling down gradient (Figure 4-1 f).
Figure 4-2 shows four examples of tracer measurement
of hydrogeologic parameters using induced flow. A
tracer in surface water combined with pumping from a
nearby well can verify a connection, as shown in Figure
4-2a. Interconnections between fractures can be
mapped using tracers and inflatable packers in two
uncased wells, as shown in Figure 4-2b. Figure 4-2c
shows the measurement of a number of aquifer
parameters using a pair of wells with forced circulation
between wells. Figure 4-2d shows the evaluation of
geochemical interactions between multiple tracers and
aquifer material by alternating injection and pumping.
Tracers also can be used to determine ground-water
recharge using environmental isotopes (Ferronsky and
Polyakov, 1982; Moser and Rauert, 1985; Vogel and
others, 1974), and to date ground water (Davis and
Bentley. 1982).
Delineation of Contaminant Plumes
Any contaminant that moves in ground water acts as a
tracer; thus the contaminant itself may be mapped, or
other tracers may be added to map the velocity and
direction of the flow. Contaminant plumes are not
tracers in the sense used in this chapter and are not
discussed further here. However, Figure 4-3 shows
three examples of noncontaminant tracers used to
identify contaminant sources and flow patterns. Figure
4-3a shows the use of a tracer in a sinkhole to determine
if trash at a particular location is contributing to
contamination of a spring. Similarly, Figure 4-3b shows
that by flushing a dye tracer down a toilet one can
determine whether septic seepage is causing
contamination of a well or surface water. Figure 4-3c
shows the use of multiple tracers at multiple sources of
potential contamination to pinpoint the actual source.
67
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a. To meawra velocity o* water m cave tneam.
Sinking Straem
b . To check Murce of water it riae in Mream bed.
Sampling Point
I
77 i i n i
I I I I 1 11 I I I
ill i i /// r
C. To teat velocity of movement of dfeaotved mnrW unxtor
natural ground-wattr gradlmti.
Sampling Point
Wtt,, T.bl,
HUH
uiig
mini
Mill
Hill
urn
nun
d. To OMwmint v«*ociry arx) direction of groond-w»t»f ftow under
nnurtt condition*. Iniection tollowM) by Mmpling from same w*l
Sampling Poinu
To dturmin* 0M direction and velocity of natural ground-water
flow by drilling an array of sampling well* around a tracer injection
wall.
Muru-level Sampling
t t t
f. To tact hydrodynamfc cSeperaion in aquifer under natural
ground-water gradient*.
Figure 4-1. Common Configurations for Use of Tracer to Measure Hydrogeologic Parameters Using
Natural Gradient Flow (from Davis and others, 1985)
68
-------
L_J
7^\_ 1
Sampling Point
tl Pumping Wai
a. To verify connection between turlaca water and wen.
° ' li"* " numb*f °' •OU'*" perametefi using a pak of welte
whh forced circuUtion between VMM. ««""»««
Sampling
Point
b . To datarmina tha intarconnact f racturaa bafrvaao (wo uncaa*d
hohN. Ptckat* art inflaud with air and can b* poaitionad w
(taaintd In tha hotaa.
To teat precipitation of Mlactad conttituanti on th« aqurttf mitarial
by injacting multipia uacan into aquif ar than pumping back ttw
injactad watar.
Figure 4-2. Common Configurations for Use of Tracers to Measure Hydrogeologlc Parameters Using
Induced Flow (from Davis and others, 1985)
Tracer Selection
Overview of Types of Tracers
Ground-watertracerscan be broadly classified as natural
(environmental) tracers and injected tracers. Table 4-
1 lists 14 natural tracers and 30 injected tracers. Table
4-2 lists review papers, reports, and bibliographies that
are good sources for general information on ground-
water tracing.
The potential chemical and physical behavior of the
tracer in ground water is the most important selection
criterion. Conservative tracers, used for most purposes,
travel with the same velocity and direction as the water
and do not interact with solid material. Nonconservative
tracers, which tend to be slowed by interactions with the
solid matrix, are usedto measure distribution coefficients
and preferential flow zones in the vadose zone. For
most uses, a tracer should be nontoxic, inexpensive,
and easily detected to a low concentration with widely
available and simple technology. If the tracer occurs
naturally in ground water, it should be present in
concentrations well above background concentrations.
Finally, the tracer itself should not modify the hydraulic
conductivity or other properties of the medium being
studied.
No one ideal tracer has been found. Because natural
systems are so complex and the requirements for the
tracers themselves are so numerous, the selection and
69
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Fractured Rock
V:-.V.:.- Tracer
• To determine if trash in sinkhole contributes to
contamination of spring.
- t t ^"Sv
~~&'*£:£';«J«!S.,
Well
b . To determine if tile drain from septic tank contributes to
contamination of well.
Three Different Tracers
'Waste Water
Lagoon lOf Toil
» _jini...iiiiii'iniiiuc
rfini,..iini
'A-
Sampling Point
C . To determine source of pollution from three possibilities.
Figure 4-3. Common Configurations for Use of Tracers to Identify Contaminant Sources Using Natural
Gradient Flow (from Davis and others, 1985)
70
-------
NATURAL TRACER*
WUECTEO TRACER*
Rttloicllv*
AotlvcUkl*
Dauttrium ^
Oxygwv-IS 18O
C«rtoo-13 13C
Nbog*n-1S 15N
SronSum-M "»
Tritium
Sodun-24
Onmium-61
Cobi«-«
Cob*-60
51Cr
MIM
tonlixt SubtUMM OrlH MilMUl
S*ta: Ni'O Lynpodufn Spam
K*CT Bwlwte
IT'CT Vmam
***T Furqi
ITBT BmduU
R«dlo»etlv* Uotopfts
Trtfcjm-S SH
C*taon-14 >4C
Silioon-32 32a Phuphonw-32
Chtorin»-36 ^O
Argon-37 37Af
191,
Tnopri SBm6l(FMZZ)
Omd Vclow 96
FUxwowi
Aod Yellow 7
Krypton-81
81
Kr
Soorce; MooJIed horn Jonet. IBM
foun (Acid Red 87)
AmidDitiadwnn* e (Acid Rod 50)
Pkytlc*! ChiraetcrUllc*
Waur T*mpMM>»
FbodPUu
Table 4-1. Survey of Ground-Water Tracers
Reference
Description
Atkinson and Smart (1981)
Davis and others (1980)
Davis and others (1985)
Drew and Smith (1969)
Caspar (1987)
Grisak and others (1983)
Kairiman and Orlob (1956)
Keswicketal. (1982)
Knuttson (1968)
Mclaughlin (1982)
Mob and others (1986)
Smart and Laidlaw (1977)
Taylor and Dey (1985)
Tumor Design's
Ground Water Tracing
Series
van der Leeden (1987)
Review paper on uses of artificial tracers in hydrology.
Review paper on ground-water tracers.
Introductory EPA report on ground-water tracing. See also
discussion by Quintan (1986) and reply by Davis (1986)
Focuses on fluorescent dyes and lycopodium spores, but also
contains annotated bibliography on other tracers.
Compilation of papers on modern trends in tracer hydrology.
Report evaluating ground-water tracers for nuclear fuel waste
management studies.
Early review papar on use of radioactive and chemical tracers in
porous media.
Review paper on use of microorganisms as ground-water tracers.
Review paper on use of tracers for ground-water investigations.
Review paper on use of dyes as soil water tracers.
Focuses on aquifer tracer tests in porous media and use in
contaminant transport modeling.
Classic paper on the use of fluorescent dyes tor water tracers.
Bibliography on borehole geophysics as applied to ground-water
hydrology containing 42 references on tracers.
A series of annotated bibliographies concerning solute
movement in aquifer* and us* of dyes as tracers.
Smart et al. (1988) review 57 papers that compare dyes with other
tracers. See also Edwards and Smart (1988a. b).
The section in this bbliography on tracers and ground-water
dating contains 69 references.
Table 4-2. Sources of Information on General Ground-Water Tracing
71
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use of tracers is almost as much an art as a science.
The following sections discuss factors that should be
considered when selecting a tracer.
Hydrogeologic Considerations
The initial step in determining the physical feasibility of
a tracer test is to collect as much hydrogeologic
information about the field area as possible. The logs
of the wells at the site to be tested, or logs of the wells
closest to the proposed site, will give some idea of the
homogeneity of the aquifer, layers present, fracture
patterns, porosity, and boundaries of the flow system.
Local or regional piezometric maps, or any published
reports on the hydrology of the area (including results of
aquif ertests), are valuable, as they may give an indication
of the hydraulic gradient and hydraulic conductivity.
Major hydrogeologic factors that should be considered
when selecting a tracer include:
Lithology. Fine-grained materials, particularly clays,
have higher sorptive capacities than coarse-grained
material. The sorptive capacity must be considered
when evaluating the potential mobility of a tracer.
Flow Regime. Whether flow is predominantly
through porous media (alluvium, sandstone, soil),
solution features (karst limestone), or fractures will
influence the choice of tracer. For example,
fluorescent dyes work well in karst settings, but
because of sorption effects are less effective than
ground-water tracers in porous media.
Direction of Flow. For tracer studies using two or
more wells, the general direction of ground-water
movement must be known.
Travel Time. The equation for estimating travel
time was discussed previously. In two-well tracer
tests, travel time must be known to estimate
spacing for wells.
Dispersion. Tracertests often are used to measure
dispersion. In two-well tests, some preliminary
estimates may be required to estimate the quantity
of tracer to inject so that concentrations will be high
enough to detect.
Tracer Characteristics
Tracers have a wide range of physical, chemical, and
biological characteristics. These properties, as they
relate to hydrogeologic and other factors will determine
the most suitable tracer for the purposes desired. .
Detectability. Injected tracers should have no, or
very low, natural background levels. Lower
detection limit is for instruments (ppm, ppb, ppt),
are better. The degree of dilution is a function of
type of injection, distance, dispersion, porosity, and
hydraulic conductivity. Too much dilution may
result in failure to observe the tracer when it reaches
a sampling point because concentrations are below
the detection limit. Possible interferences from
othertracers and natural waterchemistry may have
the same effect.
Mobility. Conservative tracers used to measure
aquifer parameters such as flow direction and
velocity should be (1) stable (i.e., not subject to
transformation by biodegradation or nonbiological
processes during the length of the test and analysis);
(2) soluble in water; (3) of a similar density and
viscosity; and (4) not subject to adsorption or
precipitation. Nonconservative, nontoxic tracers
used to simulate transport of contaminants should
have adsorptive and other chemical properties
similar to the contaminant of concern.
Toxicity. Nontoxic tracers should be used if at all
possible. If a tracer may be toxic at certain
concentrations, maximum permissible levels as
determined by federal, state, or county agencies
must be considered in relation to expected dilution
and proximity to drinking water sources. Most
agencies have set no limits, partly because the
commonly used tracers are nontoxic in
concentrations usually employed, and partly
because they never considered tracers to be a
problem demanding regulation.
Other Considerations
A tracer may be suitable for the test's purpose and the
hydrogeologic setting, yet still not be suitable for reasons
of economics, technological availability or sophistication,
or public health.
Economics. The tracer or the instrumentation to
analyze samples may be expensive. In this situation,
another less-expensive tracer with somewhat less
favorable characteristics may suffice.
Technology. Some tracers may be difficult to
obtain, or may require more complicated sampling
methods. Gases, for example, will escape easily
from poorly sealed containers. Similarly.
instrumentation forgas or isotope analyses may not
be available; e.g., only one or two laboratories in
the world can perform analyses of 36CI.
Public Health. Tracer injections must involve a
careful consideration of possible health implications.
Some local or state health agencies insist on review
72
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authority priorto use of artificially introduced tracers,
but most do not. Local citizens must be informed of
the tracer injections, and usually the results should
be made available to the public. Under some
circumstances, analytical work for tracer studies
must be performed in appropriately certified
laboratories. These are job-specific decisions.
Tracing In Karst vs. Porous Media
Ground-water flow in karst terranes is characterized by
conduit flow and diffuse flow through often complex
subsurface channel systems. Ground-water
contaminants tend to move rapidly in karst and re surge
at the surface in locations that cannot be readily predicted
from the morphology of surface drainage patterns. In
contrast, ground-water flow in porous media is
characterized by slow travel times and more generally
predictable flow directions. These differences require
substantially different approaches to conducting tracer
tests, as discussed in karst and porous media sections
in this document.
Types of Tracers
Considering the full range of organic ground-water
contaminants, hundreds, and possibly thousands, of
substances have been used as tracers in ground water.
The most commonly used tracers can be grouped into
six categories: (1) water temperature, (2) particulates
(called drift material in Table 4-1), (3) ions, (4) dyes, (5)
gases, and (6) isotopes. These categories are not
mutually exclusive (i.e., isotopes may take the form of
ions or gases). Selected tracers in each category in
relation to applicability in different hydrologic settings,
field methods, and type of detection used, are discussed
in the following sections .
Water Temperature
The temperature of water changes slowly as it migrates
through the subsurface, because water has a high
specific heat capacity compared to most natural
materials. For example, temperature anomalies
associated with the spreading of warm wastewater in
the Hanford Reservation in south central Washington
have been detected more than 8 km (5 mi) from the
source.
Water-temperature tracing is potentially useful, although
it has not been used frequently. The method may be
applicable in granular media, fractured rock, or karst
regions. Keys and Brown (1978) traced thermal pulses
from the artificial recharge of playa lake water into the
Ogallala formation in Texas. They described the use of
temperature logs (temperature measurements at
intervals in cased holes) to detect hydraulic conductivity
differences in an aquifer. Temperature logs also have
been used to determine vertical movement of water in
a borehole (Keys and MacCary, 1971; Sorey, 1971).
Changes in water temperature are accompanied by
changes in water density and viscosity, which in turn
alter the velocity and direction of flow. For example,
injected ground water with a temperature of 40°C will
travel more than twice as fast in the same aquiferunder
the same hydraulic gradient as water at 5°C. Because
the warm water has a slightly lower density than cold
water, buoyant forces give rise to flow that "floats" on top
of the cold water. To minimize temperature-induced-.
convection problems, accurately measured small
temperature differences should be used if hot or cold
water is in the introduced tracer.
Davis and others (1985) used temperature as a tracer
for small-scale field tests, in shallow drive-point wells 2
feet apart in an alluvial aquifer. The transit time of the
peaktemperaturewasabout 107 min, while the resistivity
data indicated a travel time of about 120 min (Figure 4-
4). The injected water had a temperature of 38°C, while
the ground-water temperature was 20°C; the peak
temperature obtained in the observation well was 27°C.
In these tests, temperature indicated breakthrough of
Initial Twnpcrnui* of lnj*ct*d Fluid • 47.1 *C
0 10 » SO 70 90 110 130
Tim* Ahw Inaction (Mlnutw)
Figure 4-4. Results of Field Test Using a Hot
Water Tracer (from Davis and others, 1985)
73
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the chemical tracers, aiding in the timing of sampling. It
also was useful as a simple, inexpensive tracer for
determining the correct placement of sampling wells.
Water-temperature tracing also can be used to detect
river recharge in an aquifer. Most rivers have large
seasonal water temperature fluctuations. If the river is
recharging an aquifer, the seasonal fluctuations can be
detected in the ground water relative to the river
(Rorabaugh, 1956).
Partlculates
Solid material in suspension, such as spores, can be a
useful tracer in areas where waterflows in large conduits
such as in some basalt, limestone, or dolomite aquifers.
Seismic methods at the surface have been used to
detect the location of time-delayed explosives floating
through a cave system (Arandjelovic, 1969). Small
paniculate tracers, such as bacteria, can travel through
any porous media such as soils and fractured bedrock
where the pore size is larger than the size of the
microorganism. Microorganisms are probably the most
commonly used paniculate tracers. Table 4-3 compares
characteristics of microbial tracers.
Yeast. Wood and Ehrlich (1978) reported the use of
baker's yeast (Saccharomyces cerevisiae) as a ground-
water tracer in a sand and gravel aquifer. Yeast is a
single-celled fungus that is ovoid in shape. The diameter
of a yeast cell is 2 to 3 u,m, which closely approximates
the size of pathogenic bacterial cells. This tracer
probably provides most information about the potential
movement of bacteria.
Wood and Ehrlich (1978) found that the yeast penetrated
more than 7 m into a sand and gravel aquifer in less than
48 hours after injection. This tracer is very inexpensive,
as is analysis. Another advantage is the lack of
environmental concerns.
Bacteria. Bacteria are the most commonly used
microbial tracers, because they grow well and are easily
detected. Keswick and others (1982) reviewed over 20
case studies of bacteria tracers. Some bacteria that
have been used successfully are Escherichia coliform
(E. coli). Streptococcus faecalis, Bacillus
stearothermophilus, Serratia marcescens, and Serratia
indica. These bacteria range in size from 1 to 10 u.m
and have been used in a variety of applications.
A fecal coliform, E. coli. has been used to indicate fecal
pollution at pit latrines, septic fields, and sewage disposal
Tracer
Bacteria
Spores
Size
(urn)
1-10
25-33
Time
Required for
Assay (days)
1-2
1/2
Essential
Equipment
Required
Incubator*
Microscope
Plankton nets
Yeast
Viruses:
Animal (enteric)
Bacterial
2-3
0.2-0.8
0.2-1.0
1-2
3-5
1/2-1
Incubator*
Incubator
Tissue Culture
Laboratory
Incubator*
'Many may be assayed at room temperature
Source: Keswick and others (1982)
Table 4-3. Comparison of Microbial Tracers
74
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sites. A "marker such as antibiotic resistance or H2S
production is used to distinguish the tracer from
background organisms.
greatest health concern in using these tracers is
that the bacteria must be nonpathogenic to humans.
Even Ejsoli has strains that can be pathogenic. Davis
and others (1970) and Wilkowske and others (1970)
have reported that Serratia marcescens may be lite-
threatening to patients who are hospitalized with other
illnesses. Antibiotic-resistant strains are another
concern, as the antibiotic resistance can be transferred
to potential human pathogens. This problem can be
avoided by using bacteria that cannot transfer this
genetic information. As is true with most other injected
tracers, permission to use bacterial tracers should be
obtained fromtheproperfederal, state, and local health
authorities.
Viruses. Animal, plant, and bacterial viruses also have
been used as ground-water tracers. Viruses are
generally much smaller than bacteria, ranging from 0.2
to 1.0 urn (see Table 4-1). In general, human enteric
viruses cannot be used because of disease potential.
Certain vaccine strains, however, such as a type of polio
virus, have been used but are considered risky. Most
animal enteric viruses are considered safer as they are
not known to infect humans (Keswick and others, 1 982).
Neither human nor most animal viruses, however, are
tene rally considered suitable tracers for field work
cause of their potential to infect humans.
Spores. Lycopodium spores have been widely used as
tracers in karst hydrogeologic systems in Europe since
the early 1950s, and less frequently used in the United
States since the 1970s. Much of the literature on the
use of spores, however, is in obscure European and
American speleological journals. More readily
accessible references on the use of spores include
Atkinson and others (1973). Gardner and Gray (1976),
and Smart and Smith (1976).
Lycopodium is a clubmoss that has spores nearly
spherical in shape, with a mean diameter of 33 u,m. It
is composed of cellulose and is slightly denser than
water, so that some turbulence is required to keep the
material in suspension. Some advantages of using
lycopodium spores as a tracer are:
* The spores are relatively small.
* They are not affected by water chemistry or
adsorbed by clay or silt.
* They travel at approximately the same velocity
as the surrounding water.
* The injection concentration can be very high
(e.g., 8 x 106 spores per cm3).
* They pose no health threat.
* The spores are easily detectable under the
microscope.
* At least five dye colors may be used, allowing
five tracings to be conducted simultaneously in
a karst system.
Some disadvantages associated with lycopodium
spores include the large amount of time required for
their preparation and analysis, and the filtration of
spores by sand or gravel if flow is not sufficiently
turbulent.
The basic procedure involves adding a few kilograms of
dyed spores to a cave or sinking stream. The movement
of the tracer is monitored by sampling downstream in
the cave or with plankton nets installed in the stream
bed at a spring. The sediment caught in the net is
concentrated and treated to remove organic matter.
The spores are then examined under the microscope.
Tracing by lycopodium spores is most useful in open
joints or solution channels (karst terrane) where there is
minimal suspended sediment. It is not useful in wells or
boreholes unless the water is pumped continuously to
the surface and filtered. The spores survive well in
polluted water, but do not perform well in slow flow or in
water with a high sediment concentration. A velocity of
a few miles per hour has been found sufficient to keep
the spores in suspension. According to Smart and
Smith (1976), lycopodium is preferable to dyes for use
in large-scale water resource reconnaissance studies
in karst areas. Skilled personnel should be available to
sample and analyze the spores and a relatively small
number of sampling sites should be used.
Ions
Inorganic ionic compounds such as common salts have
been used extensively as ground-water tracers. This
category of tracers includes those compounds that
undergo ionization in water, resulting in their separation
into charged species possessing a positive charge
(cations) or a negative charge (anions). The charge on
an ion affects its movement through aquifers by
numerous mechanisms.
Ionic tracers have been used as tools to determine flow
paths and residence times and measure aquifer
properties. Slichter (1902,1905) was probably the first
to use ionic tracers to study ground water in the United
States. Specific characteristics of individual ions or
75
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ionic groups may approach those of an ideal tracer,
particularly dilute concentrations of certain anions.
leachate migration and the extent of dilution by receiving
waters located by a waste disposal site (Ellis, 1980).
In most situations, anions (negatively charged ions) are
not affected by the aquifer medium. Mattson (1929),
however, showed that the capacity of clay minerals for
holding anions increases with decreasing pH. Under
conditions of low pH, anions in the presence of clay,
other minerals, or organic detritus may undergo anion
exchange. Other possible effects include anion exclusion
and precipitation/dissolution reactions. Cations
(positively charged ions) react much more frequently
with clay minerals through the process of cation
exchange, which displaces other cations such as sodium
and calcium into solution. Because of their interaction
with the aquifer media, little work has been done with
cations. Natural variations in Ca and Mg concentrations,
however, have been used to separate baseflow and
stormf low components in a karst aquifer (Dreiss,1989).
One advantage of simple ionic tracers is that they do not
decompose and, therefore, are not lost from the system.
However, a large number of ions (including Cl- and
NOs') have high natural background concentrations;
thus requiring the injection of a highly concentrated
tracer. More importantly, several hundred pounds of
chloride or nitrate may have an adverse effect on water
quality and biota, thus becoming a pollutant. This also
may result in density separation and gravity segregation
during the tracer test (Grisak and Pickens, 1980b).
Density differences will alterf low patterns, the degree of
ion exchange, and secondary chemical precipitation, all
of which may change the aquifer permeability.
Comparisons of tracer mobilities under laboratory and
field conditions by Everts and others (1989) found
bromide (BR-) to be only slightly less mobile than
nitrate. The generally low background concentrations
of bromide often make it the ion of choice when a
conservative tracer is desired.
Various applications of ionic tracers have been described
in the literature. Murray and others (1981) used lithium
bromide (LiBr) in carbonate terrane to establish hydraulic
connection between a landfill and a freshwater spring,
where use of Rhodamine WT dye tracer proved
inappropriate. Mather and others (1969) used sodium
chloride (NaCI) to investigate the influence of mining
subsidence on the pattern of ground-water flow.
Tennyson and Settergren (1980) used bromide (Br-) to
evaluate pathways and transit time of recharge through
soil at a proposed sewage effluent irrigation site.
Schmotzer and others (1973) used post-sampling
neutron activation to detect a Br- tracer. Chloride (Cl-
) and calcium (Ca+) were used by Grisak and Pickens
(1980b) to study solute transport mechanisms in
fractures. Potassium (K+) was used to determine
Non-ionic organic compounds that are not dyes (see
below) have received little attention as injected tracers.
The ubiquitousness of trace levels of organic
contaminants such as methylene chloride creates some
problems in evaluating the integrity of clay liners at
waste disposal sites, llgenfritz and others (1988) have
suggested using fluorobenzene as a field monitoring
tracer because it would not be likely to occur in normal
industrial and commercial activities.
Dyes
Dyes are relatively inexpensive, simple to use, and
effective. Either fluorescent or nonfluorescent dyes
may be useful in studies of water movement in soil if the
soil material that has absorbed the dye is excavated and
visually inspected. Fluorescent dyes are preferable to
nonfluorescent varieties in ground-water tracer studies
because they are easier to detect. Dole (1906) was the
first recommended use of dyes to study ground water in
the United States by reporting the results of f luorescein
and other dyes used in France beginning around 1882.
Stiles and others (1927) conducted early experiments
using uranine (fluorescein) to demonstrate pollution of
wells in a sandy aquifer, and Meinzer (1932) described
use of fluorescein as a ground-water tracer. However,
extensive use of fluorescent dyes for ground-water
tracing did not begin until after 1960. Quinlan (1986)
provides a concise, but comprehensive, guide to the
literature on dye tracing.
The advantages of using fluorescent dyes include very
high detectability, rapid field analysis, and relatively low
cost and low toxicity. Smart and Laidlaw (1977) classified
commonly used fluorescent dyes by color: orange
(Rhodamine B, Rhodamine WT, and Sulforhodamine
B); green (fluorescein, Lissamine FF, and pyranine);
and blue—also called optical brighteners. Aley and
others (in press) classify dyes according to the detector
(also called bug) used to recover them: dyes recovered
on cotton include optical brighteners (such as Tinopal
5BM GX, and Phorwhite BBH) and Direct Yellow 96;
and dyes recovered on activated charcoal (fluorescein
and Rhodamine WT).
The literature on fluorescent dye use is plagued by a
lack of consistency in dye nomenclature (Quinlan, 1986).
The standard reference to dyes is the Colour Index (Cl)
(SDC& AATCC, 1971-1982). Most dyes are classified
according to the Cl generic name (related to method of
dyeing) and chemical structure (the Cl constitution
number). Abrahart (1968, pp. 15-43) provides a concise
guide to dye nomenclature. Dyes also are classified
according to their use in foods, drugs and cosmetics
76
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(Marmion, 1984). There are numerous commercial
names for most dyes. Consequently reported results of
dye tracing experiments should always specify (1) the
ICI generic name or Cl constitution number, and (2) the
manufacturerandthemanufacturer'scommercialname.
The full name of the dye should be mentioned at least
once to distinguish it from other dyes with the same or
similar names. For example, in 1985, four structurally
different kinds of Rhodamine were sold in the United
States under 11 different names by five manufacturers,
and there are more than 180 kinds of Direct Yellow dye
(Quinlan, 1986).
The first part of the commerical name of a dye should
not be confused with the dye itself. For example,
Tinopal and Phorwhite are trade names used for whole
series of chemically unrelated dyes made by a single
company and should be capitalized. Seven chemically
different Tinopals and 20 different Phorwhites are
currently sold in the United States as optical brighteners
(Aley and others, in press).
A particularly confusing point of dye nomenclature is
that there are two fluorescein dyes with the same Cl
name and number, although they do have different
(Drug and Cosmetic) D&C designations: fluorescein
(C20H12O5)—D&C Yellow 7—and fluorescein sodium
(C20H12O5Na2)—D&C Yellow 8. Only D&C Yellow 8
is soluble in water and, therefore, suitable for ground-
'watertracing. In the American and British literature this
is referred to as fluorescein, whereas in the European
literature it is called uranine (Quinlan, 1986).
Although fluorescent dyes exhibit many of the properties
of an ideal tracer, a number of factors interfere with
concentration measurement. Fluorescence is used to
measure dye concentration, but the amount of
fluorescence may vary with suspended sediment load,
temperature, pH, CaCOa content, salinity, etc. Other
variables that affect tracer test results are "quenching"
(some emitted fluorescent light is reabsorbed by other
molecules), adsorption, and photochemical and
biological decay. A disadvantage of fluorescent dyes in
tropical climates is poor performance because of
chemical reactions with dissolved carbon dioxide (Smart
and Smith, 1976).
Fluorescence intensity is inversely proportional to
temperature. Smart and Laidlaw (1977) described the
numerical relationship and provided temperature
correction curves. Low pH tends to reduce fluorescence.
Figure 4-5 shows that the fluorescence of Rhodamine
WT decreases rapidly at increasingly acidic pHs below
about 6.0. An increase in the suspended sediment
concentration also generally causes a decrease in
fluorescence.
— — HCI &N»OH
— HNO, ft NiOH
3.0
5.0
7.0
9.0
11.0
pH
Figure 4-5. The Effect of pH on Rhodamine WT
(adapted from Smart and Laidlaw, 1977)
Dyes travel slower than water due to adsorption, and
are generally not as conservative as radioactive tracers
or some of the ionic tracers. Adsorption can occur on
organic matter, clays (bentonite, kaolinite, etc.),
sandstone, limestone, plants, plankton, and even glass
sample bottles. However, the detected fluorescence
may decrease or actually increase due to adsorption.
Adsorption on kaolinite caused a decrease in the
measured fluorescence of several dyes, as measured
by Smart and Laidlaw (1977). If dye is adsorbed onto
suspended solids, and the fluorescence measurements
are taken without separating the water samples from
the sediment, the dye concentration is a measure of
sediment content rather than water flow.
These possible adsorption effects are a strong incentive
to choose a dye that is nonsorptive for the type of
medium tested. Different dyes vary greatly in amount of
sorption on specific materials. For example, Repogle
and others (1966) measured sorption of three orange
dyes on bentonite clay with the following results:
Rhodamine WT, .28%; Rhodamine B, 65%; and
Sulforhodamine B. 96%.
In a review of the toxicity of 12 fluorescent dyes Smart
(1984) .identified only three tracers (Tinopal CBS-X,
Fluorescein, and Rhodamine WT) with no demonstrated
carcinogenic or mutagenic hazard. Use of Rhodamine
B was not recommended because it is a known
77
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carcinogen. Use of the other dyes was considered
acceptable provided normal precautions are observed
during dye handling. Aulenbach and others (1978)
concluded that Rhodamine B should not be used as a
ground-water tracer simply on the basis of sorption
losses.
Currently, the U.S. Geological Survey has a policy of
limiting the maximum concentration of fluorescent dyes
at water-user withdrawal points to 0.01 ppm (Hubbard
and others, 1982). This is a conservative, non-obligatory
limit, and Field and others (1990) recommend that
tracer concentrations not exceed 1 ppm for a period in
excess of 24 hours in ground water. Dyes should
probably not be used where water supplies are
chlorinated because dye molecules may react with
chlorine to form chlorophenols (Smart and Laidlaw,
1977). Field and others (1990) recommend careful
evaluation of a tracer before use in a sensitive or unique
ecosystem.
General references on fluorescent dye use are three
U.S. Geological Survey publications (Hubbard and
others. 1982; Kilpatrick and Cobb, 1985; Wilson and
others, 1986), reviews by Smart and Laidlaw (1977)
and Jones (1984), and two reports prepared for EPA
(Mull, 1988; Quinlan, 1989). Aley and Fletcher (1976)
remains a classic but outdated text on practical aspects
of dye tracing; it will be replaced by The Joy of Dyeing
(Aley and others, in press) when that compendium is
published.
Fluorescein, also known as uranine, sodiumf luorescein,
and other names, has been one of the most widely used
green dyes. Like all green dyes, its use is commonly
complicated by high natural background fluorescence,
which lowers sensitivity of analyses and makes
interpretation of results more difficult. Feuerstein and
Selleck (1963) recommend that f luorescein be restricted
to short-term studies of only the highest quality water.
Lewis and others (1966) used f luorescein in a fractured
rock study. Mather and others (1969) recorded its use
in a mining subsidence investigation in South Wales.
Tester and others (1982) used f luorescein to determine
fracture volumes and diagnose flow behavior in a
fractured granitic geothermal reservoir. They found no
measurable adsorption or decomposition of the dye
during the 24-hr exposures to rocks at 392°F. At the
other extreme, Rahe and others (1978) did not recover
any injected dye in their hillslope studies, even at a
distance of 2.5 m downslope from the injection point.
The same experiment used bacterial tracers
successfully.
Anothergreenfluorescent dye, pyranine, has a stronger
fluorescent signal than does fluorescein, but is much
more expensive. It has been used in several soil
studies. Reynolds (1966) found pyranine to be the most
stable dye for use in an acidic, sandy soil. Drew and
Smith (1969) stated that pyranine is not as easily
detectable as fluorescein, but is more resistant to
decoloration and adsorption. Pyranine has a very high
photochemical decay rate, and is strongly affected by
pH in the range found in most natural waters (McLaughlin,
1982).
Rhodamine WT has been considered one of the most
usefultracersforquantitative studies.based on minimum
detectability, photochemical and biological decay rates,
and adsorption (Knuttson, 1968; Smart and Laidlaw,
1977; Wilson and others, 1986). Rhodamine WT is the
most conservative dye available for stream tracing
(Hubbard and others, 1982). Fluorescein is the most
common dye used for tracing ground water in karst.
Aulenbach and others (1978) compared Rhodamine B,
Rhodamine WT, and tritium as tracers in effluent from
a sewage treatment plant that was applied to natural
delta sand beds. The Rhodamine B was highly adsorbed,
while the Rhodamine WT and tritium yielded similar
breakthrough curves. Aulenbach and Clesceri (1980)
found Rhodamine WT very successful in a sandy
medium. Gann and Harvey (1975)fused Rhodamine
WT for karst tracing in a limestone and dolomite system
in Missouri.
Rhodamine B and Sulforhodamine B are poor tracers
for use in ground water and most surface waters; it
could be said the "B" stands for "bad." Amidorhodamine
G is a significantly better tracer; similarly, it can be said
that the "G" stands for "good" (personal communication,
James Quinlan, ATEC Environmental Consultants,
Nashville, TN, July, 1990).
Blue fluorescent dyes, or optical brighteners, have been
used in increasing amounts in the past decade in
textiles, paper, and other materials to enhance their
white appearance. Water that has been contaminated
by domestic waste entering septic tank soil absorption
fields can be used as a "natural" tracer if it contains
detectable amounts of the brighteners. Glover (1972)
was the first to describe the use of optical brighteners as
tracers in karst environments. Since then, they have
been extensively used in the United States (Quinlan,
1986). The tracer Amino G acid is a dye intermediate
used in the manufacture of dyes that is sometimes
mistakenly classified as an optical brightener (Quinlan,
1986). Amino G acid is now recognized as a carcinogenic
and should not be used in water that might be used for
drinking (personal communication, James Quinlan,
ATEC Environmental Consultants, Nashville, TN, July,
78
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1990). Smart and Laidlaw (1977) provide detailed
information on the characteristics of the optical brightener
Photine CU and Amino G acid.
Gases
Numerous natural and artificially produced gases have
been found in ground water. Some of the naturally
produced gases can be used as tracers, and gas also
can be injected into ground water where it dissolves and
can be used as a tracer. Only a few examples of gases
being used as ground-water tracers are found in the
literature, however. Table 4-4 lists possible gases to
use in hydrogeologic studies. Gases are useful tracers
in the saturated zone. They are less reliable in the
unsaturated zone because bleeding into the atmosphere
can give falsely negative results.
Inert Natural Gases. Because of their nonreactive and
nontoxic nature, noble gases are potentially useful
tracers. Helium is used widely as a tracer in industrial
processes. Carter and others (1959) studied the
feasibility of using helium as a tracer in ground water
and found that it traveled at a slightly lower velocity than
chloride. Advantages of using helium as a tracer are its
(1) safety, (2) low cost, (3) relative ease of analysis, (4)
low concentrations required, and (5) chemical inertness.
Disadvantages identified by Carter and others (1959)
include (1) relatively large errors in analysis, (2) difficulties
in maintaining a constant recharge rate, (3) time required
to develop equilibrium in unconfined aquifers, and (4)
possible loss to the atmosphere in unconfined aquifers.
Neon, krypton, and xenon are other possible candidates
for injected tracers because their natural concentrations
are very low (Table 4-4). Although the gases do not
undergo chemical reactions and do not participate in ion
exchange, the heavier noble gases (krypton and xenon)
do sorb to some extent on clay and organic material.
The solubility of the noble gases decreases with
increases in temperature. Therefore, the natural
concentrations of these gases in ground water are an
indication of surface temperatures at the time of water
infiltration. This property has been used to reconstruct
palaeoclimatic trends in a sandstone aquifer in England
using argon and krypton for age estimates (Andrews
and Lee, 1979). Sugisaki (1969) and Mazor (1972) also
have used natural inert gases in this way.
Anthropogenic Gases. Numerous artificial gases have
been manufactured during the past decade, and several
of them have been released in sufficient volumes to
produce measurable concentrations in the atmosphere
on a worldwide scale. One of the most interesting
groups of these gases is the fluorocarbons. These
gases generally pose a very low biological hazard, are
generally stable for periods measured in years, do not
react chemically with other materials, can be detected
in very low concentrations, and sorb only slightly on
most minerals. They do sorb strongly, however, on
organic matter.
Fluorocarbons have two primary applications. First,
because large amounts of fluorocarbons were not
Approximate Natural
Background Assuming
Equilibrium with
Atmosphere at 20°C
(mggas/L water)
Source: Davis and others (1985)
Maximum Amount in
Solution Assuming
100% Gas at Pressure
of 1 atm at 20°C
(mg gas/L water)
Argon
Neon
Helium
Krypton
Xenon
Carbon monoxide
Nitrous oxide
0.57
I.7x10^4
8.2 x 10'6
2.7 x10'4
5.7 X 10'5
6.0 x10'6
3.3 x 10'4
60.6
9.5
1.5
234
658
28
1.100
Table 4-4. Gases of Potential Use as Tracers
79
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released into the atmosphere until the later 1940s and
early 1950s, the presence of fluorocarbons in ground
water indicates that the water was in contact with the
atmosphere within the past 30 to 40 years (Thompson
and Hayes, 1979). The second application of
fluorocarbon compounds is as injected tracers
(Thompson and others, 1974). Because detection
limits are so low, large volumes of water can be labeled
with the tracers at a rather modest cost. Despite the
problem of sorption on natural material and especially
on organics, initial tests have been quite encouraging.
Isotopes
An isotope is any of two or more forms of the same
element having the same atomic number and nearly the
same chemical properties but with different atomic
weights and different numbers of neutrons in the nuclei.
Isotopes may be stable (they do not emit radiation) or
radioactive (they emit alpha, beta, and/or gamma rays).
There are over 280 isotopic forms of stable elements
and 40 or so radioactive isotopes (Glasstone, 1967). A
wide variety of stable and radioactive isotopes have
been used in ground-water tracer studies. There is an
extensive literature on the use of isotopes in ground-
water investigations; Table 4-5 lists 15 general sources
of information. Isotopes have beenused mainly in
porous media to study regional ground-water flow
regimes and measure aquifer parameters. Back and
Zoetl (1975) and LaMoreaux and others (1984) review
use of isotopes in karst hydrologic systems. Lack of
familiarity with techniques to analyze environmental
isotopes has limited their use by practicing field
Reference
Description
Back and Cherry (1976)
Csallany(1966)
Davis and Bentley (1982)
Ferronsky and Polyakov (1982)
Fritz and Fontes (1980.1986)
Caspar and Oncescu (1972)
IAEA (1963)
IAEA (1966)
IAEA (1967)
IAEA (1970)
IAEA (1974)
IAEA (1978)
Moserand Rauert (1985)
Wiebenga and others (1967)
Contains a brief review of use of environmental isotopes in ground-
water studies.
Early review paper on use of radioisotopes in water resources
research.
Review paper on ground-water dating techniques.
Text on use of environmental isotopes in the study of water.
Handbook on environmental isotope geochemistry (two volumes).
Text on use of radioactive tracers in hydrology (14 chapters).
Symposium on radioisotopes in hydrology.
Symposium on isotopes in hydrology with 21 papers on subsurface
hydrology.
Symposium on radioisotope tracers in industry and geophysics
contains a number of papers related to ground-water applications.
Symposium on isotopes in hydrology with 25 papers on subsurface
hydrology.
Symposium on isotopes in ground-water hydrology with 51 papers.
Symposium on isotopes in hydrology with 41 papers on subsurface
hydrology.
Review paper on use of environmental isotopes for determining
ground-water movement.
Review paper on use of radioisotopes in ground-water tracing.
Table 4-5. Sources of Information on Uses of Isotopes In Ground-Water Tracing
80
-------
hydrogeologists ground-water contamination studies.
Hendry (1988) recommends the use of hydrogen and
oxygen isotopes as a relatively inexpensive way to
estimate the age of near-surface ground-watersamples.
Stable Isotopes. Stable isotopes are rarely used for
artificially injected tracer studies in the field because (1)
it is difficult to detect small artificial variations of most
isotopes against the natural background, (2) their
analysisiscostly,and(3) preparingisotopicallyenriched
tracers is expensive. The average stable isotope
composition of deuterium (2H) and 180 in precipitation
changes with elevation, latitude, distance from the
coast, and temperature. Consequently, measurement
of these isotopes in ground water can be used to trace
the large-scale movement of ground water and to locate
areas of recharge (Gat, 1971; Ferronsky and Polyakov,
1982).
The two abundant isotopes of nitrogen (14N and 15N)
can vary significantly in nature. Ammonia (NH/t)
escaping as vapor from decomposing animal wastes,
for example, will tend to remove the lighter (14N)
nitrogen and will leave behind a residue rich in heavy
nitrogen. In contrast, many fertilizers with an ammonia
base will be isotopically light. Natural soil nitrate will be
somewhat between these two extremes. As a
consequence, nitrogen isotopes have been used to
determine the origin of unusually high amounts of
nitrate in ground water. Also, the presence of more than
about 5 mg/L of nitrate is commonly an indirect indication
of contamination from chemical fertilizers and sewage.
The stable sulfur isotopes (32S, 34S, and 36S) have
been used to distinguish between sulfate originating
from natural dissolution of gypsum (CaSO4.2H2O) and
sulfate originating from an industrial spill of suit uric acid
(H2S04).
Two stable isotopes of carbon (12C and 13C) and one
radioisotope (14C) are used in hydrogeologic studies.
Although not as commonly studied as 14C, the ratio of
the stable isotopes, 13C/12C, is potentially useful in
sorting out the origins of certain contaminants found in
water. For example, methane (CH4) originating from
some deep geologic deposits is isotopically heavier
then methane originating from near-surface sources.
This contrast forms the basis for identifying aquifers
contaminated with methane from pipelines and from
subsurface storage tanks.
Isotopes of other elements such as chlorine, strontium,
and boron are used to determine regional directions of
ground-water flow rather than to identify sources of
contamination.
Radlonuclides. Radioactive isotopes of various
elements are collectively referred to as radionuclides.
In the early 1950s there was great enthusiasm for using
radionuclides both as natural "environmental" tracers
and as injected artificial tracers. The use of artificially
injected radionuclides has all but ceased in many
countries, including the United States, however, because
of concerns about possible adverse health effects (Davis
and others, 1985). Artificially introduced radioactive
tracing mostly is confined to carefully controlled
laboratory experiments ortodeeppetroleum production
zones that are devoid of potable water. Table 4-6 lists
eight radionuclides commonly used as injected tracers,
their half-lives, and the chemical form in which they are
typically used.
Radionucllide
2H
32p
51Cr
60Co
82Br
85Kr
1311
98Au
HaB-LJte
y-year
d-day
h - hour
12.3y
14.3d
27. Bd
.25y
33.4h
10.7y
8.1d
2.7d
Chemical Compound
H2O
H32HPO4
EDTA-CrandCrC13
EDTA-Co and KaCo (CNe>
NhUBr. NaBr,LJBr
Kr(oas)
landKI
AuC)3
Source: Davis and others (1985)
Table 4-6. Commonly Used Radioactive Tracers for Ground-Water Studies
81
-------
The use of natural environmental tracers has expanded
so that they are now a major component of many
hydrochemical studies. A number of radionuclides are
present in the atmosphere from natural and artificial
sources, and many of these are carried into the
subsurface by rain water. The most common
hydrogeologic use of these radionuclides is to estimate
the average length of time ground water has been
isolated from the atmosphere. This measurement is
complicated by dispersion in the aquifer and mixing in
wells that sample several hydrologic zones.
Nevertheless, the age of water in an aquiferusually can
be established as being older than some given limiting
value. For example, detection of atmospheric
radionuclides might indicate that ground water was
recharged more than 1,000 years ago or that, in another
region, all the ground water in a given shallow aquifer is
younger than 30 years.
Since the 1950s, atmospheric tritium, the radioactive
isotope of hydrogen (3H) with a half life of 12.3 years,
has been dominated by tritium from the detonation of
thermonuclear devices. Thermonuclear explosions
increased the concentration of tritium in local rainfall to
more than 1,000 tritium units (TU) in the northern
hemisphere by the early 1960s (Figure 4-6). As a result,
ground water in the northern hemisphere with more
than about 5 TU is generally less than 30 years old.
Very small amounts of tritium, 0.05 to 0.5 TU, can be
produced by natural subsurface processes, so the
presence of these low levels does not necessarily
indicate a recent age.
The radioactive isotope of carbon. 14C (with a half-life
of 5,730 years), is also widely studied in ground water.
In practice, the use of 14C is rarely simple. Sources of
old carbon, primarily from limestone and dolomite, will
dilute the sample, and a number of processes, such as
the formation of CH4 gas orthe precipitation of carbonate
minerals, will fractionate the isotopes and alter the
apparent age. Interpreting 14C "ages" of water is so
complex that it should be attempted only by
hydrochemists specializing in isotope hydrology. Despite
the complicated nature of 14C studies, they are highly
useful in determining the approximate residence time of
old water (500 to 30,000 years) in aquifers. In certain
circumstances, this information cannot be obtained in
any other way.
Inert Radioactive Gases. Chemically inert but
radioactive 133Xe and 85Kr appear to be suitable for
many injected tracer applications (Robertson, 1969;
Wagner, 1977), provided legal restrictions can be
overcome. 222Rn, one of the daughter products from
the spontaneous fission of 238U, is the most abundant
of the natural inert radioactive gases. Radon is present
4000
3600
3000
5 2500
I 200°
*~ 1800
1000
BOO
YMT
Figure 4-6. Average Annual Tritium
Concentration of Rainfall and Snow for Arizona,
Colorado, New Mexico, and Utah (from Davis and
others, 1985, after Vuataz and others, 1984)
in the subsurface, but owing to the short half-life (3.8 d)
of 222Rn, and the absence of parent uranium nuclides
in the atmosphere, radon is virtually absent in surface
waterthat has reached equilibrium with the atmosphere.
Surveys of radon in surface streams and lakes have,
therefore, been useful in detecting locations where
ground water enters surface waters (Rogers, 1958).
Hoehn and von Gunten (1989) measured dilution of
radon in ground water to assess infiltration from surface
waters to an aquifer.
Tracer Tests In Karst
Probably no hydrogeologic system has been more
extensively studied by a more diverse group of people
with such a plethora of tracing techniques as karst
limestone terranes. Geese (Aley and Fletcher, 1976),
tagged eels (Bdgli, 1980), computerpunch-card confetti
(Davis and others, 1985), and time bombs
(Arandjetovich, 1969) are among the more exotictracers
that have been used in karst.
There is an extensive international literature on karst
tracing. Table 4-7 describes 18 major sources of
general information on this topic. There is a substantial
English- language literature in American caving journals,
such as Cave Notes/Caves and Karst (which ceased
publication in 1973), Missouri Speleology, and the
National Speleological Society Bulletin, and similar
British periodicals, such as Transactions of the Cave
Research Group (now Cave Science), and the
Proceedings of the University of Bristol Speleological
Society. The international symposia on underground
82
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water tracing (SUWT— see Table 4-7) provide the best
systematic compilations of international research on
this topic. Probably the easiest way to monitor the
international literature on dye-tracing in karst terranes
fend otherkarst and speleological literature is the annual
Speleological Abstracts published by the Union
Internationale de Speleologie in Switzerland.
Table 4-8 summarizes information on the most
commonly used water tracers in North American karst
studies. Dyes are almost ideal tracers because the
adsorption is usually not a problem in karst hydrogeologic
systems. Smart (1985) lists four applications of
fluorescent dye tracers in evaluating existing or potential
contamination in carbonate rocks: (1) confirmation of
leachate contamination, (2) determination of on site
hydrology, (3) determination of hydraulic properties of
landfill materials, and (4) prediction of leachate
contamination and dilution.
Fluorescein, Rhodamine WT, optical brighteners
(Tinopal 5BM GX), and Direct Yellow 96 are the most
commonly used dyes. The amount of dye injected
depends on whether qualitative or quantitative analysis
is planned. Qualitative tests involve simple visual
detection of dye in flowing water or captured by a
Reference
Description
Aley and Fletcher
(1976)
Aley and others (in press)
Back and Zoetl (1975)
Bogli (1980)
Brown (1972)
Gospodaric and Habic
(1976)
Gunn (1982)
Jones (1984)
LaMoreaux and others
Milanovia(1981)
Mull and others (1988)
Quinlan(1989)
Sweeting (1973)
SUWT (1966.1970.
1976. 1981. and 1986)
Thrailkill and others
(1983)
Classic guide to use of tracers in karst. Should be
replaced by Aley et al. (in press) when it is published.
Compendium of techniques for ground-water tracing focusing on karst terranes.
Review of the use of geochemical, isotopic. dye, spore, and artificial
radioisotopes as tracers in karst systems.
Pages 138-143 review use of tracers in karst hydrology.
Chapter III reviews tracer methods in karst hydrologic systems.
Pages 217-230 contain reviews of the applicability of dyes, salts, radionuclides,
drifting materials, and other tracers in karst.
Review paper on karst water tracing in Ireland.
Review paper on use of dye tracers in karst.
Pages 196-210 of the 1984 annotated bibliography focus (1984. 1989) on
isotope techniques for water tracing in carbonate rocks. The 1989 annotated
bibliography contains a section reviewing pollution assessment in
carbonate terranes.
Pages 263-309 focus on karst water tracing.
EPA report on dye-tracing techniques in karst terranes.
EPA report with recommended dye-tracing protocols for ground-water tracing in
karst terranes.
Pages 218-251 focus on karst water and karst water tracing.
Publications related to the various international
symposia on underground water tracing (SUWT) contain numerous papers on
ground-water tracing techniques, mostly focusing on karst.
Report focusing on karst dye-tracing techniques.
fable 4-7. Sources of Information on Ground-Water Tracing In Karst Systems
83
-------
Tracer A
Color
Fluorescein
Sodium
C.H.Na.0.
Yellow-Green
Xanthene
Rhodamlne
WT
C.H.N.O.CI
Red-Purple
Xanthene
Lycopodium
Spore*
Lycopodium
CaMtum
Optical
Brighteners
Colorless
normal light
Direct Yellow
(DY96)Low
Visibility
Slilbene
derivative
Salt
Nad
Coloriost
Passive
Detector
Activated
coconut
charcoal
6-14 mesh
Activated
coconut
charcoal
6-14 mesh
Plankton
ratting
nytoln-
26 micron
Unbleached
cotton
Unbleached
cotton
' Recording
specific
conductance
meter or
regular
sampling
Maximum
Test Exofcain i
(alutriant) Emission nm
Ethyl actohol 405
•ndSXKOH. SIS
Visual test or
fluorometer »
2A-47B:
2A-12.
6SA filers
Ethyl alcohol 550
ft 5% KOH or 580
1-Propanol *
NH.OH. Solution
tasted using
fluorometer
and 5*6- 590
fihars.
Spares « sad- N/A
ment are washed
from tie nets.
Microscopic ex-
amination Is used
to Identity spores.
Visual exami- 360
nation ol do- 435
teotors under UV
lghtor7-37;2A +
47B Rlters.
Visual examine- N/A
ton of detectors
under UV ight or
7-37: 2A * 47B
Filters
Either a dree* N/A
lest lor an irv
crease In chlor-
ide, or a sub-
stantial increase
in specific
conductance
Detectable
Cone.
o.t ug/i
Dependent on
background
levels. •Controls*
must be
used to deter-
mine back-
ground
.01 UQ/1.
Dependent on
background
levels and fluc-
tuation.
Dependent on
background
levels. Several
kilograms of
spores are
usually used.
Dependent on
background
tevels. but
generally at least
.1 ug/1.
1 .0 ug/1 on
cotton, and with
fluorometric
analysis.
Dependent on
background
levels. Sovoral
hundred kilo-
grams may be
needed for larger
tests.
Advantages
1) Does not require
constant monitoring
or any •pedal
equipment. 2) In-
expensive.
1)Dyeisphoto-
chernicaly stable.
2) Dye may be used
in tow pH water*.
1) Several simultan-
eous tests may be
conducted using
diflorenl colored
spores 2) No coloring
of water occur*.
1) Inexpensive. 2)
No coloring of water
occurs
1 ) Little natural
background. 2) Good
stability and low
sorption. 3) No
coloring ol water.
1) Generally con-
sidered safe for use
on public water sy-
stems. 2) Useful
where fluorescent
background condi-
tions exclude other
methods
Disadvantages
1)Dyelspholo-
chemicaDy unstable.
2) Moderate
sorption on day.
3) pH sensitive.
1) Requires fie use
of a fturometer. 2)
Moderate day
sorption.
1 ) Spores may be
prematurely filtered
out 2) Field
collodion system
elaborate. 3) Sys-
tem is generally
more expensive.
1 ) Background
readings may be
excessively high.
2)Adsorbed onto
organic*.
1) Moderate cost 2)
Sensitive to pH.
1) Large quantities
usually needed. 2)
Background specific
conductance is
often high.
Remarks
Th« is "he most
popular method
used in the USA.
Carbon detectors
tret (uggetwd by
Dunn. 1957.
Rhodamina has
been used ex-
tensively In Can-
ada* USA. This
is not a suitable
method lor ama-
teurs without ac-
cess to a
flourometer.
1) Spores have not
been used in North
America.
May be used
simultaneously
with a green ft red
dye using fluoro-
metric separation.
Has been used
extensively in
Kentucky.
Salt is occai-
tionally used by
the US Geological
Survey for tests
dealing with public
water supplies.
1 G.K. Turner Piters lor Turner 111 Filter Fluorometer.
•Dye is usually most visible in dear water, deep pools, and in bright sunlight. These figures are not exact.
* Very dilute dye solutions may be concentrated upon the detector over a period of time.
Source: Jones, 1984.
Table 4-8. Evaluation of Principal Water Tracers Used in North American Karst Studies
detector (see discussion below). Semi-quantitative
results can be obtained by using a fluorometer or
spectrofluorometer to detect amounts of dye captured
by detectors such as activated charcoal that may not be
discernible to the eye. Interpretation of values from
such measurements is limited due to lack of precise
information on the variation in ground water flow and
dye concentration between collection of detectors.
Quantitative tests involve precise measurement of dye
concentrations in grab samples of water. If the exact
amount of injected dye is known, and flow measurements
are taken along with each sample, a mass-balance
analysis allows estimation of how much dye has been
distributed through different parts of the subsurface flow
system.
In qualitative tests, enough dye must be injected for
visual detection; quantitative tests using a fluorometer
or spectrofluorometer generally require one-tenth to
one hundredth as much dye. Determination of the
84
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correct quantity to inject is as much an art as a science,
and this should be determined by, or with the assistance
of, someone with experience in karst tracer tests.
ye is recovered with detectors called bugs (cotton or
activated charcoal, depending on the tracer), that are
typically suspended in streams and springs on
hydrodynamically stable stands called gumdrops.
Detectors are placed at springs or in streams where flow
from the point of injection is suspected of reaching the
surface. At chosen time intervals related to the distance
from the source of injection, detectors are collected and
replaced with fresh detectors. Detectors are usually
collectedfrequentlyduringthefirstfewdaysafter injection
to pinpoint the most rapid dye arrival time, and then
typically on a daily basis for several weeks. Background
tests always must be run before injection, especially
with optical brighteners because sewage effluent from
individual septic tank absorption fields may increase
background levels substantially.
Qualitative tracer tests in which two dyes are injected
into two different locations are readily done by combining
a fluorescent dye and an optical brightener, which use
different detectors. Quantitative techniques are available
(developed originally in Europe) forseparating mixtures
of fluorescent dyes (Quinlan, 1986). A 5-dye tracer test
has recently been conducted using these techniques
(personal communication, James Quinlan, January
•990). Perhaps the most comprehensive karst tracing
'xperiments in a single location were carried out in
Slovenia, Yugoslavia, in the early 1970s where five
dyes, lycopodium spores, lithium chloride, potassium
chloride, chromium-51, and detergents all were used
(Gospodaric and Habic, 1976).
Reports prepared for EPA by Mull and others (1988)
and Quinlan (1989) are the most comprehensive
references currently available on procedures for dye-
tracing in karst terranes. Aley and others (in press)
should be obtained when it becomes available. Smoot
and others (1987), and Smart (1988a) describe
quantitative dye- tracing techniques in karst, and Smart
(1988b) describes an approach to the structural
interpretation of ground-water tracers in karst terrane.
Tracer Tests In Porous Media
Tracer tests in porous media are used primarily to
characterize aquif erparameters such as regional velocity
(Leap, 1985), hydraulic conductivity distributions (Molz
and others, 1988), anisotropy (Kenoyer, 1988),
dispersivity (Bumb and others, 1985), and distribution
coefficient or retardation (Pickens and others, 1981;
Rainwater and others, 1987). Smart and others (1988)
nave prepared an annotated bibliography on ground-
water tracing that focuses on use of tracers in porous
media.
The purpose and practical constraints of a tracer test
must be clearly understood prior to actual planning.
Following are a few of the questions that need to be
addressed:
* Isonly the directionofwaterflowto be determined?
* Are other parameters such as travel time, porosity,
and hydraulic conductivity of interest?
* How much time is available for the test?
* How much money is available for the test?
If results must be obtained within a few weeks, then
certain kinds of tracer tests would normally be out of the
question. Those using only the natural hydraulic gradient
between two wells that are more than about 20 m apart
typically require long time periods for the tracer to flow
between the wells. Another primary consideration is
budget. Costs for tests that involve drilling several deep
holes, setting packers to control sampling or injection,
and analyzing hundreds of samples in an EPA-certified
laboratory could easily exceed $1 million. In contrast,
some short-term tracertests may cost less than $1,000.
Choice of atracerwill depend partiallyonwhichanalytical
techniques are easily available and which background
constituents might interfere with these analyses. The
chemist or technician who will analyze the samples can
advise whetherbackground constituents might interfere
with the analytical techniques to be used. Bacteria,
isotopes, and ions are the most frequently used types of
tracers in porous media. Fluorescent dyes are less
commonly used astracers because they tendto adsorb.
A more common use of dyes in porous media is to locate
zones of preferential flow in the vadose zone. In this
application, adsorption on soil particles is desirable
because it allows visual inspection of flow patterns
when the soil is excavated.
Estimating the Amount of Tracer to Inject
The amount of tracer to inject is based on the natural
background concentrations, the detection limit for the
tracer, the dilution expected, and experience.
Adsorption, ion exchange, and dispersion will decrease
the amount of tracer arriving at the observation well, but
recovery of the injected mass is usually not less than 20
percent for two-hole tests using a forced recirculation
system and conservative tracers. The concentration
should not be increased so much that density effects
become a problem. Lenda and Zuber (1970) presented
graphs that can be used to estimate the approximate
85
-------
quantity of tracer needed. These values are based on
estimates of the porosity and dispersion coefficient of
the aquifer.
Single-Well Techniques
Two techniques, injection/withdrawal and borehole
dilution, produce parameter values from a single well
that are valid at a local scale. Advantages of single-well
techniques are:
* Less tracer is required than for two-well tests.
* The assumption of radial flow is generally valid,
so natural aquifer velocity can be ignored, making
solutions easier.
* Knowledge of the exact direction of flow is not
necessary.
Molz and others (1985) describe design and performance
of single-well tracer tests conducted at the Mobile site.
Injection/Withdrawal. The single-well injection/
withdrawal (or pulse) technique can be used to obtain a
pore velocity value and a longitudinal dispersion
coefficient. The method assumes that porosity is known
or can be estimated with reasonable accuracy. In this
procedure, a given quantity of tracer is instantaneously
added to the borehole, the tracer is mixed, and then two
to three borehole volumes of freshwater are pumped in
to force the tracerto penetrate the aquifer. Only a small
quantity is injected so as not to disturb natural flow.
After a certain time, the borehole is pumped out at a
constant rate large enough to overcome the natural
ground-water flow. Tracer concentration is measured
with time or pumped volume. If the concentration is
measured at various depths with point samplers, the
relative permeability of layers can be determined. The
dispersion coefficient is obtained by matching
experimental breakthrough curves with theoretical
curves based on the general dispersion equation. A
finite difference method is used to simulate the theoretical
curves (Fried, 1975).
Fried concluded that this method is useful for local
information (2-to 4-m radius) and for detecting the most
permeable strata. A possible advantage of this test is
that nearly all of the tracer is removed from the aquifer
at the end of the test.
Borehole Dilution. This technique, also called point
dilution, can be used to measure the magnitude and
direction of horizontal tracer velocity and vertical flow
(Fried, 1975; Caspar and Oncescu, 1972; Klotz and
others, 1978).
The procedure introduces a known quantity of tracer
instantaneously into the borehole, mixes ft well, and
then measures the concentration decrease with time.
The tracer is generally introduced into an isolated
volume of the borehole using packers. Radioactive
tracers have been most commonly used for borehole
dilution tests, but other tracers can be used.
Factors to consider when conducting a point dilution
test include the homogeneity of the aquifer, effects of
drilling (mudcake, etc.), homogeneity of the mixture of
tracer and well water, degree of tracer diffusion, and
density effects.
Ideally, the test should be conducted using a borehole
with no screen or gravel pack. If a screen is used, it
should be next to the borehole because dead space
alters the results. Samples should be very small in
volume so that flow is not disturbed by their removal.
A variant of the point dilution method allows
measurement of the direction of ground-water flow. In
this procedure, a section of the borehole is usually
isolated by packers, and a tracer (often radioactive) is
introduced slowly and without mixing. Then, after some
time, a compartmental sampler (four to eight
compartments) within the borehole is opened. The
direction of minimum concentration corresponds to the
flow direction. A similar method is to introduce a
radioactive tracer and subsequently measure its
adsorption on the borehole or well screen walls by
means of a counting device in the hole. Caspar and
Oncescu (1972) describe the method in more detail.
Another common strategy is to inject and subsequently
remove the water containing a conservative tracerf rom
a single well. If injection is rapid and immediately
followed by pumping to remove the tracer, then almost
all of the injected conservative tracer can be recovered.
If the pumping is delayed, the injected tracer will drift
downgradient with the general flow of the ground water
and the percentage of tracer recovery will decrease with
time. Successive tests with increasingly longer delay
times between injection and pumping can be used to
estimate ground-water velocities in permeable aquifers
with moderately large hydraulic gradients.
Two-Well Techniques
There are two basic approaches to using tracers with
multiple wells: one measures tracer movement in
uniform (natural) flow and the other measures movement
86
-------
by radial (induced) flow. The parameters measured
(dispersion coefficient and porosity) are assumed to be
the same for both types of flow.
Rjnlform Flow. This approach involves placing a
tracer in one well without disturbing the flow field, and
sampling periodically to detect the tracer in observation
wells. This test can be used at a local (2 to 5 m) or
intermediate (5 to 100 m) scale, but it requires much
more time than radialtests. If the direction and magnitude
of the velocity are not known, a large number of
observation wells are needed. Furthermore, local flow
directions may diverge widely from directions predicted
on the basis of widely spaced water wells. Failure to
intercept a tracer in a well just a few meters away from
the injection well is not uncommon under natural-gradient
flow conditions.
The quantity of tracer needed to cover a large distance
can be expensive. On a regional scale, environmental
tracers, including seawater intrusion, radionuclides, or
stable isotopes of hydrogen and oxygen, are used.
Manmade pollution also has been used. For regional
problems, a mathematical model is calibrated with
concentration versus time curves from field data, and is
used to predict future concentration distributions.
Local- or intermediate-scale uniform flow problems can
t solved analytically, semianalytically, or by curve-
tching. Layers of different permeability can cause
torted breakthrough curves, which can usually be
analyzed using one- ortwo-dimensional models (Gaspar
and Oncescu, 1972). Fried (1975) and Lenda and
Zuber (1970) present analytical solutions.
Radial Flow. Radial flow techniques work by altering
the flow field of an aquifer through pumping. Solutions
are generally easier if radial flow velocity greatly exceeds
uniform flow. This method yields values for porosity and
the dispersion coefficient, but not natural ground-water
velocity. Types of radial flow tests include diverging,
converging, and recirculating tests.
A diverging test involves constant injection of water into
an aquifer. The tracer is introduced into the injected
water as a slug or continuous flow and the tracer is
detected at an observation well that is not pumping.
Point or integrated samples of small volume are carefully
taken at the observation well so that flow is not disturbed.
Packers can be used in the injection well to isolate an
interval.
In a converging test, the tracer is introduced at an
observation well, while another well is pumped.
^pncentrations are monitored at the pumped well. The
tracer often is injected between two packers or below
one packer; then two to three well-bore volumes are
injected to push the tracer out into the aquifer. At the
pumping well, intervals of interest are isolated
(particularly in fractured rock), or an integrated sample
is obtained.
A recirculating test is similarto a converging test, but the
pumped water is injected back into the injection well.
This tests a significantly greater part of the formation
because the wells inject to and pump from 360 degrees.
The flow lines are longer, however, partially canceling
out the advantage of a higher gradient. Sauty (1980)
provides theoretical curves for recirculating tests.
Design and Construction of Test Wells
In many tracer tests, construction of the test wells is the
single greatest expense. Procedures for the proper
design and construction of monitoring wellsfor sampling
ground-water quality (discussed in Chapter 3) apply
equally to wells used for tracer tests.
Special considerations for designing and constructing
test wells for tracer tests include:
* Drilling muds and mud additives tend to have a
high capacity for the sorption of most types of
tracers and, therefore, should be avoided.
* Drilling methods that alter the hydrologic
characteristics of the aquifer being tested (such
as clogging of pores) should be avoided.
* Use of packers to isolate the zones being
sampled from the rest of the water in the well
(see Figure 4-2b) allows the most precise
measurements of vertical variations in
hydrologic parameters. This approach tends to
be more expensive, takes longer, and requires
more technical training than whole-well tests.
* If packers are not used, the diameter of the
sampling well should be as small as possible so
that the amount of "dead" water in the well
during sampling is minimized.
* Well casing material should not be reactive with
the tracer used.
* Well-screen slot size and gravel pack must be
selected and installed with special care when
using single-weil tests with alternating cycles of
injection and pumping large volumes of water
into and out of loose fine-grained sand. On the
other hand, if the aquifer being tested contains
a very permeable coarse gravel and the casing
87
-------
diameter is small, then numerous holes drilled
in the solid casing may be adequate.
As with any monitoring well, tracer test wells
should be property developed to remove silt,
clay, drilling mud, and other materials that
would prevent tree movement of water in and
out of the well.
Injection and Sample Collection
Choice of injection equipment depends on the depth of
the borehole and the funds available. In very shallow
holes, the tracer can be lowered through a tube, placed
in an ampule that is lowered into the hole, and broken,
or just poured in. Mixing of the tracer with the aquifer
water is desirable and important for most types of tests
and is simple for very shallow holes. For example, a
plunger can be surged up and down in the hole or the
tracer can be released through a pipe with many
perforations. Flanges on the outer part of the pipe will
mix the tracer as the pipe is raised and lowered. For
deeper holes, tracers must be injected under pressure
and equipment can be quite sophisticated.
Sample collection also can be simple or sophisticated.
For tracing thermal pulses, only a thermistor needs to
be lowered into the ground water. For chemical tracers,
a variety of sampling methods may be used. Some
special sampling considerationsfortracertests include:
Bailers should not be used if mixing of the tracer
in the borehole is to be avoided.
* Where purging is required, removal of more
than the minimum required to obtain fresh
aquifer water may create a gradient towards
the well and distort the natural movement of the
tracer.
Use of existing water wells that tap multiple
aquifers should be generally avoided in tracer
tests except to establish whether a hydrologic
connection with the point of injection exists.
Interpretation of Results
This section provides a brief qualitative introduction to
the interpretation of tracer test results. More extensive
and quantitative treatments are found in the works of
Halevy and Nir (1962). Theis (1963), Fried (1975),
Sauty (1978), and Grisakand Pickens (1980a,b). Some
more recent papers on analysis of tracer tests include
GOven and others (1985,1986), Molz and others (1986,
1987), and Bullivant and O'Sullivan (1989).
The basic plot of the concentration of a tracer as a
function of time or water volume passed through the
system is called a breakthrough curve. The
concentration either is plotted as the actual concentration
(Figure 4-7) or, quite commonly, as the ratio of the
measured tracer concentration at the sampling point, C,
to the input tracer concentration, CQ (Figure 4-8).
too
10 -
i.o •
——— AminoG Acid
fthodcmirwWt
(LJBMmin* FF in|«ct«d but
Injection Wtfl
10 r
1.0
£• o.i
o.oi t,
0.11-
0.01
At 10 F«*t
0.001
AtSOFwt
4/285/1 5/10 6/»
e/i e/io
D«U
7/1 7/10 7/20
Figure 4-7. Results of Tracer Tests at the Sand
Ridge State Forest, Illinois (from Naymik and
Slevers, 1983)
The measured quantity that is fundamental for most
tracertests is the first arrival time of the tracer as it goes
from an injection point to a sampling point. The first
arrival time conveys at least two bits of information.
First, it indicates that aconnectionforground-waterflow
actually exists between the two points. For many tracer
tests, particularly in karst regions, this is all the information
that is desired. Second, if the tracer is conservative, the
maximum velocity of ground-water flow between the
two points may be estimated.
88
-------
Ditch Hied with
Tracer Having a
Conc«ntr»tJon of C
Sampling Well with
Water Hiving •
Trace* Conetntration
o»C
* Tracer Front
A. Tree* movement from infection ditch to umpling wed.
1.0
0.6
0.0
TWncof firw
Arrival
Tknt of Minimum
Rale of Change of C
A a
8. Breakthrough Curve.
-*-T.me
Figure 4-8. Tracer Concentration at Sampling Well,
C, Measured Against Tracer Concentration at Input,
Co (from Davis and others, 1985)
Interpretations more elaborate than the two mentioned
above depend very much on the type of aquifer being
tested, the velocity of ground-water flow, the
ponfiguration of the tracer injection and sampling
systems, and the type of tracer or mixture of tracers
used in the test.
The value of greatest interest after the first arrival time
is the arrival time of the peak concentration for a slug
injection; or, for a continuous feed of tracers, the time
since injection when the concentration of the tracer
changes most rapidly as a function of time (Figure 4-8).
In general, if conservative tracers are.used, this time is
close to the theoretical travel time of an average molecule
of ground water traveling between the two points.
If a tracer is being introduced continuously into a ditch
penetrating an aquifer, as shown in Figure 4-8, then the
ratio C/CQ will approach 1.0 after the tracer starts to
pass the sampling point. The ratio of 1.0 is rarely
approached in most tracer tests in the field, however,
because waters are mixed by dispersion and diffusion
in the aquifer and because wells used for sampling will
commonly intercept far more ground water than has
been tagged by tracers (Figure 4-9). Ratios of C/Co
ranging between 10'5 and 2 x 10'1 often are reported
from field tests.
|f a tracer is introduced passively into an aquifer but it is
recovered by pumping a separate sampling well, then
various mixtures of the tracer and the native ground
water will be recovered depending on the amount of
water pumped, the transmissivity of the aquifer, the
slope of the water table, and the shape of the tracer
plume. Keely (1984) has presented this problem
graphically with regard to the removal of contaminated
water from an aquifer.
With the introduction of a mixture of tracers, possible
interactions between the tracers and the solid part of the
aquifer may be studied. If interactions take place, they
can be detected by comparing breakthrough curves of
a conservative tracer with the curves of the other tracers
being tested (Figure 4-10). Quantitative analyses of
tracer breakthrough curves are generally conducted by
curve-matching computer-generated type curves, or
by applying analytical methods.
Ditch Fiaad with
Tracer Which
Suppkea 1/4 of
Downgrediant
Ground-Water
How- Sampling WeH
A. Tracer doea not fully taturala aquifer.
O.SO
O.X
0.00
8. Breakthrough curve.
Time
Figure 4-9. Incomplete Saturation of Aquifer with
Tracer (from Davis and others, 1985)
0.10 -
o.os •
0.00
Tracer A
(Conservative)
Tracer D
(Precipitated)
Tracer B
(Some Sorptionl
Tracer C
(Largely Sorbedl
Time
Figure 4-10. Breakthrough Curves for Conservative
and Nonconservatlve Tracers (from Davis and
others, 1985)
89
-------
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Pickens, J.F.. R.E. Jackson, K.J. Inch, and W.F. Merrill,
1981, Measurement of distribution coefficients using
radial injection dual-tracer tests: Water Resource
Research, v. 17, pp. 529-544.
Quinlan, J.F., 1986, Discussionof "Ground watertracers"
by Davis and others (1985) with emphasis on dye
tracing, especially in karst terranes: Ground Water, v.
24, no. 2, pp. 253-259 and v. 24, no. 3, pp. 396-397
(References).
Quinlan, J.F., 1989, Ground-water monitoring in karst
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Assumptions: EMSL. Las Vegas, NV, EPA 600/X-89/
050.
Quinlan. J.F., R.O. Ewars, and M.S. Fiel, 1988, How to
use ground-water tracing to "prove" that leakage of
harmful materials from a site in karst terrane will not
occur: in Proc. Second Conf. on Environmental Problems
in Karst Terranes and Their Solutions (Nashville, TN),
National Water Well Association, Dublin, OH. pp. 265-
288.
Rahe, T.M., C. Hagedorn, E.L. McCoy, and G.G. Kling,
1978, Transport of antibiotic-resistant Escherichia Coli
through western Oregon hill slope soils under conditions
of saturated flow: J. Environ. Quality, v. 7, pp. 487-494.
93
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Rainwater, K.A., W.R. Wise, and R.J. Charbeneau,
1987, Parameter estimation through groundwatertracer
tests: Water Resources Research, v. 23, pp. 1901-
1910.
Repogle, J.A., L.E. Myers, and K.J. Brus, 1966, Flow
measurements with fluorescent tracers: J. Hydraulics
Division ASCE, v. 92, pp. 1-15.
Reynolds, E.R.C, 1966, The percolation of rainwater
through soil demonstrated by fluorescent dyes: J. Soil
Science, v. 17, no. 1, pp. 127-132.
Robertson, J.B., 1969, Behavior of Xenon-133 gas after
injection underground: U.S. Geological Survey Open
File Report ID022051.
Rogers, A.S., 1958, Physical behavior and geologic
control of radon in mountain streams: U.S. Geological
Survey Bulletin 1052E. pp. 187-211.
Rorabaugh, M.I., 1956, Ground water in Northeastern
Louisville, Kentucky: U.S. Geological Survey Water-
Supply Paper 1360-B, pp. 101-169.
Sauty.J.P., 1978, Ident if icationofhydrodispersive mass
transfer parameters in aquifers by interpretation of
tracer experiments in radial converging or diverging
flow (in French): J. Hydrology, v. 39, pp. 69-103.
Sauty.J.P., 1980, An analysis of hydrodispersive transfer
in aquifers: Water Resources Research, v. 16, no. 1, pp.
145-158.
Slichter, C.S., 1902, The motions of underground waters:
U.S. Geological Survey Water-Supply and Irrigation
Paper 67.
Slichter, C.S., 1905, Field measurement of the rate of
movement of underground waters: U.S. Geological
Survey Water-Supply and Irrigation Paper 140, pp. 9-
34.
Schmotzer, J.K., W.A. Jester, and R.R. Parizek, 1973,
Groundwater tracing with post sampling activation
analysis: J. Hydrology, v. 20, pp. 217-236.
SDC & AATC (Society of Dyers & Colorists and American
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Index, 3rd ed.
Smart, C.C.. 1988a, Quantitative tracing of the Maligne
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185-204.
Smart, C.C., 1988b, Artificial tracer techniques for the
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Water, v. 26, no. 4, pp. 445-453.
Smart, P.L., 1976, Catchment delimitation in karst areas
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Symp. of Underground Water Tracing, Bled, Yugoslavia,
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Smart, P.L., 1984, A review of the toxicity of twelve
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Smart, P.L. and I.M.S. Laidlaw. 1977, An evaluation of
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Smoot, J.L., D.S. Mull, and T.D. Liebermann, 1987,
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Stiles, C.W., H.R. Crohurst, and G.E. Thomson, 1927,
Uranin test to demonstrate pollution of wells: in
Experimental Bacterial and Chemical Pollution of Wells
via Ground Water and the Factors Involved, U.S. Public
Health Service Hygienic Laboratory Bulletin No. 147,
pp. 84-87.
Sugisaki, R., 1969, Measurement of effective flow
velocity of groundwater by means of dissolved gases:
American Jour. Science, v. 259, pp. 144-153.
Sweeting, M.M., 1973, Karst landforms: Columbia
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Symposium on Underground Water Tracing (SUWT),
1966,1stSUWT(Graz,Austria): inSteirischesBeitraege
zur Hydrogeolgie Jg. 1966/67.
94
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Symposium on Underground Water Tracing (SUWT).
1970, 2nd SUWT (Freiburg/Br.. West Germany):
Published JQ Steirisches Beitraege zur Hydrogeolgie
22(1970) :5-165, and Geologisches Jahrbuch, Reihe C.
2(1972):1-382.
Symposium on Underground Water Tracing (SUWT),
1976,3rd SUWT (Ljubljana-Bled, Yugoslavia), published
by Ljubljana Institute for Karst Research: Volume 1
(1976), 213 pp., Volume 2 (1977) 182 pp. See also
Gospodaric and Habic (1976).
Symposium on Underground Water Tracing (SUWT),
1981, 4th SUWT (Bern, Switzerland): in Steirisches
Beitraege zur Hydrogeologie 32(1980):5-100;
33(l981):1-264; and Beitraege zur Geologie der
Schweiz—Hydrologie 28 pt.1(1982):1-236; 28
pt.2(1982):1-213.
Symposium on Underground Water Tracing (SUWT),
1986, SthSUWT (Athens, Greece), published by Institute
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Taylor, T.A. and J.A. Dey, 1985, Bibliography of borehole
geophysics as applied to ground-water hydrology. U.S.
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Tennyson, L.C. and C.D. Settergren, 1980, Percolate
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effluent irrigation sites: Water Resources Bulletin, v. 16,
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Tester, J.W., R.L. Bivens, and R.M. Potter. 1982,
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Atomic Energy Agency. Vienna, Austria (as cited by
Davis and others, 1985).
Thompson, G.M. and ~J.M. Hayes, 1979,
Trichlorofluoromethane in ground water, a possible
tracer and indicator of ground-water age: Water
Resources Research, v. 15. no. 3, pp. 546-554.
Thompson, G.M., J.M. Hayes, and S.N. Davis. 1974,
Fluorocarbon tracers in hydrology: Geophysical
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Thrailkill, J., and others, 1983, Studies in dye-tracing
techniques and karst hydrogeology: Univ. of Kentucky,
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van der Leeden, F., 1987. Geraghty & Miller's
groundwater bibliography, 4th ed: Water Information
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Vogel. J.C., L. Thilo, and M. Van Dijken. 1974,
Determination of groundwater recharge with tritium:
Jour. Hydrology, v. 23, pp. 131-140.
Vuataz. F.D.. J. Stix, F. Goff, and C.F. Pearson, 1984.
Low-temperature geothermal potential of the Ojo
Caliente warm spring area in Northern New Mexico: Los
Alamos National Laboratory Publication LA-10105-
OBES/VC-666.
Wagner, O.R., 1977. The use of tracers in diagnosing
interwell reservoir heterogeneities: J. Petroleum
Technology, v. 11, pp. 1410-1416.
Wiebenga, W.A.. W.R. Ellis, B.W. Seatonberry, and
J.T.G. Andrew, 1967, Radioisotopes as ground-water
tracers: Jour. Geophysical Research, v. 72, pp. 4081-
4091.
Wilkowske, C.J., J.A. Washington II, W.J. Martin, and
R.E. Ritts, Jr., 1970, Serratiamarcescens: Biochemical
characteristics, antibiotic susceptibility and clinical
significance: Jour. American Medical Association, v.
214, no. 12, pp. 2157-2162.
Wilson, Jr. J.F., E.D. Cobb, and F.A. Kilpatrick. 1986,
Fluorometric procedures for dye tracing (Revised): U .S.
Geological Survey TWI3-A12. (updates report with the
same title by J.F. Wilson, Jr. published in 1968).
Wood, W.W. and G.G. Ehriich, 1978, Use of baker's
yeast to trace microbial movement in ground water:
Ground Water, v. 16, no. 6, pp. 398-403.
95
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Chapter 5
INTRODUCTION TO AQUIFER TEST ANALYSIS
Cone of Depression
Both wells and springs can be ground-water supply
sources. However, most springs with yields large
enough to meet municipal, industrial, and large
commercial and agricultural needs are located only in
areas underlain by cavernous limestones and lava
flows. Most ground-water needs, therefore, are met by
withdrawals from wells.
An understanding of the response of aquifers to
withdrawals from wells is important to an understanding
of ground-water hydrology. When withdrawals start and
water is removed from storage in the well, the water
level in the well begins to decline. The head in the well
falls below the level in the surrounding aquifer, and
water begins to move from the aquifer into the well. As
pumping continues, the water level in the well continues
to decline, and the rate of flow into the well from the
aquifer continues to increase until the rate of inflow
equals the rate of withdrawal.
When water moves from an aquifer into a well, a cone
Lond turfoce
of depression is formed (Figure 5-1). Because water
must converge on the well from all directions and
because the area through which the flow occurs
decreases toward the well, the hydraulic gradient must
get steeper toward the well.
There are several important differences between cones
of depression in confined and unconfined aquifers.
Withdrawals from an unconfined aquifer cause drainage
of water from the rocks, and the water table declines as
the cone of depression forms (Figure 5-1 a). Because
the storage coefficient of an unconfined aquifer equals
the specific yield of the aquifer material, the cone of
depression expands very slowly. On the other hand.
dewatering of the aquifer resufts in a decrease in
transmissivity, which causes, in turn, an increase in
drawdown both in the well and in the aquifer.
Withdrawals from a confined aquifer cause a drawdown
in artesian pressure but normally do not cause a
dewatering of the aquifer (Figure 5-1 b). The water
withdrawn from a confined aquifer is derived from
expansion of the water and compression of the rock
Land surface
of depression ~\/i ?«i«iMioi»it'.c •«>(•£•
x''>VX X^'cone °'
Oro.do«n \^ /^ depression
Confining bed
rsss////s/sssssst
Confined aquifer
Confining bed
(a) (b)
Figure 5-1. Cone of Depression In an Unconfined and a Confined Aquifer
96
-------
skeleton of the aquifer. The small storage coefficient of
confined aquifers results in a rapid expansion of the
cone of depression. Consequently, the mutual
interference of expanding cones around adjacent wells
occurs more rapidly in confined aquifers than it does in
unconfined aquifers.
SOURCE OF WATER DERIVED FROM WELLS
Both the economic development and the effective
management of any ground-water system require an
understanding of the system's response to withdrawals
from wells. The first concise description of the hydrologic
principles involved in this response was presented by
Theis(1940).
Theis pointed out that the aquifer's response to
withdrawals from wells depends on:
1. The rate of expansion of the cone of depression
caused by the withdrawals, which depends on
the transmissivity and the storage coefficient of
the aquifer.
2. The distance to areas in which the rate of water
discharging from the aquifer can be reduced.
3. The distance to recharge areas in which the rate
of recharge can be increased.
Over a sufficiently long period of time and under natural
conditions—that is, before the start of withdrawals—the
discharge from every ground-water system equals the
recharge to it (Figure 5-2a). This property is expressed
by the equation:
natural discharge (D)= natural recharge (R)
In the eastern United States and in the more humid
areas in the West, the amount and distribution of
precipitation are such that the period of time over which
discharge and recharge balance may be less than a
year or, at most, a few years. In the drier parts of the
country—that is, in the areas that generally receive less
than about 500 mmof precipitation annually—the period
over which discharge and recharge balance may be
several years or even centuries. Over shorter periods of
time, differences between discharge and recharge
involve changes in ground-water storage. When
discharge exceeds recharge, ground-water storage (S)
is reduced by an amount (AS) equal to the difference
between discharge and recharge:
D = R + AS (1.)
Conversely, when recharge exceeds discharge, ground-
water storage is increased:
D = R - AS (2)
When withdrawal through a well begins, water is removed
from storage in the well's vicinity as the cone of
depression develops (Figure 5-2b).Thus, the withdrawal
(Q) is balanced by a reduction in ground-water storage:
Q = AS (3)
As the cone of depression expands outward from the
pumping well, it may reach an area where water is
discharging from the aquifer. The hydraulic gradient will
be reduced toward the discharge area, and the rate of
natural discharge will decrease (Figure 5-2c). To the
extent that the decrease in natural discharge
compensates for the pumpage, the rate at which water
is being removed from storage also will decrease, and
the rate of expansion of the cone of depression will
decline. If and when the reduction in natural discharge
(AD) equals the rate of withdrawal (Q), a new balance
will be established in the aquifer. This balance is
represented as:
(D-AD)+Q=R (4)
Conversely, if the cone of depression expands into a
recharge area rather than into a natural discharge area,
the hydraulic gradient between the recharge area and
the pumping well will increase. If, under natural
conditions, more water was available in the recharge
area than the aquifer could accept (the condition that
Theis referred to as rejected recharge), the increase in
the gradient away from the recharge area will permit
more recharge to occur, and the rate of growth of the
cone of depression will decrease. If the increase in
recharge (AR) equals the rate of withdrawal (Q), a new
balance will be established in the aquifer, and expansion
of the cone of depression will cease. The new balance
is represented as:
D + Q = R + AR (5)
In the eastern United States, gaining streams are
relatively closely spaced, and areas in which rejected
recharge occurs are relatively unimportant. In this region,
the growth of cones of depression first commonly causes
a reduction in natural discharge. If the pumping wells
are near a stream or if the withdrawals are continued
long enough, ground-water discharge to a stream may
be stopped entirely in the vicinity of the wells, and water
may be induced to move from the stream into the aquifer
(Figure 5-2d). The tendency in this region is for
withdrawals to change discharge areas into recharge
areas. This consideration is important where the streams
contain brackish or polluted water or where the
streamf low is committed or required for other purposes.
In summary, withdrawal of ground water through a well
reduces the water in storage in the source aquifer
97
-------
^^--rrCT^7^ . .. Lend surfoce ^"""^~—^ ^-—^^ Stream^
• Unconfined • pquifer.' •.'•/•. •.• .• .' . ^ •. '. .»*.'• -^-^\
-_Conf ining -^
Dischorge (0) = Rcchorge(R)
Withdrowol (0)= Reduction in storoge (As)
Wilhdrowol (0)- Reduction in storage (As) + Reduction in discharge (Ao)
Withdrawal (0): Reduction in discharge (AD) + Increase in recharge (Af?)
Figure 5-2. Source of Water Derived From Wells
98
-------
during the growth of the cone of depression. If the cone
of depression ceases to expand, the rate of withdrawal
is being balanced by a reduction in the rate of natural
discharge and (or) by an increase in the rate of recharge.
Under this condition,
Q = AD + AR (6)
AQUIFER TESTS
Determining the yield of ground-water systems and
evaluating the movement and fate of ground-water
pollutants require, among other information, knowledge
of:
1. The position and thickness of aquifers and
confining beds.
2. The transmissivity and storage coefficient of
the aquifers.
3. The hydraulic characteristics of the confining
beds.
4. The position and nature of the aquifer
boundaries.
5. The location and amounts of ground-water
withdrawals.
6. The locations, kinds, and amounts of pollutants
and pollutant practices.
Acquiring knowledge of these factors requires both
geologic and hydrologic investigations. One of the most
important hydrologic studies involves analyzing the
change, with time, in water levels (or total heads) in an
aquifer caused by withdrawals through wells. This type
of study is referred to as an aquifer test and, in most
cases, includes pumping a well at a constant rate for a
period ranging from several hours to several days and
measuring the change in water level in observation
wells located at different distances from the pumped
well (Figure 5-3).
Successful aquifer tests require, among other things:
1. Determination of the prepumping water-level
trend (that is, the regional trend).
2. A carefully controlled constant pumping rate.
3. Accurate water-level measurements made at
precise times during both the drawdown and
the recovery periods.
Drawdown is the difference between the water level at
any time during the test and the position at which the
water level would have been if withdrawals had not
started. Drawdown is very rapid at first. As pumping
continues and the cone of depression expands, the rate
of drawdown decreases (Figure 5-4).
The recovery of the water level under ideal conditions is
a mirror image of the drawdown. The change in water
level during the recovery period is the same as if
withdrawals had continued at the same rate from the
pumped well but, at the moment of pump cutoff, a
recharge well had begun recharging water at the same
point and at the same rate. Therefore, the recovery of
the water level is the difference between the actual
measured level and the projected pumping level (Figure
5-4).
Figure 5-3. Map of Aquifer Test Site
Figure 5-4. Change of Water Level in Well B
In addition to the constant-rate aquifer test mentioned
above, analytical methods also have been developed
for several other types of aquifer tests. These methods
include tests in which the rate of withdrawal is variable
and tests that involve leakage of water across confining
beds into confined aquifers. The analytical methods
available also permit analysis of tests conducted on
both vertical wells and horizontal wells or drains.
The most commonly used method of aquifer-test-data
99
-------
analysis—that for a vertical well pumped at a constant
rate from an aquifer not affected by vertical leakage and
lateral boundaries—is discussed below. The method of
analysis requires the use of a type curve based on the
values of W(u.) and l/u, listed in Table 5-1. Preparation
and use of the type curve are covered in the following
discussion.
3. The discharging well penetrates the entire
thickness of the aquifer, and its diameter is
small in comparison with the pumping rate, so
that storage in the well is negligible.
These assumptions are most nearly met by confined
aquifers at sites remotefromtheirboundaries. However,
1/K
10 7.69 5.88 5.00 4.00 3.33 2.86
2.5
2.22 2.00
1.67
1.43 1.25
1.11
10 '
1
10
\0>
10'
HI4
10s
Uf
to-
IfV
I09
10'°
10"
10"
10"
I014
0.219
1.82
4.04
6.33
B.63
10.94
13.24
15.54
17.84
20.15
22.45
24.75
27.05
29.36
31.66
33.96
0.135
1.59
3.78
6.07
8.37
10.67
12.98
15.28
17.58
19.88
22.19
24.49
26.79
20.09
31.40
33.70
0.075
1.36
3.51
5.80
8.10
10.41
12.71
15.01
17.31
19.62
21.92
24.22
26.52
28.83
.31.13
33.43
0.049
1.22
3.35
5.64
7.94
10.24
12.55
14.85
17.15
19.45
21.76
24.06
26.36
28.66
30.97
33.27
0.025
1.04
3.14
5.42
7.72
10.02
12.32
14.62
16.93
19.23
21.53
23.83
26.14
28.44
30.74
33.05
0.013
.91
2.96
5.23
7.53
9.84
12.14
14.44
16.74
19.05
21.35
23.65
25.%
28.26
30.56
32.86
0.(X)7
.79
2.81
5.08
7.38
9.68
11.99
14.29
16.59
18.89
21.20
23.50
25.80
28.10
30.41
32.71
O.tXM
.70
2.68
4.95
7.25
;9.55
11. 8S
14.15
16.46
18.76
21.06
23.36
25.67
27.97
30.27
32.58
0.002
.63
2.57
4.83
7.13
9.43
11.73
14.04
16.34
18.64
20.94
23.25
25.55
27.85
30.15
32.46
0.001
.56
2.47
4.73
7.02
9.33
11.63
13.93
16.23
18.54
20.84
23.14
25.44
27.75
30.05
32.35
o.noo
.45
2.30
4.54
6.84
9.14
11.45
13.75
16.05
18.35
20.66
22.96
25.26
27.56
29.87
32.17
O.ttlO
.37
2.15
4.39
6.69
8.99
11.29
13.60
15.90
18.20
20.50
22.81
25.11
27.41
29.71
32.02
o.ono
.31
2.03
4.26
6.55
8.R6
11.16
13.46
15.76
18.07
20.37
22.67
24.97
27.28
29.58
31.88
0.000
.26
1.92
4.14
6.44
8.74
11.O4
13.34
15.65
17.95
20.25
22.55
24.86
27.16
29.46
31.76
C*jmpl«: When 1/u-IOxlO '. VVM-0.219; when 1/u-J.JJx I03. W(u)-5.23.
Table 5-1. Selected Values of W(u) for values of lu
Analysis of Aquifer-Test Data
In 1935, C. V. Theisof the New Mexico Water Resources
District of the U.S. Geological Survey developed the
first equation to include time of pumping as a factor that
could be used to analyze the effect of withdrawals from
a well. The Theis equation permitted, for the first time,
determination of the hydraulic characteristics of an
aquifer before the development of new steady-state
conditions resulting from pumping. This capacity is
important because, under most conditions, a new steady
state cannot be developed or, if it can, many months or
years may be required.
In the development of the equation, Theis assumed
that:
1. The transmissivity of the aquifer tapped by the
pumping well is constant during the test to the
limits of the cone of depression.
2. The water withdrawn from the aquifer is derived
entirely from storage and is discharged
instantaneously with the decline in head.
if certain precautions are observed, the equation also
can be used to analyze tests of unconfined aquifers.
The forms of the Theis equation used to determine the
transmissivity and storage coefficient are
T=(Q x W(u))/(4 x T: x s). (7)
S=(4 x T x t x u)/r2 (8)
where T is transmissivity, S is the storage coefficient, Q
is the pumping rate, s is drawdown, t is time, r is the
distance from the pumping well to the observation well,
W(u) is the well function of u, which equals
-.577216 - logeu + u. - u2/(2x2!) + u3/(3x3!) - u4/(4x4!) + ...
and u=(r2S)/(4Tt).
O)
The Theis equation is in a form that cannot be solved
directly. To overcome this problem, Theis devised a
convenient graphic method of solution that uses a type
curve (Figure 5-5). To apply this method, a data plot of
drawdown versus time (or drawdown versus t/r^) is
100
-------
matched to the type curve of W(u) versus l/u (Figure 5-
6). At some convenient point on the overlapping part of
the sheets containing the data plot and type curve,
values of s, t (ort/r2), W(u), and l/u are noted (Figure 5-
6). These values are then substituted in the equations,
which are solved for T and S, respectively.
A Theis type curve of W(u) versus l/u can be prepared
10
a
i
O.I
0.01
to
' V
t. In minuMi
I
10 »
10s
Figure 5-5. Thels Type Curve
f. In mlnulM
10 I01 10'
10' 10'
3
i
/
u
!•«
H
/
ItCti
iM
S
^
/
.-•
KATC
Wv| •
V, •
M -POINT c
i. » t.j
i, >• 1.1
DATA
O-- 1 t m
'•' '",
50»DINAT£S
0«
PLOT
»•(.-'
*
=1
Ty»» Cufvi
O.I I 10 10'
meters per day and s is in meters, T will be in square
meters per day. Similarly, if T is in square meters per
day,tis in days, andris in meters,Swill be dimensionless.
Traditionally, in the United States.Thas been expressed
in units of gallons per day per loot. The common
practice now is to report transmissivity in units of square
meters per day or square feet per day. If Q is measured
in gallons per minute, as is still normally the case, and
drawdown is measured in feet, as is also normally the
case, the equation is modified to obtain T in square feet
per day as follows:
Figure 5-6. Data Plot of Drawdown Versus Time
Matched to Thels Type Curve
from the values given in Table 5-1. The data points are
plotted on logarithmic graph paper—that is, graph paper
having logarithmic divisions in both the x and y directions.
The dimensional units of transmissivity (T) are L2t, where
L is length and t is time in days. Thus, if Q is in cubic
=(QxW(u))/(4ns) = (gal/min) x (1.440 min/d) x
(U3/7.48 gal) x 1/ft x W(u)/(4xn) i
(10)
or
T(inft2 d"1) = (15.3 x Q x W(u))/s (11)
(when Q is in gallons per minute and s is in feet). To
convert square feet per day to square meters per day,
divide by 10.76.
The storage coefficient is dimensionless. Therefore, if T
is in square feet per day, t is in minutes, and r is in feet,
then,
S=(4Ttu)/r2=(4/1) x U2/d x min/ft2 x d/1440 min (12)
or
S=(Ttu)/360r2) (13)
(when T is in square feet per day, t is in minutes,
andris in feet).
Analysis of aquifer-test data using the Theis equation
involves plotting both the type curve and the test data on
logarithmic graph paper. If the aquifer and the conditions
of the test satisfy Theis' assumptions, the type curve
has the same shape as the cone of depression along
any line radiating away from the pumping well and the
drawdown graph at any point in the cone of depression.
There are two considerations for using the Theis
equation for unconfined aquifers. First, if the aquifer is
relatively fine grained, water is released slowly over a
period of hours or days, not instantaneously with the
decline in head. Therefore, the value of S determined
from a short-period test may be too small.
Second, if the pumping rate is large and the observation
well is nearthe pumping well, dewatering of the aquifer
may be significant, and the assumption that the
101
-------
transmissivity of the aquifer is constant is not satisfied.
The effect of dewatering of the aquifer can be eliminated
with the following equation:
s'=s-s2/(2b) (14)
where s is the observed drawdown in the unconfined
aquifer, b is the aquifer thickness, and s' is the drawdown
that would have occurred if the aquifer had been confined
(that is, if no dewatering had occurred).
To determine the transmissivity and storage coefficient
of an unconfined aquifer, a data plot consisting of s
versus t (or t/r2) is matched with the Theis type curve of
W(u) versus 1/u. Both s and b must be in the same
units, either feet or meters.
As noted above, Theis assumed in the development of
his equation that the discharging well penetrates the
entire thickness of the aquifer. However, because it is
not always possible, or necessarily desirable, to design
a well that fully penetrates the aquifer under
development, most discharging wells are open to only
a part of the aquifer that they draw from. Such partial
penetration creates vertical flow in the vicinity of the
discharging well that may affect drawdowns in
observation wells located relatively close to the
discharging well. Drawdowns in observation wells that
are open to the same zone as the discharging well will
be larger than the drawdowns in wells at the same
distance from the discharging well but open to other
zones. The possible effect of partial penetration on
drawdowns must be considered in the analysis of
aquifer-test data. If aquifer-boundary and other
conditions permit, the problem can be avoided by
locating observation wells beyond the zone in which
vertical flow exists.
Time-Drawdown Analysis
The Theis equation is only one of several methods that
have been developed for the analysis of aquifer-test
data. Another somewhat more convenient method,
was developed from the Theis equation by C.E.Jacob.
The greater convenience of the Jacob method derives
partly from its use of semilogarithmic graph paper
instead of the logarithmic paper used in the Theis
method, and from the fact that, under ideal conditions,
the data plot along a straight line rather than along a
curve.
However, it is essential to note that, whereas the Theis
equation applies at all times and places (if the
assumptions are met), Jacob's method applies only
under certain additional conditions. These conditions
also must be satisfied in orderto obtain reliable answers.
To understand the limitations of Jacob's method, the
changes that occur in the cone of depression during an
aquifer test must be considered. The changes that are
of concern involve both the shape of the cone and the
rate of drawdown. As the cone of depression migrates
outward from a pumping well, its shape (and, therefore,
the hydraulic gradient at different points in the cone)
changes. We can refer to this condition as unsteady
shape. At the start of withdrawals, the entire cone of
depression has an unsteady shape (Figure 5-7a). After
a test has been underway for some time, the cone of
depressionbeginsto assume a relatively steady shape,
first at the pumping well and then gradually to greater
and greater distances (Figure 5-7b). If withdrawals
continue long enough for increases in recharge and /or
reductions in discharge to balance the rate of withdrawal,
drawdowns cease, and the cone of depression is said
to be in a steady state (Figure 5-7c).
o
Land surface i F
-
Cons of depression— ~"H[H
(unsteady shape)
%^^««5«««5^^«»%^»5««:
- ' '
Confining bed
llv«r
H
River
'////////////////////////y///r//^^^
Figure 5-7. Development of Cone of Depression
from Start of Pumping to Steady-State
The Jacob method is applicable only to the zone in
which steady-shape conditions prevail or to the entire
cone only after steady-state conditions have developed.
For practical purposes, this condition is met when
u=(r2S)/(4Tt) is equal to or less than about 0.05.
102
-------
Substituting this value in the equation for u and solving
fort, we can determine the time at which steady-shape
conditions develop at the outermost observation well.
Thus,
tc «r (7,200 r2S)/T (15)
where tc is the time, in minutes, at which steady-shape
conditions develop, r is the distance from the pumping
well, in feet (or meters), S is the estimated storage
coefficient (dimensionless), and T is the estimated
transmissivity, in square feet per day (or square meters
per day).
After steady-shape conditions have developed, the
drawdowns at an observation well begin to fall along a
straight line on semilogarithmic graph paper, as Figure
5-8 shows. Before that time, the drawdowns plot below
the extension of the straight line. When a time-drawdown
graph is prepared, drawdowns are plotted on the vertical
(arithmetic) axis versus time on the horizontal
(logarithmic) axis.
one log cycle, to is the time at the point where the
straight line intersects the zero-drawdown line, and r is
the distance from the pumping well to the observation
well.
These equations are in consistent units. Thus, if Q is in
cubic meters per day and s is in meters, T is in square
meters per day. S is dimensionless, so that if r is in
square meters per day, then r must be in meters and to
must be in days.
It is still common practice in the United States to express
Q in gallons per minute, s in feet, t in minutes,! in feet,
and T in square feet per day. The equations can be
modified for direct substitution of these units as follows:
T=(2.3Q)/(47iAs) = (2.3/4n) x (gal/min) x (1,440 min/d)
x(ft3/74.8gal)x(1/ft) . (18)
T = (35Q)/S (19)
TIME-DRAWDOWN GRAPH
0,
_.
tr
UJ
l— •)
£ c
«JB
A
^
•y C
* °
o
0 o
£ °
<
cc
o 10
1?
y— ^q
~ t * i
lo
Ac - \
S • 1.
-
-
r- 75 m
" O = 9.3 m 3
t0 = 2.5x
"
^HCTfc
^v^~— — ^
2.,: ».
III *-
min"1 ( 24!
IO'5 d
i i i i i ii i
""-• — -— .
— •*«.
cycle
55 gal min"1
i i i 1 1 1 1 1
"~~*"~— -*_
*->.
)
i t i 1 1 1 ii
u r a w o
/^m e o so
-^L
I 1 I 1 i I II
-
r crncn 1 5
*-K
-
i i 1 1 1 1 il
10-5
10-3 10-2 0.
TIME, IN DAYS
10
Figure 5-8. Time-Drawdown Graph
The slope of the straight line is proportional to the where T is in square feet per day. Q is in gallons per
pumping rate and to the transmissivity. Jacob derived minute and s is in feet, and
thefolbwingequationsfordeterminationof transmissivity
andstorageooefficientfromthetime-drawdowngraphs: S=(2.25Ttrj/r2) = (2.25/1) x (ft2/d) x (min/ft2)
x (d/1 .440 min)
x (d/1.440 min) (20)
S=(Ttrj)/(640r2) (21)
T = (2.3Q)/(4nAs) (16)
S = (2.25Tto)/r2 W
where T is in square feet per day, t0 is in minutes, and
where Q is the pumping rate, As is the drawdown across r is in feet.
103
-------
Distance-Drawdown Analysis
Aquifer tests should have at least three observation
wells located at different distances from the pumping
well (Figure 5-9). Drawdowns measured at the same
time in these wells can be analyzed with the Theis
equation and type curve to determine the aquifer
transmissivity and storage coefficient.
DISTANCE-DRAWDOWN CRAPH
Observation wells
B A
Pumping well
D to V
^^^^^^
o ;
1 ^_
I
N
X
.
\
— r
fs,\,s
\
^Static water level
X%L Pumping water
X level
Confining bed
[ Confined
aquifer
>
Confining bed
Dotum Plonc
Figure 5-9. Desirable Location for Observation
Wells In Aquifer Tests
After the test has been underway long enough,
drawdowns in the wells also can be analyzed by the
Jacob method, eitherthroughthe use of a time-drawdown
graph using data from individual wells or through the
use of a distance-drawdown graph using simultaneous
measurements in all of the wells. To determine when
sufficient time has elapsed, see the discussion of time-
drawdown analysis earlier in this chapter.
In the Jacob distance-drawdown method, drawdowns
are plotted on the vertical axis versus distance on the
horizontal axis (Figure 5-10). If the aquifer and test
conditions satisfy the Theis assumptions and the
limitation of the Jacob method, the drawdowns measured
at the same time in different wells should plot along a
straight line (Figure 5-10).
The slope of the straight line is proportional to the
pumping rate and to the transmissivity. Jacob derived
the following equations for determination of the
transmissivity and storage coefficient from distance-
drawdown graphs:
*-
UJ
^ ^
*• £
z
* G
o °
o
i,o
«r IU
o
12
t * 4 Ooyi
- 'o = 30.0
-
-
^^
<"
30 ft
B _
x""
Jr^
^
*^ *\
Log
cyelt
^V-
=2.4 II _
-
-
-
1 t
10 100 1000
DISTANCE. IN FEET
10,000
Figure 5-10. Distance-Drawdown Graph
T = (2.3Q)/(2nAs) (22)
S = (2.25Tt)/r02 <23)
where Q is the pumping rate, As is the drawdown across
one log cycle, t is the time at which the drawdowns were
measured, and r0 is the distance from the pumping well
to the point where the straight line intersects the zero-
drawdown line.
These equations are in consistent units. For the
inconsistent units still in relatively common use in the
United States, the equations should be used in the
following forms:
T = (70Q)/As (24)
where T is in square feet per day, Q is in gallons per
minute, and s is in feet and
S = (Tt)/(640r02) (25)
where T is in square feet per day, t is in minutes, and ro
is in feet.
The distance r0 does not indicate the outer limit of the
cone of depression. Because nonsteady-shape
conditions exist in the outer part of the cone, before the
development of steady-state conditions, the Jacob
method does not apply to that part. If the Theis equation
were used to calculate drawdowns in the outer part of
the cone, it would be found that they would plot below
the straight line. In other words, the measurable limit of
the cone of depression is beyond the distance TO-
If the straight line of the distance-drawdown graph is
104
-------
extended inward to the radius of the pumping well, the
drawdown indicated at that point is the drawdown in the
aquifer outside of the well. If the drawdown inside the
well is found to be greater than the drawdown outside,
the difference is attributable to well loss. (See Single-
Well Tests.)
The hydraulic conductivities and, therefore, the
transmissivities of aquifers may be different in different
directions. These differences may cause differences in
drawdowns measured at the same time in observation
wells located at the same distances but in different
directions from the discharging well. Where this condition
exists, the distance-drawdown method may yield
satisfactory results only where three ormore observation
wells are located in the same direction but at different
distances from the discharging well.
Single-Well Tests
The most useful aquifer tests are multiple-well tests,
which are those that include water-level measurements
in observation wells. It also is possible to obtain useful
data from production wells, even where observation
wells are not available. These single-well tests may
consist of pumping a well at a single constant rate, or at
two or more different but constant rates or, if the well is
not equipped with a pump, by instantaneously introducing
a known volume of water into the well. The following
discussion is limited to tests involving a single constant
rate.
In order to analyze the data, the nature of the drawdown
in a pumping well must be understood. The total
drawdown (st) in most, if not all, pumping wells consists
of two components (Figure 5-11). One is the drawdown
(sa) in the aquifer, and the other is the drawdown (sw)
that occu rs as water mo ves from the aquifer into the well
and up the well bore to the pump intake. Thus, the
drawdown in most pumping wells is greater than the
drawdown in the aquifer at the radius of the pumping
well.
The total drawdown (st) in a pumping well can be
expressed in the form of the following equations:
st = sa + Sw
st = BQ + CQ2 (26)
where 83 is the drawdown in the aquifer at the effective
radius of the pumping well, sw is well loss, Q is the
pumping rate, B is a factor related to the hydraulic
characteristics of the aquifer and the length of the
pumping period, and C is a factor related to the
characteristics of the well.
The factor C is normally considered to be constant, so
that, in a constant rate test, CQ2 is also constant. As a
result, the well loss (sw) increases the total drawdown
in the pumping well but does not affect the rate of
change in the drawdown with time. It is, therefore,
possible to analyze drawdowns in the pumping well with
Land surface
Static potentiometric surface __
/ / / / / s~/
Confining bed.
L////////S///S//
Confined
aquifer
Effective well radius
////////////////ss///////////////////
Confining bed
Figure 5-11. Two Components of Total Drawdown in a Pumping Well
105
-------
the Jacob time-drawdown method using semilogarithmic
graph paper. (See Time-Drawdown Analysis" earlier in
this section.) Drawdowns are plotted on the arithmetic
scale versus time on the logarithmic scale (Figure 5-
12), and transmissivity is determined from the slope of
the straight line by using the following equation:
T = (2.3Q)/(4nAs) (27)
Where well loss is present in the pumping well, the
storage coefficient cannot be determined by extending
the straight line to the line of zero drawdown. Even
where well loss is not present, the determination of the
storage coefficient from drawdowns in a pumping well
likely will be subject to large error because the effective
radius of the well may differ significantly from the
nominal radius.
- *,- Aquifer lost
- *«•= Well lots
I I
10
Pumping Rate, In
Cubic Meters per Minute
•*— I log e»eie-
i i i 1 1 1
il
O.I
I 10
Time, In Minutes
100
Figure 5-12. Time-Drawdown Plot With and Without
Well Loss
In this equation, drawdown in the pumping well is
proportional to the pumping rate. The factor B in the
aquifer-loss term (BQ) increases with time of pumping
as long as water is being derived from storage in the
aquifer. The factor C in the well-loss term (CQ2) is a
constant if the characteristics of the well remain
unchanged, but, because the pumping rate in the well-
lossterm is squared, drawdown due to well loss increases
rapidly as the pumping rate is increased. The relation
between pumping rates and drawdown in a pumping
well, if the well was pumped for the same length of time
at each rate, is shown in Figure 5-13. The effect of well
toss on drawdown in the pumping well is important both
for pumping wells data analysis, and supply well
design.
Well Interference
Pumping a well causes a drawdown in the ground-water
Figure 5-13. Relation of Pumping Rate and
Drawdown
level in the surrounding area. The drawdown in water
level forms a conical-shaped depression in the water
table or potentiometric surface which is referred to as a
cone of depression. (See "Cone of Depression" at the
beginning of this section.) Similarly a well through which
water is injected into an aquifer (that is a recharge or
injection well) causes a buildup in ground-water level in
the form of a conical-shaped mound.
The drawdown (s) in an aquifer caused by pumping at
any point in the aquifer is directly proportional to the
pumping rate (Q) and the length of time (t) that pumping
has been in progress and is inversely proportional to the
transmissivity (T), the storage coefficient (S), and the
square of the distance (r2) between the pumping well
and the point. This is represented by the equation:
s = (Q.t)/T,S. r2 <28)
Where pumping wells are spaced relatively close
together, pumping of one will cause a drawdown in the
others. Because drawdowns are additive, the total
drawdown in a pumping well is equal to its own drawdown
plus the drawdowns caused at its location by other
pumping wells (Figures 5-14 and 5-15). The drawdowns
in pumping wells caused by withdrawals from other
pumping wells are referred to as well interference. As
Figure 5-15 shows, a divide forms in the potentiometric
surface (or the water table in the case of an unconfined
aquifer) between pumping wells.
At any point in an aquifer affected by both a discharging
well and a recharging well, the change in water level is
106
-------
Well
A
Wei
8
Cone of
depression with
well A pumping
Static Potentiometric surface
•*" Cone of
depression if well B were
pumping ond well A were idle
Confined aquifer
Figure 5-14. Cone of Depression When Well A or B is Pumped
Cone of
depression with both
well A and B pumping
Figure 5*15. Total Drawdown Caused by Overlapping Cones of Depression
equal to the difference between the drawdown and the
buildup. If the rates of discharge and recharge are the
same and if the wells are operatedonthe same schedule,
the drawdown and the buildup will cancel midway
between the wells and the water level at that point will
remain unchanged from the static level (Figure 5-16).
(See "Aquifer Boundaries" below.)
From the functional equation above, it can be seen that,
iin the absence of well interference, drawdown in an
aquifer at the effective radius of a pumping well is
directly proportional to the pumping rate. Conversely,
the maximum pumping rate is directly proportional to
the available drawdown. Forconfined aquifers, available
drawdown is normally considered to be the distance
between the prepumping water level and the top of the
aquifer. For unconfined aquifers, available drawdown is
normally considered to be about 60 percent of the
saturated aquifer thickness.
Where the pumping rate of a well is such that only a part
of the available drawdown is utilized, the only effect of
well interference is to lower the pumping level and,
thereby, increase pumping costs. In the design of a well
107
-------
Discharging
well
Recharging
well
Static potent iometric
^.
Drawdown
f f f ffffl/ff f 7 ff
Figure 5-16. Cones of Depression and Buildup Surrounding Discharging and Recharging Wells
field, the increase in pumping cost must be evaluated
along with the cost of the additional waterlines and
poweriines that must be installed if the spacing of wells
is increased to reduce well interference.
Because well interference reduces the available
drawdown, it also reduces the maximum yield of a well.
Well interference is, therefore, an important matter in
the design of well fields where it is desirable for each
well to be pumped at the largest possible rate. For a
group of wells pumped at the same rate and on the
same schedule, the well interference caused by any
well on another well in the group is inversely proportional
to the square of the distance between the two wells (r2).
Therefore, excessive well interference is avoided by
increasing the spacing between wells and by locating
the wells along a line rather than in a circle or in a grid
pattern.
Aquifer Boundaries
One of the assumptions inherent in the Theis equation
(and in most other fundamental ground-water flow
equations) is that the aquifer to which it is being applied
is infinite in extent. Obviously, no such aquifer exists on
Earth. However, many aquifers are areally extensive,
and, because pumping will not affect recharge or
discharge significantly for many years, most water
pumped is fromground-waterstorage; as a consequence
water levels must decline for many years. An excellent
example of such an aquifer is that underlying the High
Plains from Texas to South Dakota.
All aquifers are vertically and horizontally bounded.
Vertical boundaries may include the water table, the
plane of contact between each aquifer and each confining
bed, and the plane marking the lower limit of the zone
of interconnected openings—in other words, the base
of the ground-water system.
Hydraulically, aquifer boundaries are of two types:
recharge boundaries and impermeable boundaries. A
recharge boundary is a boundary along which flow lines
originate. Under certain hydraulic conditions, this
boundary will serve as a source of recharge to the
aquifer. Examples of recharge boundaries include the
zones of contact between an aquifer and a perennial
stream that completely penetrates the aquifer or the
ocean.
An impermeable boundary is a boundary that flow lines
do not cross. Such boundaries exist where aquifers
terminate against impermeable material. Examples
include the contact between an aquifer composed of
sand and a laterally adjacent bed composed of clay.
The position and nature of aquifer boundaries are
critical to many ground-waterproblems, including those
involved in the movement and fate of pollutants and the
response of aquifers to withdrawals. Depending on the
directionofthehydraulicgradient.astream.forexample,
may be eitherthe source orthe destination of a pollutant.
Lateral boundaries within the cone of depression have
a profound effect on the response of an aquifer to
108
-------
withdrawals. To analyze or predict the effect of a lateral
boundary, it is necessary to "make" the aquifer appear
to be of infinite extent by using imaginary wells and the
theory of images. Figures 5-17 and 5-18 show, in both
plan view and profile, how image wells are used to
compensate hydraulically for the effects of both
recharging and impermeable boundaries. (See "Well
Interference" earlier in this section.)
The key feature of a recharge boundary is that
withdrawals from the aquifer do not produce drawdowns
across the boundary. A perennial stream in intimate
contact with an aquifer represents a recharge boundary
because pumping from the aquifer will induce recharge
from the stream. The hydraulic effect of a recharge
boundary can be duplicated by assuming that a
recharging image well is present on the side of the
boundary opposite the real discharging well. Water is
injected into the image well at the same rate and on the
same schedule that water is withdrawn from the real
well. In the plan view in Figure 5-17, flow lines originate
at the boundary and equipotentiat lines parallel the
boundary at the closest point to the pumping (real) well.
The key feature of an impermeable boundary is that no
watercan cross it. Such a boundary, sometimes termed
a "no-flow boundary," resembles a divide in the water
table orthe potentiometric surface of a confined aquifer.
The effect of an impermeable boundary can be duplicated
by assuming that a discharging image well is present on
the side of the boundary opposite the real discharging
well. The image well withdraws water at the same rate
and on the same schedule as the real well. Flow lines
tend to parallel an impermeable boundary and
equipotential lines intersect it at a right angle.
The image-well theory is an essential tool in the design
of well fields near aquifer boundaries. To minimize
lowering water levels, apply the following conditions:
1. Pumping wells should be located parallel to and
as close as possible to recharging boundaries.
2. Pumping wells should be located perpendicular
to and as far as possible from impermeable
boundaries.
Figures 5-17 and 5-18 illustrate the effect of single
REAL SYSTEM
REAL SYSTEM
HYDRAULIC CONTERPART OF REAL SYSTEM
0
PLAN VIEW OF THE HYDRAULIC CONTERPART
\
HYDRAULIC CONTERPART OF REAL SYSTEM
I
-------
boundaries and show how their hydraulic effect is
compensated for through the use of single image wells.
It is assumed in these figures that other boundaries are
so remote that they have a negligible effect on the areas
depicted. At many places, however, pumping wells are
affected by two or more boundaries. One example is an
alluvial aquifer composed of sand and gravel bordered
on one side by a perennial stream (a recharge boundary)
and on the other by impermeable bedrock (an
impermeable boundary).
Contrary to first impression, these boundary conditions
cannot be satisfied with only a recharging image well
and a discharging image well. Additional image wells
are required, as Figure 5-19 shows, to compensate for
the effect of the image wells on the opposite boundaries.
Because each additional image well affects the opposite
boundary, it is necessary to continue adding image
wells until their distances from the boundaries are so
great that their effect becomes negligible.
CR
^N
OSS SECTION THROUGH AQUIFER
Landaurtac* Purapln9«nl^ SUMI
^M/aHHUbki
Aqutar
1 ,
lontintng malarial
PLAN VIEW OF BOUNDA
AND IMAC
Boundmiy \
o o • • p
•„ .... y, f
Mcrtarglng knag*1 /
Mfl 1
* Rapeal. tt Mnty P«n>
«Ml
BALANCING OF WELLS
kipwmaabki
bounda>|r
It
1,
>!
i»
••
•I
RIES. PUMPING WE
5E WELLS
boundary
-4 4 -
E> « • O
W l\ 1, 1.
Radhargino, imaga
"8 RapatfiloH
1
TI
LLS.
+--H
- 4
0 •
l> It
Mr — ».
ACROSS BOUNDARIES
Racharg*
boundaqr
•W
i!
i.
i.
«1
il
i,
Figure 5-19. A Sequence of Image Wells
Tests Affected By Lateral Boundaries
When an aquifer test is conducted near one of the
lateral boundaries of an aquifer, the drawdown data
depart from the Theis type curve and from the initial
straight line produced by the Jacob method. The
hydraulic effect of lateral boundaries is assumed, for
analytical convenience, to be due to the presence of
other wells. (See "Aquifer Boundaries" earlier in this
section.) Thus, a recharge boundary has the same
effect on drawdowns as a recharging image well located
across the boundary and at the same distance from the
boundary as the real well. The image well is assumed
to operate on the same schedule and at the same rate
as the real well. Similarly, an impermeable boundary
has the same effect on drawdowns as a discharging
image well.
To analyze aquifer-test data affected by either a recharge
boundary or an impermeable boundary, the early
drawdown data in the observation wells nearest the
pumping well must not be affected by the boundary.
These data, then, show only the effect of the real well
and can be used to determine the transmissivity (T) and
the storage coefficient (S) of the aquifer. (See "Analysis
of Aquifer-Test Data" and Time-Drawdown Analysis"
earlier in this section.) In the Theis method, the type
curve is matched to the early data and a "match point"
is selected to calculate the values of T and S. The
position of the type curve in the region where the
drawdowns depart from the type curve is traced onto the
data plot (Figures 5-20 and 5-21). The trace of the type
curve shows where the drawdowns would have plotted
if there had been no boundary effect. The differences in
drawdown between the data plot and the trace of the
type curve show the effect of an aquifer boundary. The
direction in which the drawdowns depart from the type
curve—that isin the direction of eithergreaterdrawdowns
or lesser drawdowns—shows the type of boundary.
Drawdowns greater than those defined by the trace of
the type curve indicate the presence of an impermeable
boundary because, as noted above, the effect of such
boundaries can be duplicated with an imaginary
discharging well. Conversely, a recharge boundary
causes drawdowns to be less than those defined by the
trace of the type curve.
In the Jacob method, drawdowns begin to plot along a
straight line after the test has been underway for some
time (Rgures 5-22 and 5-23). The time at which the
straight-line plot begins depends on the values of T and
S of the aquifer and on the square of the distance
between the observation well and the pumping well.
(See Time-Drawdown Analysis" earlier in this section.)
Values of T and S are determined from the first straight-
line segment defined by the drawdowns after the start
of the aquifer test. The slope of this straight line depends
on the transmissivity (T) and on the pumping rate (Q).
If a boundary is present, the drawdowns will depart from
the first straight-line segment and begin to fall along
another straight line.
According to image-well theory, the effect of a recharge
boundary can be duplicated by assuming that water is
110
-------
10
TIME. IN MINUTES
J5> 10' 10' 10-
10-
XTtare of Theis
type rnrve
0.01
TIME. IN MINUTES
10 10' 10'
10-
o
«/>
"'• o,
u< 0 ?
UJ
2 nj
Z
o
1
o I0
1 ?
• .^\.
t
1
•^
"-,
•»•..
^ -
"H
S. " * X , K
^*>w 9,
Figure 5-20. Thels Time-Drawdown Plot Showing
a Negative Boundary
Figure 5-23. Jacob Time-Drawdown Plot Showing
a Positive Boundary
TIME. IN MINUTES
10-
r.uiv«
IU
OC
UJ
tL
2 ,.o
Z
z"
5
§0.1
$
2
o
0.01
//
•C »
• -J...
II
—5$
Jk .^
*/
•
/ *
_J j
wv*^.
^n
10'
Figure 5-21. Thels Time-Drawdown Plot Showing
a Positive Boundary
to
£ 0.7
UJ
2 04
Z
Z" 0.6
*
§ 0.8
1"
1.2
- « , \^
— ^-.—^^
- -]
N. *'
*»M
^
TIME. IN MINUTES
10'
10-
Figure 5-22. Jacob Time-Drawdown Plot Showing
a Negative Boundary
injected into the aquifer through a recharging image
well at the same rate that water is being withdrawn from
the real well. It follows, therefore, that, when the full
effect of a recharge boundary is felt at an observation
well, there will be no further increase in drawdown and
the water level in the well will stabilize. At this point in
both the Theis and the Jacob methods, drawdowns plot
along a straight line having a constant drawdown.
Conversely, an impermeable boundary causes the rate
of drawdown to increase. In the Jacob method, as a
result, the drawdowns plot along a new straight line
having twice the slope as the line drawn through the
drawdowns that occurred before the boundary effect
was felt.
The Jacob method should be used carefully when it is
suspected that an aquifer test may be affected by
boundary conditions. In many cases, the boundary
begins to affect drawdowns before the method is
applicable, the result being that T and S values
determined from the data are erroneous and the effect
of the boundary is not identified. When it is suspected
that an aquifer test may be affected by boundary
conditions, the data should, at least initially, be analyzed
with the Theis method.
The position and the nature of many boundaries are
obvious. For example, the most common recharge
boundaries are streams and lakes; possibly, the most
common impermeable boundaries are the bedrock
walls of alluvial valleys. The hydraulic distance to these
boundaries, however, may not be obvious. A stream or
lake may penetrate only a short distance into an aquifer
and their bottoms may be underlain by fine-grained
material that hampers movement of water into the
aquifer. Hydraulically, the boundaries formed by these
surface-water bodies will appear to be farther from the
111
-------
pumping well than the near shore. Similarly, if a small
amount of water moves across the bedrock wall of a
valley, the hydraulic distance to the impermeable
boundary will be greater than the distance to the valley
wall.
Fortunately, the hydraulic distance to boundaries can
be determined from aquifer-test data analysis. According
to the Theis equation, for equal drawdowns caused by
the real well and the image well (in other words,
if sr= Sj),then
rr2/^ = rj2/tj (29)
where rr is the distance from the observation well to the
real well, n is the distance from the observation well to
the image well, t is the time at which a drawdown of s is
caused by the real well at the observation well, and tj is
the time at which a drawdown of sj is caused by the
image well at the observation well.
Solving this equation for the distance to the image well
from the observation well, results in
ri = rr(tj/tr)1/2 (30)
The image well is located at some point on a circle
having a radius of n centered on the observation well
(Rgure 5-24). Because the image well is the same
distance from the boundary as the real well, the
boundary must be located halfway between the image
well and the pumping well (Figure 5-24).
Circll olong which Ihe
imoge well it
locoltd
Circle olong which o poinl
on Ihe boundory it
locoltd
Figure 5-24. Method for Determining Location of
Boundary
If the boundary is a stream or valley wall or some other
feature whose physical position is obvious, its "hydraulic
position" may be determined by using data from a single
observation well. If, on the other hand, the boundary is
the wall of a buried valley or some other feature not
obvious from the land surface, distances to the image
well from three observation wells may be needed to
identify the position of the boundary.
Tests Affected By Leaky Confining Beds
In the development of the Theis equation for aquifer-
test data analysis, it was assumed that all water
discharged from the pumping well was derived
instantaneously from storage in the aquifer. (See
"Analysis of Aquifer-Test Data" earlier in this section.)
Therefore, in the case of a confined aquifer, at least
during the period of the test, the movement of water into
the aquifer across its overlying and underlying confining
beds is negligible. This assumption is satisfied by many
confined aquifers. Many other aquifers, however, are
bounded by leaky confining beds that transmit water
into the aquifer in response to the withdrawals and
cause drawdowns to differ from those that would be
predicted by the Theis equation. The analysis of aquifer
tests conducted on these aquifers requires the use of
the methodsthat have beendevelopedforsemi confined
aquifers (also referred to in ground-water literature as
"leaky aquifers").
Rgures 5-25, 5-26, and 5-27 illustrate three different
conditions commonly encountered in the field. Figure 5-
25 shows a confined aquifer bounded by thick,
impermeable confining beds. Water initially pumped
from such an aquifer is from storage, and aquifer-test
data can be analyzed by using the Theis equation.
Figure 5-26 shows an aquifer overlain by a thick, leaky
confining bed that, during an aquif ertest, yields significant
waterf com storage. The aquif er in this case may properly
be referred to as a semiconfined aquifer, and the
release of water from storage in the confining bed
affects the analysis of aquifer-test data. Figure 5-27
shows an aquifer overlain by a thin confining bed that
does not yield significant water from storage but that is
sufficiently permeable to transmit water from the
overlying unconfined aquifer into the semiconfined
aquifer. Methods have been devised largely by Madhi
HantushandC. E.Jacob, 1955, for use in analyzing the
leaky conditions illustrated in Figures 5-26 and 5-27.
These methods use matching data plots with type
curves, as the Theis method does. The major difference
is that, whereas the Theis method uses a single type
curve, the methods applicable to semiconfined aquifers
involve "families" of type curves, each curve of which
reflects different combinations of the hydraulic
characteristics of the aquifer and the confining beds.
112
-------
Land surfaces
Discharging wellx
Confined
aquifer
Figure 5-25. Nonleaky Artesian Conditions
Water table
Unconfinod
^^II^ZI^IZrTr^jrLealcy confining^ bed -^.-T--—-TT^
Semiconfined
aquifer
Figure 5-26. Semiconfined Aquifer with Leakage From Confining Bed
1
:—-^-——
V,
I
—^^-rr—r
V
^-—
T
•^JEfcE/e
^
,- i
.
II? ^^IJnconfined
j 1 i aquifor
-^.•^n^jr^ LeMkylfcoofininQ J:J)«i— l—Zr^H-iTfZ^
^ ^ / Semiconfined
1 ^ ^- aquifer
Figure 5-27. Semiconfined Aquifer with Leakage Through a Confining Bed
Data plots of s versus t on logarithmic graph paper for
aquifer tests affected by release of water from storage
in the confining beds are matched to the family of type
curves illustrated in Figure 5-28. For convenience,
these curves are referred to as Hantush type. Four
match-point coordinates are selected and substituted
into the following equations to determine values of T
andS:
aquifer tests affected by leakage of water across
confining beds are matched to the family of type curves
shown in Figure 5-29. These type curves are based on
equations developed by Hantush and Jacob and, for
convenience, will be referred to as the Hantush-Jacob
curves. The four coordinates of the match point are
substituted into the following equations to determine T
andS:
T = (QH(U,P))/(47CS) (31) T=QW(u.r/B)/(47tS) (33)
S = (4Ttu)/r2 (32) S = 4Ttu/r2 (34)
Data plots of s versus t on logarithmic graph paper for In planning and conducting aquifer tests, hydrologists
113
-------
i
-------
express W(u)/4rc as a constant. To do so it is first
Eecessary to determine values for u and, using a table
f values of u (or l/u) and W(u), determine the
corresponding values for W(u).
Valuesof u are determined by substituting in the equation
values of T, S, r, and t that are representative of
conditions in the area. For example, assume that in an
area under investigation and for which a large number
of values of specific capacity are available, that:
1 . The principal aquifer is confined and aquifer tests
indicate that it has a storage coefficient of about
2 x IO'4 and a transmissivity of about 1 1 ,000 ft2
-------
McClymonds, N. E., and O.L. Franke,1972, Water-
transmining properties of aquifers on Long Island, New
York: U.S. Geol. Survey Professional Paper 627-E,
24p.
Meinzer.O.E.,1923,The occurrence of groundwaterin
the United States, with a discussion of principles: U.S.
Geol. Survey Water-Supply Paper 489, 321p.
Moulder, E. A., 1963, Locus circles as an aid in the
location of a hydrogeologic boundary in Bentall, Ray,
comp., Shortcuts and special problems in aquifer tests:
U.S. Geol. Survey Water-Su;>ply Paper 1545-C, pp.
C110-C115.
Stallman.R.W., 1971, Aquifer-test design,observations,
and data analysis: U.S. Geol. Survey Techniques of
Water-Resources Investigations, Book 3, Chapter B1,
26p.
Theis, C. V., 1935, The relation between the lowering of
the piezometric surface and the rate and duration of
discharge of a well using ground-water storage: Trans.
of the Amer. Geophysical Union, v. 16, pp. 519-524.
Theis, C. V., 1940, The source of water derived from
wells, essential factors controlling the response of an
aquifer to development: Civil Engineering, v. 10, no. 5,
pp. 277-280.
Todd, D. K., 1980, Groundwater hydrology, 2d ed.: New
York, John Wiley. 535p.
Walton, W. C., 1970, Groundwater resource evaluation:
New York, McGraw-Hill, 664p.
116
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Chapter 6
MODELS AND COMPUTERS IN GROUND-WATER INVESTIGATIONS
Models, in the broadest sense, are simplified descriptions
of an existing physical system. Any ground-water
investigation that does more than simply collect and
tabulate data involves modeling. A preliminary model,
or hypothesis, describing the ground-water system is
tested by collecting data. If the data fit the hypothesis,
the model is accepted; otherwise, the model must be
revised. Models can be (1) qualitative descriptions of
how processes operate in a system; (2) simplified
physical representations of the system such as "sand
tank" physical aquifer models and laboratory batch
experiments to measure adsorption isotherms; and (3)
mathematical representations of the physical system.
lis chapter focuses on models that can be expressed
|mathematical form and adapted for use in computer
jes. The American Society forTesting and Materials
(ASTM) defines model and computer code as follows
(ASTM.1984):
A model is an assembly of concepts in the form of a
mathematical equation that portrays understanding of a
natural phenomenon.
A computer code is the assembly of numerical
techniques, bookkeeping, and control languages that
represents the model from acceptance of input data and
instruction to delivery of output.
Modeling with computers is a specialized field that
requires considerable training and experience. In the
last few decades, literally hundreds of computer codes
for simulating various aspects of ground-water systems
have been developed. Refinements to existing codes
and development of new codes proceed at a rapid pace.
This chapter provides a basic understanding of modeling
and data analysis with computers, including (1) their
uses; (2) basic hydrogeologic parameters that define
their type and capabilities; (3) classification according
to mathematical approach and major types of
fedrogeologic parameters simulated; (4) special
management considerations in their use; and (5) their
limitations.
Uses of Models and Computers
The great advantage of the computer is that large
amounts of data can be manipulated quickly, and
experimental modifications can be made with minimal
effort, so that many possible situations for a given
problem can be studied in great detail. The danger is
that without proper selection, data collection and input,
and quality control procedures, the computer's
usefulness can be quickly undermined, bringing to bear
the adage "garbage in, garbage out."
Computer codes involving ground water can be broadly
categorized as (1) predictive, (2) resource optimizing,
or (3) manipulative. Predictive codes simulate physical
and chemical processes in the subsurface to provide
estimates of how far, how fast, and in what directions a
contaminant may travel. These are the most widely
used codes and are the focus of most of this chapter.
Resource-optimizing codes combine constraining
functions (e.g., total pumpage allowed) and optimization
routines for objective functions (e.g., optimization of
well field operations for minimum cost or minimum
drawdown/pumping lift) with predictive codes. The U.S.
Forest Service's multiple-objective planning process
for management of national forests makes extensive
use of resource-optimizing codes (Iverson and Alston,
1986). The availability of such codes for ground-water
management is limited and is not a very active area of
research and development (van der Heijde and others,
1985).
Manipulative codes primarily process and format data
for easier interpretation or to assist in data input into
predictive and resource-optimizing codes. A specific
computer code may couple one or more of these types
117
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of codes. For example, codes that facilitate data entry
(preprocessors) and data output (postprocessors) are
becoming an increasingly common feature of predictive
codes.
Government Decision-Making
Computers can assist government decisions concerning
ground-water evaluation/protection in the areas of (1)
policy formulation, (2) rule-making, and (3) regulatory
action.
A study by the Holcomb Research Institute (1976) of
environmental modeling and decision-making in the
United States provides a good overview of modeling for
policy formulation, although most of the case studies
involve surface water and resources other than ground
water. The Office of Technology Assessment (1982)
more specifically addresses the use of water resource
models for policy formulation.
The U.S. EPA's Underground Injection Control Program
regulations on restrictions and requirements for Class I
wells exemplify the use of modeling to assist in rule-
making (Proposed Rules: 52 Federal Register 32446-
32476, August 27,1987; Final Rules:53 Federal Register
28118-28157. July 26, 1988). The 10,000-year no-
migration standard in 40 CFR 128.20(a)(1) for injected
wastes is based, in part, on numerical modeling of
contaminant transport in four major hydrogeologic
settings by Ward and others (1987). Furthermore,
worst-case modeling of typical injection sites by EPA
formed the basis for the decision not to require routine
modeling of dispersion in no-migration petitions.
Ground-water flow and, possibly, solute transport
modeling are required to obtain a permit to inject
hazardous wastes into Class I wells. Permitting decisions
involving activities that may pose a threat to ground-
water quality, such as landfills and surface storage of
industrial wastes, commonly require ground-water
simulations to demonstrate that no hazard exists. U.S.
EPA (1987) provides a good overview of the use of
models in managing ground-waterprotection programs.
Site Assessment and Remediation
Use of modeling and computer codes can be valuable
in three phases of site-specific ground-water
investigations: (1) site characterizaton, (2) exposure
assessment, and (3) remediation assessment.
V"
Site Characterization, Relatively simple models (such
as analytic solutions) may be useful at the early stage
for roughly defining the possible magnitude of a
contaminant problem. Solute transport models that
account for dispersion but not retardation may be useful
in providing a worst-case analysis of the situation. They
may help in defining the size of the area to be studied
and in siting of monitoring wells. If more sophisticated
computer modeling is planned, the specific code to be
used will, to a certain extent, guide site characterization
efforts by the aquifer parameters required as inputs to
the model. Site characterization, particularly where
water-quality samples are tested for possible organic
contaminants, can generate large amounts of data.
Computers are invaluable in compiling and processing
these data.
Exposure Assessment. There is growing use of exposure
assessments across EPA's regulatory programs (U.S.
EPA, 1987). In the case of ground-water contamination,
the results of an exposure assessment will often
determine whether remediation will be required.
Remediation. Predictive models can be particularly
valuable in estimating the possible effectiveness of
alternative approaches to remediating ground-water
contamination (Boutwell and others, 1985). Table 6-1
summarizes the types of modeling required for various
remediation design features.
Hydrogeologic Model Parameters
All modeling involves simplifying assumptions
concerning parameters of the physical system that is
being simulated. Furthermore, these parameters will
influence the type and complexity of the equations that
are used to represent the model mathematically. There
are six major parameters of ground-water systems that
must be considered when developing or selecting a
computer code for simulating ground-water flow and six
additional parameters for contaminant transport.
Ground-Water Flow Parameters
Type of Aquifer. Confined aquifers of uniform thickness
are easier to model than unconfined aquifers because
the transmissivity remains constant. The thickness of
unconfined aquifers varies with fluctuations in the water
table, thus complicating calculations. Similarly,
simulation of variable-thickness confined aquifers is
complicated by the fact that velocities will generally
increase in response to reductions and decrease in
response to increases in aquifer thickness.
Matrix Characteristics. Flow in porous media is much
easier to model than in rocks with fractures or solution
porosity. This is because (1) equations governing
laminar flow are simpler than those for turbulent flow,
118
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ble6-1. Modeling Designed-S
Design Feature
Capping, grading and
revegetation
Effects on
Ground Water
Reduction of
infiltration
Reduction of
successive
leachate
generation
Type of
Model Required
Unsaturated zone
model, vertical
layered
Typical Modeling Problems
Parameters related to leaching
characteristics of reworked soil
(O
a
1
3
o
Changes in heads,
direction of flow,
and contaminant
migration
Controlled plume
removal
Changes in heads and
direction of flow
Plume generation
Saturated zone
model, two-
dimensional area),
axisymmetric or
three-dimensional;
well or series of
wells assigned to
individual node
Saturated zone
model, two-
dimensional area),
axisymmetric or
three-dimensional;
density-dependent
flow; temperature
difference effects
Representing partial penetration
Representing density-dependent effects
Interceptor trenches
Changes in heads,
direction of flow,
and contaminant
migration
Plume removal
Saturated zone
model, two-
dimensional areal
or cross-
sectional, or
three dimensional;
trenches are
represented by
line of notes with
assigned heads
Representing partial penetration,
resolution near trenches
-------
0)
CT
-------
which may occur in fracture; and (2) effective porosity
and hydraulic conductivity can be more easily estimated
for porous media.
Homogeneity and Isotropy. Homogeneous and isotropic
aquifers are easiest to model because their properties
do not vary in any direction. If hydraulic properties and
concentrations are uniform vertically, and in one of two
horizontal dimensions, a one-dimensional simulation is
possible. Horizontal variations in properties combined
with uniform vertical characteristics can be modeled
two-dimensionally. Most aquifers, however, show
variation in all directions and, consequently, require
three-dimensional simulation, which also necessitates
more extensive site characterization data. The spatial
uniformity or variability of aquifer parameters such as
recharge, hydraulic conductivity, effective porosity,
transmissivity, and storativity will determine the number
of dimensions to be modeled.
Phases. Flow of ground water and contaminated ground
water in which the dissolved constituents do not create
a plume that differs greatly from the unpolluted aquifer
in density orviscosity are fairly easy to simulate. Multiple
phases, such as water and air in the vadose zone and
NAPLs in ground water, are more difficult to simulate.
Numberof Aquifers. Asingleaquiferiseasiertosimulate
than multiple aquifers.
Flow Conditions. Steady-state flow, where the
magnitude and direction of flow velocity are constant
with time at any point in the flow field, is much easier to
simulate than transient flow. Transient, or unsteady
flow, occurs when the flow varies in the unsaturated
zone in response to variations in precipitation, and in
the saturated zone when the water table fluctuates.
Contaminant Transport Parameters
Type of Source. For simulation purposes, sources can
be characterized as point, line, area, or volume. A point
source enters the ground water at a single point, such
as a pipe outflow or injection well, and can be simulated
with either a one-, two-, or three-dimensional model.
An example of a line source would be contaminants
leaching from the bottom of a trench. An area source
enters the ground waterthrough a horizontal or vertical
plane. The actual contaminant source may occupy
three dimensions outside of the aquifer, but contaminant
entry into the aquifer can be represented as a plane for
modeling purposes. Leachate from a waste lagoon or
an agricultural field are examples of area sources. A
volume source occupies three dimensions within an
aquifer. A DNAPL that has sunk to the bottom of an
aquifer would be a volume source. Line and area
sources may be simulated by either two- or three-
dimensional models, whereas a volume source would
require a three-dimensional model. Figure 6-1 illustrates
the type of contaminant plume that results from a landfill
in the following cases: (1) area -source on top of the
aquifer, (2) area source within the aquifer and
perpendicular to the direction of flow, (3) vertical line
source in the aquifer, and (4) point source on top ol the
aquifer.
Type of Source Release. Release of an instantaneous
pulse, or slug, of contaminant is easier to model than a
continuous release. A continuous release may be
either constant or variable.
Dispersion. Accurate contaminant modeling requires
incorporation of transport by dispersion. Unfortunately,
the conventional convective-dispersion equation often
does not accurately predict field-scale dispersion (U.S.
EPA, 1988).
Adsorption. It is easiest to simulate adsorption with a
single distribution or partition coefficient. Nonlinear
adsorption and temporal and spatial variation in
adsorption are more difficult to model.
Degradation. As with adsorption, simulation of
degradation is easiest when using a simple first-order
degradation coefficient. Second-order degradation
coefficients, which result from variations in various
parameters, such as pH, substrate concentration, and
microbial population, are much more difficult to model.
Simulation of radioactive decay is complicated but
easier to simulate with precision because decay chains
are well known.
Density/Viscosity Effects. If temperature or salinity of
the contaminant plume is much different than that of the
pristine aquifer, simulations must include the effects of
density and viscosity variations.
Types of Models and Codes
Ground-water models and codes can be classified in
many different ways, including the mathematical
approaches used to develop computer codes, as
computer prediction codes, and as manipulative codes.
Mathematical Approaches
Models and codes are usually described by the number
of dimensions simulated and the mathematical
approaches used. At the core of any model or computer
code are governing equations that represent the system
being modeled. Many different approaches to
formulating and solving the governing equations are
possible. The specific numerical technique embodied
in a computer code is called an algorithm. The following
121
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a. various ways to represent source.
b. horizontal spreading resulting from
various source assumptions.
Figure 6-1. Definition of the Source Boundary Condition Under a Leaking Landfill (numbers 1 to 4
refer to cases 1 to 4) (from van der Heljde and others, 1988)
122
-------
c. detailed view of 3D spreading for
various ways to represent source
boundary.
Case V.
horizontal 2D-areal source at top
of aquifer (for 3D modeling)
Case 2: vertical 2D-source in aquifer
(for 2D horizontal, vertically
averaged, or 3D modeling)
Case 4: point source at top of aquifer
(for 2D or 3D modeling)
Case 3:
1D vertical line source in aquifer
(for 2D horizontal, vertically
averaged, 2D cross-sectional, or
3D modeling)
kFIgure
6-1. Continued
123
-------
discussion compares and contrasts some of the most
important choices that must be made in mathematical
modeling.
Deterministic vs. Stochastic Models. A deterministic
model presumes that a system or process operates so
that a given set of events leads to a uniquely definable
outcome. The governing equations define precise cause-
and-effect or input-response relationships. In contrast,
a stochastic model presumes that a system or process
operates so that a given set of events leads to an
uncertain outcome. Such models calculate the
probability, within a desired level of confidence, of a
specific value occurring at any point.
Most available models are deterministic. However, the
heterogeneity of hydrogeologic environments,
particularly the variability of parameters, such as effective
porosity and hydraulic conductivity, plays a key role in
influencing the reliability of predictive ground-water
modeling (Smith, 1987; Freeze and others, 1989).
Stochastic approaches to characterizing variability with
the use of geostatistical methods, such as kriging, are
being used with increasing frequency to characterize
soil and hydrogeologic data (Hoeksma and Kitandis,
1985; Warrick and others, 1986). The governing
equations for both deterministic and stochastic models
can be solved either analytically or numerically.
Analytical vs. Numerical Models. A model's governing
equation can be solved either analytically or numerically.
Analytical models use exact closed-form solutions of
the appropriate differential equations. The solution is
continuous in space and time. In contrast, numerical
models apply approximate solutions to the same
equations.
Analytical models provide exact solutions, but employ
many simplifying assumptions in order to produce
tractable solutions; thus placing a burden on the user to
test and justify the underlying assumptions and
simplifications (Javendel and others, 1984).
Numerical models are much less burdened by these
assumptions and, therefore, are inherently capable of
addressing more complicated problems, but they require
significantly more data, and their solutions are inexact
(numerical approximations). For example, the
assumptions of homogeneity and isotropicity are
unnecessary because the model can assign point (nodal)
values of transmissivity and storativity. Likewise, the
capacity to incorporate complex boundary conditions
provides greater flexibility. The user, however, faces
difficult choices regarding time steps, spatial grid designs,
and ways to avoid truncation errors and numerical
oscillations (Remson and others, 1971; Javendel and
others, 1984). Improper choices may result in errors
unlikely to occur with analytical approaches (e.g., mass
imbalances, incorrect velocity distributions, and grid-
orientation effects).
Grid Design. Afundamental requirement of the numerical
approach is the creation of a grid that represents the
aquifer being simulated (see Figures 6-2 and 6-3). This
grid of interconnected nodes, at which process input
parameters must be specified, forms the basis for a
matrix of equations to be solved. A new grid must be
designed for each site-specific simulation based, on
data collected during site characterization. Good grid
design is one of the most critical elements for ensuring
accurate computational results.
Figure 6-2. Typical Ground-Water Contamination
Scenario. Several Water-Supply Production Wells
are Located Downgradlent of a Contaminant Source
and the Geology Is Complex.
The grid design is influenced by the choice of numerical
solution technique. Numerical solution techniques
include (1) finite-difference methods (FD); (2) integral
finite-difference methods (IFDM); (3) Galerkin and
variational finite element methods (FE); (4) collocation
methods; (5) boundary (integral) element methods
(BIEM or BEM); (6) particle mass tracking methods,
such as the RANDOM WALK (RW) model; and (7) the
method of characteristics (MOC) (Huyakorn and Finder,
1983; Kinzelbach, 1986). Figure 6-4 illustrates grid
designs involving FD, FE, collocation, and boundary
methods. Finite-difference and finite-element methods
are the most frequently used and are discussed further
below.
124
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VetuMferneturelpi
i parameter* would tw
*p*cHtod et Me* nod* of the grid In performing
•imulrtoo*. The grid d*n«hy h grMtm it th» source
end «t poMntW impact location*.
Figure 6-3. Possible Contaminant Transport
Model Grid Design for the Situations Shown In
Figure 6-2
Finite Difference vs. Finite Element. The finite-element
method approximates the solution of partial differential
equations by using finite-difference equivalents, whereas
the finite-difference method approximates differential
equations by an integral approach. Figure 6-5 illustrates
the mathematical and computational differences in the
two approaches. Table 6-2 compares the relative
advantages and disadvantages of the two methods. In
general, finite-difference methods are best suited for
relatively simple hydrogeologic settings, whereas finite-
element methods are required where hydrogeology is
complex.
Ground-Water Computer Prediction Codes
Terminology for classifying computer codes according
to the kind of ground-water system they simulate is not
uniformly established. There are so many different
ways that such models can be classified (i.e., porous vs.
fractured-rock flow, saturated vs. unsaturated flow,
mass flow vs. chemical transport, single phase vs.
multiphase, isothermal vs. variable temperature) that a
systematic classification cannot be developed that would
not require placement of single codes in multiple
categories.
Defining discrete elements
Finite-difference net
Domain
boundary
Discrete-element
boundary
finite-difference
node
Domain
boundary
Discrete-element
boundary
Collocation
Finite
element
node
Finite
element
Domain
boundary
Discrete-element
boundary
Triangular finite-element net
Domain
boundary
Discrete-element
boundary
•Boundary
element node
Collocation
finite element
\>. Jr ^Bo
Collocation node
Boundary element
seomem
Collocation finite-element net
Boundary element net
: Eaoi noo* rapnMnti o-» rauuon p»
iaeimHOi em csussn. Ths ssmry
nwubM. *icte( « in* eta* el u*< »m in one* MC* mucjtun port
. sr« "»'.i iiyy* nt*oi«a> edtr tmttun n o»u«««cnc nenwrut
Figure 6-4. Influence of Numerical Solution Technique on Grid Design (from Plnder, 1984)
125
-------
Concepts ol the
physical system
Translate to
Partial differential equa-
tion, boundary and Initial
conditions
Subdivide region
into a grid and
apply finite-
difterence approx-
imations to space
and time derivatives
Finite-difference
approach
Finite-element
approach
. Transform to
X-
*
ives
i
Subdivide
into elem
and integ
/
Integral equation
region
ents
rate
i
First-order dilterential
equations
f Apply finite-difference
f approximation to
time derivative
System ol algebraic
equations
I
Solve by direct or
iterative methods
Solution
Figure 6-5. Generalized Model Development by
Finite-Difference and Finlte-Elment Methods (from
Mercer and Faust, 1981)
Table 6-3 identifies four major categories of codes and
11 major subdivisions, which are discussed below. This
classification scheme differs from others (see, for
example, Mangold and Tsang, 1987; van der Heijde
and others, 1988), by distinguishing among solute
transport models that simulate (1) only dispersion, (2)
chemical reactions with a simple retardation or
degradation factor, and (3) complex chemical reactions.
The literature on ground-water codes often is further
confused by conflicting terminology. For example, the
term "hydrochemical" has been applied to completely
different types of codes. Van der Heidje and others
(1988) used the term hydrochemical for codes listed in
the geochemical category in Table 6-3, whereas Mangold
and Tsang (1987) used the same term to describe
coupled geochemical and flow models (chemical-
reaction transport codes in Table 6-3).
Porous Media Flow Codes. Modeling of saturated flow
in porous media is relatively straightforward;
consequently, by far the largest number of codes are
available in this category. Van der Heijde and others
(1988) summarize 97 such models. These models are
not suitable (or modeling contaminant transport if
dispersion is a significant factor, but they may be
required for evaluating hydrodynamic containment of
contaminants and pump-and-treat remediation efforts.
Modeling variably saturated flow in porous media (most
Advantages
Disadvantages
Finite-Difference Method
Intuitive basis
Easy data entry
Efficient matrix techniques
Programming changes easy
Low accuracy for some problems
Regular grids required
Finite-Element Method
Flexible grid geometry
High accuracy possible
Evaluates cross-product terms
better
Complex mathematical basis
Difficult data input
Difficult programming
Source: Adapted from Mercer and Faust (1981).
Table 6-2. Advantages and Disadvantages of FDM and FEM Numerical Methods
126
-------
typically soils and unconsolidated geologic material) is
more difficult because hydraulic conductivity varies with
changes in water content in unsaturated materials.
Such codes typically must model processes, such as
capillarity, evapotranspi ration, diffusion, and plant water
uptake. Van der Heijde and others (1988) summarized
29 models in this category.
Solute Transport Codes. The most important types of
codes in the study of ground-water contamination
simulate the transport of contaminants in porous media.
This isthe second largest category (73 codes) identified
by van der Heidje and others (1988) as being readily
available. Solute transport codes fall into three major
categories (see Table 6-3 for descriptions): (1) dispersion
codes, (2) retardation/degradation codes, and (3)
chemical-reaction transport codes.
Dispersion codes differ from saturated flow codes only
in having a dispersion factor, and they have limited
utility except perhaps for worst-case analyses, since
few contaminants act as conservative tracers.
Retardation/degradation codes are slightly more
sophisticated because they add a retardation or
degradation factor to the mass transport and diffusion
equations. Chemical reaction-transport codes are the
most complex (but not necessarily the most accurate)
because they couple geochemical codes with flow
codes. Chemical reaction-transport codes may be
classified as integrated or two-step codes.
Geochemical Codes. Geochemical codes simulate
chemical reactions in ground-water systems without
considering transport processes. These fall into three
major categories (see Table 6-3): (1) thermodynamic
Type of Code
Description/Uses
Flow fPorous Medial
Saturated
Variable saturated
Simulates movement of water in saturated porous media. Used
primarily for analyzing ground-water availability.
Simulates unsaturated flow of water in the vadose (unsaturated)
zone. Used in study of soil-plant relationships, hydrologic cycle
budget analysis.
Solute Transport (Porous Medial
Dispersion
Retardation/
Degradation
Chemical-reaction
Simulates transport of conservative contaminants (not subject to
retardation) by adding a dispersion factor into flow calculations.
Used for nonreactive contaminants such as chloride and for
worst-case analysis of contaminant flow.
Simulates transport contaminants that are subject to partitioning
of transformation by the addition of relatively simple retardation or
degradation factors to algorithms for advection-dispersion flow.
Used where retardation and degradation are linear with respect to
time and do not vary with respect to concentration.
Combines an advection-dispersion code with a transport
geochemical code (see below) to simulate chemical speciation
and transport. Integrated codes solve all mass momentum,
energy-transfer, and chemical reaction equations simultaneously
for each time interval. Two-step codes first solve mass
momentum and energy balances for each time step and then
requilibrate the chemistry using a distribution-of-species code.
Used primarily for modeling behavior of inorganic contaminants.
Table 6-3. Classification of Types of Computer Codes
127
-------
Type of Code
Description/Uses
Geochemical Codes
Thermodynamic
Distributio n-of-
species
(equilibrium)
Reaction progress
(mass-transfer)
Specialized Codes
Fracture rock
Heat transport
Multiphase flow
Processes empirical data so that thermodynamic data at a
standard reference state can be obtained for individual species.
Used to calculate reference state values for input into
geochemical speciation calculations.
Solves a simultaneous set of equations that describe equilibrium
reactions and mass balances of the dissolved elements.
Calculates both the equilibrium distribution of species (as with
equilibrium codes) and the new composition of the water, as
selected minerals are precipitated of dissolved.
Simulates flow of water in fractured rock. Available codes cover
the spectrum of advective flow, advection-dispersion, heat, and
chemical transport.
Simulates flow where density-induced and other flow variations
resulting from fluid temperature differences invalidate
conventional flow and chemical transport modeling. Used
primarily in modeling of radioactive waste and deep-well injection.
Simulates movement of immiscible fluids (water and nonaqueous
phase liquids) in either the vadpse or saturated zones. Used
primarily where contamination involves liquid hydrocarbons or
solvents.
Source: Adapted from van der Heijde and others (1988) and U.S. EPA (1989).
Table 6-3. Continued
codes, (2) distribution-of-species codes, and (3) reaction
progress codes. Thermodynamic codes perhaps would
be classified more properly as manipulative codes, but
are included here because of their special association
with geochemical codes. Such codes are especially
important for geochemical modeling of deep-well
injection where temperatures and pressures are higher
than near-surface conditions for which most geochemical
codes were developed. Apps (1989) reviews the
availability and use of thermodynamic codes
By themselves, geochemical codes can provide
qualitative insights into the behaviorof contaminants in
the subsurface. They also may assist in identifying
possible precipitation reactions that might adversely
affect the performance of injection wells in pump-and-
treat remediation efforts. Chemical transport modeling
of any sophistication requires coupling geochemical
codes with flow codes. Over 50 geochemical codes
have been described in the literature (Nordstrom and
Ball, 1984), but only 15 are cited by van der Heijde and
others (1988) as passing their screening criteria for
reliability and usability.
Specialized Codes. This category contains special cases
of flow codes and solute transport codes (see Table 6-
3), including (1) fractured rock, (2) heat transport, and
(3) multiphase flow. Fractured rock creates special
problems in the modeling of contaminant transport for
several reasons. First, mathematical representation is
more complex due to the possibility of turbulent flow and
the need to consider roughness effects. Furthermore,
128
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precise field characterization of fracture properties that
influence flow, such as orientation, length, and degree
of connection between individual fractures, is extremely
difficult. In spite of these difficulties, much work is being
done in this area (Schmelling and Ross, 1989). Van der
Heijde and others (1988) identified 27 fractured rock
models.
Heat transport models have been developed primarily
in connection with enhanced oil-recovery operations
(Kayser and Collins, 1986) and programs assessing
disposal of radioactive wastes. Van der Heijde and
others (1988) summarized 36 codes of this type. Early
work in multiphase flow centered in the petroleum
industry focusing on oil-water-gas phases. In the last
decade, multiphase behavior of nonaqueous phase
liquids in near-surface ground-water systems has
received increasing attention. However, the number of
codes capable of simulating multiphase flow is still
limited.
Manipulative Codes
Manipulative codes that may be of value in ground-
water investigations include (1) parameter identification
codes, (2) data processing codes, and (3) geographic
information systems.
Parameter Identification Codes. Parameteridentification
codes most often are used to estimate the aquifer
parameters that determine fluid flow and contaminant
transport characteristics. Examples of such codes
include annual recharge (Pettyjohn and Henning, 1979;
Pun, 1984), coefficients of permeability and storage
(Shelton, 1982; Khan, 1986a and 1986b), and
dispersivity (Guven and others, 1984; Strecker and
Chu, 1986).
Data Processing Codes. Data manipulation codes
specifically designed tofatilitateground-watermodeling
efforts have received little attention until recently. They
are becoming increasingly popular, because they
simplify data entry (preprocessors) to other kinds of
models and facilitate the production of graphic displays
(postprocessors) of the data outputs of other models
(van der Heijde and Srinivasan, 1983;Srinivasan, 1984;
Moses and Herman, 1986). Other software packages
are available for routine and advanced statistics,
specialized graphics, and database management needs
(Brown, 1986).
Geo-EAS (Geostatistical Environmental Assessment
Software} is a collection of interactive software tools for
performing two-dimensional geostatistical analyses of
spatially distributed data. It includes programs for data
file management, data transformations, univariate
statistics, variogram analysis, cross validation, kriging,
contour mapping, post plots, and line/scatter graphs in
a user-friendly format. This package can be obtained
from the Arizona Computer Oriented Geological Society
(ACOGS), P.O. Box 44247, Tucson. AZ. 85733-4247.
Geographic Information Systems. Geographic
information systems (GIS) provide data entry, storage.
manipulation, analysis, and display capabilities for
geographic, environmental, cultural, statistical, and
political data in a common spatial framework. EPA's
Environmental Monitoring System Laboratory in Las
Vegas (EMSL-LV) has been piloting use of GIS
technology at hazardous waste sites that fall under
RCRA and CERCLA guidance. The American Society
for Photogrammetry and Remote Sensing is a primary
source of information on GIS.
Management Considerations for Code Use
The effective use of ground-water models is often
inhibited by a communication gap between managers
who make policy and regulatory decisions and technical
personnel who develop and apply the models (van der
Heijde and others, 1988). This section focuses on the
follow! ng management considerations for using models
and codes: personnel and communication
requirements, cost of hardware and software options,
selection criteria, and quality assurance.
Personnel/Communication
The successful use of mathematical models depends
on the training and experience of the technical support
staff applying the model to a problem, and on the degree
of communication between these technical persons
and management. Managers should be aware that a
fair degree of specialized training and experience are
necessary to develop and apply mathematical models,
and relatively few technical support staff can be expected
currently to have such skills (van der Heijde and others,
1985). Technical personnel need to be familiar with a
numberof scientific disciplines, so that they can structure
models to faithfully simulate real-world problems.
A broad, multidisciplinary team is mandatory for
adequate modeling of complex problems, such as
contaminant transport in ground water. No individual
can master the numerous disciplines involved in such
an effort; however, staff should have a working
knowledge of many sciences so that they can address
appropriate questions to specialists, and achieve some
integration of the various disciplines involved in the
project. In practice, ground-water modelers should
become involved in continuing education efforts, which
managers should expect and encourage. The benefits
129
-------
of such efforts are likely to be large, and the costs of not As a general rule, costs are greatest for personnel,
engaging in them may be equally large. moderate for hardware, and minimal for software. An
optimally outfitted business computer (e.g., VAX 117
Technical staff also must be able to communicate 785 or IBM 3031) costs about $100,000, but it can
effectively with management. As with statistical rapidly pay for itself in terms of dramatically increased
analyses, an ill-posed problem yields answers to the speed and computational power. In contrast, a well-
wrong questions. Tables 6-3 through 6-5 list some complemented personal computer (e.g..IBM-PC/AT or
useful questions managers and technical support staff DEC Rainbow) may cost $10,000, but the significantly
should ask each other to ensure that the solution being slower speed and limited computational power may
developed is appropriate to the problems. Table 6-3 incur hidden costs in terms of its inability to perform
consists of "screening level" questions, Table 6-4 specific tasks. For example, highly desirable statistical
addresses correct conceptualizations, and Table 6-5 packages like SAS and SPSS are unavailable or
contains questions of sociopolitical concern. available only with reduced capabilities for personal
computers. Many of the most sophisticated
Cost of Hardware and Software Options mathematical models are available in their fully capable
The nominal costs of the support staff, computing form only on business computers.
facilities, and specialized graphics' production equipment
associated with numerical modeling efforts can be high. Figure 6-6 compares typical software costs fordiff erent
In addition, quality control activities can result in levels of computing power. Obviously, the software for
substantial costs, depending on the degree to which a less capable computers is less expensive, but the
manager must be certain of the model's characteristics programs are not equivalent; managers need to seriously
and accuracy of output. consider which level is appropriate. If the modeling
Assumptions and Limitations
What are the assumptions made, and do they cast doubt on the model's projections for this
problem?
What are the model's limitations regarding the natural processes controlling the problem? Can
the full spectrum of probable conditions be addressed?
How far in space and time can the results of the model simulations be extrapolated?
Where are the weak spots in the application, and can these be further minimized or
eGminated?
input Parameters and Boundary Conditions
How reliable are the estimates of the input parameters? Are they quantified within accepted
statistical bounds?
What are the boundary conditions, and why are they appropriate to this problem?
Have the initial conditions with which the model is calibrated been checked for accuracy and
internal consistency?
Are the spatial grid design(s) and time-steps of the model optimized for this problem?
Quality Control and Error Estimation
Have these models been mathematically validated against other solutions to this kind of
problem?
Has anyone field verified these models before, by direct applications or simulation of
controlled experiments?
How do these models compare with others in terms of computational efficiency, and ease of
use or modification?
What special measures are being taken to estimate the overall errors of the simulations?
Source: Keely(l987).
Table 6-4. Conceptualization Questions for Mathematical Modeling Efforts
130
-------
Demographic Considerations
Is there a larger population endangered by the problem than we are able to provide sufficient
responses to?
Is It possible to present the model's results in both nontechnical and technical formats, to
reach all audiences?
What role can modeling play in public information efforts?
How prepared are we to respond to criticism of the model(s)?
Political Constraints
Are there nontechnical barriers to using this model, such as tainted by association" with a
controversy elsewhere?
Do we have the cooperation of all involved parties in obtaining the necessary data and
Implementing the solution?
Are similar technical efforts for this problem being undertaken by friend or foe?
Can the results of the model simulations be turned against us? Are the results ambiguous or
equivocal?
legal Concerns
Will the present schedule allow all regulatory requirements to be met in a timely manner?
If we are dependent on others for key inputs to the model(s), how do we recoup losses
stemming from their nonperformance?
What liabilities are incurred for projections that later turn out to be misinterpretations
originating in the model?
Do any of the issues relying on the applications of the model(s) require the advice of
attorneys?
Source: Keely (1987).
Table 6-5. Sociopolitical Questions for Mathematical Modeling Efforts
decisions will be based on very little data, it may not accessed and the results retrieved with no more than a
make sense to insist on the most elegant software and phonecall. Mostimportantlyforground-watermanagers.
hardware. If the intended use involves substantial many of the mathematical models and data packages
amounts of data, however, and sophisticated analyses have been "down-sized" from mainframe computers to
are desired, it would be unwise to opt for the least personal computers; many more are now being written
expensive combination. directly for this market. Rgure 6-7 provides some idea
of the costs of available software and hardware for
There is an increasing trend away from both ends of the personal computers.
hardware and software spectrum and toward the middle;
that is, the use of powerful personal computers is Code Selection Criteria
increasing rapidly, whereas the use of small Technical criteria for selecting ground-water modeling
programmablecalculatorsandlargebusinesscomputers codes have been formulated by U.S. EPA (1988) in the
alike isdeclining. In part, this trend stems from significant form of a decision tree (Rgure 6-8). These technical
improvements in the computing power and quality of criteria correspond roughly to the hydrogeologic model
printed outputs obtainable from persona! computers. It parameters discussed earlier. Table 6-6 summarizes
also is due to the improved telecommunications information with respect to these technical criteria f or 4S
capabilities of personal computers, which are now able analytical and numerical ground-water codes. More
to emulate the interactive terminals of large business detailed information about these codes can be found in
computers so that vast computational power can be U.S. EPA (1988).
131
-------
100
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12345
Ground-Water Modeling Software Categories
Categoric*
1 Mainframe /business computer models
2 Personal computer version* of mainframe models
3 Original IBM-PC and compatibles' model*
4 Handheld microcomputer models (e.g.. Sharp
PC1500)
5 Programmable calculator models (e.g., HP41-CV)
Prices include software and all available
documentation, reports, etc.
A code might meet all of the above technical criteria and
still not be suitable for use due to deficiencies in the
code itself. An ongoing program at the International
Ground Water Modeling Center evaluates codes using
performance standards and acceptance criteria (van
der Heijde, 1987). The Center has rated 296 codes in
seven major categories using a variety of usability and
reliability criteria (van der Heijde and others, 1988).
Favorable ratings for the usability criteria include:
Pre- and Postprocessors. Code incorporates one
or more of this type of code.
Documentation. Code has an adequate description
of user's instructions and example data sets.
Support. Code is supported and maintained by the
developers or marketers.
Hardware Dependency. Code is designed to
function on a variety of hardware configurations.
Figure 6-6. Average Price per Category for Ground-
Water Models from the International Ground Water
Modeling Center
&
ui
1500
12SO
1000
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5 8 7 8 9 10
V«ndora of Gnxjntf-Wiur Mocteto
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Homogeneous or Heterogeneous?
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Select the Appropriate Analytical or 1
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X
X
X
X
X
1 X X
1 X X
1.2 X X
1.2J X X
3 X X
1.2 X X
1 X
3 X X
2 X
2 X X
1.2.3 X X
2 X
3 X X
1.2J X X
2 X X
1.2 X X
2 X X
3 X X
3 X X
1
R
I.R
0.1
1
1
acH
t
i
B
I.R.
CH
R
R
I.R.
CH
1
1
1
R
1
ACH
1
X
y
x
X
X
X
,x
X
FE
ff
r t
FE
FD
FD
FE
FE
FE
FE
FO
FD
FD
FD
FD
FD
NR
FE
FE
FE
IFD
FE
FO
FE
FE
FE
FD.
NR
cr
RW
RW
FE
FD
FD.
HOC
FD
RW
FE
FE
FO
FC
FE
FD
FO
LEGEND
AN - Analytical
Degradation
0 -Zero Order De
1 - First Order Dei
2 - Second Order
R - Radioactive D<
CH - Radioactive ch
Dimensionality
1 - One Dimensior
2 - Two Dimensior
3 - Three Dimensk
FD - Rnhe Difference
FE - Finite Element
IFD - Integrated Finite
MOC • Method of Char
NR - Newton Raphson
Source Type
A - Area! Source
L - Line Source
P - Point Source
Source: U.S. EPA. 1988
Table 6-6. Analytical and Numerical Models Worksheet
134
-------
Favorable ratings for the reliability criteria include:
Review. Both theory behind the coding and the
coding itself are peer reviewed.
Verification. Code has been verified.
Reid Testing. Code has been extensively field
tested for site-specific conditions for which extensive
datasets are available.
Extent of Use. Code has been used extensively by
other modelers.
Quality Assurance/Quality Control
The increasing use of modeling and computer codes in
regulatory settings where decisions may be contested
in court requires careful attention to quality assurance
and quality control in both model development and
application. The American Society for Testing and
Materials (ATSM) defines several important terms that
relate to QA/QC procedures for computer code modeling
(ASTM, 1984):
Verification involves examination of the numerical
technique in the computer code to ascertain that it
truly represents the conceptual model and that
there are no inherent numerical problems associated
with obtaining a solution.
Validation involves comparison of model results
with numerical data independently derived from
experiments or observations of the environment.
Calibration is a test of a model with known input and
output information that is used to adjust or estimate
factors for which data are not available.
Sensitivity is the degree to which the model result is
affected by changes in a selected input parameter.
Huyakom and others (1984) identified three major
levels of quality control in the development of ground-
water models:
1. Verification of the model's mathematics by
comparison of its output with known analytical
solutions to specific problems.
2. Validation of the general framework of the model by
successful simulation of observed field data.
3. Benchmarking of the model's efficiency in solving
problems by comparison with other models.
These levels of quality control address the soundness
and utility of the model alone, but do not treat questions
of its application to a specific problem. Hence, at least
two additional levels of quality control appear justified:
1. Critical review of the problem's conceptualization to
ensure that the modeling effort considers all physical
and chemical aspects that may affect the problem.
2. Evaluation of the specifics of the application, e.g.,
appropriateness of the boundary conditions, grid
design, time steps, etc. Calibration and sensitivity
analysis to determine if the model outputs vary.
greatly with changes in input parameters are
important aspects of this process.
Verification of the mathematical framework of a numerical
model and of a code for internal consistency is relatively
straightforward. Field validation of a numerical model
consists of first calibrating the model using one set of
historical records (e.g., pumping rates and water levels
from a certain year), and then attempting to predict the
next set of historical records. In the calibration phase,
the aquifer coefficients and other model parameters are
adjusted to achieve the best match between model
outputs and known data; in the predictive phase, no
adjustments are made (excepting actual changes in
pumping rates, etc.). Presuming that the aquifer
coefficients and other parameters were known with
sufficient accuracy, a mismatch means that either the
model is not correctly formulated or that it does not treat
all of the important phenomena affecting the situation
being simulated (e.g., does not allowf or leakage between
two aquifers when this is actually occurring).
Field validation exercises usually lead to additional data
gathering efforts, because existing dataforthe calibration
procedure commonly are insufficient to provide unique
estimates of key parameters. Such efforts may produce
a "black box" solution that is so site-specific that the
model cannot be readily applied to another site. For this
reason, the blind prediction phase is an essential check
on the uniqueness of the parameter values used. Field
verification is easiest if the model can be calibrated to
data sets from controlled research experiments.
Benchmarking routines to compare the efficiency of
different models in solving the same problem have only
recently become available (Ross and others. 1982;
Huyakom and others, 1984). Van derHeijde and others
(1988) discuss, in some detail, proceduresfor developing
QA plansforcode development/maintenance and code
application.
Limitations of Computer Codes
Mathematical models are useful only within the context
of the assumptions and simplifications on which they
135
-------
are based and according to their ability to approximate
the field conditions being simulated. Faust and others
(1981) rated the predictive capabilities of available
models with respect to 10 issues involving quantity and
quality of ground water (Table 6-7). A four-tiered
classification scheme for models is shown in Table 6-7:
(1) geographic scope (site, local, regional); (2) pollutant
movement (flow only, transport without reactions, and
transport with reactions); (3) type of flow (saturated or
unsaturated); and (4) type of media (porous orf ractured).
The rating scale by Faust and others (1981) in Table 6-
7 also can be viewed as stages of model development:
0 - No model exists.
1 - Models are still in the research stage.
2 * Models can serve as useful conceptual
tools for synthesizing complicated
hydrologic and quality data.
3 - Models can make short-term predictions (a
few years) with a moderate level of
credibility, given sufficient data.
4 «= Models can make predictions with a high
degree of reliability and credibility, given
sufficient data.
The most advanced model is only able to simulate
available supplies and conjunctive use at the local level.
Contaminant transport modeling is generally at stage 3
for transport without reactions in saturated porous flow
at the site and local level. Models at the stage 2 level of
development generally include transport without
reactions (saturated fractured, unsaturated porous),
and transport with reactions (saturated porous) at the
site and local level. Models at the earliest stage of
development involve transport with reactions in
saturated, fractured media.
Advances have been made in all areas of modeling
since the ratings in Table 6-7 were made, but the basic
relationships are essentially unchanged. This is
illustrated in Table 6-8, which shows the percentage of
computer codes in seven categories that received
favorable usability and reliability ratings by van der
Heijde and others (1988). The heat transport and
geochemical model categories do not have direct
SpftW considffitiant:
^Dw/ljnf /nowpFnwif,
Hmy:
flew condition:
laua
Quantity
Available wppliei
Quantity
Conjunctive UM
Quality
Accidental
Petroleum product!
Quality
Accidental
Roedull
Quality
Accidental
Induttriel chemicjh
OinlllV
Aoriculture
Peiticidn * herbicidei
Quality
Agriculture
Salt buildup
Duality
Wane dispoul
Landfill!
Quality
Watta diieoMl
Injection
Quality
See-water mtniiion
Model Typa
Silt
Flow only
*»f
*
3
1
ul
f
2
f
IXUft
•
mall!
fluid
1
3
Tnmport
w/onaciion*
tmt
3
3
3
3
3
3
3
3
«rr
2
2
2
2
2
2
2
2
untml
1
2
2
2
2
2
2
2
Transport
w/ ruction
Mf
2
2
2
2
CM
1
1
1
1
unar
0
0
0
0
tool
flow
only
ml
4
4
1*1
3
3
Tnmport
w/b
nmciiont
•ft
3
3
3
3
3
3
3
or
1
2
9
2
2
2
2
Trmntpon
w/
ntfctiont
at
2
2
2
2
or
0
0
0
0
Htfion*l
flow
only
aar
3
3
(at
3
3
Tantoorl
*Vb
/•acrrem
aai
2
a»i
2
Table 6-7. Matrix Summarizing Reliability and Credibility of Models Used In Ground-Water Resource
Evaluation
136
-------
Kay to Matrix
Aowi issue and tubissue areas.
Columm model types and scale of ipplicitions; for example, sixth
column applies to a site-scale problem in which pollutant
movement is described by a transport model without reactions
and with saturated flow in fractured media.
Ampliation sctlt
Site area modeled less than a few square mitos.
Local area modeled greater than a few square miles but less than a
lew thousand square miles.
Regional area modeled greater than a few thousand square miles.
Abbreviations
w/ with.
w/o without.
sat saturated ground-water flow conditions.
untat unsaturated flow conditions..
P porous media.
F fractured, fissured, or solution cavity media.
Entries
4 a useable predictive tool having a high degree of reliability
and credibility given sufficient data.
3 a reliable conceptual tool capable of short-term (a few years)
prediction with a moderate level of credibility given sufficient
data.
2 a useful conceptual tool for helping the hydrologist synthesize
complicated hydrologic and quality data.
1 a model thai is still in the research stage.
0 no model exists.
blank model type not applicable to issue area.
Table 6-7. Continued
counterparts in Table 6-7. The multiphase flow category
m closest to the accidental petroleum products quality
category in Table 6-8.
Not surprisingly, the largest number of codes are in the
saturated flow category (97), followed by the saturated
solute-transport category (73). The more limited
availability of models for unsaturated flow, fractured
rock, multiphaseflow, and geochemistry primarily reflects
the difficulties in mathematical formulation due to
complexity of processes, process interactions, and field
heterogeneities.
Table 6-8 also provides an overview of the status of
ground-water modeling from a quality assurance
perspective. In general, a high percentage of codes
have been peer reviewed in terms of the basic theory.
The exceptions are f ractured-rock (44%) and multiphase
flow models (21%). In contrast, relatively few models
have been reviewed in terms of actual coding. Only the
geochemical model category has more than half its
models (60%) meeting this criterion. As was noted
earlier, model verification is a relatively straightforward
procedure, which is demonstrated in Table 6-8 where
high percentages of all categories have been verified.
In contrast, very few codes have had any significant
bmount of field testing. Less than a third of the codes
In the saturated flow category have been extensively
field tested, and field testing of codes in the other
categories ranges from none for fractured rock and
geochemical to 21% for variable saturated flow. The
percentages in Table 6-8 should be viewed with the
following caveats: (1) many codes received an
"unknown" rating, which means that the percentages
may underestimate the number of codes with actual
favorable ratings; and (2) many of the codes have been
subjected to limited field testing.
A number of possible pitfalls will doom a ground-water
modeling effort to failure (OTA, 1982; van der Heijde
and others, 1985):
1. Inadequate conceptualization of the physical
system, such as flow in fractured bedrock
2. Use of insufficient or incorrect data
3. Incorrect use of available data
4. Use of invalid boundary conditions
5. Selection of an inadequate computer code
6. Incorrect interpretation of the computational
results
7. Imprecise or wrongly posed management
problems
137
-------
Type of code
Saturated flow
Solute transport
Heat transport
Variable saturated flow
Fractured rock models
Multiphase flow
Geochemical
Total
97
73
36
29
27
19
15
Support
65%
67%
78%
48%
7%
5%
33%
Theory
Rev.
74%
68%
78%
72%
44%
21%
60%
Code
Rev.
12%
29%
42%
21%
33%
11%
60%
Verifi-
cation
90%
96%
97%
83%
100%
89%
100%
Field
Tested
32%
14%
6%
21%
0%
11%
0%
Source: Adapted from van der Heijde and others (1988).
Table 6-8. Percentage of Computer Codes with Favorable Usability and Reliability Ratings
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