United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA625'6-900l6a
September 1990
xvEPA Handbook
Ground Water
Volume I:
Ground Water and
Contamination
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EP A/625/6-90/016a
September 1990
Handbook
Ground Water
Volume I: Ground Water and Contamination
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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.
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EP A/625/6-90/016a
September 1990
Handbook
Ground Water
Volume I: Ground Water and Contamination
D.S. Environmental Protecti-jn Age-ncy
Region 5, Llhrury (5FL-16)
23C S. Dearborn Street, Boom 1670
Chicago, IL 60604
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Cincinnati, OH 45268
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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
Chaper 2. 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
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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
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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 ground
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 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.
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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 pastonly 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
pit or the bluffs along a 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 intothelake 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:
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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 (CaCOa), 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 (FeaOaHaO), 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 (CaSO4-2H2O), 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-illite, 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.
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Type of Clay3
Property
Lattice type0
Expanding?
Specific surface
(m2/g)
External surface
Internal surface
Swelling capacity
Cation exchange
capacity (meq/100g)
Other similar
Clays
Montmorillonite Verriculite
(Smetite)
2:1
Yes
700-800
High
Very High
High
80-150
Beidellite
Nontrorite
Saporite
Bentonrte^
2:1
Slightly
700-800
High
High
Med-High
100-150+
Illite
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
a Clays 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.
dBentonite is a clay formed from weathering of volcanic ash and is made up mostly of montmorillonite and
beidellite.
6 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, aleadsulfide(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 coalfields
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 that
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
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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
anddikestend 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 lithification. 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
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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.
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. In 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.
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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 fartherdownstream.
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 duringthelast
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, boththrough erosion and deposition. The
debris associated with glacial activity is collectively
termed glacial drift. Unstratif led 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 lit ho logy 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 flecture in which the
rocks are horizontal, or nearly so, on either side of the
f lecture.
-------
Map View
Anticline
Syncline
Cross Section
The arrow indicates the direction of dip. In an anticline,
the rocks dip away from the crest and in a syncfine 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, carryingthemtootherplacesof deposition. The
final topography is related to the erodibility 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 andtheirdimensions appear asunusual 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
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Nonconformity An9ular Unconformity
Disconformity
/ I .»
1 I \
I i i 1 i IT
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 waterborne
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 orthrust 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.
Foot Wall
Fault
Cross-Section
of
Normal Fault
vu:.-:.'..-;-;^
~~A"~~~- Hanging Wall
Hanging Wall
FootWaH-
Cross-Section
of
Reverse Fault
Graben
Plan View
of
Lateral Fault
-Normal Fault
Cross-Section
Of
Graben
Figure 1-4. Cross Sections of Normal, Reverse and Lateral Faults
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Millions of
Eia
Cenozoic
Mesozoic
Paleozoic
Eanfld
Quaternary
Tertiary
Cretaceous
Jurassic
Trias sic
Permian
Pennsylvanian
Mississipplan
Devonian
Silurian
Ordovician
Cambrian
Eoash
Recent
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Pateocene
Years Aflfl
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 least 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 glacialtill, 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
-------
Qgm
Scale (miles)
Qal = aluvium
Qgm - ground moraine
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 of
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 easier to 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 surface
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
Test
Hole
Test
Hole
Test
Hole
!iHJi-';. I!"--: ":::*£ »
V «" ..» i _ _., -- if*:*-'
'i ta - . * ^Xj i ollt » ^ . t/i ' *» --
*;.-.--V.\<' ::^Tr7>~-^ . . ' .-r- - rT"^°:-'5^> ...:, 1
;"''"'' - ''°' "*' ° " ° *'"'' "" "f " '
v'".%X-'.. : Gravel «->;
Scale (feet)
, .. .. .
) ' i'-'t> /.? .;'"> ".':"- °1 vVv'/'
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, silty, yellow brown, poorly consolidated 1-5
Clay, silty, yellow or»v, soft, moderately compacted 5-10
Clay, silty, as above, silty layers, soft 10-15
Silty, clayey, gray, soft, uniform drilling 15-20
Clay, silty, some fine to medium sand, gray 20-30
Clay, gray to black, soft, very tight 30-40
Clay, as above, gravelly near top 40-50
Clay, as above, no gravel 50-60
Clay, as above, very silty in spots, gray 60-70
Clay and silt, very easy drilling 70-80
Clay, as above to gravel, fine to coarse,
sandy, thin clay layers, taking lots of water 80-90
Gravel, as above, some clay near top, very
rough drilling, mixed three bags of mud, lots
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 clay, gravelly and rocky, rough drilling,
poor sample return (till) 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 180-190
Clay, as above, no sand, good sample return 190-200
Clay, as 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 Metamorphic
Rocks
Nearly all of the porosity and permeability of igneous
and metamorphic rocks are the result of secondary
openings, such as fractures, faults, and the dissolution
of certain minerals. A few notable exceptions include
large lava tunnels present in some flows, interflow or
coarse sedimentary layers between individual lavaf lows,
and deposits of selected pyroclastic materials.
Because the openings in igneous and metamorphic
rocks are, volumetrically speaking,quite small, rocksof
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 nearthe surf ace to 15to35 feet at depth inthe 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
-------
Material
Depth (ft)
Test Hole 1
Fill 0-3
Silt, olive-gray 3-14
Sand, fine-medium 14-21
SW, «andy, gray 21-25
Clay, gray 25-29
Sand, fine-coarse 29-47
Clay, gray 47-62
Gravel, fine to coarse, losing water 62-92
Silt, sandy, gray 92-100
Observation well depth 80 feet
Test 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, coarse, 2 bags bentontte and bran 80-100
Observation well depth 88 feet
Test Hole 3
SHt. yellow 0-5
Clay, silty, black 6-15
Sand, fine to coarse 15-29
Clay, silty, 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
Sand, fine 55-61
Clay, sandy, gray 61-66
Sand, fine-coar»e, some gravel 66-103
Silt, gray 103-120
Observation well depth 96 feet
Test Hole 5
Clay, silty, brown 0-10
Silt, clayey, gray 10-80
Gravel, fine-coarse, sandy, taking lots of water
3 bags bentonite 80-120
Sand, fine to coarse, gravelly 120-130
Clay, gravelly and rocky (till) 130-150
Sand, fine. Fort Union Group 150-180
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.
Unless some special circumstance exists, water obtained
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 anunfracturedpartofthesame
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 after burial. 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 (CaMg[CO32]), 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. Inaddition
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-waterquality
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
500 mg/L and 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
-------
Dissolved solids concentrations, mg/l
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 southward from the 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. Thewateris 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
500to 1,500 mg/L range. Much of the water from
glacial drift and bedrock formations is very hard and
15
-------
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 isfound 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, sutfate 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.
A water-quality problem of growing concern, particularly
in irrigated regions, is nitrate, which is derived from
fertilizers, sewage, and through naturalcauses. 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 and geomorphology: 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.
16
-------
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, Guidetothe hydrology of carbonate rocks:
UNESCO, Studies and Reports in Hydrology No. 41.
Pettyjohn, W.A., J.R.J. Studlick, and R.C. Bain, 1979,
Quality 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 surface, an
introductiontogeomorphology: 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 openingsof 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 or insoluble, (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-
watersystems and stream systems are intimately related,
as shown in the following discussions of each of the
ground-water regions.
1. Western Mountain Ranges
(Mountains withthinsoilsoverfractured rocks, alternating
with narrow alluvial and, in part, glaciated valleys)
The Western Mountain Ranges, shown in Figure 2-3,
encompass three areas totaling 278,000 mi2- The largest
area extends in an arc from the Sierra Nevada in
California, north through 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
-------
Feature
Aspect
Range in Conditions
Significance of Feature
Component of the
system
Unconfined 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 for 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 hydrologically insignificant.
Thin or not highly productive.
Multiple thin aquifers interbedded with
nonproductive zones.
The dominant aquiferthick and productive.
A single, unconfirmed aquifer.
Two interconnected aquifers of 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
Pores in unconsoiidated 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. Affect
disperson and dilution of
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
Transmissivity
Large, as in well-sorted, unconsoiidated
deposits.
Moderate, as in poorly-sorted unconsoiidated
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
of wells. Affect 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 of 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 flood 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
-------
2. Alluvial BMin
Nonglaciated
Central
region
^~~~~\
s \
s \
1 V
i .// "\
i
*Zr\.*:/*r
V^\o .
13.
HAWAII
' ,.,
'"D Cr7-i
15. PUERTC
0
1 p.
3 RICO °
- «ij.>» Vo
V\
^
500 Miles
i , ^ . i
800 Kilometers
/£**&
%
AND
VIRGIN ISLANDS
Figure 2-1. Ground-Water Regions Used in This Report [The Alluvial Valleys Region (region 12) is
shown on figure 2-2]
PUERTO RICO AND
VIRGIN ISLANDS
Figure 2-2. Alluvial Valleys Ground-Water Region
20
-------
NORTH
DAKOTA
SOUTH
DAKOTA
NEBR.
CANADA
UNITED STATES
WASHINGTON
OREGON
San Juan Mountains
COLO.
100
I
100
200
300
200
I
300
I
400
T
500
400
T
600
I
700
500 Miles
I
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 o r 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
Consolidated
sedimentary rocks.JX
Water-bearing
fractures
Alluvial
deposits
Granitic and metamorpmc
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
yields of 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 parts 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 f rom af ew 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
inthegranitic 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 finertoward the centersof 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
Survey 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 coverof 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." Water discharges
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 at the 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
-------
m
100
200 300 400 500 Miles
in~
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 than 160 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 nearthe 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
lower parts of the flows. The axes of sharp folds and the
offset of the interflow zones alongfaults 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 connected hydraulically
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 barrierstoground-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(l976) have estimated
that the hydraulic conductivity along the flow-contact
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
losses that occur as water moves vertically through the
26
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OKANOGAN
WASHINGTON } HIGHLANDS
Explanation
Chiefly sedimentary rocks
Chiefly volcanic rocks
Sedimentary and volcanic rocks
Major aquifers thin or absent
Figure 2-8. Generalized Distribution and Types of Major Aquifers of the Columbia Lava Plateau Region
(Modified from U.S. Geological Survey Professional Paper 813-S)
27
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Older mountains
River canyon
Lava |
Flows j
X * X X
1 1 1 III
Explanation
-Present soil zone
-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 openhole (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 whencondit ions are mostfavorable for recharge.
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
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200-
500 250 0 250 500
Hole Radius in Millimeters
750
Figure 2-10. Well In a Recharge Area in the
Columbia River Group (Modified from Luzier and
Burt, 1974)
regulatory restrictions or other changes that have
reduced withdrawals. Declines are still occurring, at
rates as much as a few feet 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 mi2 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 ratherwidely 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 sulfate).
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
Sandstone
Limestone
Metamorphic
rocks
Figure 2-11. Topographic and Geologic Features of the Colorado Plateau and Wyoming Basin Region
29
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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 supplies of grou nd
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 water in 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)
waterthat is, water containing more than 1,000 mg/L
of dissolved solidsis widespread. Most of the shales
and siltstones contain mineralized waterthroughout the
region and below altitudes of about 6,500 ft.
Freshwaterwater containing less than 1,000 mg/L of
dissolved solidsoccurs 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 co ntainingfreshwater,
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 mi2
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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Plane River
Figure 2-12. JopograpMc 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
Quarternary 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, silty 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 corner
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.
31
-------
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 arid 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 from the aquifer occurs to 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 of
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. Nonglaciated 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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40'
35'
105°
WYOMING
100"
MEXICO
SOUTH DAKOTA
Explanation
900 Altitude of the water table in
meters, winter 1978
Contour interval 300 meters
Datum is National Geodetic
Vertical Datum of 1929
50 100
200 Miles
I
0 50 100
I
200 Kilometers
Figure 2-13. Altitude of the Water Table of the High Plains Aquifer
33
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Wichita IpTa'teaus/ .£ J g/g
ounta,ns\ /&* frl '*[* x>
Figure 2-14. Location of Geographic Features Mentioned In the Discussions or Regions Covering the
Central and Eastern Parts of the United States
Regolith
^-^^£^-:
.-...^.-.t:-l
T" : :jJLj t*-^~: Ks?f>yt&.
fractures. . ... .
-t^w.-j. - . . >frr-
Figure 2-15. Topographic and Geologic Features of the Nonglaciated Central Region
34
-------
Explanation
Sandstone
Shale
Metamorphic rocks
Figure 2-16. Topographic and Geologic Features Along the Western Boundary of the Nonglaciated
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
theBalcones 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 of salty water at
relatively shallow depths. In most of the 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
-------
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"area in 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 regionwestwardfromOhiotothe
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 enlarged 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
Loess
Fresh water
Salty water
Figure 2-17. Topographic and Geologic Features of the Glaciated Central Region
36
-------
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. Both the 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 of 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 per day. 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.
All 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
dependsonthenumberand 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 outcrops
Best well sites indicated with X's
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
f ailu re to apply existing technology to the caret u I 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 rangesf rom afewf eet on the higher mountains,
which also have large expanses of barren rock, to more
thanSOO 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 oropenend 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. Twoof 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 of 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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Figure 2-20. Glacial Features of the United States (Adapted from U.S. Geological Survey, 1970, p. 76)
40
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determined. The second problemthat of de-icing
saltsaffects 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
geologicunitsintowhichthey have been divided contain
layers of the different types of sediment that underlie the
region. These named geologic units dip toward the
coast ortoward 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
underlainby thick sequences of unconsolidated 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 nearpumping centers inthe 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 than13,000 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
2
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 unconsolidated 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 surficial deposits rest on
formations, primarily of middle to late Miocene age,
composed of interbedded clay, sand, and limestone.
Theformations of middle to late Miocene age or surficial
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 unconfined 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 surficial aquifers
serve as sources of small ground-water supplies
42
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GEORGIA \
FLORIDA! .
Explanation
10 Altitude of the water level in wells in meters above
sea level. May 1980, Contour Interval 10 meters
VjVA Principal recharge areas
Figure 2-22. Potentiometrlc Surface for the Floridan 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
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The most spectacular discharge from the 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 floodplain and adjacent terraces. In other
words, the width of the deposit is small or very small
compared with its length.
According to these criteria, the valleys of streams that
were not affected by glacial meltwater are not considered
alluvial valleys. The floodplains 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
water table to several feet below the bottom of streams.
This surficial deposit of sand and gravel is commonly
underlain by interbedded silt 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 M ississippi,
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 from the 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
waterformunicipalities, 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
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Mississippi Riv>
Explanation
Gravel
Sand
Silt and clay
Limestone
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. 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. 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.
, 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
systems of the United States:
no. 4, July-August 1982.
Ground Water, v. 20,
Mann, W.B., IV, and others, 19831, Estimated water use
in the United States, 1980: U.S. Geological Survey
Circular 1001.
McGuiness, C.L., 1963, The role of ground water in the
national watersituation." 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., 1951, The conservation of ground water.
McGraw-Hill, New York.
,1952,Ground-water regionsof the United
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Cong. House Committee on Interior and Insular Affairs,
pp 3-78.
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
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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. 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.
Nace, R.L., 1958, Hydrology of the Snake Riverbasalt:
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, Groundwaterhydrology: 2ded. 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.
LeGrand, 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, Hydrochemical 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 LR. 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,
Quarternary 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 with stream hydrographs, channel characteristics,
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
hydrologiststendto 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-water component 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 surface
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 watertable 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 in 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 profound 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 River flows 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. Asaresu It 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
A S O
White 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 watertable is above the base of the stream channel.
Of course the position of the water table fluctuates
throughout the year in response todifferences 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 streamf 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-waterrunoff. That is, it is againing stream. Inthe
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
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Losing stream
(A-A')
Gaining in spring
Losing m fall
(B-B')
Gaining stream
(C-C-)
A'
Water table
(S) in spring
(F) in fall
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 itsflow 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 blockground-
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
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13
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01
01
(0
O
/ Peak 3
0 150
Began 1700 hours
13
Z 11
o>
'5
o>
o>
a
O
Land surface
200 400 600 800
Horizontal Distance, in Feet
1000 1200
Land Surface
3 2 1\ « Gage
Well number
Stage D
Stage E
200
400 600 800 1000 1200
Horizontal Distance, in Feet
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 of 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 stream channel 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 of the previous two
54
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55
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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
aquifersone 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 preparedto represent intermediate
conditions.
Depletion curves are the basis tor 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
-------
40
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
aquifer 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
streamftow. 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
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Peak
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 specif ic conductance of ground water, su rf ace 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 (CCs)/(CgCs) (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 amoff. 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-water discharge
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
solids content of 50-68 mg/L, white that from the surf icial
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)/(CaCsd) 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
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, = ? C. =50
Q=18 C =43
50 Qa + 10 Q^ = 43 x 18
-10Q,-10QW|= -180
50Qt + IQQ^ = 774
40Q.
= 594
Q. = 14.85 fs
OR
50-10 40
Figure 3-8. Contribution to Econfina 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 m2- 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 =
(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=2.3 cf s, C=410;Cg=520 and Cs = 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
Cn = 520
Loess
Glacial till
30'±
Figure 3-9. Four Mile Creek, Iowa
1000
500
u
2
200
20
10
VJ
I
I
o
c
O)
n 1
I I I
Specific conductance
Discharge hydrograph
Ground-water runoft
computed from
conductivity
22 23 24 25 26 27 28 29 30 1 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 streamtlow 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
Pettyjohn and 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
-------
750J23OTOO OUR CKEK AT HOUNT STEALING. OHIO
5 OBI FIICO IH7EAVAL
226.0
S^.HI.
tDTRl OISCHRRGE I.39SC 9 CF OR 17.62 INCHES
GIHUNO WlrEA MNOFF !.J?SE 9 CF OR 9.15 INCHES
GROUND MAFER AS I 56.0
RECHARGE RATE «9000 CPO /SO. HI.
!
-I-
-r-
I-
r-
"7~
OATS
1MMMMO DEER CKEK RT HdUNT STERLING, OHIO
s DDT SLIDING INTERVAL
22B.O St.Ml.
T«TAL DISCHARGE l.HSE 9
GROUND HATER Al'HCFf U.9USC 9
MOUND NATEA RS Z $3.0
MCNRRGt RBTE 4WOO CPO /St. HI.
CF CR 17. (2 H.CHES
CF OR 9. }» IfcCCf!
DOTS
7503230(00 CEER CREEK At BOON! SWUNG. OHIO
S DRT LOCAL HINIllR
IL.
221.0 SO."I.
-t-
-t-
TOTAL DISCHARGE l.HSE <
GAGimt NRTER RONOFF
CAaUNO MUTER AS 1 U6.S
MCWMCE RATE 390000
CF OR 17.62 INCHES
CF OR t.il INCHES
cro /so. HI.
OAT)
Figure 3-11. Deer Creek Hydrographs Separated
by Three Methods and Statistical Data
intervals when streamflow consists entirely of ground-
water runoff. Selected water-level measurements are
plotted on a graph with the mean daily streamflow and
a smooth curve is drawn through the points.
The graph is used by determining, eitherfrom individual
measurements orwater-level recorder data, the ground-
water stage, reading across the graph to the curve, and
then reading down to the stream discharge. For
example, in Figure 3-12 when the mean ground-water
stage is 44.5 feet, ground-water runoff is 10 cfs. Any
flow in excess of this amount is surface runoff. Daily
values of ground-water runoff are plotted on the stream
hydrograph, eventually forming a continuous line
throughout the period of record.
Although wells produce only limited yieldsf romcrystalline
rocks in the Piedmont Upland part of the Delaware River
Basin, streams have unusually high base flows. Olmsted
and Hely (1962) used a ground-water rating curve to
study this apparent inconsistency in a 287 m2 part of the
Brandywine Creek basin in southeastern Pennsylvania,
as illustrated in Figure 3-13.
Bedrock units in the dissected upland basin, which
consists largely of folded Precambrian and Paleozoic
igneous and metamorphic rocks, have similar hydrologic
characteristics. Weathered material of variable thickness
mantles the area and the water table lies largely within
this zone. Precipitation averages about 44 inches.
The 16 observation wells used in this study ranged from
12 to 40 feet in depth; all tapped a weathered schist
aquifer. Six or seven wells were measured weekly or
immediately after storms and wells De-3, Ch-13, and
Ch-14 were selected as index wells. The average depth
to water in all of the wells was 17.45 feet and the annual
fluctuation was 5.75 feet.
Figure 3-14 shows a composite hydrograph of the three
index wells and the discharge of Brandywine Creek.
The curves have similartrends, differing only in amplitude
following runoff events. This is to be expected because
of the quick response time of a stream. Certainly the
closer an observation well is to a stream, the more
nearly the hydrographs will approach a similar shape.
The rating curve in Figure 3-15 shows the relation
between ground-water runoff and ground-water stage
in the Brandywine Creek basin. Notice the elliptical
pattern of the data, which approach a straight line from
October through March but then loop back during
spring, summer, and early fall. Although confusing at
first glance, the significance of the loop becomes readily
apparent when the changes that occur in a ground-
water reservoir throughout a 12-month period in a
humid area are considered. From late fall to spring, the
ground-water stage rises because there are little or no
losses to evapotranspiration, soil moisture may be at or
above field capacity, and ground-water recharge occurs.
The water table reaches its peak during the spring
runoff. From April to October, however, large quantities
of ground water are removed by evapotranspiration, the
soil moisture becomes so depleted there is little or no
recharge, and the quantity of water in storage decreases
because ground-water runoff exceeds recharge. Thus,
61
-------
JO 40
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 (Meinzerand Stearns, 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
evapot ranspiratio n.
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 90 percent flow-duration
period, depending on geographic location and climate,
commonly occurs during the late summer and fall when
62
-------
Schist
Geology generalized from Bixom
and Siese (1932, 1938)
Gneiss and granitic
to ultramafic rocks
Contact approximately
located
Stream-gaging station
O
Precipitation-gaging
station
Temperature-measuring
station
Ch-3
Observation well
Index well
Generalized boundary
of basin
Bd-10
Figure 3-13. Sketch Map of Brandywine Creek Basin, Showing Generalized Geology and Location of
Hydrologic and Meteorologic Stations Used in Report
63
-------
round-Water Statge, in Inches Above Datum 28 Feet
Below Land Surface
S88SSS3S88
n Inches
O W
3 0
Precipitation i
0 O i
Jtnuaiy
February
kx
fm \
'- ..-'/
ill Ji
\
I
Match
/
/
p/x
J
llj
April
AA
i
.1
May
"s. .
V
V ./
\J
l.ll
,
June
' \
\
\
\
\
\
j in
July
"~\
A
\
1 .i .
AUQUSI
".,
V
K\
September
Average gr
B«M Il0
Snow
X.
/N
L..J J,
October
Novembef
Kind-water stag* in ind
*-3, Ch12. and Cn t4
w of BrindywirM Cr*ek
Sno
*-.s
^*^
*****!
hi
ex
w arid Rain
^ "^
^*» ,
L L
D«c*mber
,-'"
/x v
'
II
1
900
800
700 S
600 ^
500 1
u
^
400 0
_c
300 |
200 a
m
100
0
Figure 3-14. Composite Hydrograph of Three Index Wells and the Discharge of Brandywine Creek
100
June +
Mayo
Mar +
Feb.o
Feb.
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 Brandywine
Creek Basin
soil moisture is depleted, there is little or no ground-
water recharge, and the watertable, 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 corner 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
-------
5000
Daily discharge, affected
by regulation at low flow
Estimated base flow, not including
effects of regulation
10
1953
Figure 3-16. Hydrograph of Brandywine Creek at Chadds Ford, Pennsylvania, 1952-53
2 r-
4 -
Explanation
« Data for Periods When Evapotransplration is Very Small
o Data for Periods When Evapotranspiration Is Great
14
40
80 120 160 200
Ground-Water Runoff in Cubic Feet per Second
240
280
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 reservoir on 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, etc.)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 cubicf eet 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 Otentangy 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 widelyfrom 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 afew 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 combiningthe seepage
data and well yields with a map showing the areal 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-wateryield 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 u,mhos. 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 u,mhos),
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
.0500
Q
Upper number is low flow, mgd.
Lower number is low flow, mgd/sq mi
Area of surficial out wash; well yields
may exceed 1000 gpm.
Area of outwash covered by a few teet
of alluvium, well yields commonly
between 500 and 1000 gpm.
Chillicothe
Generally fine-grained alluvium along
flood plain: well yields usually less than
25 gpm
10
15
20
I
Scale (miles)
Figure 3-18. Discharge Measurements in the Scioto River Basin, Ohio
67
-------
(0
>.
3
o
I
o
a
&
c
(0
(0
m
o>
o>
oo
2
Q
O
O
o>
3
68
-------
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 u/nhos, while that from the alluvium
and upstream-derived baseflow average 475 u,mhos.
After mixing, the surface water had a conductivity of
550 umhos.
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 + Qb = QO
where Qi, Qa, Qb, QO are the discharge from upstream
(inflow), from the alluvium, from the limestone, andf rom
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
855Qb = 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 cfs.
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
8 miles
C = 550
Q = 29.8 Cfs
5 miles
C = 550
Q =37.0 cfs
QO
7 miles
O a
o O
G
Qa . 40 ft
ft o
° ° Oa 0° o
^J. -~^, Gravel o
f^ 0 o 0 i "^^ 0 o 0 *
o
7~~~
I ,
I
- *
i
o
"
s*
O 0
, I
I
0
I
I
L^\* - « °
^ je
Till
I I
Limestone C
Potentiometric surface
in limestone
Conductivity of alluvium = 475;
limestone = 1330
1330
inflow
bedrock Inflow
Qa = alluvium Inflow
Oo = outflow
475 Q, -t- 475Q, + 1330Qb = C Q0
Q, + Q. + QB = Q0
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, reflectingthe mean 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 extremesfreezing 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 altitude 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 ail 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 ground-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
glacial till; ground-water runoff from this 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-water storage 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
-------
Discharge in Cubic Feet per Second per Square Mile
i 2 s s
^\
\V^
i\V
\V *i
1 \
\ \
\
\
\
\
\
\
\
Low
^
v\
1
\
\
V
\
V
I
t
\
er Extr
/
?\
\N
\\
^
t
\
\
\
I
I
erne
Up
\
\
\
^
\v
\N
\ '
\
\
\
t
l
\
I
1
^^"
>er
^
\
'\
\v
V;.
\
1
t
1
\
I
1
1
\
Extr
s,
^N
v\
i
1
1
\
\
\
1
\
1
9(716
^_
\
\ \
\N
\
y
A
\\
\\
V
\
l
\
l
t
1
l
1
I
1
l
i
^:
\
\\
\ *
\;
\
\\
\
\
\
~*<.:~*
Little
\y
NX1.
S'
\
Note
^
Mad
Beave
/
H
s. /
'V,
i, . 'r
\
-Curves ore
1921-45, Ire
adjustment
River Nea
r Creek Nc
locking Ri
lor rtormol per
m records or
or short-term i
' Springfielc
!ar East Liv
i/er at Ather
Scioto River at Chillicothe
L
V
\ v\\
1
1
I
1
i
1
1
1
1
1
\°
\ \
\ i
\WI
\
\
\
Home
/
and River
/ |
Near Madi;
iod
>i
ecords.
1
erpool
s
on
liteoak Creek Near Georgetown
/
/
Creek Near New Philadelphia
5 10 2030*090*0 70 (0 «0*9
Percent of Time Discharge per Square Mile Equalled or Exceeded That Shown
Figure 3-21. Flow-Duration Curves for Selected Ohio Streams
71
-------
90°
X'Tytertowr,/
///»///////////////
' MISSISSIPPI *
Jtckson groufi
Gaging station
Figure 3-22. Geologic Map of Area in Southern
Mississippi Having Approximately Uniform
Climate and Altitude
=
2
I
8
^
\N
\\
t r^ iri i i
Explanation
1 Bogua CMto rtM Tytwtown
2 Bnw« Hdlliaabutg
SOikahcyCrMkM Mm
It JWrnng ftftn it D'k)
8^^888 8 8 8 § § 8 8 §8 888^328
0 = do-«^s gisgggg t i » i t it
Percent of Time 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 streamf low 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 tow
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 components inEconfina 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 f tow, by use of specif ic 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
streamflow 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 Brandywine 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.
Goldthwait, 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
-------
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, andthe relation betweenground
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 surf ace is variable in time, area!
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 hydrogeologic 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 snowmelt, 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 snowmelt (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. Thisoccurs.forexample, in the Pacific Northwest,
74
-------
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
obtained from 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 18
36 38 40 42 44
-46
50
48
52
20 22 24 26 28 30 32 34
Unas of equal (irecipflation
(inches)
Figure 4-1. Distribution of Annual Average Precipitaiton 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.
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. Asandysoilhas 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
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
f fictional increases due to the increasing length of flow
channels and the general decrease in permeability
owing to swelling clays. If precipitation isgreaterthan
infiltrationcapacity, surface runoff occurs. If precipitation
is less than the infiltration capacity, all moisture is
absorbed.
Average inches depth of 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 orstormflow, 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
!
880
878
! °
I 877
'£
1
Well A-4
.X" _
/ x'" Well D-5
/' Barometric
-..pressure. ...--. ..-
2 6 12 6
July 14
July 15
Figure 4-4. Response of the Water Table in a
Fine-Grained, Unconfined Aquifer to a High
Intensity Rain
DwchafB* (cubic fwt/wcond)
Figure 4-5. A Generalized Stream Stage vs.
Discharge Rating Curve
77
-------
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.
CfMt
TVrxXdayi)
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 itishiddenfromview, 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 under a 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 watertable must
be determined with care, and many such
measurements have been incorrectly taken. The
position of the water table 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 BASIN
07176500 BIRD CHEEK NEAR AVANT. OX
LOCATION. Lat 3*'29'12", long 96*03'50", In SW 1/4 Nt 1/4 sec.7 (revised), T.ZJ N., R.12 E., Osage County,
Hydrologlc Unit 11070107, ISO ft upstreM fro« county road bridge at Avant, 1.5 ml upstrean fro* Candy Creek,
and at mile 54.2.
DRAINAGE AREA.--36* ml1.
PERIOD OF RECORD.--August IMS to current year.
CACr..--»ater-stage recorder. Datun of gage Is 651.28 ft above National Geodetic Vertical Oatu* of 1929.
REMARKS.--Records fair. Several unpublished observations of water temperature, specific conductance, and pH were
oade during the year and are available at the District Office. Flow slightly regulated since 1958 by Bluesten
Lake. Some regulation since March 1777 by 8Iron Lake (station 07176*60), located on Birch Creek, 12.1 ml
upstream. S«all diversions upstrea* for Municipal water supply for the cities of Pawhuska and Barnsdall.
AVERAGE DISCHARGE. 43 years, 221 ft'/s, 1*0,100 acre-ft/yr.
EXTREMES FOR PERIOD OF RECORD. Maxima discharge, 32,MX) ft'/s, Oct. 2, 1959, gage height, 31.40 ftl maximum gage
height, 32.03 ft, Mar. 11, 197k; no flow at tines.
EXTREMES FOR CURRENT VEAR.--Peak discharges greater than base discharge of £,000 ft'/s and Maximum ():
Date Tine
Date
Nov. 24
Dec. 19
Tine
1415
2115
Discharge
(ft3/s)
8,270
9,820
Cage Height
(ft)
11.25
13.65
Discharge Gage Height
(ft3/*) (ft)
Mar. 3
Apr. 1
1215
20*5
«,3SO
16,200
8.77
"23.47
dally discharge, 3.1 ft'/s, Oct. 17, 18.
DISCHARGE, CUBIC FEET PER SECOND, WATER YEAR OCTOBER 1987 TO SEPTEMBER 1988
MEAN VALUES
DAY
OCT
NOV
DEC
3AN
FEB
HAR
APR
NAY
JUN
3UL
AUC
SEP
1
2
)
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2*
25
2<
27
2*
29
30
31
TOTAL
MEAN
MAX
MIN
AC-FT
CAL YR
WTR YR
116
332
303
295
277
268
23*
132
2t
7.0
6.3
6.2
5.8
5.6
4.8
3.8
3.1
3.1
3.3
3.<
4.1
4.7
8.5
9.6
8.3
7.6
7.4
8.4
9.6
9.6
13
2*11.4
77.8
416
3.1
4780
1987
1988
14
13
)2
12
12
10
10
10
to
10
10
9,»
9.6
9.6
80
93
94
51
36
26
16
10
8.z
3370
1450
336
207
227
240
200
...
6596.2
220
3170
8.2
13080
TOTAl. 123598
TOTAL 111432
157
111
115
92
45
40
36
29
26
23
20
15
14
20
30
47
52
141
5400
4770
943
778
595
525
876
1110
2160
1540
7S8
752
938
22178
715
5400
14
43990
.2 MEAN
.0 MEAN
457
341
297
257
163
138
132
120
8f
120
120
120
142
165
165
923
1540
519
1020
785
397
344
253
?21
191
129
91
120
120
146
120
9737
314
15*0
81
19310
339 MAX
304 MAX
101
101
120
81
118
107
105
106
11*
116
111
104
102
62
43
41
41
42
175
212
147
117
96
80
73
70
69
69
69
...
2792
96.3
212
41
5540
7340
13200
69
522
4680
1460
1860
1890
1520
898
638
534
478
349
322
268
156
137
170
555
405
333
272
232
209
196
188
175
170
166
553
528
1170
21103
681
4680
69
41860
MIN 3.1
MIN 3.1
13200
7040
922
633
813
711
662
629
608
2180
1090
645
404
311
252
152
1*5
2570
831
48*
383
317
186
157
200
220
200
166
151
1*6
36*08
121*
13200
145
72220
AC-FT 2*5200
AC-FT 221000
146
144
137
133
129
125
120
120
120
108
91
54
35
28
22
21
23
24
24
24
24
24
29
33
59
41
29
20
18
17
15
1937
62.5
1*6
15
38*0
12
142
52
55
46
26
23
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
1*2
9.6
1310
8.8
9.6
20
22
24
24
22
18
16
16
22
112
62
34
26
22
3*
26
234
158
50
41
31
27
26
24
22
27
277
123
56
1614.4
52.1
277
8.8
3200
36
30
27
21
20
16
16
16
16
16
16
15
15
15
15
15
15
15
15
15
15
15
15
15
16
16
16
16
16
16
16
537
17.3
36
15
1070
16
17
20
17
16
16
14
It
9.8
9.6
9.6
9.6
9.6
9.6
9.6
146
208
1410
1540
450
219
160
327
268
168
108
80
66
61
54
...
5459.4
182
1540
9.6
10830
Figure 4-7. Stream Discharge Record for Bird Creek near Avant, Oklahoma (From U.S. Geological
Survey Water Resources Data for Oklahoma Water Year 1987, p. 90)
79
-------
We»
Grade leva)
Hooded Basement
Untaturated Zoo*
(So» Moiature)
Saturated Zone
(Ground Water)
Opening* Largely
Filled with Air
CapMary Fringe
Water Table
Openinga Rtod with
Water
Stream
Figure 4-8. The Water Table Generally Conforms to the Surface Topography
Water Table
234
'I
5-5->
ftftft
.
f;f;f:f;f:(Mt:t;t;t:(i
';t;f:tifit;t;f;f;i
itit:(if:(ifi(itif
tififitl
4£v Gravel pack
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 asconfining 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
conditionsunconfined 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. The water level in an unconfined aquifer
is referred to as the water table; in confined aquifers
the water level is called the potentiometric surface.
. . *..^r _ _
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
orcanal, 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 headorpotentiometric surface sufficiently for leakage
to occur. Underflow, which is the normal movement
81
-------
of water through an aquifer, also will transmit ground
waterto 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 time the rock was deposited and
secondary if they developed after lithification. Examples
of the latter include fractures and solution openings.
Porosity and Hydraulic Conductivity
Porosity, expressedasapercentageordecimalfractfon,
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 determinesthe total volume of waterthatarock
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 gpd/ft2 (gallons per
day per square foot) will be used in this section; see
Table 4-2 for conversion factors) and both refer to the
ease with which water can pass through a rock unit. It
Material
Soil
Clay
Sand
Gravel
limestone
Sandctorw, aemiconsolidatad
Granite
Basalt, young
Porocity
56
50
25
20
20
11
0.1
It
Specific Yield
(%byvo»
40
2
22
19
18
6
0.09
8
Specific
Retention
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 per unit 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
fromthree 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
-------
Hydraulic CM***** 0'
2.12x10«
7.46
1
Square melera per day
tm»d-«)
1
.0929
.0124
Square leet per day
(ll*d-»)
10.W
1
.134
Gallon* per day
per fa*
(gald-'ll-1)
W.S
7.46
1
McMret MM*
Unit depth _ Volume _
per year |m* d - ' Inn - ») (tl*d-'
-------
Igneous and Metamorphic Rocks
Unfroct ured
Froctured
Basalt
Unfroctured
Fractured
Sandstone
Lovo flow
Frocfured
Semiconsolidoted
Shale
Unfroctured
Froetured
Carbonate Rocks
Froetured
Cavernous
Clay
Silt, Loess
Silty Sand
Clean Sand
Fine
Coarse
Glacial Till
Gravel
I
.0"'
i
lO"7
I
lO'6
I
I0"s
-. . 1 ....
10-"
IO"S 10"* 10"'
10
10
10
10
m
IO'7 10"* I0~5 10"* IO"3 10"* 10"'
ftd '
10 10
10
10
10
IO"7 IO"6 IO"5 IO"4 I0"s 10"* 10"'
10 10 z 10 5 10' 4 10 5
gal d-' ft'
Figure 4-11. General Range In Hydraulic Conductivity for Various Rock Types
elevation. In the case of a confined aquifer, however,
the water may have the potential to rise to a certain
elevation, but it cannot actually do so until the confining
unit is perforated by a well. Therefore, a potentiometric
surface map of a confined aquifer represents an
imaginary surface.
A potentiometric surface map can be developed into a
flow net by constructing flow lines that intersect the
equipotential lines at right angles. Flow lines are
imaginary paths that would be followed by particles of
water as they move through the aquifer. Although
there is an infinite number of both equipotential and
flow lines, the former are constructed with uniform
differences in elevation between them and the latter so
that they form, in combination with equipotential lines,
a series of squares. A carefully prepared flow net in
conjunction with Darcy's Law (discussed below) can
be used to estimate the quantity of water flowing
through an area.
A plan view flow net of an unconfined aquifer is shown
in Figure 4-16. Notice that all of the water-table contours
point upstream, and that the flow lines originate in the
central part of the interstream divide (recharge area)
and terminate at the streams (discharge line). A vertical
84
-------
i
Total
head
(ht)
i
t
r
«»*»
^ Measuring
*~ point
Potentiometric
i
i
t
i
i surface
Pressure
head (hp)
f
' 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. Inthiscase, 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.
Dirtction of Ground- i
Water Movement ~\|
Segments of
Water Tabte Contours
Water Table Altitude
27 2
27.0
26.8
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 flows 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, lateralflow in units of tow 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
otherflow lines. Where laminar flow does not occur, as
85
-------
\
Aquifer Bcxirxfcry
Bluffs along River Valley
638 We* Location and Altitude of Water Leva! (feet)
Zone of Springs and Seeps -»
Figure 4-15. A Potentiometric Surface Map Representing the Hydraulic Gradient in an Aquifer that
Crops Out Along the Bluffs of a River Valley
Flow line^, ^. - , ^Equipotential line
Land surface
\
Horizontal Scate. in feet
. i 4v° _ L_
80,
IJOO
Figure 4-17. Vertical Flow Net of an Unconfined
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 Net of an Unconfined exits- Tne water level declines along the length of the
Aquifer (Modified from Heath, 1983) flow Path
-------
Gaining stream
Gaining stream
1
1
1
1
'" .
1
\
1
1
1
- -^.
_ix\ ^
1
1
1
1
-~J^
1
1
1
1
r"^^ i
i
i
1
>
t
912
910
908
906
904
Losing stream
Figure 4-18. Plan View Flow Net of an Unconfined
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. It is important to keep in
mind the fact that the head loss occurs in the direction
of flow.
B Land surface
Waterjabje. _. - * ">r^9o2»
B'
904
902
900
898
- 896
894
r 892
-890
3000
60,00
90.00
Horizontal scale, in feet
Figure 4-19. Vertical Flow Net of an Unconfined
Aquifer with a Losing Stream (Modified from
Heath, 1983)
In Figure 4-21B, 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 whenthe 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 wellsof 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
Unconfined -^
aquifer
Confined -/---_ -~ r -
aquifer
Figure 4-20. Flow Lines in Aquifers Tend to Parallel Boundaries but in Confining Units They are Nearly
Perpendicular to Boundaries (Modified from Heath, 1983)
87
-------
A. Horizontal land-filed tube.
Gradient - H/L - I. the energy required
to move the vwrar dattanoa L
0 - Quantity of flow, gpd
A - CroM action*! araa of flow, ft*
K » Hydraulic conductivity « gpd/ft*
B. Vartical tub* with flow
from bottom to top.
O
Vertical tuba wtth flow
from top to bottom.
]T
II
I
Q
-Hi
C. Reid condftiorw.
Water Level
Diacharge
Area
45 feet below land surface. The difference in water
level in two we Us 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)
The quantity of flow from one aquifer to anotherthrough
a confining unit can be calculated by a slightly modified
form of Darcy's Law.
where
Q|_
K'
m'
A
H
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
Q|_=(KVm')AH (13)
quantity of leakage, in gpd
vertical hydraulicconductivity of theconfining
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 tower 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 = (2 gpd/ft2/10 ft) * (5,280ft*2,000 ft)* 2 ft (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 ahydraulic 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?
v = (2,000 gpd/ft2 *(10 ft/5,280 ft))/7.48*. 20
= 2.5 ft/day (16)
Time = 5,280 ft/2.5 ft/day
= 2,112 days or 5.8 years
This velocity value is crude at best and can only be used
as an estimate. Hydrodynamic dispersion, for
Spill
1525'
1 mile
1515*
K - 2000 gpd/sq.tt ££
Unconfined aquifer
. .' -.> /. ./ V- v> ''.. '. *'. -V -V
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.
Transmissivity and Storativity
Hydrogeologists commonly use thetermtransmissivity
(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 importanttermisstorativity(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 aquifer or an increase in discharge from an overlying
stream. Other water-level fluctuations are related to
changes in barometric pressure, tides, earthtides, 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 ortwo long,
4M
4)4
«JJ
4M
4»
. ILLINOIS RIVER STAGE _
AT BEAROSTOWN
.
«-*
*\
~~
=
t
^
*
4)4
4M
4)0
41i
-w
M**
ELL
=
1 1 1 1
CSSI8NI2W-I!
- «.
^HMV
*N
>.4fl
^,
^
X
X""
-*s
tO (9
JUNE
Figure 4-24. Effect of Increasing River Stage on
the Water Level In a Well Tapping a Confined
Aquifer (From Walton, 1970)
TV
[4.00
3.00
2.00
1.00
0.00
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-
water storage can be estimated by the following equation:
Vw = 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.
Evapotranspiratton effects on a surficial or shallow
aquifer are both seasonal and daily. Trees, each
serving as a minute pump, remove water from the
90
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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.
-9.8T
45678
Time, in days
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
unconfined 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 surf ace
Limits of cone
of depression.
Land surface
Confining unit
Figure 4-27. Cones of Depression in Unconfined and Confined Aquifers (From Heath, 1983)
91
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Well A
WellB
-A
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
ff St It f J *
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
^ojemipjmtric^ui^e_(^oj2p^mping)_
T
Cone of depression
l^x^
Producing zone
Length
of
screen
Drawdown in
aquifer -
Well loss
«- 'Nominal* radius
**
o
\ Effective radius
Confined
X X X / /
donffnfnd unlT7XX /x/ xxxxxxxxxxx-x
XXXXXXXXXXXXX
Figure 4-29. Values of Transmissivity Based on Specific Capacity Commonly are Too Low Because of
Well Construction Details that Increase Well Loss (Modified from Heath, 1983)
92
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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.
Specific capacity = Q/s (18)
where
Q = the discharge rate, in gpm
s = 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
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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). Thesources
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 28.0percentof 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 health 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 difficultto 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 watergenerally contains less dissolved
solids than ground water, although at certain times
where ground-water runoff is the major source of
streamf low, 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
94
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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 surface-water quality 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 waterfrom surrounding strata to the well. In
coastal areas pumping has caused seawaterto invade
fresh-water aquifers. In parts of coastal 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. Salt spreading on roads
7. Animal feedlots
8. Fertilizers and pesticides
9. Accidental spills
10. Participate 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 that Originate on
the Land Surface
Infiltration of Contaminated Surface Water. The yield of
many wells tapping streamside aquifers is sustained by
95
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Contaminated
stream
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).
Other stockpiles include coal, metallicores, 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 wastewater 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
96
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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 waterto
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 of these, 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, used for cattle, 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 matter originating
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 waterborne
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 soilthus 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 roadssnow
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 watertable, and into them if they are inthe
saturated zone. In many places the watertable fluctuates
to such a degree that the sewer is gaining in discharge
part 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 inthe deeperwell. Even
the lowest concentrations near the sewer are 50 percent
or more higher than the average background
concentration, which is less than 25 mg/L.
in
ISO
I 100
WM D-S (10.5 ft)
WM D-4 (14 H)
\
MM A-4 (t< ft)
April May June July
' Od Nov DK
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, streamflow, 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 Dry 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, aconsiderable numberof 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 sou rce of water supply. G raveyards
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 water from several of the wells showed high
concentrations of dissolved solids, iron, suit ate, 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-saltwaterinterface 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
intothe deepened channel. Underthese circumstances,
reduction of the depth to fresh water can result in a rise
in the level of saline water several times greater than 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 freshwaterfor
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 200drainage
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 forf luids 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.
Althoughconfined 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, agreat 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 f reshwaterpotentiometric
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 four
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, andoxidation. The greaterthe 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 majorcontrol 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,
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oxidation, and microbial degradation, aswell asdilution.
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 flowthrough
open holes.
The final control is the nature of the contaminant. This
includes its physical, chemical, and biological
characteristics and, particularly, its stability undervarying
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 watertable and the thickness of the
unsaturated zone. Generally speaking, the watertable
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
pro videaconsiderablevolumeof 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 watertable. Much of the annual
precipitation, including snowmelt, is removed by surface
runoff and evapotranspiration; it is only the remainder
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Plan View
Figure 5-3. Infiltration from a Surface Impoundment Will Create a Mound on the Water Table
103
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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
Total iron,
Sulfate, mg/L
Specific Conductance,
H-mhos
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-waterflow
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.
o
*
1200
1000
800 -
600
400 H
200
1000 2000 3000
Tlme.days
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
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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.
In contrastto confined 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
882
I
880-
878
§ 876
874 .
872'
100
200
Tlnw, day*
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-water quality.
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 urnhos. 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
800to1.175u.mhos.
1200
11100
i
I 1000 -
900 -
800
100
200
Tims, day»
300
400
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-1964, 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
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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 Contaiminated
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, 1987b, 1988), Pettyjohn
and others (1986), Nelson (1989), and Froneberger
J«10NOJFU«UJJA«OND
1988 1«M
J«n.
May S»fX.
IBM
Figure 5-9. Variations In Chloride Concentration in
Cluster Wells at the Olentangy River Site
106
-------
CMcxtdt eonemtallon In mg«
i
S
10
!
S"
20
II i §
I* S «
1 § II | II I 11 I
« S jjj R if j}« «? |Jw »
Oct'65;
Oct. Nov. Dec. Jan. Fib. Mw. April
19M
19M
Figure 5-10. Vertical Distribution of Chloride
During 1965 and 1966
Chlorltto concwitritlon In mgfl
3 10
Jin. F«6. Mir. April Ml? Aug. S«pt Oct.
196»
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
Jan'69;
Figure 5-12. Conceptual Model Showing Leaching
and Recontamination 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
inSeptember, 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 14 (A-4) feet deep. 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 depth of 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
- 10
0-
I -10'
2
-20
1300
10 20
Time, In days (September. 1985)
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
waterdecreasesfromaround 1,000 to about 400 umhos
(fig. 5-16).
30
20-
10
Nlrate in Wei A-1 (8.5ft)
Nitrate In Well A-4 (14 ft)
10
Time, in days
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.Cood
Nitrate
30
20 .E
10
10
Time, In 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
-------
1800
1400
1000
600
WrilE-4
(14 «)
Aprfl Hay Jum July Aug S«p Oct Nov Dtc
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 ( 1 972) 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
watertable 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.
Ecologic conditions in fractures 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 andf ractures
than in the bulk soil matrix. Coupled withtheirfargreater
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 characteristicsofthewasteorleachate,
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 hydraulicgradient. Fromthis point on, the
109
-------
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
continuouscontaminat ion source. As Figure 5-l7shows,
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
500
i
1000
1
1500
Scale In Feel
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 lower Plume Velocity is 0.5 feet/day
110
-------
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 Dy = 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 (Rd). 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 areal 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 behaviorof 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
aplume might develop at asite 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, Tritium-hydrologic research, some
results of the U.S. Geological Survey Research Program:
Science, v. 143, no. 3608.
900-
450-
0-
450-
900-
1 1 -J
0 900 1800
1. Chlorobenzen* R^ - 35
2. Unknown -15
3. Chloroform - 3
4. Chloride - 1
Plumes after 2800 days
~T
2700
I
3600
4500
Figure 5-18. Constituents Move at Different Rates Because of Retardation
111
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Deutsch, 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 Activitieson
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, Water pollution 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 pollutionan
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.,1987a, 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 Internal. 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
112
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tanks: Office of Underground Storage Tanks, EPA 6007
M-86/020.
Williams, J.S., 1984, Road saltsilent 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
113
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Chapter 6
GROUND-WATER INVESTIGATIONS
Introduction
Within the last decade, a substantial number of ground-
water investigations have been conducted. Many ot
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. Ingeneral,
most of the sites consist of only several acres or a few
square miles, but a number of 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 areal 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,
114
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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
trespass or negligence, the information 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 Superfund. In these cases particular attention must
be given to the handling of samples as well as the
documentation of field 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, generalized flow direct ions
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
in 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 cover af ew tens 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, lexicological 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 ordetailof 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.
115
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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 costly 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 how directly the investigation
pursues those goals.
I n 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 areal 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 af lexible 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 forthe 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.
Evenf airly 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 communications to 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 pan! 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
logs, 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 with the 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 departments 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 systems. 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 hydrologic
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
areal 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 areal 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-
tabte 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 bythe 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 aground-watermanagement and aquifer
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 onsite 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-waterf low 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 inflow 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 waterf rom 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 atracer and measuring the dilution with time
caused by the inflow of water 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 majorzones of water transmission 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 RCRA 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 for a 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 at 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 thatthe shallow watertable 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 thatthe 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 findingsmost 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,
Miller and others, 1974, and Fuhriman and 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 achange
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 co mparedto geologic
maps, which allowed some general identification of the
physical system that was or appeared to be impacted
(fig. 6-2).
>10mg/l NO,
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, or poor siting 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. Inareasof extensive irrigatfonwhere 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 methemogtobinemia 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 monthsto years. The increase in afew 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 local investigation covered an area of
about 576 mi2. 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 that
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 partteularchemical 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)
w Sandstone outcrop area
Aquifer thickness exceeds 125 ft
Figure 6-3. Generalized Geologic Map of a Local
Investigation
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1100
A
900
700
600
300
100
-100
-300
f = Freth
B - Brackish
S = Salty
Sandstone
Shale
12 miles
-H
Figure 6-4. Geologic Cross-Section Showing Downdip 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. Rather all 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 sums of 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 RCRA sites, the regulatory
124
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investigator probably will be required to work with or at
least use data collected by consultants forthe 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 of 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 pluggedtothe surface with a bentonite
and cement slurry. The borehole data were used to
determine drilling sites for 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 shale, their purpose being to monitor the
water table, 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
estimateground-watervelocity and f tow direction. During
thedrilling phases,cores of the aquifer andthe 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 alongthe western margin of 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'
Sandstone'.
Sandstorm
£3 Shale
Potantiomatric
Surface
Uppermost
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, thatthe quality was within
Figure 6-7. Potentiometric 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
problemshydrocarbons in the unsaturated zone and
ground-water contamination in the vicinity of the surface
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 orderto
126
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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, and T.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 aground-water
management aquifer protection plan: Underground
Injection Practices Council and Texas Water Comm.
Scalf, 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. Cerrillo, 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 problema 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 watersupplies. 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 orto 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-waterquality 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 or perhaps even decades
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-water investigator with the basic information
needed for the determination of a viable approach and
for selecting and designing a cost-effective restoration
scheme.
This chapter provides 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 Superfund 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 miscible character of
the contaminants in ground water. When a material,
particularly a hydrocarbon, is released to the soil, capillary
128
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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 continueto move downward in the saturated zone.
In both cases, the contaminants tend to migrate in the
direction of ground-waterflow. 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 another consideration 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 aquifer material, 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
hydrogeotogic 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 alternativesforground-waterremediation.
Many ground-water texts and reports, particularly the
older ones, show ground-water flow nets to be
homogeneous in both the horizontal and vertical
dimensionsat 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
129
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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
considerations for contaminants. 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 hydrotogic cross
section, that is, a vertical flow net, constructed along the
line A-A'. Notice in this example that in the upper 50 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
tow
10M
10(7
1001
10(7
1077
10(0 10M
(M
1104
'»»..
1127
10M
IMS
1070
V?£3 ("
;ii££i? \
)
Seal*, ft
A1
10SL
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 grou nd-water flow. I n 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 hydrotogic 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, in either pump-and-treat 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,
bituminousconcrete, or asphalt, acombination 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 PVCtopseal, 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 low into, through, orf romacertain location.
Barriers keep fresh ground water from coming into
contact with a contaminated aquifer zone or ground
131
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X v^
£- (mien
Key
Control Point
Specific Conductance Contour
(micromnos/cin)
Figure 7-4. Distribution of Specific Conductance,
May 19,1981
Pond 3
Pond5
Key
t Control Point
-x Specific Conductance Contour
C. (mlcromhOB/cm)
Figure 7-5. Distribution of Specific Conductance,
November 12,1981
waterfrom 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 co ntaminants. 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 bentonite priorto 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 per row 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, orotheropenings. 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). I ncomplete penetration of the grout into the vo ids
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
beam technique 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
Cleary (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. Injectionwellswerethenused
to force a soil bentonite slurry outward along the bottom
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
for excavation 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. Arlotta and
others (1983) described a system that 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.
Hydrodynamic Controls
Hydrodynamic controls are used to isolate a plume of
contamination! romthe normal ground-water flow 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 similar to 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 hydrogeological 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 liner 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 water tend 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 liquid, 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 foraperiod 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 lowerthe
watertable beneath a contamination source orto collect
contaminated ground water from 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 atrench also can be usedto 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 a burning 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. Leachate from the landfill flows into
the filled trench, seeps into the perforated pipe, and
then is collected for treatment. In this case, the main
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Upflradlant Monitoring
Well
Figure 7-6. She Layout
Pert orated *,:
Pipe
Figure 7-7. Site Cross Section
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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.
In 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 Physical/Chemical Treatment
Organic and inorganic contaminants may be treated
chemically to cause immobilization, mobilization for
extraction, or detoxification. 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 metalsororganics, ordetoxification 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). Polymerization 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 fordetoxif ication. 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, andthendownwardthrough
a storm sewertrench 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, infournarrow trenches thattrended
across the plume. Ariser and manifold headerconnected
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 mo nomer contaminant was solidified,
and in some places it exceeded 99 percent
polymerization. It was assumed that the remaining
material 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
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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
tetrachloride, 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 sorbed 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.
I n 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 based on the solvent solubilizing or chemically 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.
Inthe 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 airf rom
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
Figure 7-8. Schematic of a Vaccum Extraction System
injection wells to adjust the pH to the desired level.
Tolman 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 neutralizationovera 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 toxic than the targeted compound. Esters, amides,
carbamates, phosphoric and phosphonic acid esters,
and pesticides are potentially degradable by hydrolysis
(Wagner and others, 1986).
Biodegradation
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, however some 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 uncontaminated 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
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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 ortransformed 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), Parsonsandothers(1985), Parsons and others
(1984), Sulflita and Gibson (1985), Sulflita 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
biodegradationortransformation 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 sitestreated 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 inthis mannerhave 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
for treatment above the watertable. 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 bforestoration 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 volumesofrecirculated water required.
Hydrogen peroxide is an alternative source of oxygen in
biorestoration and Raymond and others (1986) have
patented a process of treat me nt 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 hydrogeotogic 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 fortreatment 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 Coarse Gravel
Gravel to Coarse Sand
Come to Medium Sand
Medium to Fine Sand
Fine Sand to Sift
Hydrocarbons
(when drained)
0.006
0.008
0.015
0.02S
0.040
Air
(when drained)
0.4
0.3
0.2
0.2
0.2
Water
(when flooded)
0.4
0.4
0.4
0.4
0.5
Volume* Required
to meet
Hydrocarbons
Oxygen Demand
Air
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
141
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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. Athorough 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 Controling 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 surf ace 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 attempt ing 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 water supply, 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.
References
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143
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Williams, E.B., 1982, Contaminant containment by in
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*U.S. GOVERNMENT PRINTING OFFICE: 1990 748-159/20501
144
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