Untied States
Environmental Protection
Agency
Roberts Kerr Environmental
Research Laboratory
Ada OK 74820
EPA/600/2-85/018
May 1985
Research and Development
vvEPA
DRASTIC: A
Standardized System for
Evaluating Ground Water
Pollution Potential Using
Hydrogeologic Settings
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EPA/600/2-85/018
May 1985
DRASTIC: A Standardized
System for Evaluating
Ground Water Pollution
Potential Using
Hydrogeologic Settings
by
Linda Aller, Truman Bennett, Jay H. Lehr, and Rebecca J. Petty
National Water Well Association
Worthington, Ohio 43085
Cooperative Agreement
CR-810715-01
Project Officer
Jerry Thornhill
Applications and Assistance Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
U.S. Environmental Protection Agency
Region V, Library
230 South Dearborn Street
Chicago, Illinois 60604
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Notice
The information in this document has been funded wholly or in part by the U.S.
Environmental Protection Agency under Cooperative Agreement CR-810175-
01 to the National Water Well Association. It has been subject to the Agency's
peer and administrative review and approved for publication as an EPA
document.
U,S. Environmental Protection Agency
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Foreword
The U.S. Environmental Protection Agency was established to coordinate
administration of the major Federal programs designed to protect the quality of
our environment.
An important part of the Agency's effort involves the search for information
about environmental problems, management techniques and new technologies
through which optimum use of the Nation's land and water resources can be
assured and the threat pollution poses to the welfare of the American people can
be minimized.
EPA's Office of Research and Development conducts this search through a
nationwide network of research facilities.
As one of the facilities, the Robert S. Kerr Environmental Research Laboratory is
the Agency's center of expertise for investigation of the soil and subsurface
environment. Personnel at the laboratory are responsible for management of
research programs to: (a) determine the fate, transport and transformation rates
of pollutants in the soil, the unsaturated zone and the saturated zones of the
subsurface environment; (b) define the processes to be used in characterizing
the soil and subsurface environment as a receptor of pollutants; (c) develop
techniques for predicting the effect of pollutants on ground water, soil and
indigenous organisms; and (d) define and demonstrate the applicability and
limitations of using natural processes, indigenous to the soil and subsurface
environment, for the protection of this resource.
This report contributes to that knowledge which is essential in order for EPA to
establish and enforce pollution control standards which are reasonable, cost
effective and provide adequate environmental protection for the American
public.
Clinton W. Hall
Director
Robert S. Kerr Environmental
Research Laboratory
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Abstract
A methodology is described that will allow the pollution potential of any
hydrogeologic setting to be systematically evaluated anywhere in the United
States. The system has two major portions: the designation of mappable units,
termed hydrogeologic settings, and the superposition of a relative rating system
called DRASTIC.
Hydrogeologic settings are described for different regions in the United States.
These settings incorporate the major hydrogeologic factors which affect and
control ground-water movement including depth to water table, net recharge,
aquifer media, soil media, topography, impact of the vadose zone and hydraulic
conductivity of the aquifer. These factors, which form the acronym DRASTIC, are
used to infer the potential for contaminants to enter ground water. These
settings form the basis for the entire system and create units which can be
graphically displayed on a map.
The relative ranking scheme uses a combination of weights and ratings to
produce a numerical value, called the DRASTIC INDEX, which helps prioritize
areas with respect to ground-water contamination vulnerability. The entire
system optimizes the use of existing data and provides an evaluation which can
be used to direct resources and waste disposal activities to appropirate areas.
This report was submitted in partial fulfillment of Cooperative Agreement No.
CR-810715-01 by the National Water Well Association under the sponsorship of
the Robert S. Kerr Environmental Research Laboratory, Ada, Oklahoma. This
report covers a period from October 1983 to February 1985, and work was
completed as of February 1985.
IV
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Contents
Page
Disclaimer ii
Foreword iii
Abstract iv
Figures ix
Tables xi
Acknowledgements xiii
Section
1. Introduction 1
Objectives and scope 1
Project background 2
Classificatidn systems 2
Some existing systems which evaluate ground-water
pollution potential 3
2. Development of the System and Overview 5
Hydrogeologic settings 5
DRASTIC 7
Agricultural DRASTIC 10
Integration of hydrogeologic settings and DRASTIC 11
3. DRASTIC: A Description of the Factors 14
Ground-water contamination and DRASTIC 14
Depth to water 15
Net recharge 16
Aquifer media 16
Soil media 17
Topography 18
Impact of the vadose zone 19
Hydraulic conductivity of the aquifer 20
Interaction between parameters 20
4. How to Use Hydrogeologic Settings and DRASTIC 23
Organization of the document 23
Where to obtain information on DRASTIC parameters 23
Steps for use of the system 24
How to use the range in media ratings 27
How to evaluate confined aquifers 27
Single factors overrides 28
Build-your-own-settings 29
How to interpret a DRASTIC INDEX 29
5. Impact—Risk Factors 30
6. Hydrogeologic Setting of the United States by Ground Water
Regions 31
1. Western Mountain Ranges 40
1 Aa East Mountain Slopes 41
1 Ab West Mountain Slopes 42
1 Ba East Alluvial Mountain Valleys 42
1 Bb West Alluvial Mountain Valleys 43
1 Ca East Mountain Flanks 44
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Contents (continued)
1 Cb West Mountain Flanks 44
1 D Glacial Mountain Valleys 45
1EA East Wide Alluvial Valleys (External Drainage) 46
1 Eb West Wide Alluvial Valleys (External Drainage) 46
1 F Coastal Beaches 47
2. Alluvial Basins 48
2A Mountain Slopes 49
2B Alluvial Mountain Valleys 50
2C Alluvial Fans 51
2D Alluvial Basins (Internal Drainage) 52
2E Playa Lakes 52
3. Columbia Lava Plateau 54
3A Mountain Slopes 56
3B Alluvial Mountain Valleys 57
3C Hydraulically Connected Lava Flows 57
3E Alluvial Fans 59
4. Colorado Plateau and Wyoming Basin 60
4A Resistant Ridges * 61
4B Consolidated Sedimentary Rocks 62
4C River Alluvium 63
4D Alluvium and Dune Sand 63
5. High Plains 65
5A Ogallala 67
5B Alluvium 67
5C Sand Dunes 68
5D Playa Lakes 68
5E Braided River Deposits 69
6. Non-Glaciated Central 71
6A Mountain Slopes 73
6B Alluvial Mountain Valleys 73
6C Mountain Flanks 74
6Da Alternating SS, LS, SH-Thin Soil 75
6Db Alternating S, LS, SH-Deep Regolith 75
6E Solution Limestone 76
6Fa River Alluvium With Overbank 77
6Fb River Alluvium Without Overbank 77
6G Braided River Deposits 78
6H Triassic Basins 78
7. Glaciated Central 80
7Aa Glacial Till Over Bedded Sedimentary Rock 82
7Ab Glacial Till Over Outwash 82
7Ac Glacial Till Over Solution Limestone 83
7Ad Glacial Till Over Sandstone 84
7Ae Glacial Till Over Shale 84
7Ba Outwash 85
7Bb Outwash Over Bedded Sedimentary 86
7Bc Outwash Over Solution Limestone 86
7C Moraine 87
7D Buried Valley 88
7Ea River Alluvium With Overbank Deposit 88
7Eb River Alluvium Without Overbank Deposit 89
7F Glacial Lake Deposits 90
7G Thin Till Over Bedded Sedimentary 90
7H Beaches, Beach Ridges, and Sand Dunes 91
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Contents (continued)
8. Piedmont and Blue Ridge 92
8A Mountain Slopes 93
8B Alluvial Mountain Valleys 94
8C Mountain Flanks 94
8D Thick Regolith 95
8E River Alluvium 96
8F Mountain Crests 96
9. Northeast and Superior Uplands 98
9A Mountain Slopes 99
9B Alluvial Mountain Valleys 100
9C Mountain Flanks 100
9Da Glacial Till Over Crystalline Bedrock 101
9Db Glacial Till Over Outwash 102
9E Outwash 102
9F Moraine 103
9Ga River Alluvium With Overbank 104
9Gb River Alluvium Without Overbank 104
10. Atlantic and Gulf Coastal Plain 106
10Aa Confined Regional Aquifers 108
10Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer 108
lOBa River Alluvium With Overbank Deposit 109
10Bb River Alluvium Without Overbank Deposit 110
10C Swamp 110
11. Southeast Coastal Plain 112
11A Solution Limestone 113
11B Coastal Deposits 114
11C Swamp 114
11D Beaches & Bars 115
12. Hawaii 116
12A Mountain Slopes 117
12B Alluvial Mountain Valleys 118
12C Volcanic Uplands 119
12D Coastal Beaches 119
13. Alaska 121
13A Alluvium 121
13B Glacial and Glaciolacustrine Deposits of the
Interior Valleys 123
13C Coastal Lowland Deposits 124
13D Bedrock of the Uplands and Mountains 124
References 127
Appendices
A. Processes and Properties Affecting Contaminant Fate
Fate and Transport 131
Density 131
Solubility 131
Sorption 132
Ion exchange 133
Oxidation-reduction 133
Biodegradation 134
Hydrolysis 135
Volatilization 135
Buffering and neutralization 136
Dilution 136
vii
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Contents (continued)
Dispersion 1 36
Viscosity 137
Mechanical filtration 137
B. Characteristics of Ground-Water Contaminants 140
Inorganic metals 140
Inorganic non-metals 142
Organic compounds 143
C. Sources of Ground-Water Contamination 1 56
Ground-water quality problems that originate on the
land surface 157
Ground-water quality problems that originate in the
ground above the water table 1 59
Ground-water quality problems that originate in the
ground water below the water table 161
VIII
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Figures
Number Page
1 Ground water regions of the United States 6
2 Format of hydrogeologic setting 6
3 Graph of ranges and ratings for depth to water 9
4 Graph of ranges and ratings for net recharge 9
5 Graph of ranges and ratings for aquifer media 10
6 Graph of ranges and ratings for soil media 11
7 Graph of ranges and ratings for topography 11
8 Graph of ranges and ratings for impact of the vadose zone 12
9 Graph of ranges and rating for hydraulic conductivity 13
10 Travel of contaminant with same density as water in the aquifer 14
11 Travel of contaminant that is denser than water in the aquifer 14
12 Travel of contaminant that is less dense than water
in the aquifer 15
1 3 Travel of contaminant that is denser than water and sinks
in the aquifer 15
14 Travel of contaminant that is denser than water in the aquifer in a
direction opposed to the ground-water flow direction 15
1 5 Depth to water in a confined and unconfined aquifer 16
1 6 Soil textural classification chart 18
17 Description and illustration for setting 7Aa—glacial till over
bedded sedimentary rocks 25
18 Description and illustration for setting 6Da—alternating
sandstone, limestone and shale-thin soil 25
19 Ground-water regions of the United States 31
20 Map Legend 39
A-1 Schematic of pathlines showing longitudinal and transverse
dispersion 137
A-2 Plume configuration based on contaminant input 138
B-1 Covalent bonding arrangements of carbon atoms 145
IX
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Tables
Number Page
1 Sources of hydrogeologic information 7
2 Assigned weights for DRASTIC features 8
3 Assigned weights for agricultural DRASTIC features 8
4 Ranges and ratings for depth to water 8
5 Ranges and ratings for net recharge 8
6 Ranges and ratings for aquifer media 8
7 Ranges and ratings for soil media 9
8 Ranges and ratings for topography 9
9 Ranges and ratings for impact of vadose zone media 9
10 Ranges and ratings for hydraulic conductivity 9
11 Potential sources of ground-water contamination and
mode of replacement 14
12 Range of values of hydraulic conductivity and permeability 24
13 Conversion factors for permeability and hydraulic
conductivity units 25
14 DRASTIC and agricultural DRASTIC charts for setting 7Aa—glacial
till over bedded sedimentary rocks 26
15 DRASTIC and agricultural DRASTIC charts for setting
6Da—alternating sandstone, limestone and shale-thin soil 26
16 DRASTIC rating for Maco I 28
17 DRASTIC rating for Maco II 28
18 Hydrogeologic settings and associated DRASTIC INDEX by region 32
19 Hydrogeologic settings and associated DRASTIC INDEX
sorted by rating 32
20 Hydrogeologic settings and associated DRASTIC INDEX
sorted by setting title 33
21 Hydrogeologic settings and associated agricultural DRASTIC
INDEX by region 34
22 Hydrogeologic settings and associated agricultural DRASTIC
INDEX sorted by rating 34
23 Hydrogeologic settings and associated agricultural DRASTIC
INDEX sorted by setting title 35
24 Summary of the principal physical and hydrologic characteristics
of the ground-water regions of the United States 37
25 Common ranges for the hydraulic characteristics of ground-water
regions of the United States 38
26 Ranges and ratings for depth to water 39
27 Ranges and ratings for net recharge 39
28 Ranges and ratings for aquifer media 39
29 Ranges and ratings for soil media 39
XI
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Tables (continued)
30 Ranges and ratings for topography 39
31 Ranges and ratings for impact of the vadose zone 39
32 Ranges and ratings for hydraulic conductivity 39
B-1 EPA list of 129 priority pollutants and the relative frequency of
these materials in industrial waste waters 144
B-2 Substances known to occur in ground water, ranges of detected
concentrations, exceeded standards, examples of uses and
quantitative estimates of carcinogenic potency and
noncarcinogenic toxicity 147
XII
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A cknowledgements
This document creates a standardized system which can be used to evaluate
ground-water pollution potential. At the inception of the project, the implications
for use of such a system were realized and a technical advisory committee was
assembled. Prominent individuals with ground-water expertise respresented
federal and state agencies, the Canadian government and private consultants.
Throughout the development of the system, the committee provided guidance
and direction. The document is a resu It of the synthesis of many approaches and
opinions of individual committee members. Although each of the individuals
contributed positively and effectively to the process, this report is a product of the
National Water Well Association and is not endorsed entirely by each of the
committee members. Successful completion of the project is due to the time and
effort which an unusually able advisory committee was willing to devote to this
activity. To the following named persons, grateful acknowledgement of their
contributions is made:
Michael Apgar, Delaware Department of Natural Resources
Jim Bachmaier, USEPA, Office of Solid Waste
William Back, USGS
Harvey Banks, Consulting Engineer, Inc.
Truman Bennett, Bennett & Williams, Inc.
Robert E. Bergstrom, Illinois State Geological Survey
Stephen Born, University of Wisconsin
Keros Cartwright, Illinois State Geological Survey
Stuart Cohen, USEPA, Hazard Evaluation Division
Steve Cordle, USEPA, Office of Research and Development
George H. Davis, USGS, retired
Stan Davis, University of Arizona
Art Day, USEPA, Land Disposal Branch—OSW
Norbert Dee, USEPA, Office of Ground Water Protection
Donald A. Duncan, South Carolina Department of Health and
Environmental Control
Catherine Eiden, USEPA, Hazard Evaluation Division
Grover Emrich, SMC Martin, Inc.
Glen Galen, USEPA, Land Disposal Branch
Phyllis Garman, Consultant, Tennessee
Jim Gibb, Illinois State Water Survey
Todd Giddings, Todd Giddings & Associates
Ralph Heath, USGS, retired
Ron Hoffer, USEPA, Office of Ground Water Protection
George Hughes, Ontario Ministry of the Environment
Jack Keeley, USEPA, Kerr Environmental Research Laboratory
Jerry Kotas, USEPA, Office of Waste Programs Enforcement
Harry LeGrand, Consultant, North Carolina
Fred Lindsey, USEPA, Waste Management and Economics Division
Paula Magnuson, Geraghty & Miller, Inc.
Martin Mifflin, University of Nevada
Walter Mulica, IEP, Inc.
John Osgood, Pennsylvania Bureau of Water Quality
Wayne Pettyjohn, Oklahoma State University
Paul Roberts, Stanford University
XIII
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John Robertson, Weston Designers & Consultants
Dave Severn, USEPA, Hazard Evaluation Division
Frank Trainer, USGS, retired
Warren Wood, USGS
The basic conceptual foundation for this system is modeled after a waste
disposal site evaluation technique developed by Harry LeGrand. The geographic
framework for the presented system is developed within ground-water regions
as defined by Ralph C. Heath. A special note of acknowledgement and gratitude
is made to those two individuals for their inspiration and assistance in
developing this document.
XIV
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Section 1
Introduction
Objectives and Scope
The purpose of this project is to create a methodology
that will permit the ground-water pollution potential
of any hydrogeologic setting to be systematically
evaluated with existing information anywhere in the
United States. This document has been prepared to
assist planners, managers, and administrators in the
task of evaluating the relative vulnerability of areas to
ground-water contamination from various sources of
pollution. Once this evaluation is complete, it can be
used to help direct resources, waste disposal, and
other land-use activities to the appropriate areas.
This document will also be useful to industry per-
sonnel who desire to understand the relationship
between various practices and the ground-water
pollution potential associated with them and to
university personnel who teach the fundamentals of
hydrogeology and ground-water contamination. It
has been assumed that the reader has only a basic
knowledge of hydrogeology and the processes which
govern ground-water contamination. However, the
greater the experience of the user, the more useful
the system will become because the system can
expand to be beneficial at any level of expertise. This
report is not designed or intended to replace on-site
inspections, or specifically to site any type of industrial
facility or practice. Rather, it is intended to provide a
basis for comparative evaluation of areas with respect
to potential for pollution of ground water.
The scope of this project includes not only the
development of a standardized system for evaluating
pollution potential but also the creation of a system
which can be readily displayed on maps. For purposes
of mapping, the United States has been divided into
hydrogeologic settings. These settings incorporate
the many hydrogeologic factors which will influence
the vulnerability of that setting to ground-water
pollution. The settings have been chosen to represent
areas larger than 100 acres in size, thereby limiting
the system to use as a screening tool and not as a site
assessment methodology.
The scope of this project does not include producing
pollution potential maps of the entire United States.
Rather, a set of demonstration maps will be prepared
to show howthe system could display the information
on a map for ease of use and reference. Inherent in
this demonstration is the idea that the standardized
system cannot be finalized until it has been exten-
sively tested in a wide variety of representative
settings. Therefore, this system and the setting
descriptions will be continually evolving until the
demonstration project is complete.
In the formulation of this document an attempt was
made to try to assimilate the thought processes of
knowledgable professional hydrogeologists when
evaluating the ground-water pollution potential of
any area. From this thought process a simple-to-use
and easy-to-understand methodology has been de-
veloped. It is important to remember that this
document is intended to be used as a screening tool
and is not intended to replace the need for profes-
sional expertise and field work in assessing the
pollution potential in specific areas.
The system has been designed to use information
which is available through a variety of sources.
Information on the parameters, including the depth to
water in an area, net recharge, aquifer media, soil
media, general topography or slope, vadose zone
media, and hydraulic conductivity of the aquifer is
necessary to evaluate the ground-water pollution
potential of any area using hydrogeologic settings.
Although much of this information is available in
existing reports, some might require estimation. In
choosing parameters for which information is already
available in some form, this system does not include
many parameters and types of information which
would be available from a more detailed site invest-
igation. Therefore, it is important to realize that this
document provides only a general, broad assessment
to be used to evaluate sites for potential pollution.
To help illustrate two potential uses of this document,
examples have been included: (1) When a profes-
sional hydrogeologist is asked to recommend the
most hydrogeologically acceptable site for municipal
waste disposal in a county area, he begins by
reviewing many types of different information. From
the information, he immediately rejects sites which
are obviously unsuitable and continues to narrow his
focus until a number of the most promising areas are
identified. Hewill usually then recommendthat more
detailed information be obtained and/or site invest-
igations be made on the most promising areas before
any type of further action is taken. This is analogous
to the purpose of this document. It provides the user
with an idea of where to direct resources for further
evaluation. (2) When state or local administrators
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have limited resources available to devote to ground-
water protection, they are forced to focus these
resources in certain areas. The system presented in
this document helps identify areas which are more or
less vulnerable than others to contamination. This
delineation allows administrators to direct their
resources to those more vulnerable areas most critical
to the management problems, thereby making the
most of the limited resources which are available.
Project Background
With the scope of the project in mind it is necessary to
understand the importance of this document. Ground
water is clearly regarded to be one of our nation's
most valuable resources. Americans have long
depended on ground water for many uses, but the
primary use has been as a source of drinking water.
Over 90 percent of the nation's public water supplies
obtain their source water from ground water (Lap-
penbusch, 1984). Additionally, 97 percent of the
water needs for domestic use in rural areas is served
by ground-water resources (Solley et al., 1983).
National reliance on ground water has increased
dramatically over the past 20 years. In the last 10
years alone, ground-water use has increased almost
30 percent while surface water withdrawals have
increased only 15 percent (Solley et al., 1983). It is
anticipated that the nation's reliance on ground water
will continue to increase as demand for water
increases in the future.
Concomitant with our reliance on ground water has
come the need to protect our ground-water resources
from contamination. Although contamination due to
man has occurred for centuries, only in the past few
years has the nation become aware of the dangers of
ground-water contamination and of the many ways in
which ground water can become contaminated.
Moreover, in recent decades, the diversity of potential
pollutants produced and used by man has increased
dramatically. Since 1974, the Congress of the United
States has been making an attempt to protect the
nation's ground-water resources through legislation.
The Safe Drinking Water Act (SDWA) (Public Law
93-523) mandated the establishment of drinking
water standards to protect the public health, estab-
lished the underground injection control (UIC) pro-
gram to protect underground sources of drinking
water from subsurface injection of wastes through
wells, and established the Sole-Source Aquifer
program. The Resource Conservation and Recovery
Act (RCRA) (Public Law 94-580), passed in October
1976 and amended in November 1982, is the legisla-
tion which controls the management and disposal of
solid and hazardous waste in such a manner that
ground water will not be contaminated. The amended
Federal Insecticide, Fungicide and Rodenticide Act
(FIFRA) (Public Law 92-516) as first passed in October
1972 and amended in 1 975 and 1 978, allows EPA to
prohibit or mitigate ground-water contamination by
pesticides by denying registrations, by modifying
application methods, and through cancellations and
suspensions of pesticides registrations. FIFRA also
explicitly requires EPA to monitor environmental
pollution. The Toxic Substances Control Act (TSCA)
(Public Law 94-469), signed into law in October 1976,
has no direct impact on ground-water protection but
has the potential to be used as a mechanism in
ground-water protection because the Act provides
EPA with the power to regulate the use and manu-
facture of specific chemicals, some of which may
pose ground-water contamination potential. The
Surface Mining Control and Reclamation Act
(SMCRA) of 1977 is the legislation which controls
environmental impacts resulting from all mining
activities: By establishing standards for these facil-
ities, ground water may once again be protected.
Finally, the Comprehensive Emergency Response
Compensation and Liability Act(CERCLA) also known
as "Superfund" provides a mechanism for the clean-
up of ground water which has been contaminated at
abandoned hazardous waste sites. A more complete
discussion of these acts and their provisions which
relate to ground water is given by Lehr, et al. (1984).
This host of legislative measures has sought to help
prevent the pollution of ground water in the future
andto help mitigatesomeofthe problems which have
been created in the past.
Because prevention is the key to helping ensure that
future practices do not result in ground-water con-
tamination, it is now more important than ever to use
planning and management tools to help recognize the
places where certain activities pose a higher risk. This
document addresses this need by providing an
approach which can be used to help direct resources
to protect ground water for future generations.
Classification Systems
One of the fundamental needs of any natural science
is the development of an effective system to group
similar entities into categories. Well-established
systems exist in the fields of botany, geology, and
many other sciences (Joel, 1926). These systems
permit an appropriately trained person to gain certain
insight about an entity simply by knowing the
appropriate category in which it is grouped.
This systematic and logical way of imposing an
artificial system on natural entities has long been
used in the field of geology also. For example, rocks
have been classified according to origin and minerals
grouped according to crystal systems. However, as a
science expands and changes, so must the types of
systems used to describe those characteristics which
need to be studied. The field of hydrogeology is one
area of geology which has only been overtly recog-
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nized since the term was coined by Lucas in 1879
(Davis and Dewiest, 1966). Since that time hydro-
geology has expanded, from a discipline devoted to
water occurrence and availability, to include the
broad aspect of water quality and solute chemistry.
Definition of water quality is fundamental to the
protection of the ground-water resource from pollu-
tion.
The idea of an organized way to describe ground
water systems is not new. Meinzerf 1923) prepared a
small-scale map of the United States showing general
ground-water provinces. Thomas (1952) and Heath
(1984) prepared similar but more detailed maps and
descriptions which grouped aquifers mainly on their
water-bearing characteristics within certain geo-
graphic areas. Blank and Schroeder( 1973) attempted
to classify aquifers based on the properties of rocks
which affect ground water. Of all these systems,
geographic ones have been more widely accepted as
ways to describe the quantity of water which is
available in various regions.
Some Existing Systems Which Evaluate
Ground-Water Pollution Potential
Within the last 20 years the need to expand these
systems or to create a new system to address ground-
water quality has become evident. Many different
systems have been developed to address site selection
for waste disposal facilities such as sanitary landfills
or liquid waste ponds. Among these, the LeGrand
System (LeGrand, 1983) and the modified version
used by the U.S. EPA in the Surface Impoundment
Assessment (SIA) are probably the most well known.
The LeGrand system uses numerical weighting to
evaluate ground-water pollution potential from a
given waste disposal site. By evaluating the site
through a series of four stages, a description of the
hydrogeology of the site, the relative aquifer sensi-
tivity combined with the contaminant severity, the
natural pollution potential presented at that site, and
the engineering modifications which might change
that potential are all evaluated.
The LeGrand system presupposes only a limited
technical knowledge but encourages the user to
become familiar with the concepts presented in the
manual so that skilled judgements can be made inthe
subjective portion of the system. The similarities
between sites are emphasized and the uniqueness of
each site is downplayed.
The U.S. EPA methodology (U.S. EPA, 1983) uses the
basic LeGrand System to define the hydrogeologic
framework, but modifies the system to place empha-
sis on establishing a monitoring priority for the
facility. Once the hydrogeologic characteristics have
been rated, a table is used to define the monitoring
priority. This priority may be adjusted by the rater
using prescribed techniques. Once again only a
limited technical knowledge is presupposed.
Other systems have been designed to tailor the
results to more specific purposes. Thornthwaite and
Mather (1957) and Fenn et al. (1975) developed
water-balance methods to predict the leachate gen-
eration at solid waste disposal sites. This approach is
based on the premise that by knowing the amount of
infiltration into the landfill and the design of the cell,
the leachate quantity for the landfill can be deter-
mined. The system is intended as a tool to be used by
engineers in the early design phase of a facility.
Gibb et al. (1983) devised a rating scheme to establish
priorities for existing waste disposal sites with respect
to their threat to human health by ground water. Via
ranking the site through four factors, (1) health risk of
the waste and handling mode, (2) population at risk,
(3) proximity to wells or aquifers, and (4) susceptibility
of aquifers, a number that ranges from 0-100 was
used to display the relative risk. The system was used
in a specific 2-county assessment by technically
qualified individuals.
Another rating scheme, developed by the Michigan
Department of Natural Resources (1983), is designed
to rank large numbers of sites in terms of risk of
environmental contamination. By evaluating the five
categories: (1) release potential, (2) environmental
exposure, (3) targets, (4) chemical hazard, and (5)
existing exposure, the user obtains a number ranging
from 0 to 2000 points which evaluates the relative
hazard of that site with respect to other sites in
Michigan.
Seller and Canter (1980) evaluated seven empirical
methods to determine their usefulness in predicting
the ground-water pollution effects of a waste disposal
facility at a particular site. The methods they reviewed
included rating schemes, a decision tree approach, a
matrix and a criteria-listing method. They determined
that each method took into account the natural
conditions and facility design and construction, but
that each method was best applied to the specific
situation for which it was designed.
This brief review of selected existing systems reveals
that there are a number of methods that can be
applied to site-specific situations or to evaluation of
the pollution potential of existing sites. However, a
planning tool is needed for use before the site-
specific methods are employed. The system must (1)
function as a management tool, (2) be simple and
easy-to-use, (3) utilize available information, and (4)
be able to be used by individuals with diverse
backgrounds and levels of expertise. This document
contains a system which attempts to meet these
needs and to provide the planning tool necessary
before site-specific evaluations.
-------
References
Blank, Horace R. and Melvin C. Schroeder, 1973.
Geologic classification of aquifers; Ground Water,
Vol. 11, No. 2, pp. 3-5.
Davis, S. N. and R. J. DeWiest, 1966. Hydrogeology;
John Wiley & Sons, 463 pp.
Fenn, Dennis G., Keith J. Hanley, and Truett V.
DeGeare, 1975. Use of the water balance method
for predicting leachate generation from solid waste
disposal sites; U.S. EPA Solid Waste Report No.
168, Cincinnati, Ohio, 40 pp.
Gibb, James P., Michael J. Barcelona, Susan C.
Schock, and Mark W. Hampton, 1983. Hazardous
waste in Ogle and Winnebago Counties: potential
risk via ground water due to past and present
activities; Illinois Department of Energy and Natural
Resources, Document No. 83/26, 66 pp.
Heath, Ralph C., 1984. Ground water regions of the
United States; United States Geological Survey
Water Supply Paper 2242, 78 pp.
Joel, A. H., 1926. Changing viewpoints and methods
in soil classification; reprinted in Soil Classification,
Charles W. Finkl, Jr., editor (1982), Hutchinson
Ross Publishing Co., Stroudsburg, Pennsylvania,
pp. 52-59.
Lappenbusch, W. L, 1984. Health effects of drinking
water contaminants; Proceedings of theThirty-first
Ontario Industrial Waste Conference, Ontario
Ministry of the Environment, Ontario, Canada, pp
271-291.
LeGrand, Harry E., 1 983. A standardized system for
evaluating waste-disposal sites; National Water
Well Association, Worthington, Ohio, 49 pp.
Lehr, Jay H., David M. Nielsen, and John J. Mont-
gomery, 1984. U.S. federal legislation pertaining to
ground water protection; Groundwater Pollution
Microbiology, Gabriel Bitton and Charles P. Gerba,
editors, John Wiley & Sons, pp. 353-371.
Meinzer, Oscar E., 1923. Outline of ground-water
hydrology; United States Geological Survey Water
Supply Paper 494, 71 pp.
Michigan Department of Natural Resources, 1983.
Site assessment system (SAS) for the Michigan
priority ranking system under the Michigan Envi-
ronmental Response Act; Michigan Department of
Natural Resources, 91 pp.
Seller, L. E. and L. W. Canter, 1980. Summary of
selected ground water quality impact assessment
methods; National Center for Ground Water Re-
search Report No. NCGWR 80-3, Norman, Okla-
homa, 142 pp.
Solley, Wayne B., Edith B. Chase, and William B.
Mann, 1983. Estimated use of water in the United
States in 1980; United States Geological Survey
Circular 1001, 56 pp.
Thomas, Harold E., 1952. Ground water regions of
the United States—their storage facilities; Interior
and Insular Affairs Committee, U.S. House of
Representatives, 76 pp.
Thornthwaite, S. W. and J. R. Mather, 1957. Instruc-
tions and tables for computing potential evapo-
transpiration and the water balance; Drexel Insti-
tute of Technology, Laboratory of Climatology,
Publications in Climatology, Centerton, New Jersey,
Vol. 10, No. 3, 311 pp.
United States Environmental Protection Agency,
1983. Surface impoundment assessment national
report; U.S. EPA-570/9-84-002, 200 pp.
-------
Sect/on 2
Development of the System and Overview
The focus of this project is to create a system which
can be used to evaluate the ground-water pollution
potential of any area in the United States. The system
presented herein has two major portions: the designa-
tion of mappable units, termed hydrogeologic set-
tings; and the application of a scheme for relative
ranking of hydrogeologic parameters, called DRAS-
TIC, which helps the user evaluate the relative
ground-water pollution potential of any hydrogeologic
setting. Although the two parts of the system are
interrelated, they are discussed separately in a logical
progression.
At the inception of the project, the far-reaching
implications of a standardized system for evaluating
ground-water pollution potential were realized, and a
broadly-based, highly qualified technical advisory
committee was assembled to help direct this effort.
Through the direction and help of many, and discus-
sion of opinions and suggestions, this system has
evolved to represent a compromise approach. Further
reference to the roleofthecommitteewill be made in
the section discussing the development of the
DRASTIC Index. A list of committee members can be
found in the acknowledgement section.
Hydrogeologic Settings
This document has been prepared using the concept
of hydrogeologic settings. A hydrogeologic setting is a
composite description of all the major geologic and
hydrologic factors which affect and control ground-
water movement into, through, and out of an area. It
is defined as a mappable unit with common hydro-
geologic characteristics, and as a consequence,
common vulnerability to contamination by introduced
pollutants. From these factors it is possible to make
generalizations about both ground-water availability
and ground-water pollution potential.
In order to assist users who may have a limited
knowledge of hydrogeology, the entire standardized
system for evaluating ground-water pollution poten-
tial has been developed within the framework of an
existing classification system of ground-water re-
gions of the United States. Heath (1984) divided the
United States into 15 ground-water regions based on
the features in a ground-water system which affect
the occurrence and availability of ground water
(Figure 1). These regions include:
1. Western Mountain Ranges
2. Alluvial Basins
3. Columbia Lava Plateau
4. Colorado Plateau and Wyoming Basin
5. High Plains
6. Nonglaciated Central Region
7. Glaciated Central Region
8. Piedmont and Blue Ridge
9. Northeast and Superior Uplands
10. Atlantic and Gulf Coastal Plain
11. Southeast Coastal Plain
12. Alluvial Valleys
13. Hawaiian Islands
14. Alaska
15. Puerto Rico and Virgin Islands
Region 12, Alluvial Valleys, is "distributed" through-
out the United States.
For the purposes of the present system. Region 12
(Alluvial Valleys) has been reincorporated into each of
the other regions and Region 15 (Puerto Rico and
Virgin Islands) has been omitted. Since the factors
which influence ground-water occurrence and avail-
ability also influence the pollution potential of an
area, this regional framework is used to help familiar-
ize the user with the basic hydrogeologic features of
the region. An annotated description of each of the
regions and the significant hydrogeologic factors are
included in Section 9, Hydrogeologic Settings of the
United States by Ground-Water Regions.
Because pollution potential cannot be determined on
a regional scale, smaller "hydrogeologic settings"
were developed within each of the regions described
by Heath (1984). These hydrogeologic settings create
units which are mappable and, at the same time,
permit further delineation of the factors which affect
pollution potential.
Each hydrogeologic setting is described in a written
narrative section and illustrated in a block diagram.
Figure 2 shows the format which is used throughout
the document. The descriptions are used to help
orient the user to typical geologic and hydrologic
configurations which are found in each region and to
help focus attention on significant parameters which
are important in pollution potential assessment. The
block diagram enables the user to visualize the
described setting by indicating its geology, geomor-
phology, and hydrogeology.
-------
Figure 1. Ground-water regions of the United States (After Heath, 1984).
2. Alluvial Basins
7.
Glaciated
Central
Region
vo^fr'6. Nonglaciated
Central
Region
9 Northeast and
Superior Uplands
Nonglaciated Central "
Region
9 Northeast and
Superior Uplands
1. Western Mountain
6 Nonglaciated
Central
Region
. Colorado/J1^?
Plateau
and
Wyoming
Basin
6. Nonglaciated
Central Region
6. Nonglaciated
Central Region
a coastal Plain
A set of hydrogeologic settings has been developed
for each region. The document is designed so that
once the broad geographic area is located, the user
does not have to refer to other hydrogeologic settings
in other regions. This means that similar hydrogeo-
logic settings may appear more than once in the
document, but that they have been tailored to reflect
the typical hydrogeologic conditions within each
individual region.
Hawaii
(12C) Volcanic Uplands
This hydrogeologic setting is characterized by mod-
erately rolling topography, at medium elevations, and
rich, dark, soils developed from the basaltic bedrock.
The soils are permeable, rainfall is high, and recharge
is high. Bedrock is composed primarily of alternating
extrusive basaltic lava flows and interlayered weath-
ered zones formed between flows. Ground water
occurs at moderate to deep depths, and aquifer yield
is controlled by fracture zones, vesicular zones (both
primarily cooling features), and the inter-flow weath-
800 kilometers
ered zones. Hydraulic conductivity is high. As with
other settings in Hawaii, heavy pumping stresses
often result in salt-water intrusion. This is a reflection
Figure 2. Format of hydrogeologic setting.
-------
of the fact that each island is surrounded by and
underlain by salt water, with the fresh water occur-
ring in a lenticular body that floats on the salt water.
Ground water yield is therefore limited quite specif-
ically to the amount of water recharged annually.
DRASTIC
Inherent in each hydrogeologic setting are the
physical characteristics which affect the ground-
water pollution potential. A wide range of technical
positions was considered regarding the relative
importance of the many physical characteristics that
affect pollution potential. Factors including aquifer
chemistry, temperature, transmissivity, tortuosity,
gaseous phase transport, and others were evaluated.
The availability of mappable data has also been
considered. As a result of this evaluation, the most
important mappable factors that control the ground-
water pollution potential were determined to be:
D—Depth to water
R—(Net) Recharge
A—Aquifer Media
S—Soil Media
T—Topography (Slope)
I —Impact of the Vadose Zone
C—Conductivity (Hydraulic) of the Aquifer
These factors have been arranged to form the
acronym, DRASTIC, for ease of reference. A complete
description of the important mechanisms considered
within each factor and a description of the signif-
icance of the factor are included in Section 3,
DRASTIC: A Description of the Factors. While this list
is not all inclusive, these factors, in combination,
were determined to include the basic requirements
needed to assess the general pollution potential of
each hydrogeologic setting. The DRASTIC factors
represent measurable parameters for which data are
generally available from a variety of sources without
detailed reconnaissance. Sources of this information are
listed in Table 1.
A numerical ranking system to assess ground-water
pollution potential in hydrogeologic settings has been
devised using the DRASTIC factors. The system
contains three significant parts: weights, ranges, and
ratings. A description of the technique used for
weights and ratings can be found in Dee et at., (1973).
Table 1. Sources of Hydrogeologic Information
Source
U.S. Geological Survey
State Geological Surveys
Depth
to
Water
Table
X
X
Net
Recharge
X
X
Aquifer
Media
X
X
Soil
Media
Topography
X
Impact
of the
Vadose
Zone
X
X
Hydraulic
Conductivity
of the
Aquifer
X
X
State Department of Natural/
Water Resources
U.S. Department of Agriculture-
Soil Conservation Service
State Department of
Environmental Protection
Clean Water Act "208" and
other Regional Planning
Authorities
County and Regional Water
Supply Agencies and
Companies (private water
suppliers)
Private Consulting Firms
(hydrogeologic, engineering)
Related Industry Studies
(mining, well drilling,
quarrying, etc.)
Professional Associations
(Geological Society of
America, National Water Well
Association, American
Geophysical Union)
Local Colleges and Universities
(Departments of Geology,
Earth Sciences, Civil
Engineering)
Other Federal/State Agencies
(Army Corps of Engineers,
National Oceanic and
Atmospheric Administration)
X
X
X
X
X
X
X
X
X
X
-------
(1) Weights—Each DRASTIC factor has been eval-
uated with respect to the other to determine the
relative importance of each factor. Each DRASTIC
factor has been assigned a relative weight ranging
from 1 to 5 (Table 2). The most significant factors have
weights of 5; the least significant, a weight of 1. This
exercise was accomplished by the committee using a
Delphi (consensus) approach. These weights are a
constant and may not be changed. A second weight
has been assigned to reflect the agricultural usage of
herbicides and pesticides (Table 3). These weights are
also constants and cannot be changed. A description
of the usage of this second system can be found in
Section 2 under the heading, "Agricultural DRAS-
TIC."
Table 2. Assigned Weights for DRASTIC Features
Feature Weight
Depth to Water Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact of the Vadose Zone
Hydraulic Conductivity of the Aquifer
Table 3.
Assigned Weights for Agricultural DRASTIC
Features
Feature
Agricultural
Weight
Depth to Water Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact of the Vadose Zone
Hydraulic Conductivity of the Aquifer
5
4
3
5
3
4
2
(2) Ranges—Each DRASTIC factor has been divided
into either ranges or significant media types which
have an impact on pollution potential (Tables 4-10). A
discussion of the media types is included in Section 3,
Aquifer Media, Soil Media, and Impact of the Vadose
Zone. The ranges and media types are graphed to
show the linearity and non-linearity of the factor
(Figures 3-9).
(3) Ratings—Each range for each DRASTIC factor
has been evaluated with respect to the others to
determine the relative significance of each range
with respect to pollution potential. Based on the
graphs, the range for each DRASTIC factor has been
assigned a rating which varies between 1 and 10
(Tables 4-10). The factors of D, R, S, T, and C have
been assigned one value per range. A and I have been
assigned a "typical" rating and a variable rating. The
variable rating allows the user to choose either a
typical value or to adjust the value based on more
specific knowledge. The ratings are the same for both
the DRASTIC Index and the modified Agricultural
DRASTIC Index.
This system allows the user to determine a numerical
value for any hydrogeologic setting by using an
additive model. The equation for determining the
DRASTIC Index is:
DRD
RW
+ TRT
RW
+ !R!W + CRCW = Pollution Potential
where:
R = rating
W = weight
Table 4. Ranges and Ratings for Depth to Water
Depth to Water
(feet)
Range
Rating
0-5
5-10
15-30
30-50
50-75
75-100
100 +
Weight: 5
10
9
7
5
3
2
1
Agricultural Weight:
5
Table 5. Ranges and Ratings for Net Recharge
Net Recharge
(inches)
Range
0-2
2-4
4-7
7-10
10 +
Weight: 4
Rating
1
3
6
8
9
Agricultural Weight:
4
Table 6. Ranges and Ratings for Aquifer Media
Aquifer Media
Range
Massive Shale
Metamorphic/lgneous
Weathered Metamorphic/lgneous
Thin Bedded Sandstone,
Limestone, Shale Sequences
Massive Sandstone
Massive Limestone
Sand and Gravel
Basalt
Karst Limestone
Weight: 3
Rating
1-3
2-5
3-5
5-9
4-9
4-9
6-9
2-10
9-10
Typical Rating
2
3
4
6
6
6
8
9
10
Agricultural Weight: 3
8
-------
Table 7. Ranges and Ratings for Soil Media
Soil Media
Range
Rating
Thin or Absent
Gravel
Sand
Shrinking and/or Aggregated Clay
Sandy Loam
Loam
Silty Loam
Clay Loam
Nonshrinking and Nonaggregated Clay
Weight: 2
10
10
9
7
6
5
4
3
1
Agricultural Weight: 5
Table 8. Ranges and Ratings for Topography
Topography
(percent slope)
Range
0-2
2-6
6-12
12-18
18 +
Weight: 1
Rating
10
9
5
3
1
Agricultural Weight: 3
Table 9. Ranges and Ratings for Impact of Vadose
Zone Media
Impact of Vadose Zone Media
Range
Silt/Clay
Shale
Limestone
Sandstone
Bedded Limestone, Sandstone, Shale
Sand and Gravel with
significant Silt and Clay
Metamorphic/lgneous
Sand and Gravel
Basalt
Karst Limestone
Weight: 5
Rating Typical Rating
1-2
2-5
2-7
4-8
4-8
4-8
2-8
6-9
2-10
8-10
Agricultural
1
3
6
6
6
6
4
8
9
10
Weight: 4
Table 10. Ranges and Ratings for Hydraulic Conductivity
Hydraulic Conductivity
(GPD/FT2)
Range
1-100
100-300
300-700
700-1000
1000-2000
2000 +
Weight: 3
Rating
1
2
4
6
8
10
Agricultural Weight:
2
Once a DRASTIC Index has been computed, it is
possible to identify areas which are more likely to be
susceptible to ground-water contamination relative
to one another. The higher the DRASTIC Index, the
greater the ground-water pollution potential. The
DRASTIC Index provides only a relative evaluation
tool and is not designed to provide absolute answers.
Therefore, the numbers generated in the DRASTIC
Index and in the agricultural DRASTIC Index cannot
be equated.
Figure 3 Graph of ranges and ratings for depth to water.
10-
Q«
8-
7-
6-
c
oc
6 10 20 30 40 50 60 70 80 90 100
Feet
Figure 4. Graph of ranges and ratings for net recharge.
5 6 7 8 9 10
0123
Inches
9
-------
Agricultural DRASTIC
Agricultural DRASTIC is designed to be used where
the activity of concern is the application of herbicides
and pesticides to an area. It represents a special case
of the DRASTIC Index. The only way in which
Agricultural DRASTIC differs from DRASTIC is in the
assignment of relative weights for the seven DRASTIC
Figure 5. Graph of ranges and ratings for aquifer media.
10-
9-
8-
7-
6-
I 5H
CD
DC
4-H
3-
2-
1 —
Relative ranges of ease of pollution for the
principal aquifer types.
Ranges are based upon consideration of:
a) route length and tortuosity
b) potential for consumptive sorption
c) dispersion
d) reactivity and
e) degree of fracturing
Primary factors affecting rating:
1 Reactivity (solubility and fracturing)
2. Fracturing
3. Route length and tortuosity, sorption,
dispersion. All essentially determined by grain size,
sorting, and packing
4 Route length and tortuosity as determined
by bedding and fracturing
5. Sorption and dispersion
6 Fracturing, route length and tortuosity,
influenced by intergranular relationships
7. Reactivity (solubility) and fracturing
8 Fracturing and sorption
CO
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CD
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en
en
CD
o
ffi
c
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u
^
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O
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c
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iu
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Primary Media
10
-------
factors. All other parts of the two indexes are
identical; the ranges, ratings, and instructions for use
are the same. If the user is concerned with the
ground-water pollution potential of an area by
herbicides and pesticides, then the weights for
Agricultural DRASTIC should be used.
Agricultural DRASTIC was created to address the
important processes which specifically offset the fate
and transport of herbicides and pesticides in the soil.
These processes, however, may not be as significant
when assigning weights to the other DRASTIC factors
for non-agricultural activities. Thus, by comparing
Tables 2 and 3, it can be seen that for non-agricultural
activities. Soil Media is assigned a weight of 2, while
for the modified Agricultural DRASTIC, the Soil Media
is assigned a weight of 5. Topography, Impact of the
Vadose Zone, and Hydraulic Conductivity of the
Aquifer are also slightly different. By making these
adjustments, the committee addressed the special
conditions which influence the potential for ground-
water contamination by pesticides and herbicides. It
is important to note that the relative relationship
between the DRASTIC factors was not deemed
Figure 6. Graph of ranges and ratings for soil media.
10-
9-
8-
"7 _
6-
03 5 —
IT
4 -
3 -
2-
1 -
-o
c
C 03
O CD
* i
E
co
o
O
-a
c
O 03
E CD
^ 0)
E o>
-o
c
o
c
.c
Figure 7. Graph of ranges and ratings for topography.
10-
9-
8-
7-
B-\
4-
3-
2-
1 -
I
CO
I
03
(O
CN
I
rsl
6
Primary Media
Percent Slope
significantly different enough to warrant the devel-
opment of any other modified DRASTIC indexes. The
user should be reminded that weights may not be
changed for any of the DRASTIC factors. These
relative weights form the basis for the system and any
changes will make the system invalid.
Integration of Hydrogeologic
Settings and DRASTIC
The mappable hydrogeologic units and the DRASTIC
Index have been combined to provide the user with a
relative pollution potential for all typical hydrogeologic
settings in the United States. A "typical" range for
each DRASTIC factor is assigned to each hydro-
geologic setting and a DRASTIC Index is determined
for each typical hydrogeologic setting. These settings
are developed as guides and are not designed to be
representative of each and every area. The ranges for
each factor may be adjusted by the user and the rating
adjusted accordingly when available data indicate
different conditions. These hydrogeologic settings
-------
Figure 8. Graph of ranges and ratings for impact of the vadose zone
10-
9-
8-
7-
6-
§ 5-I
CD
oc
3-
2 —
1-
Relative impact of the principal Vadose Zone
Media types. Range based upon
a) Path length and tortuosity
b) potential for dispersion and consequent
dilution
c) reactivity (solubility)
d) consumptive sorption
e) fracturing
Primary factors affecting rating
1. Consumptive sorption and fracturing
2. Fracturing and reactivity
3 Fracturing; path length as influenced by
mtergranular relationships
4 Fracturing; path length and tortuosity as
influenced by bedding planes, sorption,
and reactivity
5. Path length and tortuosity as impacted
by bedding gram size, sorting and
packing, sorption
6. Path length and tortuosity as influenced
by gram size, sorting, and packing
7. Reactivity and fracturing
_cu
CO
.c
in
tn
c
o
CO
o
SI
CO
CD
C
O
CO
CD
E
_i
CD
C
0
CO
•a
c
CO
c/j
VJ
CD
C
O
05
CD
E
T3
CD
•a
•a
CD
CO
g
"CD
CD
^
tj
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t/3 CO
0
CD
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SI
a
o
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2
S
5
T3
C.
ID
CO
ro
m
•r- «- CM
Primary Media
12
-------
Figure 9. Graph of ranges and ratings for hydraulic
conductivity.
10-
9-
8-I
7-
6-
5-
4-
3-
2-
I
o
o
co
I I
o
o
P"*" O
8 8
o
CN
6
o
o
o
o
o
gpd/ff2
provide units which are mappable and permit the
drafting of pollution potential maps. Thus, the user
can use hydrogeologic settings as a mappable unit,
define the area of interest by modifying the ranges
within a setting to reflect specific conditions within
an area, choose corresponding ratings, and calculate
a pollution potential DRASTIC Index or a specialized
index for agricultural pesticides and herbicides.
References
Dee, Norbert, Janet Baker, Neil Drobny, Ken Duke, Ira
Whitman, and Dave Fahringer, 1973. An environ-
mental evaluation system for water resource
planning; Water Resources Research, Vol. 9, No. 3,
pp. 523-535.
Heath, Ralph C., 1984. Ground-water regions of the
United States; U.S. Geological Survey Water Supply
Paper 2242, 78 pp.
13
-------
Section 3
DRASTIC: A Description of the Factors
Ground-Water Contamination and
DRASTIC
Ground-water pollution is caused by a variety of
substances originating from many different activities.
In general, man-influenced contaminants enter
ground water through three pathways: (1) the placing
or spreading of liquids or water soluble products on
the land surface, (2) the burial of substances in the
ground above the water table, or (3) the emplacement
or injection of materials in the ground below the
water table (Lehr et al., 1976). Table 11 lists the
activities which cause contamination through one or
more of these pathways. A brief description of each of
these activities is included in Appendix C, Sources of
Ground-Water Contamination.
After release at the land surface, the contaminant
may infiltrate downward through the soil, vadose
zone, and saturated zone finally reaching the aquifer.
If the volume of contaminant is not great, the con-
taminant may be flushed toward the water table by
infiltrating precipitation or additional amounts of
contaminant. Once within the aquifer, the contam-
inant may: (1) travel at the velocity of and in the
direction of ground water (Figure 10), (2) travel slower
than the ground water (Figure 11), (3) float on the
surface of the water table (Figure 12), (4) "sink"
through the aquifer to the bottom (Figure 13) or (5)
under some conditions, may actually move in a
direction against the flow of the ground water (Figure
14). Generally, the majority of contaminants travel in
the direction of ground-water flow at a velocity
somewhat less than that of the ground water.
As the contaminant travels through this system,
attenuation of the contaminant may take place. Atten-
Figure 10. Travel of contaminant with same density as water
in the aquifer.
Source
Ground Water
Flow Direction
Figure 11. Travel of contaminant that is denser than water in the
aquifer.
Source
Ground Water
Flow Direction
Water Table
2
uation includes those mechanisms which reduce the
velocity of the contaminant through processes such
as dilution, dispersion, mechanical filtration, volatil-
ization, biological assimilation and decomposition,
precipitation, sorption, ion exchange, oxidation-
reduction, and buffering and neutralization (Pye and
Table 11. Potential Sources of Ground-Water Contamination and Mode of Emplacement (After Lehr et al., 1976)
On the Land Surface
In the Ground Above the Water Table
In the Ground Below the Water Table
1. Land disposal of either solid or liquid
waste materials
2. Stockpiles
3. Disposal of sewage and water-treatment
plant sludge
4. Salt spreading on roads, airport runways
and parking lots
5. Animal feed lots
6. Fertilizers and pesticides
7. Accidental spills of hazardous materials
8. Paniculate matter from airborne sources
1. Leaching tile fields, cesspools and
privies
2. Holding ponds and lagoons
3. Sanitary 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
1. Waste disposal in wet excavations
2. Drainage wells and canals
3. Abandoned/improperly constructed
wells
4. Exploratory wells
5. Water supply wells
6. Waste disposal walls
7. Mines
8. Salt water intrusion
14
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Figure 12. Travel of contaminant that is less dense than
water in the aquifer.
Source
Water Table
y
Ground Water
Flow Direction
Figure 13. Travel of contaminant that is denser than water and
sinks in the aquifer.
Source
Water Table
X
Ground Water
Flow Direction
Figure 14.
Travel of contaminant that is denser than water in
the aquifer in a direction opposed to the water
flow direction.
Source
Ground Water
Flow Direction
Kelley, 1984; Fetter, 1980). Dilution is accomplished
by hydrodynamic dispersion. The degree of attenua-
tion which can occur is a function of (1) the time that
the contaminant is in contact with the material
through which it passes, (2) the grain size and
physical and chemical characteristics of the material
through which it passes, and (3) the distance which
the contaminant has traveled. In general, for any
given material the longer the time and greater the
distance, the greater the effects of attenuation. In a
similar manner, the greater the surface area of the
material through which the contaminant passes, the
greater the potential for sorption of the contaminant
and hence for attenuation. The greater the reactivity
of the material through which the contaminant
passes, the greater the potential for attenuation.
Any combination of these processes may be active
depending on the hydrogeologic conditions and the
contaminant. It is therefore necessary to have a
general idea of these processes and whether they are
active. A discussion of the mechanisms which control
contaminant movement is included in Appendix A,
Processes and Properties Affecting Contaminant Fate
and Transport.
The effectiveness of dilution and attenuation proc-
esses is largely determined by (1) the rate and loading
of the applied contaminant, (2) the characteristics of
the contaminant, and (3) the physical characteristics
of the area. Ultimately, it is these factors which
control the ground-water pollution potential of any
area. The rate and loading factor which generally is of
site-specific character is discussed briefly in Section
5, IMPACT—Risk Factors. The characteristics of the
contaminants are discussed in more detail in Ap-
pendix B, Characteristics of Ground-Water Contam-
inants. However, it is the physical properties char-
acterized by the hydrogeologic setting of the area
which determine the extent to which the attenuation
mechanisms may have the potential to be active.
Because it is neither practical nor feasible to obtain
quantitative evaluations of these intrinsic mechan-
isms from a regional perspective, it is necessary to
look at the broader physical parameters which
incorporatethe many processes. Each of the DRASTIC
parameters includes various mechanisms which will
help to evaluate the vulnerability of ground water. A
description of each DRASTIC feature and the included
processes is contained in the following sections.
Depth to Water
The water table is the expression of the surface below
the ground level where all the pore spaces are filled
with water. Above the water table, the pore spaces
are partially filled with water and air. The water
table may be present in any type of media and may be
either permanent or seasonal. For purposes of this
document, depth to water refers either to the depth to
the water surface in an unconfined aquifer or to the
top of the aquifer where the aquifer is confined
(Figure 15). Depth to water does not include saturated
zones which have insufficient permeability to yield
significant enough quantities of water to be con-
sidered an aquifer.
The depth to water is important primarily because it
determines the depth of material through which a
contaminant must travel before reaching the aquifer,
and it may help to determine the amount of time
during which contact with the surrounding media is
15
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Figure 15. Depth to water in a confined and unconfined aquifer.
Depth to Water
\
Water Table
Unconfined Aquifer
-,—7—7—7—7—r Confining Layer-
Confmed Aquifer
maintained. The depth to water is also important
because it provides the maximum opportunity for
oxidation by atmospheric oxygen. In general, there is
a greater chance for attenuation to occur as the depth
to water increases because deeper water levels infer
longer travel times. Figure 3 shows the relative
importance of depth to water. The ranges in depth to
water as defined in the DRASTIC system have been
determined based on what are considered to be
depths where the significance for pollution potential
changes.
Net Recharge
The primary source of ground water is precipitation
which infiltrates through the surface of the ground
and percolates to the water table. Net recharge
indicates the amount of water per unit area of land
which penetrates the ground surface and reaches the
water table. This recharge water is thus available to
transport a contaminant vertically to the water table
and horizontally within the aquifer. In addition, the
quantity of water available for dispersion and dilution
in the vadose zone and in the saturated zone is
controlled by this parameter. In areas where the
aquifer is unconfined, rechargetotheaquifer usually
occurs more readily and the pollution potential is
generally greater than in areas with confined aquifers.
Confined aquifers are partially protected from contam-
inants introduced at the surface by layers of low
permeability media which retard water movement to
the confined aquifer. In some parts of some confined
aquifers, head distribution is such that movement of
water is through the confining bed from the confined
aquifer into the unconfined aquifer. In this situation,
there is little opportunity for local contamination of
the confined aquifer. For a better understanding of
how to deal with this situation, refer to the Section 4,
How to Evaluate Confined Aquifers. The principal
recharge area for the confined aquifer is often many
miles away. Many confined aquifers are not truly
confined and are partially recharged by migration of
water through the confining layers. The more water
that leaks through, the greater the potential for
recharge to carry pollution into the aquifer. Recharge
water, then, is a principal vehicle for leaching and
transporting sol id or liquid contaminants to the water
table. Therefore, the greater the recharge, the greater
the potential for pollution. This is true until the
amount of recharge is great enough to cause dilution
of the contaminant andthe pollution potential ceases
to increase and may actually decrease. For purposes
of this document, this phenomena has been acknowl-
edged but the ranges and associated ratings do not
reflect the dilution factor.
One additional factor which must be considered is
augmentation of natural recharge through artificial
recharge or by irrigation. When a range for net
recharge is assigned, these additional sources of
water must be considered.
Aquifer Media
Aquifer media refers to the consolidated or uncon-
solidated medium which serves as an aquifer (such as
sand and gravel or limestone). An aquifer is defined
as a medium which will yield sufficient quantities of
water for use. Water is held by aquifers in the pore
spaces of granular and clastic rock and in the frac-
tures and solution openings of non-clastic and non-
granular rock. Rocks which yield water from pore
spaces have primary porosity; rocks where the water
is held in openings such as fractures and solution
openings which were created after the rock was
formed have secondary porosity. The aquifer medium
exerts the major control over the route and path
length which a contaminant must follow. The path
length is an important control (along with hydraulic
conductivity and gradient) in determining the time
available for attenuation processes such as sorption,
reactivity, and dispersion and also the amount of
effective surface area of materials contacted in the
aquifer. The route which a contaminant will take can
be strongly influenced by fracturing or by any other
feature such as an interconnected series of solution
openings which may provide pathways for easier
flow. In general, the larger the grain size and the more
fractures or openings within the aquifer, the higher
the permeability and the lower the attenuation
capacity; consequently the greater the pollution
potential.
For purposes of this document, aquifer media have
been designated by descriptive names. Each medium
is listed in the order of increasing pollution potential.
A discussion of each medium follows:
(a) Massive Shale—Thick bedded shales, claystone
or clays which typically yield only small quanti-
ties of water from fractures and which have a
low pollution potential. Pollution potential is
influenced by the degree of fracturing.
16
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(b) Metamorphic/lgneous—Consolidated bedrock
of metamorphic or igneous origin which con-
tains little or no primary porosity and which
yields water only from fractures within the rock.
Typically well yields are low and the relative
pollution potential is a function of the degree of
fracturing.
(c) Weathered Metamorphic/lgneous—Unconsoli-
dated material, commonly termed regolith or
saprolite, which is derived by weathering of the
underlying consolidated bedrock, and which
contains primary porosity. The pollution poten-
tial is largely influenced by the amount of clay
material present: the higher the clay content,
the lower the pollution potential.
(d) Bedded Sandstone, Limestone, and Shale—
Typically thin bedded sequences of sedimentary
rocks which contain primary porosity but where
the controlling factor in determining pollution
potential is the degree of fracturing.
(e) Massive Sandstone—Consolidated sandstone
bedrock which contains both primary and sec-
ondary porosity and is typified by thicker deposits
than the Bedded Sandstone Limestone and
Shale sequences. Pollution potential is largely
controlled by both the degree of fracturing and
the primary porosity of the sandstone.
(f) Massive Limestone—Consolidated limestone or
dolomite bedrock which is characterized by
thicker deposits than Bedded Sandstone, Lime-
stone, and Shale sequences. Pollution potential
is largely affected by the degree of fracturing
and the amount of solution of the limestone.
(g) Sand and Gravel—Unconsolidated mixtures of
sand- to gravel-sized particles which contain
varying amounts of fine materials. Sands and/or
gravels which have only small amounts of fine
material are termed "clean." In general, the
cleaner and more coarse-grained the aquifer,
the greater the pollution potential.
(h) Basalt—Consolidated extrusive igneous bedrock
which contains bedding planes, fractures, and
vesicular porosity. The term is used herein in a
generic sense, even though it is actually a rock
type. Pollution potential is influenced by the
amount of interconnected openings which are
present in the lava flow materials.
(i) Karst Limestone—Consolidated limestone bed-
rock which has been dissolved to the point
where large, open, interconnected cavities and
fractures are present. This is a special case of
Massive Limestone.
A graphic display of the ratings which have been
assigned to each media is contained in Figures.
This graph also contains a more complete listing
of the mechanisms which affect the pollution
potential of that media. Because this DRASTIC
parameter is less quantifiable, the user will be
instructed to choose a rating based on the above
discussion and available information on the
geology of the area (Section 4, How to Use the
Range in Media Ratings).
Soil Media
Soil media refers to that uppermost portion of the
vadose zone characterized by significant biological
activity. For purposes of this document, soil is
commonly considered the upper weathered zone of
the earth which averages three feet or less. Soil has a
significant impact on the amount of recharge which
can infiltrate into the ground and hence on the ability
of a contaminant to move vertically into the vadose
zone. Moreover, where the soil zone is fairly thick, the
attenuation processes of filtration, biodegradation,
sorption, and volatilization may be quite significant.
Thus, for certain on-land surface practices such as
agricultural applications of pesticides or application
of herbicides, soil can be a primary influence on
pollution potential. In general, the pollution potential
of a soil is largely affected by the type of clay present,
the shrink/swell potential of that clay, and the grain
size of the soil. In general, the less the clay shrinks
and swells and the smaller the grain size, the less the
pollution potential. The quantity of organic material
present in the soil may also be an important factor.
Soil media are best described by referring to the basic
soil types as classified by the Soil Conservation
Service. A description of the soil media in order of
increasing pollution potential follows:
(a) Nonshrinking Clay—Illitic or Kaolinitic clays
which do not expand and contract with the
addition of water and therefore do not form
vertical secondary permeability which increases
the pollution potential.
(b) Clay Loam—A soil textural classification which
is characterized by 15-55 percent silt, 27-40
percent clay, and 20-45 percent sand (Figure
16). Because of the high amounts of clay and
restrictive permeabilities, it has a low pollution
potential.
(c) Silty Loam—A soil textural classification char-
acterized by 50-85 percent silt, 12-27 percent
clay, and 0-50 percent sand (Figure 16). The
pollution potential is still low, but higher than a
clay loam because of typically lower percentages
of clay.
(d) Loam—A soil textural classification character-
ized by 25-50 percent silt, 7-27 percent clay,
and 0-50 percent sand(Figure 16). The pollution
potential is still low, but higher than a silt loam
because of lower percentages of clay and silt.
(e) Sandy Loam—A soil textural classification char-
acterized by 0-50 percent silt, 0-20 percent clay,
and 15-50 percent sand (Figure 16). The pollu-
77
-------
Figure 16. Soil textural classification chart (Soil Conservation Service, 1951).
100
o
Percent Sand
tion potential is greater than a loam due to the
higher percentage of sand.
(f) Shrinking Clay—Characterized by montmorillon-
itic clays or smectites which have an expanding
lattice that swell and contract with alternating
wetting and drying. Although the cracks formed
on drying, swell as the clay hydrates, the ability
of pollutants to move rapidly upon initial wetting
is documented. Although usually of low perme-
ability, this medium can have a seemingly high
pollution potential based on the secondary
vertical permeability created by the cracking of
the media upon drying.
(g) Sand—A size-based delineation of angular or
rounded particles ranging in size from 1 716 mm
to 2 mm. Sands are typically free of silts and
clays and therefore have a high pollution
potential.
(h) Gravel—A particle-based size classification
typified by particles larger than 2 mm in size and
commonly a mixture of sand, silt, clay, and
gravel with a preponderance of large-sized
particles. Permeability is rapid and pollution
potential is high.
(i) Thin or Absent—If a soil layer is not present or if
the layer is so thin as to be considered inef-
fective, the pollution potential is very high and
this category should be used. Figure 6 contains
a graphic representation of the pollution poten-
tial of soil media.
Topography
As used here, "topography" refers to the slope and
slope variability of the land surface. Basically, topo-
graphy helps control the likelihood that a pollutant
18
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will run off or remain on the surface in one area long
enough to infiltrate. This is particularly important in
activities such as application of pesticides and
herbicides where the effect of the contaminant tends
to be cumulative. Therefore, the greater the chance of
infiltration, the higher the pollution potential as-
sociated with the slope. Topography influences soil
development and therefore has an effect on attenua-
tion. Topography is also significant from the stand-
point that the gradient and direction of flow often can
be inferred for water table conditions from the general
slope of the land. Typically, steeper slopes signify
higher ground-water velocity.
Figure 7 contains the ranges which were chosen as
significant for pollution potential. These ranges
correspond to the ranges identified by the Soil
Conservation Service for percent slope. The ranges
are assigned ratings assuming that 0-2 percent slope
provides the greatest opportunity for a pollutant to
infiltrate because neither the pollutant nor much
precipitation exits the area as runoff. Conversely, 18+
percent slope affords a high runoff capacity and
therefore a lesser probability of infiltration with
subsequent lower pollution potential. However, steep
slopes are more conducive to rapid erosion and
contamination of surface water.
Impact of Vadose Zone
The vadose zone is defined as that zone above the
water table which is unsaturated. For purposes of this
document, this strict definition can be applied to all
water table aquifers. However, when evaluating a
confined aquifer, the "impact" of the vadose zone is
expanded to include both the vadose zone and any
saturated zones which overlie the aquifer. The
significantly restrictive zone above the aquifer which
forms the confining layer is used as the type of media
which has the most significant impact.
The type of vadose zone media determines the
attenuation characteristics of the material below the
typical soil horizon and above the water table.
Biodegradation, neutralization, mechanical filtration,
chemical reaction, volatilization, and dispersion are
all processes which may occur within the vadose
zone with a general lessening of biodegradation and
volatilization with depth. The media also control the
path length and routing, thus affecting the time
available for attenuation and the quantity of material
encountered. The routing is strongly influenced by
any fracturing present. The materials at the top of the
vadose zone also exert an influence on soil develop-
ment.
Vadose zone media have been designated by descrip-
tive names. Each medium, listed in order of increasing
pollution potential, is discussed as follows:
(a) Silt/Clay—A deposit of silt- and clay-sized
particles which serves as a barrier to retard
movement of liquids. The high clay content
provides a low pollution potential. Shrinking
clays and higher silt concentrations increase
the pollution potential.
(b) Shale—A consolidated, thick-bedded clay rock
which may be fractured. Pollution potential is
low but increases with the degree of fracturing.
(c) Limestone—Consolidated massive limestone or
dolomite which typically contains fewer bedding
planesthan Bedded Limestone, Sandstone, and
Shale sequences (see "e" below). Pollution
potential is influenced by the degree of fractur-
ing, with a high density of fracturing increases
the chance for pollutant migration.
(d) Sandstone—A consolidated sand rock which
contains both primary and secondary porosity
and is typified by thicker bedding than compared
to Bedded Limestone, Sandstone, Shale se-
quences. Pollution potential is largely controlled
by the degree of fracturing and the primary
porosity of the sandstone.
(e) Bedded Limestone, Sandstone, Shale—Typically
thin-bedded sequences of sedimentary rocks
which contain primary porosity but where the
controlling factor in determining pollution po-
tential is the degree of fracturing.
(f) Sand and Gravel with Significant Silt and Clay—
Unconsolidated mixtures of sand and gravel
which contain an appreciable amount of fine
material affect pollution potential by having a
high concentration of clay or by reducing the
permeability of the deposit. These deposits are
commonly referred to as "dirty" and have a
lower pollution potential than "clean" sands
and gravels. In general, finer-grained and
"dirtier" sands have a lower pollution potential
than coarser-grained "dirtier" gravels.
(g) Metamorphic/lgneous—Consolidated rock of
metamorphic or igneous origin which contain
no significant primary porosity and which permit
movement of liquids through fractures. The
relative pollution potential is a function of the
degree of fracturing.
(h) Sand and Gravel—Unconsolidated mixtures of
sand- to gravel-sized particles which contain
only small amounts of fine materials. The range
in rating reflects principally a grain size distri-
bution where unsorted smaller grained deposits
have a lower pollution potential and larger
grained, well-sorted deposits have a higher
pollution potential.
19
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(i) Basalt—Consolidated extrusive igneous bedrock
which contains bedding planes, fractures, and
vesicular porosity. This is a special case of
Metamorphic/lgneous. The term is used herein
in a generic sense, even though it is actually a
rock type. Pollution potential is influenced by the
number and amount of interconnected openings
present in the lava flow materials. Pollution
potential is typically high because there is little
chance for attenuation once a pollutant enters
the fracture system.
(j) Karst Limestone—Consolidated limestone bed-
rock which has been dissolved to the point
where large open interconnected cavities and
fractures are present. This is a special case of
Limestone, where pollution potential is high
based on the amount of open area in the rock.
A graphic display of the ratings which have been
assigned to each medium is contained in Figure 8.
This graph also contains a more complete listing of
the mechanisms which affect the pollution potential
of that media. Because this DRASTIC parameter is
less quantifiable, the user is instructed to choose a
rating based on the above discussion and available
information on the geology of the area (Section 4,
How to Use the Range in Media Ranges).
Hydraulic Cbnductivity of the Aquifer
Hydraulic conductivity refers to the ability of the
aquifer materials to transmit water, which in turn,
controls the rate at which ground water will flow
under a given hydraulic gradient. The rate at which
the ground water flows also controls the rate at which
a contaminant will be moved away from the point at
which it enters the aquifer. Hydraulic conductivity is
controlled by the amount and interconnection of void
spaces within the aquifer which may occur as a
consequence of intergranular porosity, fracturing,
bedding planes, etc. For purposes of this document,
hydraulic conductivity is divided into ranges where
high hydraulic conductivities are associated with
higher pollution potential. This is because the pol-
lutant hasthe potential for moving quickly awayfrom
the point in the aquifer where it is introduced.
Obviously, a wide range of hydraulic conductivities
are present in all areas. The values assigned are
considered to be typical for the settings described.
Figure 9 shows the relative importance of the ranges.
Interaction Between Parameters
From the above discussion and in the application of
the DRASTIC Index, it will be recognized that there is
apparent redundance between some of the param-
eters. The depth to the water, for example, affects the
quantity of material that will be encountered by a
pollutant moving downward toward an aquifer. The
thicker the section in a given setting, the greater the
effect may be upon the degradation, retardation, or
attenuation of the pollutant.
However, in considering the impact of the vadose
zone, degradation, retardation, and other significant
attenuation processes are all varied according to the
nature of the materials present, and their condition
within the vadose zone. If, for instance, the vadose
zone is moderately fractured granite, the materials
within the vadose zone will have only a slight impact
on most pollutants entering the vadose zone. The
protection provided will be a function of depth and the
failure of critical fractures to interconnect.
If, however, the vadose zone is comprised of unfrac-
tured glacial till (silt/clay), it can be anticipated that
consumptive sorption will be moderately high; infil-
tration will be moderately low; retardation will be
significant; and with any substantial thickness of till,
considerable time will be required for most (conserv-
ative) pollutants to penetrate the till. Thus it can be
seen that the redundant consideration of degradation,
retardation, and attenuation within the context of
both depth to water and impact of the vadose zone is
useful in the comparative evaluation of sites.
Net recharge determines, on an annual basis, the
quantity of water from precipitation that is available
for vertical transport, dispersion, and dilution of a
pollutant from a specific point of application. Net
recharge exemplifies how some parameters can have
both positive and negative effects. For example,
greater recharge typically means more rapid transport
of a pollutant and therefore less time for attenuation.
However, in this situation, dilution is also greater
thereby exerting a positive influence because the
concentration of an introduced contaminant will be
lessened. It is also evident that a thick unsaturated
zone, with a layered sequence of a bedded and
fractured shales, sandstones, and limestones, can
have a profound impact on all three of the same
factors (transport, dispersion, dilution) that are of
primary importance to net recharge.
Topography and soil media also influence net re-
charge. Topography has site-specific influence which
determines whether the capacity for recharge is high
or low at a given point. The permeability of the surface
soils has a similar impact. However, the nature of the
surface soil materials has an additional impact upon
potential pollutant attenuation, consumptive sorp-
tion, route length anddirection, and time available for
penetration.
In addition to its direct influence upon recharge,
topography exerts a significant influence upon soil
thickness, drainage characteristics, and profile de-
velopment. These factors, in turn, influence soil
media as well as the previously-mentioned factors. \r\
20
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addition, topography usually bears a predictable
relationship to hydraulic gradient, and direction of
probable pollutant movement under water table
conditions, with a consequent impact on dispersion
and dilution.
The upper portion of the vadose zone exerts influence
on the type of soils developed on the surface. The
vulnerability of an aquifer to a given pollution event
varies in responsetothe nature of the materials in the
vadose zone; grain size, sorting, reactivity, bedding,
fracturing, thickness, sorptive character. In general,
finer grain-size materials, i.e. clays and silt, have
lower hydraulic conductivity, greater capacity for the
temporary and long-term attenuation of pollutants,
and greater sorptive capacity. If expandable clay
minerals are present, the sorptive capacity is further
enhanced. If a material (seven moderately cemented,
then grain size and sorting may be less significant
than the degree of cementation.
If the material in the vadose zone is reactive to the
pollutant, or soluble in it, then there may be two
different effects. First, the pollutant may be retarded
(a positive effect) or second, the solution of the vadose
zone material may actually increase permeability and
allow subsequent introduction of pollutants to pass
through more quickly with less retardation (a negative
effect). In the case of reactive pollutants, the impor-
tance of secondary by-products must be considered. It
is here that the risks associated with gaseous phase
transport are most likely to have an impact on ground
water.
The thickness of the vadose zone and the degree of
fracturing and frequency of bedding planes in it all
impact upon the tortuosity, route length, dispersion,
and consequent travel time that is required for a
pollutant to move through the vadose zone. This is not
only of time-delay importance but is important as the
control of contact time for reactions to occur.
The vadose zone, including the surf icial soil, is also of
great importance as the zone where most of the
biologic activity occurs. There are natural organisms
found in this zone that break down many polluting
substances into secondary by-products, both harm-
less and harmful. For many chemicals these reactions
are very poorly understood, if at all, but it is known
that with sufficient time the eventual results are
generally beneficial. Among the best known of these
processes at present are the bacterial fixation of iron
and the bacterial breakdown of non-chlorinated
hydrocarbons under natural conditions. Both of these
processes occur in the vadose zone and in the aerobic
portion of shallow aquifers.
The hydraulic conductivity, together with gradient
and porosity of the aquifer beneath a site, influences
the rate of movement of an introduced pollutant away
from the point of introduction. In conjunction with
hydraulic gradient, conductivity also controls the
direction of movement. These are, in turn, affected
with regard to dispersion, by grain size, bedding,
fracturing, and tortuosity.
It is evident that all of the DRASTIC parameters are
interacting, dependent variables. Their selection is
based not on available data quantitatively developed
and rigorously applied, but on a subjective under-
standing of "real world" conditions at a given area.
The value of the DRASTIC parameters is in the fact
that they are based on information that is readily
available for most portions of the United States, and
which can be obtained and meaningfully mapped in a
minimum of time and at minimum cost. The DRASTIC
ranking scheme can then be applied by enlightened
laymen for valid comparative evaluations with ac-
ceptable results.
If the vulnerability of a site, or sites, to pollution were
to be evaluated with regard to travel time, flux, and
concentration associated with the incidence of a
pollutant introduced at the site, the DRASTIC param-
eters would be distributed as follows:
A. Travel Time
— Depth to Water
— Soil Media
— Impact of Vadose Zone
— Net Recharge
— Conductivity
B. Flux
— Aquifer Media
— Conductivity (Existence of Gradient As-
sumed)
C. Concentration
— Depth to Water
— Net Recharge
— Aquifer Media
— Soil Media
— Topography
— Impact of Vadose Zone
— Conductivity
It should be noted that although the DRASTIC
parameter of hydraulic conductivity of the aquifer is
mapped as a function of the ability of a pollutant to be
moved from a point of incidence, the direction of
migration is a function of gradient and rate depends
on both conductivity and gradient.
References
Fetter, C. W., 1980. Applied hydrogeology; Charles E.
Merrill Publishing Company, 448 pp.
Lehr, Jay H., WayneA. Pettyjohn, Truman W. Bennett,
James R. Hanson, and Laurence E. Sturtz, 1 976. A
21
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manual of laws, regulations, and institutions for
control ofground water pollution; U.S. EPA-440/9-
76-006.
Pye, Veronica I. and Jocelyn Kelley, 1984. The extent
of groundwater contamination in the United States;
Groundwater Contamination, National Academy
Press, pp. 23-33.
Soil Conservation Service, 1951. Soil survey manual;
U.S. Department of Agriculture, 503 pp.
22
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Section 4
How to Use Hydrogeologic Settings and DRASTIC
The system described in this document presents a
simple and easy-to-use approach to assess the
ground-water pollution potential of any area. Al-
though the final system appears simplistic, the sys-
tem actually includes many complex concepts and
relationships. Before an attempt is made to make full
use of this system, the user needs to develop an
appreciation for the complexity of evaluating ground-
water pollution potential. It is not necessary to
understand every concept in detail, but the greater
the depth of understanding, the more useful the
system becomes. DRASTIC provides mappable results
which can be used to provide a quick reference of
relative pollution potential of different areas. DRAS-
TIC is designed to be used as a planning or screening
tool. DRASTIC and associated maps cannot be used in
site-specific evaluations because of local complex-
ities in geologic conditions.
Organization of the Document
As described in Section 2, Development of the
System and Overview, the entire United States has
been divided into 13 geographic regions and each
region subdivided into hydrogeologic settings. Sec-
tion 6, Hydrogeologic Settings of the United States by
Ground-Water Regions, contains an annotated des-
cription of each region, a geographic location map for
each region, and an illustration of the major hydro-
geologic features of the region. Each hydrogeologic
setting contains a written narrative, an illustration,
and two DRASTIC charts. These charts contain a
listing of the seven DRASTIC features, an example of
typical ranges for each feature which might be
encountered in the region, a listing of the weight
which has been assigned to the DRASTIC feature, a
rating which corresponds to the associated range
(from Tables 4-10), a column which reflects the
weight multiplied by the rating for each factor, and a
total DRASTIC Index. The same information is con-
tained in the second agricultural DRASTIC chart. The
significant difference between the charts is in the
weights assigned to each DRASTIC feature, thus
yielding a separate specialized agricultural DRASTIC
Index.
Table 18 contains a complete listing of all hydro-
geologic settings and their associated DRASTIC I ndex.
Table 19 arranges the hydrogeologic settings by
increasing DRASTIC Index values. Table 20 arranges
the hydrogeologic settings alphabetically so that
comparison of similar settings in different regions
can be accomplished. Tables 21 -23 contain the same
information computed for the modified agricultural
DRASTIC Index. These lists have been prepared to
assist the user in evaluating the relative pollution
potential for many hydrogeologic settings. Following
these listings are Tables 24 and 25 which provide a
summary of principal physical and hydrologic charac-
teristics and common ranges of the hydraulic charac-
teristics of the ground-water regions as defined by
Heath (1984). These values may assist the user in
evaluating hydrogeologic settings.
The other important information necessary to use
DRASTIC is contained in the ranges and rating tables
for each factor. For ease of reference, Tables 4-10
have been reprinted in Section 6 as Tables 26-32.
This set of tables consists of a complete listing of the
DRASTIC factors, the ranges and associated rating for
each factor, and the weights for both the DRASTIC
and modified agricultural DRASTIC system.
Where to Obtain Information on
DRASTIC Parameters
Before an area can be evaluated using the DRASTIC
system, the basic information on each factor must be
found. DRASTIC has been designed to use informa-
tion which is available from a variety of sources. Table
1 contains a listing of possible sources of hydro-
geologic information and the types of information
which may be available from each. The most common
source of information for each parameter is listed
below:
(1) Depth to Water—Well logs or hydrogeologic
reports;
(2) Net Recharge—Water resource reports com-
bined with data on precipitation from the
National Weather Service;
(3) Aquifer Media—published geologic and hydro-
geologic reports;
(4) Soil Media—published soil survey reports or
local mapping projects conducted by the Soil
Conservation Service;
23
-------
(5) Topography—published U.S. Geological Survey
topographic maps (various scales);
(6) Impact of the VadoseZone—published geologic
reports;
(7) Hydraulic Conductivity of the Aquifer—pub-
lished hydrogeologic reports. (Of all the factors,
this information may be the most difficult to find.
Because it is related very closely to aquifer
media, if necessary, hydraulic conductivity may
be estimated using Table 12). Conversion factors
for permeability and hydraulic conductivity are
found in Table 13.
It should be noted that the more accurate the data
used to compute the index, the more reliably the
pollution potential can be assessed. There may be
many gaps in the data, of course. These gaps can be
filled with careful interpolation if such interpolation is
reasonable.
Steps for Use of the System
In order to use the DRASTIC system, the user must
follow a few simple steps. The following example
illustrates how to use the system. A decision-maker
wishes to evaluate the pollution potential of two
areas in a county. The county is located along the
glacial boundary such that part of the county lies in
the Glaciated Central Region and the other part lies in
the Non-Glaciated Central Region. Precipitation in
the area averages 42 inches per year. Area I is typified
by 5 to 20 feet of glacial till deposits, which overlie
fractured sandstones and shales with hydraulic
conductivities ranging from 100 to 300 gpd/ft2. The
terrain is rolling, and depth to the water in the
sandstones averages 30 feet below land surface.
Typical soils have mixtures of sand, silt, and clay with
predominant clay fractions. Area II is typified by
alternating sequences of sandstone, limestone, and
Table 12. Range of values of hydraulic conductivity and permeability (Freeze and Cherry, 1979)
Unconsohdated k k k k k
Rocks
;
2 ±j
: «
J CD 1
n -D T3 1
- co c tn
a CD co •*:
<: E « §
CD o
Q- 0) O
c £
ra Q.
i- TJ
-a o c
£ E to „
U CO CD ^ CD
0 CD £ £ £
ro g 2 .0 S
"- 1 $ Q -a
-J t/3
T3
Deposits
(darcy) (cm2) (cm/s) (m/s) (gal/day/ft2)
0
CD
f
^.
1
TJ
Iw
CD
QJ
"^ CJ
03 J
^ 1
£ \
•\
J
V) 1
(
r/
—
|f
CO
o
CO
>- ^3
T,l 1^
C W ™
^ 0 « « I
" ^ ir "S c
i H = w
"Mi
— J o oi
I
CD
C
i
3 m
\ 1
1
r105
-104
-103
-102
-10
- 1
-10'1
-10'2
-10'3
-10'4
-10-5
-10'6
-TO'7
-io-3
-io-4
-10-5
-io-6
-io-7
-10-8
-10-9
-10-'°
-10-"
-10-'2
-10-13
-10-'4
-1Q-15
-102
-10
-1
-io-1
^10~2
-10-3
-10-4
-io-5
-10-6
-io-7
-10-8
-10-9
-10-'°
-1
r-106
-io-1
-io-2
-IO-3
— 10'4
-10 5
-io-6
-io-7
-TO'8
-TO-9
-10-10
-10-"
-10'12
l_1Q-8 i-1Q-'6 l-10~11 1— 1Q-13
-105
-10"
-103
-102
-10
- 1
-10-'
-io-2
-TO-3
-10-"
-10-5
-10-"
-io-7
-------
Table 13. Conversion Factors for Permeability and Hydraulic Conductivity Units (Freeze and Cherry, 1979)
Permeability, k* Hydraulic conductivity, K
cm2
ft2
darcy
m/s
ft/s
U.S. gal/day/ft2
cm2
1
9.29 x 102
9.87 x 10-"
1.02 x 1CT3
3.11 x 10-
5.42 x 10-10
ft2
1.08 x 10°
1
1.06 x 10-"
1.10 x 10-"
3.35 x 10-'
5.83 x 10-13
darcy
1.01 x 10=
9.42 x 1010
1
1.04 x 10B
3.15 x 10"
5.49 x 10-7
m/s
9.80 x 102
9.11 x 106
9.66 x 10-"
1
3.05 x 10-'
4.72 x 10'7
ft/s
3.22 x 103
2.99 x 10s
3.17 x 10-5
3.28
1
1.55 x 10-6
U.S. gal/day/ft2
1.85 x 10"
1.71 x 1012
1.82 x 10'
2.12 x 10"
6.46 x 105
1
*To obtain k in ft2, multiply k in cm2 by 1.08 x 10~3
shale with moderate fracturing and hydraulic con-
ductivity averaging 300 gpd/ft2. Relief is low and
slopes are commonly 2 percent. Depth to water
averages 40 feet. Soil is thin but significant with soils
reflecting equal mixtures of sand, silt, and clay.
Average net recharge is 8 inches per year.
(1) Identify the Region in which the area is located.
Become familiar with the hydrogeology of the
region. Area I is in the Glaciated Central Region
and Area II is in the Non-Glaciated Central
Region.
(2) Identify which hydrogeologic setting most close-
ly approximates the conditions of the area. Area
I most closely approximates Setting 7Aa—Gla-
cial Till Over Bedded Sedimentary Rocks; Area
II, 6Da—Alternating Sandstone, Limestone and
Shale—Thin Soil. For ease of reference, these
setting descriptions are included as Figures 17
and 18 and Tables 14 and 15.
(3) Evaluate available area information for each
DRASTIC parameter against the ranges chosen
for each DRASTIC parameter listed in the top
table (Tables 14 and 15). In Area I (Table 14), the
depth to water averages 30 feet; the assigned
range of 30-50 would seem appropriate. There-
fore, the associated rating of 5 (Table 4) does not
need to be changed. No value for Net Recharge
was available; however, precipitation in the
region is 42 inches per year and recharge will
typically be restricted due to the presence of
clayey till; the assigned range of 4-7 seems
appropriate. Therefore, the associated rating of
6 (Table 5) does not need to be changed. The
aquifer media are fractured sandstones and
shales; thin bedded sandstone, limestone, and
shale sequences are present, so this is appro-
priate. Therefore, the associated rating of 6
(Table 6) does not need to be changed. Soils
have a predominant clay fraction but contain silt
and sand; clay loam is the prevalent soil, so the
chart designation would be appropriate. There-
fore, the associated rating of 3 (Table 7) does not
need to be changed. Terrain is rolling; 2-6
percent slopes are predominant. The listed
range is acceptable. Therefore, the associated
rating of 9 (Table 8) does not need to be changed.
The vadose zone is comprised of glacial till; silt
and clay is the most significant portion of the
Figure 17. Description and illustration for setting 7 Aa— Figure 18.
glacial till over bedded sedimentary rocks.
Description and illustration for setting 6Da—
alternating sandstone, limestone and shale-thin
soil.
25
-------
Table 14. DRASTIC and Agricultural DRASTIC Charts for
for Setting 7Aa — Glacial Till Over Bedded
Sedimentary Rocks
Setting 7Aa Glacial Till Over
Bedded Sedimentary Rock
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
Range
30-50
4-7
Thin
Bedded SS,
LS, SH
Sequences
Clay Loam
2-6%
Silt/Clay
100-300
Weight
5
4
3
2
1
5
3
DRASTIC
General
Rating
5
6
6
3
9
1
2
Index
Table 15. DRASTIC and Agricultural DRASTIC Charts
for Setting 6Da — Alternating Sandstone,
Limestone and Shale-Thin Soil
Setting 6Da Alternating SS,
LS, SH-Thin Soil
Number
25
24
18
6
9
5
6
93
Setting 7Aa Glacial Till Over
Bedded Sedimentary Rock
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
Range
30-50
4-7
Thin
Bedded SS,
LS, SH
Sequences
Clay Loam
2-6%
Silt/ Clay
100-300
Agricultural
Weight
5
4
3
5
3
4
2
Rating
5
6
6
3
9
1
2
Number
25
24
18
15
27
4
4
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin
Bedded SS,
LS, SH
Sequences
Loam
2-6%
Bedded
LS, SS, SH
1-100
Weight
5
4
3
2
1
5
3
DRASTIC
General
Rating
7
6
6
5
9
6
1
Index
Number
35
24
18
10
9
30
3
129
Setting 6Da Alternating SS,
LS, SH-Thin Soil
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin
Bedded SS,
LS, SH
Sequences
Loam
2-6%
Bedded
LS, SS, SH
1-100
Agricultural
DRASTIC
Index
117
Agricultural
Weight
5
4
3
5
3
4
2
Rating
7
6
6
5
9
6
1
Number
35
24
18
25
27
24
2
Agricultural
DRASTIC
Index
155
glacial till and is appropriate. Therefore, the
associated rating of 1 (Table 9) does not need to
be changed. Hydraulic conductivity values for
the bedrock range from 100-300 gpd/ft2 as
listed on the chart. Therefore, the associated
rating of 2 (Table 10) does not need to be
changed. Since all the ranges in the hydro-
geologic setting are acceptable, no values need
to be modified for this area. The DRASTIC Index
has already been computed for the user by
multiplying each rating by the assigned weight
to obtain the value listed in the "number"
column. The sum of the "numbers" is the
DRASTIC Index. In this instance, the DRASTIC
Index of 93 is simply read from the chart. It
should be noted here that weights are never
changed. These were determined by the com-
mittee and are the essence of the system.
In Area II (Table 15), depth to water averages 40 feet.
The range on the chart indicates 10-30 feet. This
range is not acceptable. The user should refer toTable
4 to find the correct range which most closely
approximates the area. In this case, 30-50 would be
appropriate. Note the corresponding rating would
now be 5 instead of 7 and the resultant weight of 5
multiplied by a rating of 5 is 25 instead of 35. Net
recharge is 8 inches per year; 4-7 is an acceptable
range and the associated rating of 6 does not need to
be changed. The aquifer is alternating sequences of
shale with moderate fracturing; the media listed on
the chart is accurate and the associated rating of 6
does not need to be changed. Soils are typified by
equal mixtures of sand, silt, and clay; this is the
definition of loam, so the range is adequate and the
associated rating of 5 does not need to be changed.
Topography is low(2 percent slope); the range is listed
as 2-6 percent. The user may, based on observation,
choose 0-2 percent, and change the rating as before,
or may accept the range of 2-6 if correct. For
demonstration purposes, the user can refer to Table
8, choose a 0-2 percent range, change the rating from
26
-------
9 to 10, and multiply by the weight of 1 to obtain an
answer of 10 instead of 9. The vadose zone media are
fractured limestones, sandstones, and shales; this is
acceptable. Therefore, the associated rating of 6 does
not need to be changed. Hydraulic conductivity
averages 300 gpd/ft2; the range indicates 1-100
gpd/ft2. Refer to Table 10 to choose the appropriate
range. In this case, 100-300 gpd/ft2 is chosen, the
associated rating of 2 is substituted and multiplied by
3 to obtain 6. The DRASTIC Index cannot be read off
the chart because not all the ranges were appropriate
for the setting. Calculate the correct DRASTIC Index
by adding the numbers 25+ 32 + 18 + 10+10 + 30 + 6 =
131. The decision maker can then compare the two
areas relative to one another.
From the above discussion, it is evident that the
hydrogeologic settings serve as a guide to the user in
evaluating the appropriate range for each DRASTIC
factor. Each range has an associated rating which can
then be integrated into DRASTIC by combining it with
the weighting factor. The information to evaluate
each DRASTIC factor and choose the appropriate
range may not always be expressed in exactly the
sameterms which are used in this document. Section
3, DRASTIC: A Description of the Factors, contains a
brief description of what is included in each of the
media terms so that the most accurate DRASTIC
range can be chosen.
How to Use the Range in Media Ratings
Because geologic media are more highly variable
than many of the quantifiable DRASTIC factors, the
associated rating for each of the ranges is a number
which can vary within the rating indicated on the
chart. For example, Table 6 contains the rating for
aquifer media. Basalt has a rating which can vary
from 2-8. By referring to Section 3, Aquifer Media,
Impact of the VadoseZone, which contains a descrip-
tion of the variables which affect the rating, the user
may choose an appropriate rating for the area being
evaluated. For example, the basalts in the Columbia
Lava Plateau are extremely variable in the degree of
interconnection of openings. In one area, the open-
ings may be highly connected and be assigned a
rating of 10. If data indicated a moderate degree of
interconnection, a rating of 7 might be chosen. In this
way the user can more closely approximate the actual
conditions in the area. For purposes of the hydro-
geologic settings, a typical rating is assigned to each
of the ranges. The user has the option of using this
typical rating where applicable.
How to Evaluate Confined Aquifers
Confined aquifers are treated, in the DRASTIC Index,
by evaluating the relative importance of their con-
finement. Although the confined aquifer may have
either an upward or downward leakage component,
hydraulic gradients which result in upward flow are
not taken into consideration because (a) the aquifer
already has a degree of protection and (b) upward
gradients are easily reversed by local pumpage.
Therefore, for purposes of the DRASTIC Index, the
worst case scenario of a gradient into the aquifer is
always assumed. A judgement must be made in
several of the DRASTIC factors as to the proper way to
evaluate that factor in the specific setting. Factors
that must be varied and the guidance for making the
judgement of variation are as follows:
Depth to Water—This factor can be evaluated as
either the depth to the water surface in an
unconfined aquifer, or as the depth to the top of
the aquifer where the aquifer is confined.
Net Recharge—varies with the degree of confine-
ment.
Aquifer Media—no change in judgement calls.
Soil Media—varies with the degree of confinement,
but is less sensitive than Net Recharge. When
there is significant confinement, this rating is not
read off the chart but is assigned a value of 1.
Topography—similar in impact to Soil Media,
except that the relationship is inverse. When
there is significant confinement, this rating is not
read off the chart but is assigned a value of 1.
Impact of VadoseZone—Since the "impact" of the
vadose zone is now, in effect, considered as a
functional aquitard, this zone is treated as a
Silt/Clay media, and is assigned the associated
chart rating of 1.
Hydraulic Conductivity—no change in judgement.
From this discussion it can be seen that the vulner-
ability of a specific aquifer to pollution varies with its
degree of confinement, but that the comparative
vulnerability of two protected aquifers is a function of
their ability to disperse a pollutant from the point of
application. To illustrate this, compare setting 7Ac,
Glacial Till Over Solution Limestone, with setting
10Aa, Unconsolidated and Semi-Consolidated Re-
gional Aquifers (Section 6). Setting 7Ac is typified by
conditions in northeastern Indiana, and Setting 10Aa
is typified by the Tidewater area in Virginia. Both
aquifers are confined. In the first example (7Ac),
piezometric levels rise above the carbonate aquifer
and saturate the lower portion of the surficial till. The
most important depth is the depth to the top of the
saturated zone. In spite of the confinement, average
recharge over the area is relatively high. Soil media
and topography are still significant, even though it
could be argued that their importance should be
somewhat reduced. This would give the setting a
lower rating.
27
-------
In the second example, setting 10Aa, the deep
aquifers are clearly confined by an overlying aquitard
that separates the shallow water table aquifer (setting
10Ab) from the deeper zone. While the shallow
aquifer provides recharge to the deeper, the rate of
leakage is very, very low. U nder this circumstance the
Depth to Water is interpreted to be the depth to the top
of the principal aquifer being considered. Net re-
charge to the deeper zone is almost negligible. Soil
media has no real significance, nor does topography,
so both ratings are reduced to a minimum value, or 1.
The "impact" of the vadose zone is considered an
effective aquitard, and is therefore rated as a silt/clay,
or 1, regardless of its actual composition.
From a comparison of the two ratings, 129 (Setting
7Ac) versus 53 (Setting 10Aa), it is apparent that the
deeper, highly confined, coastal aquifer is much more
protectedthan the shallow, partially-confined aquifer.
The deep coastal aquifers are actually only highly
vulnerable to injected pollutants and widespread
pollution of the overlying shallow aquifer, from which
recharge is derived over a substantial period of time.
Thus, in all DRASTIC settings the total hydraulic
condition must be considered in order to evaluatethe
degree of protection provided to the aquifer being
considered.
Single Factor Overrides
In some instances, it will be found that the DRASTIC
Index cannot adquately compensate for a single
parameter that is so dominant that it overrides all
other parameters. This may be a consideration that is
glaringly apparent, as in a highly-fractured surficial
karstarea, as is evident in parts of Florida, Indiana, or
Kentucky; or, it may be a much more subtle consid-
eration that involves design decisions and perhaps
policy decisions.
Tables 16 and 17 provide the DRASTIC ratings for two
actual sites, referenced as Maco I and Maco II. These
sites are both located in the till plains portion of the
Glaciated Central Region about five miles apart.
Based on the available data, both sites are underlain
by 25 to 40 feet of dense till containing a few
discontinuous lenses of dirty sand and gravel that
rarely exceed four inches in thickness. In the absence
of fracturing or stratification, the horizontal and
vertical permeabilities of the tills tend to fall in the
10~6 to 10~7gpd/ft2 range.
At site Maco I, the till overlies fractured limestone
which serves as a regional aquifer and has a hydraulic
conductivity usually in the 300-700 gpd/ft2 range.
Water in the limestone is confined, with the regional
piezometric surface at about 30feet. The overlying till
is saturated only in association with the occasional
discontinuous lense of sand and gravel. These zones
can be considered "perched."
Table 16. DRASTIC Rating for MACO I
MACO
Feature
Depth to Water
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
I
Range
15-30
4-7
Massive
Limestome
Clay Loam
2-6%
Silt/Clay
300-700
Weight
5
4
3
2
1
5
3
General
Rating
7
6
4
3
9
1
4
DRASTIC Index
Number
35
24
12
6
9
5
12
103
Table 17. DRASTIC Rating for MACO
MACO II
General
Feature
Depth to Water
Net Recharge
Aquifer Media
Soil Media
Topography
Impact
Vadose Zone
Hydraulic
Conductivity
Range
5-15
4-7
Massive
Shale
Clay Loam
2-6%
Silt/Clay
1-100
Weight
5
4
3
2
1
5
3
DRASTIC
Rating
9
6
2
3
9
1
1
Index
Number
45
24
6
6
9
5
3
98
At the Maco II site, the till overlies dense, fractured
shale. The hydraulic conductivities of the shale are
less than 1 gpd/ft2. As a consequence of the relative
impermeability of the shale bedrock, the overlying till
is saturated from the depth of about five feet, even
though the elevation, topography, and soils are
similar at the two sties.
It can be seen by comparing Tables 16 and 17 that
Maco II has a slightly more favorable rating than
Maco I. The principal reason is because there is no
significant aquifer at risk at site Maco II. However, site
Maco II has a "water table," albeit a saturated till at a
depth of five feet. At site Maco I a landfill, for example,
could be properly designed and operated at a max-
imum depth of 15 feet and be well within the
unsaturated zone, with a substantial thickness of
dense, low permeability material at the base to
protect the regional aquifer. Construction of a landfill
at Maco II (with the more favorable rating) involves
operating a saturation zone landfill, which often
requires a serious policy decision from the permitting
agency.
With regard to the proper application of the DRASTIC
Index to this situation, the question is "Is the shallow,
five-foot depth to saturation of sufficient significance
to 'override' all of the other favorable aspects of the
site." This should be considered for all parameters
28
-------
that are very highly-rated, i.e., in the rating range of
8-10.
Another single factor override would be exposed,
highly-fractured or dissolved bedrock which would
provide a direct conduit to an aquifer. Knowledge of
the area being mapped is usually required in order to
know when overrides must be applied.
Build-Your-Own-Settings
From the above discussion it should become obvious
that for any given area in the United States, the
ground-water pollution potential can be estimated by
choosing appropriate ranges for each DRASTIC
parameter without referring to any hydrogeologic
setting described in Section 6, Hydrogeologic Settings
of the United States by Ground-Water Regions. In
essence this is true. However, for purposes of creating
a mappable unit, the settings create units which are
mappable and which can be evaluated by super-
imposing DRASTIC. The geographic relationship also
helps the user evaluate more thoroughly the char-
acteristics of an area thereby helping create sound
judgement calls and a more realistic DRASTIC Index.
How to Interpret a DRASTIC Index
The culmination of the evaluation of any hydro-
geologic setting is a numerical value termed the
DRASTIC Index. The higher the DRASTIC Index, the
greater the ground-water pollution potential. DRAS-
TIC is designed to yield a relative numerical value
which can readily be compared to a value obtained for
another setting either in the same region or in a
different region. A numerical value of 160, for
example, has no intrinsic meaning. That number is of
value only with respect to other numbers generated
by the same DRASTIC Index.
Because this document addresses a DRASTIC Index
and a modified agricultural DRASTIC Index, the
natural tendency is to compare the two indexes
generated for one site and try to draw a conclusion.
The numerical values, in and of themselves, have no intrinsic
meaning so comparison between indexes should not
be made; only invalid conclusions will be drawn.
References
Freeze, R. A. and J. A. Cherry, 1979. Groundwater;
Prentice-Hall, 604 pp.
Heath, Ralph C., 1984. Ground-water regions of the
United States; U.S. Geological Survey Water Supply
Paper 2242, 78 pp.
29
-------
Section 5
impact—Risk Factors
The DRASTIC Index estimates the vulnerability of any
setting to pollution on the basis of determinable
geologic parameters. It does not, however, indicate a
variety of other parameters that often point out the
significance of the DRASTIC Index under the influ-
ence of cultural and physical modifications. For
example, a site with a low DRASTIC Index, indicating
moderate or low vulnerability to contamination, may
be located very near to a large population center. The
proximity to a population that can be exposed greatly
increases the risk, or impact, of an incidence of
pollution at the prospective site. Thus it can quickly be
noted that not only the size of the population exposed,
and the human/non-human nature of that popula-
tion, but the time required for the pollutant to travel
from the point of incidence to the population at risk, is
a serious consideration within a given setting.
Travel time is considered only tangentially by the
DRASTIC Index. It is implied by "hydraulic conductiv-
ity," but becomes interpretable, and meaningful, only
when the distance to be traveled from a source of
contamination to a point of concern is known, and
when the gradient, or inclination of the water table is
considered. Thus, the travel time of a pollutant from
point of introduction until it reaches a population is
not given by the DRASTIC Index, but must be
evaluated separately, by persons with adequate data
and expertise for each specific site.
In a similar manner, the risk to a given population is
dependent on the toxicity of the pollutant being
introduced. Obviously, if the pollutant being intro-
duced is non-toxic to the population exposed, there is
little or no riskto that population as a consequence of
the exposure. When the pollutant is quite toxic, it is
obvious that minimal exposure of the population may
be very serious, even where travel time as controlled
by gradient, distance, and hydraulic conductivity is
great.
Essentially, the DRASTIC Index for a given setting is
derived on the basis of the vulnerability of the site to
an invasion of water, hence the name "hydrogeo-
logic setting." Actually, the concern is not about the
vulnerability of a setting to water, but rather with the
vulnerability of that setting to contaminants. Water
forms the common baseline, but the site vulnerability
varies with the specific properties of the contaminant
being applied. Obviously all settings cannot be
mapped for all potential contaminants, so in many
instances critical judgements have to be made about
the risks involved. Where accidental spills are in-
volved, these judgements must be made rapidly,
conservatively, and on the basis of the best data
available. Where design judgements are to be made,
they should be made on the basis of adequate field
and laboratory testing. It should always be kept in
mind that some substances are so toxic that there are
no "safe" settings available.
In addition to travel time, toxicity, and population
exposed, the risk is influenced by "loading" factors.
Whether the application rate is a slug application, as
in an accidental spill; an intermittent application, as
with herbicides, pesticides, and fertilizers; or a
continuous application, such as a leaking tank or
lagoon, has an obvious bearing upon the total load of
material reaching an exposed population. Loading is
also influenced by the concentration of the polluting
substance. If the incident pollutant is highly con-
centrated, it is apparent that the exposed population
is at much greater risk than would be the case if the
pollutant were less concentrated. All of the atten-
uating factors, dilution, dispersion, sorption, filtration,
reaction, etc. are more effective at lesser loading
rates.
In order to assist in the understanding of the basic risk
factors, travel time, population exposed, loading and
toxicity, and how these risk factors impact the
DRASTIC Index, the following acronym is suggested:
I Inclination of the water table (gradient)
Direction of slope in ft/ft (feet per foot)
M Measured horizontal distance
Distance to point of exposure infeetor miles
P Population exposed
Human or non-human
A Application rate
Slug, intermittent, or continuous
C Concentration
Concentration of pollutant, often in mg/l
T Toxicity
Degree of toxicity to the population exposed
When the DRASTIC Index of a particular setting is
evaluated with regard to these parameters of impact,
as a consequence of a particular pollutant, a reason-
able judgement can be made with respect to the risk
to the population exposed.
30
-------
Section 6
Hydrogeologic Settings of the United States by Ground-Water Regions
Figure 19. Ground-water regions of the United States (After Heath, 1984).
2 Alluvial Basins
7
Glaciated
Central
Region
6. Nonglaciated
Central
Region
9. Northeast and
Superior Uplands
Region "
V
Vl. Western Mountain
- „ Nonglaciated Central ^>
? ^V rV Dnn.nn &''
9. Northeast and
Superior Uplands
3 Columbia
Lava Plateau
6. Nonglaciated
\ ^
Region
2. Alluvial
Basins
Plateau
and
Wyoming
Basin
6. Nonglaciated
Central Region
6 Nonglaciated
Central Region
500 miles
i ' i i' l *l l ' i t
0 800 kilometers
31
-------
Table 18. Hydrogeologic Settings and Associated DRASTIC
Index by Region
Number
1Aa East
1Ab West
1Ba East
1Bb West
1Ca East
ICbWest
1D
1Ea East
1Eb West
1F
2A
2B
2C
2D
2E
3A
3B
3C
3D
3E
4A
4B
4C
4D
5A
5B
5C
5D
5E
6A
6B
6C
6Da
6Db
6E
6Fa
6Fb
6G
6H
7Aa
7Ab
7Ac
7Ad
7Ae
7Ba
7Bb
7Bc
7C
7D
7Ea
7Eb
7F
7G
7H
8A
8B
8C
8D
8E
8F
9A
9B
9C
9Da
9Db
9E
9F
Title
Mountain Slopes
Mountain Slopes
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Mountain Flanks
Mountain Flanks
Glaciated Mountain Valleys
Wide Alluvial Valleys (External Drainage)
Wide Alluvial Valleys (External Drainage)
Coastal Beaches
Mountain Slopes
Alluvial Mountain Valleys
Alluvial Fans
Alluvial Basins (Internal Drainage)
Playa Lakes
Mountain Slopes
Alluvial Mountain Valleys
Hydraulically Connected Lava Flows
Lava Flows Not Connected Hydraulically
Alluvial Fans
Resistant Ridges
Consolidated Sedimentary Rocks
River Alluvium
Alluvium and Dune Sand
Ogallala
Alluvium
Sand Dunes
Playa Lakes
Braided River Deposits
Mountain Slopes
Alluvial Mountain Valleys
Mountain Flanks
Alternating SS, LS, SH-Thin Soil
Alternating SS, LS, SH-Deep Regolith
Solution Limestone
River Alluvium with Overbank
River Alluvium without Overbank
Braided River Deposits
Triassic Basins
Glacial Till Over Bedded Sedimentary Rock
Glacial Till Over Outwash
Glacial Till Over Solution Limestone
Glacial Till Over Sandstone
Glacial Till Over Shale
Outwash
Outwash Over Bedded Sedimentary
Outwash Over Solution Limestone
Moraine
Buried Valley
River Alluvium with Overbank Deposit
River Alluvium without Overbank Deposit
Glacial Lake Deposits
Thin Till Over Bedded Sedimentary
Beaches, Beach Ridges and Sand Dunes
Mountain Slopes
Alluvial Mountain Valleys
Mountain Flanks
Thick Regolith
River Alluvium
Mountain Crests
Mountain Slopes
Alluvial Mountain Valleys
Mountain Flanks
Glacial Till Over Crystalline Bedrock
Glacial Till Over Outwash
Outwash
Moraine
Rating
65
70
128
146
83
106
180
158
180
196
74
132
122
122
110
98
168
156
105
105
88
87
162
102
109
107
150
110
185
103
152
105
129
131
196
136
187
190
106
93
127
129
99
78
176
156
186
125
156
124
191
135
111
202
75
162
106
100
176
70
75
180
106
103
129
190
166
Table 18. (Continued)
Number
Title
Rating
9Ga River Alluvium with Overbank 136
9Gb River Alluvium without Overbank 191
lOAa Confined Regional Aquifers 53
10Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer 184
10Ba River Alluvium with Overbank Deposit 132
10Bb River Alluvium without Overbank Deposit 187
10C Swamp 202
11A Solution Limestone 218
11B Coastal Deposits 191
11C Swamp 224
11D Beaches and Bars 190
12A Mountain Slopes 164
12B Alluvial Mountain Valleys 184
12C Volcanic Uplands 165
12D Coastal Beaches 201
13A Alluvium 140
13B Glacial & Glaciolacustrine Deposits of the
Interior Valleys 131
13C Coastal Lowland Deposits 140
13D Bedrock of the Uplands and Mountains 92
Table 19. Hydrogeologic Settings and Associated DRASTIC
Index Sorted by Ratings
Number
10Aa
1Aa East
8F
1Ab West
2A
9A
8A
7Ae
1 Ca East
4B
4A
13D
7Aa
3A
7Ad
8D
4D
9Da
6A
3D
3E
6C
ICbWest
6H
8C
9C
5B
5A
2E
5D
7G
2D
2C
7Ea
7C
7Ab
1 Ba East
7Ac
9Db
Title
Confined Regional Aquifers
Mountain Slopes
Mountain Crests
Mountain Slopes
Mountain Slopes
Mountain Slopes
Mountain Slopes
Glacial Till Over Shale
Mountain Flanks
Consolidated Sedimentary Rocks
Resistant Ridges
Bedrock of the Uplands and Mountains
Glacial Till Over Bedded Sedimentary Rock
Mountain Slopes
Glacial Till Over Sandstone
Thick Regolith
Alluvium and Dune Sand
Glacial Till Over Crystalline Bedrock
Mountain Slopes
Lava Flows Not Connected Hydraulically
Alluvial Fans
Mountain Flanks
Mountain Flanks
Triassic Basins
Mountain Flanks
Mountain Flanks
Alluvium
Ogallala
Playa Lakes
Playa Lakes
Thin Till Over Bedded Sedimentary
Alluvial Basins (Internal Drainage)
Alluvial Fans
River Alluvium with Overbank Deposit
Moraine
Glacial Till Over Outwash
Alluvial Mountain Valleys
Glacial Till Over Solution Limestone
Glacial Till Over Outwash
Rating
53
65
70
70
74
75
75
78
83
87
88
92
93
98
99
100
102
103
103
105
105
105
106
106
106
106
107
109
110
110
111
122
122
124
125
127
128
129
129
32
-------
Table 19. (Continued)
Table 20. (Continued)
Number
Title
Rating Number
Title
Rating
6Da Alternating SS, LS, SH-Thin Soil 129
13B Glacial & Glaciolacustrine Deposits of the
Interior Valleys 131
6Db Alternating SS, LS, SH-Deep Regolith 131
10Ba River Alluvium with Overbank Deposit 132
2B Alluvial Mountain Valleys 132
7F Glacial Lake Deposits 135
9Ga River Alluvium with Overbank 136
6Fa River Alluvium with Overbank 136
13A Alluvium 140
13C Coastal Lowland Deposits 140
IBbWest Alluvial Mountain Valleys 146
5C Sand Dunes 150
6B Alluvial Mountain Valleys 152
3C Hydraulically Connected Lava Flows 156
7Bb Outwash Over Bedded Sedimentary 156
7D Buried Valley 156
1Ea East Wide Alluvial Valleys (External Drainage) 158
4C River Alluvium 162
SB Alluvial Mountain Valleys 162
12A Mountain Slopes 164
12C Volcanic Uplands 165
9F Moraine 166
3B Alluvial Mountain Valleys 168
7Ba Outwash 176
8E River Alluvium 176
1EbWest Wide Alluvial Valleys (External Drainage) 180
1D Glaciated Mountain Valleys 180
9B Alluvial Mountain Valleys 180
10Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer 184
12B Alluvial Mountain Valleys 184
5E Braided River Deposits 185
7Bc Outwash Over Solution Limestone 186
6Fb River Alluvium without Overbank 187
10Bb River Alluvium without Overbank Deposit 187
6G Braided River Deposits 190
11D Beaches and Bars 190
9E Outwash 190
11B Coastal Deposits 191
9Gb River Alluvium without Overbank 191
7Eb River Alluvium without Overbank Deposit 191
6E Solution Limestone 196
1F Coastal Beaches 196
12D Coastal Beaches 201
7H Beaches, Beach Ridges and Sand Dunes 202
10C Swamp 202
11A Solution Limestone 218
11C Swamp 224
Table 20. Hydrogeologic Settings and Associated DRASTIC
Index Sorted by Setting Title
Number
2D
2C
3E
12B
1Ba East
IBbWest
2B
3B
6B
SB
9B
Title
Alluvial Basins (Internal Drainage)
Alluvial Fans
Alluvial Fans
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Rating
122
122
105
184
128
146
132
168
152
162
180
13A Alluvium 140
5B Alluvium 107
4D Alluvium and Dune Sand 102
6Db Alternating SS,LS,SH-Deep Regolith 131
6Da Alternating SS.LS,SH-Thin Soil 129
11D Beaches and Bars 190
7H Beaches, Beach Ridges and Sand Dunes 202
13D Bedrock of the Uplands and Mountains 92
5E Braided River Deposits 185
6G Braided River Deposits 190
7D Buried Valley 156
1F Coastal Beaches 196
12D Coastal Beaches 201
11B Coastal Deposits 191
13C Coastal Lowland Deposits 140
10Aa Confined Regional Aquifers 53
4B Consolidated Sedimentary Rocks 87
13B Glacial & Glaciolacustrine Deposits of the
Interior Valleys 131
7F Glacial Lake Deposits 135
7Aa Glacial Till Over Bedded Sedimentary Rock 93
9Da Glacial Till Over Crystalline Bedrock 103
7Ab Glacial Till Over Outwash 127
9Db Glacial Till Over Outwash 129
7Ad Glacial Till Over Sandstone 99
7Ae Glacial Till Over Shale 78
7Ac Glacial Till Over Solution Limestone 129
1D Glaciated Mountain Valleys 180
3C Hydraulically Connected Lava Flows 156
3D Lava Flows Not Connected Hydraulically 105
7C Moraine 125
9F Moraine 166
8F Mountain Crests 70
1Ca East Mountain Flanks 83
6C Mountain Flanks 105
ICbWest Mountain Flanks 106
8C Mountain Flanks 106
9C Mountain Flanks 106
12A Mountain Slopes 164
1Aa East Mountain Slopes 65
1AbWest Mountain Slopes 70
2A Mountain Slopes 74
3A Mountain Slopes 98
6A Mountain Slopes 103
8A Mountain Slopes 75
9A Mountain Slopes 75
5A Ogallala 109
7Ba Outwash 176
9E Outwash 190
7Bb Outwash Over Bedded Sedimentary 156
7Bc Outwash Over Solution Limestone 186
2E Playa Lakes 110
5D Playa Lakes 110
4A Resistant Ridges 88
4C River Alluvium 162
8E River Alluvium 176
6Fa River Alluvium with Overbank 136
9Ga River Alluvium with Overbank 136
10Bb River Alluvium with Overbank Deposit 132
7Ea River Alluvium with Overbank Deposit 124
6Fb River Alluvium without Overbank 187
9Gb River Alluvium without Overbank 191
10Bb River Alluvium without Overbank Deposit 187
7Eb River Alluvium without Overbank Deposit 191
5C Sand Dunes 150
11A Solution Limestone 218
6E Solution Limestone 196
-------
Table 20. (Continued)
Table 21. (Continued)
Number
10C
11C
8D
7G
6H
10Ab
12C
1Ea East
1Eb West
Title
Swamp
Swamp
Thick Regolith
Thin Till Over Bedded Sedimentary
Triassic Basins
Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer
Volcanic Uplands
Wide Alluvial Valleys (External Drainage)
Wide Alluvial Valleys (External Drainage)
Rating
202
224
100
111
106
184
165
158
180
Number
Title
Rating
Table 21.
Number
1Aa East
1Ab West
1Ba East
1Bb West
1Ca East
ICbWest
ID
1Ea East
lEbWest
1F
2A
2B
2C
2D
2E
3A
3B
3C
3D
3E
4A
4B
4C
4D
5A
5B
5C
5D
5E
6A
6B
6C
6Da
6Db
6E
6Fa
6Fb
6G
6H
7Aa
7Ab
7Ac
7Ad
7Ae
7Ba
7Bb
7Bc
7C
7D
7Ea
Hydrogeologic Settings and Associated
Agricultural DRASTIC Index by Region
Title
Mountain Slopes
Mountain Slopes
Alluvial Mountain Valleys
Alluvial Mountain Valleys
Mountain Flanks
Mountain Flanks
Glaciated Mountain Valleys
Wide Alluvial Valleys (External Drainage)
Wide Alluvial Valleys (External Drainage)
Coastal Beaches
Mountain Slopes
Alluvial Mountain Valleys
Alluvial Fans
Alluvial Basins (Internal Drainage)
Playa Lakes
Mountain Slopes
Alluvial Mountain Valleys
Hydraulically Connected Lava Flows
Lava Flows Not Connected Hydraulically
Alluvial Fans
Resistant Ridges
Consolidated Sedimentary Rocks
River Alluvium
Alluvium and Dune Sand
Ogallala
Alluvium
Sand Dunes
Playa Lakes
Braided River Deposits
Mountain Slopes
Alluvial Mountain Valleys
Mountain Flanks
Alternating SS, LS, SH-Thin Soil
Alternating SS, LS, SH-Deep Regolith
Solution Limestone
River Alluvium with Overbank
River Alluvium without Overbank
Braided River Deposits
Triassic Basins
Glacial Till Over Bedded Sedimentary Rock
Glacial Till Over Outwash
Glacial Till Over Solution Limestone
Glacial Till Over Sandstone
Glacial Till Over Shale
Outwash
Outwash Over Bedded Sedimentary
Outwash Over Solution Limestone
Moraine
Buried Valley
River Alluvium with Overbank Deposit
Rating
91
97
166
184
99
122
214
192
214
221
105
165
155
157
139
122
202
182
143
123
117
108
186
131
136
135
177
139
216
132
176
126
155
160
216
156
209
221
135
117
145
145
121
103
196
182
206
148
178
149
7Eb River Alluvium without Overbank Deposit 224
7F Glacial Lake Deposits 165
7G Thin Till Over Bedded Sedimentary 135
7H Beaches, Beach Ridges and Sand Dunes 225
8A Mountain Slopes 102
8B Alluvial Mountain Valleys 185
8C Mountain Flanks 123
8D Thick Regolith 117
8E River Alluvium 198
8F Mountain Crests 113
9A Mountain Slopes 102
9B Alluvial Mountain Valleys 202
9C Mountain Flanks 122
9Da Glacial Till Over Crystalline Bedrock 134
9Db Glacial Till Over Outwash 153
9E Outwash 210
9F Moraine 180
9Ga River Alluvium with Overbank 156
9Gb River Alluvium without Overbank 213
lOAa Confined Regional Aquifers 53
lOAb Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer 206
lOBa River Alluvium with Overbank Deposit 157
10Bb River Alluvium without Overbank Deposit 220
10C Swamp 233
11A Solution Limestone 243
11B Coastal Deposits 224
11C Swamp 251
11D Beaches and Bars 225
12A Mountain Slopes 177
12B Alluvial Mountain Valleys 192
12C Volcanic Uplands 174
12D Coastal Beaches 230
13A Alluvium 164
13B Glacial & Glaciolacustrine Deposits of the
Interior Valleys 158
13C Coastal Lowland Deposits 164
13D Bedrock of the Uplands and Mountains 118
Table 22.
Number
10Aa
1Aa East
1Ab West
1Ca East
8A
9A
7Ae
2A
4B
8F
7Aa
4A
8D
13D
7Ad
ICbWest
9C
3A
3E
8C
6C
Hydrogeologic Settings and Associated
Agricultural DRASTIC Index Sorted by Ratings
Title Rating
Confined Regional Aquifers
Mountain Slopes
Mountain Slopes
Mountain Flanks
Mountain Slopes
Mountain Slopes
Glacial Till Over Shale
Mountain Slopes
Consolidated Sedimentary Rocks
Mountain Crests
Glacial Till Over Bedded Sedimentary Rock
Resistant Ridges
Thick Regolith
Bedrock of the Uplands and Mountains
Glacial Till Over Sandstone
Mountain Flanks
Mountain Flanks
Mountain Slopes
Alluvial Fans
Mountain Flanks
Mountain Flanks
53
91
97
99
102
102
103
105
108
113
117
117
117
118
121
122
122
122
123
123
126
34
-------
Table 22. (Continued)
Number
Title
Rating
4D Alluvium and Dune Sand 131
6A Mountain Slopes 132
9Da Glacial Till Over Crystalline Bedrock 134
5B Alluvium 135
6H Tnassic Basins 135
7G Thin Till Over Bedded Sedimentary 135
5A Ogallala 136
5D Playa Lakes 139
2E Playa Lakes 139
3D Lava Flows Not Connected Hydraulically 143
7Ab Glacial Till Over Outwash 145
7Ac Glacial Till Over Solution Limestone 145
7C Moraine 148
7Ea River Alluvium with Overbank Deposit 149
9Db Glacial Till Over Outwash 153
2C Alluvial Fans 155
6Da Alternating SS,LS,SH-Thm Soil 155
6Fa River Alluvium with Overbank 156
9Ga River Alluvium with Overbank 156
2D Alluvial Basins (Internal Drainage) 157
10Ba River Alluvium with Overbank Deposit 157
13B Glacial ft Glaciolacustrine Deposits of the
Interior Valleys 158
6Db Alternating SS,LS,SH-Deep Regolith 160
13A Alluvium 164
13C Coastal Lowland Deposits 164
2B Alluvial Mountain Valleys 165
7F Glacial Lake Deposits 165
1Ba East Alluvial Mountain Valleys 166
12C Volcanic Uplands 174
6B Alluvial Mountain Valleys 176
5C Sand Dunes 177
12A Mountain Slopes 177
7D Buried Valley 178
9F Moraine 180
3C Hydraulically Connected Lava Flows 182
7Bb Outwash Over Bedded Sedimentary 182
IBbWest Alluvial Mountain Valleys 184
8B Alluvial Mountain Valleys 185
4C River Alluvium 186
12B Alluvial Mountain Valleys 192
1Ea East Wide Alluvial Valleys (External Drainage) 192
7Ba Outwash 196
8E River Alluvium 198
3B Alluvial Mountain Valleys 202
9B Alluvial Mountain Valleys 202
10Ab Unconsolidated ft Semi-Consolidated Shallow
Surficial Aquifer 206
7Bc Outwash Over Solution Limestone 206
6Fb River Alluvium without Overbank 209
9E Outwash 210
9Gb River Alluvium without Overbank 213
1EbWest Wide Alluvial Valleys (External Drainage) 214
1D Glaciated Mountain Valleys 214
6E Solution Limestone 216
5E Braided River Deposits 216
10Bb River Alluvium without Overbank Deposit 220
1F Coastal Beaches 221
6G Braided River Deposits 221
11B Coastal Deposits 224
7Eb River Alluvium without Overbank Deposit 224
7H Beaches, Beach Ridges and Sand Dunes 225
11D Beaches and Bars 225
12D Coastal Beaches 230
10C Swamp 233
11A Solution Limestone 243
11C Swamp 251
Table 23.
Number
Hydrogeologic Settings and Associated
Agricultural DRASTIC Index Sorted by Setting
Title
Title
Rating
2D Alluvial Basins (Internal Drainage) 157
2C Alluvial Fans 155
3E Alluvial Fans 123
8B Alluvial Mountain Valleys 185
2B Alluvial Mountain Valleys 165
6B Alluvial Mountain Valleys 176
3B Alluvial Mountain Valleys 202
IBbWest Alluvial Mountain Valleys 184
1Ba East Alluvial Mountain Valleys 166
9B Alluvial Mountain Valleys 202
12B Alluvial Mountain Valleys 192
5B Alluvium 135
13A Alluvium 164
4D Alluvium and Dune Sand 131
6Db Alternating SS,LS,SH-Deep Regolith 160
6Da Alternating SS,LS,SH-Thin Soil 155
11D Beaches and Bars 225
7H Beaches, Beach Ridges and Sand Dunes 225
13D Bedrock of the Uplands and Mountains 118
5E Braided River Deposits 216
6G Braided River Deposits 221
7D Buried Valley 178
1F Coastal Beaches 221
12D Coastal Beaches 230
11B Coastal Deposits 224
13C Coastal Lowland Deposits 164
10Aa Confined Regional Aquifers 53
4B Consolidated Sedimentary Rocks 108
13B Glacial ft Glaciolacustrine Deposits of the
Interior Valleys 158
7F Glacial Lake Deposits 165
7Aa Glacial Till Over Bedded Sedimentary Rock 117
9Da Glacial Till Over Crystalline Bedrock 134
7Ab Glacial Till Over Outwash 145
9Db Glacial Till Over Outwash 153
7Ad Glacial Till Over Sandstone 121
7Ae Glacial Till Over Shale 103
7Ac Glacial Till Over Solution Limestone 145
1D Glaciated Mountain Valleys 214
3C Hydraulically Connected Lava Flows 182
3D Lava Flows Not Connected Hydraulically 143
7C Moraine 148
9F Moraine 180
8F Mountain Crests 113
1Cb West Mountain Flanks 122
6C Mountain Flanks 126
8C Mountain Flanks 123
9C Mountain Flanks 122
1Ca East Mountain Flanks 99
12A Mountain Slopes 177
9A Mountain Slopes 102
2A Mountain Slopes 105
6A Mountain Slopes 132
1Ab West Mountain Slopes 97
1Aa East Mountain Slopes 91
3A Mountain Slopes 122
8A Mountain Slopes 102
5A Ogallala 136
7Ba Outwash 196
9E Outwash 210
7Bb Outwash Over Bedded Sedimentary 182
7Bc Outwash Over Solution Limestone 206
5D Playa Lakes 139
2E Playa Lakes 139
4A Resistant Ridges 117
8E River Alluvium 198
-------
Table 23. (Continued)
Number
Title
Rating
4C River Alluvium 186
6Fa River Alluvium with Overbank 156
9Ga River Alluvium with Overbank 156
10Ba River Alluvium with Overbank Deposit 157
7Ea River Alluvium with Overbank Deposit 149
6Fb River Alluvium without Overbank 209
9Gb River Alluvium without Overbank 213
10Bb River Alluvium without Overbank Deposit 220
7Eb River Alluvium without Overbank Deposit 224
5C Sand Dunes 177
6E Solution Limestone 216
11A Solution Limestone 243
11C Swamp 251
10C Swamp 233
8D Thick Regolith 117
7G Thin Till Over Bedded Sedimentary 135
6H Triassic Basins 135
10Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer 206
12C Volcanic Uplands 174
1EbWest Wide Alluvial Valleys (External Drainage) 214
1Ea East Wide Alluvial Valleys (External Drainage) 192
36
-------
Table 24. Summary of the Principal Physical and Hydrologic Characteristics of the Ground-Water Regions of the United States
(after Heath, 1984)
Characteristics of dominant aquifers
Region
No. Name
1 Western
Mountain
Ranges
2 Alluvial Basins
3 Columbia Lava
Plateau
4 Colorado
Plateau and
Wyoming
Basin
5 High Plains
6 Nonglaciated
Central Region
7 Glaciated
Central Region
8 Piedmont and
Blue Ridge
9 Northeast and
Superior
Uplands
10 Atlantic and
Gulf Coastal
Plain
1 1 Southeast
Coastal Plain
12 Hawaii
13 Alaska
Lomponen
Upnc0"- Confining
flnefd beds
aquifer
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X X
X X
X X
X X
X X
X X
X X
X X
X X
X X
X X
X X
X X
its ot the system Water-bearing Composi- Storage and Recharge and
openings tion transmission properties discharge conditions
Confined Presence and Secon- Degree of _ . Trans- _ u „. .
aquifers aT^ PrmarY darv solubility PorosltV missivity Recharge Discharge
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X XXXXXX X X X X
X XXXX X XXX
X XXXX XXXX
X XX XX X X XXXX
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X X XXXX XXX
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37
-------
Table 25. Common Ranges for the Hydraulic Characteristics of Ground-Water Regions of The United States (after Heath, 1984)
Region
No
1
2
3
4
5
Common ranges in hydraulic characteristics of the dominant aquifers
Transmissivity Hydraulic conductivity Recharge rate
Region Geologic situation m2day~' ft^ay1 m day"1 ft day"1 mm yr1 in yr1
Western Mountains with 100 5 5,000,000 0 0003 15 0 001 50 3 50 0 1 2
Mountain thin soils over
Ranges fractured rocks.
alternating with
narrow alluvial
and, in part,
glaciated valleys
Alluvial Basins Thick1 alluvial 20 20,000 2,000 200,000 30 600 100 2,000 003 X 0 001 1
(locally glacial)
deposits in basins
and valleys
bordered by
mountains
Columbia Lava Thick sequence of 2,000 500,000 20,000 5,000,000 200 3,000 500 10,000 5 300 02 10
Plateau lava flows
interbedded with
unconsolidated
deposits and
overlain by thin
soils
Colorado Thin1 soils over 0.5 100 5 1,000 0003 2 0.01 503 50 0.01 2
Plateau and fractured
Wyoming Basin sedimentary rocks
High Plains Thick alluvial 1,000 10,000 10,000 100,000 30 300 100 1,000 5 80 0.2 3
deposits over
fractured
sedimentary rocks
Well yield
m'min"1 gal min~1
0.04 0.4 10 100
004 20 100 5,000
04 80 100 20,000
0.04 2 10 1,000
0.4 10 100 3,000
Nonglaciated Thin regoltth over
Central region fractured
sedimentary rocks
Glaciated Thick glacial
Central region deposits over
fractured
sedimentary rocks
Piedmont and Thick regollth over
Blue Ridge fractured
crystalline and
meta morphosed
sedimentary rocks
Northeast and Thick glacial
Superior deposits over
Uplands fractured
crystalline rocks
300 10,000 3,000 100,000
300 10
100
2,000 1,000 20,000
1,000
1,000
200
50
100
500
2,000 0.001
5,000
0.003
X
30
100 30
50002 2004 20 100 5,000
30002 10 02 2 50 500
300 1 10 02 2 50 500
300 1 10 0.1 1 20 200
10
11
12
13
Atlantic and
Gulf Coastal
Plain
Southeast
Coastal Plain
Hawaiian
Islands
Alaska
Complexly inter- 500 10,000 5,000 100,000 3 100 10 400 50 600 2
bedded sands.
silts, and clays
Thick layers of 1,000 100,000 10,000 1,000,000 30 3,000 100 10,000 30 500 1
sand and clay over
semiconsolidated
carbonate rocks
Lava flows 10,000 100,000 100,000 1,000,000 200 3,000 500 10,000 30 1,000 1
segmented by
dikes, interbedded
with ash deposits.
and partly overiatn
by alluvium
Glacial and alluvial 100 10,000 1,000 100,000 30 600 100 2,000 3 300 0.1
deposits in part
perennially frozen
and overlying
crystalline.
metamorphic, and
sedimentary rocks
20 04 20 100 5,000
20 4 80 1,000 20,000
40 0.4 20 100 5,000
10 0.04 4 10 1,000
38
-------
Table 26. Ranges and Ratings for Depth to Water
Depth to Water
(feet)
Range Rating
0-5 10
5-10 9
15-30 7
30-50 5
50-75 3
75-100 2
100+ 1
Weight: 5 Agricultural Weight: 5
Table 27. Ranges and Ratings for Net Recharge
Net Recharge
(inches)
Range Rating
0-2 1
2-4 3
4-7 6
7-10 8
10+ 9
Weight: 4 Agricultural Weight: 4
Table 28. Ranges and Ratings for Aquifer Media
Aquifer Media
Range Rating Typical Rating
Massive Shale 1-3 2
Metamorphic/ Igneous 2-5 3
Weathered Metamorphic/ Igneous 3-5 4
Thin Bedded Sandstone,
Limestone, Shale Sequences 5-9 6
Massive Sandstone 4-9 6
Massive Limestone 4-9 6
Sand and Gravel 6-9 8
Basalt 2-10 9
Karst Limestone 9-10 10
Weight: 3 Agricultural Weight: 3
Table 29. Ranges and Ratings for Soil Media
Soil Media
Range Rating
Thin or Absent 10
Gravel 10
Sand 9
Shrinking and/or Aggregated Clay 7
Sandy Loam 6
Loam 5
Silty Loam 4
Clay Loam 3
Nonshrinking and Nonaggregated Clay 1
Weight: 2 Agricultural Weight: 5
Table 30. Ranges and Ratings for Topography
Topography
(percent slope)
Range Rating
0-2 10
2-6 9
6-12 5
12-18 3
18+ 1
Weight: 1 Agricultural Weight: 3
Table 31. Ranges and Ratings for Impact of Vadose
Zone Media
Impact of Vadose Zone Media
Range Rating Typical Rating
Silt/Clay 1-2 1
Shale 2-5 3
Limestone 2-7 6
Sandstone 4-8 6
Bedded Limestone, Sandstone, Shale 4-8 6
Sand and Gravel with
significant Silt and Clay 4-8 6
Metamorphic/ Igneous 2-8 4
Sand and Gravel 6-9 8
Basalt 2-10 9
Karst Limestone 8-10 10
Weight: 5 Agricultural Weight: 4
Table 32. Ranges and Ratings for Hydraulic Conductivity
Hydraulic Conductivity
(GPD/FT2)
Range Rating
1-100 1
100-300 2
300-700 4
700-1000 6
1000-2000 8
2000+ 10
Weight: 3 Agricultural Weight: 2
Figure 20. Map legend.
* Sands and Silts
yf/^/r-1?!" Igneous, Metamorphic _; - ; ~__T. Sand and Silt
(V.i'.i'iii
39
-------
/. Western Mountain Ranges
(Mountains with thin soils over fractured rocks, alternating with narrow alluvial and, in part, glaciated valleys)
The Western Mountain Ranges encompass three
areas totaling 708,000 km2. 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 in South Dakota in which
Precambrian rocks are exposed. Summits in the
Rocky Mountains and Sierra Nevada exceed 3,500 m.
The general appearance of the Western Mountain
Ranges, with the exception of the Black Hills, is tall,
massive mountains alternating with relatively nar-
row, steep-sided valleys. The summits and sides of
the mountains in much of the region have been
carved into distinctive shapes by mountain glaciers.
The ranges that comprise the southern Rocky Moun-
tains are separated by major lowlands that include
North Park, Middle Park, South Park, and the Wet
Mountain Valley. These lowlands occupy downfolded
or down-faulted structural troughs as much as 70 km
wide and 160 km long. The mountains in the Black
Hills are lower in altitude than most of the mountains
in other parts of the region.
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 under-
lain by 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
meters 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
down-faulted structural troughs are underlain by
moderately thick deposits of coarse-grained alluvium
transported by streams from the adjacent mountains.
The Western Mountain Ranges and the mountain
ranges in adjacent regions are the principal sources
of water supplies developed at lower altitudes in the
western half of the conterminous United States. As
McGuinness( 1963) noted, the mountains of the west
are moist "islands" in a sea of desert or semidesert
that covers the western half of the Nation. The
mountains force moisture-laden air masses moving
eastward from the Pacific to rise to higher and cooler
altitudes. As the air cools, moisture condenses into
clouds and precipitates. The heaviest precipitation
falls on the western slopes; thus, these slopes are the
major source of runoff and are also the most densely
40
-------
vegetated. Much of the precipitation falls as snow
during the winter, and its slow melting, starting at the
lower altitudes in early spring, maintains streamflow
at large rates until late June or early July. Small
glaciers occur in the higher mountain ranges, especi-
ally in the northern Rocky Mountains, the Cascades,
and the Sierra Nevada; locally, as in northern
Washington they also provide significant sources of
summer runoff.
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 their small 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 region underlain by crystalline rocks,
relatively little opportunity exists for development of
ground-water supplies. The best opportunities exist
in 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 wells drawing water from
small, thin deposits of alluvium are generally ade-
quate 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 per-
meable sedimentary or volcanic rocks.
Western Mountain Ranges
(1Aa) Mountain Slopes—East
This hydrogeologic setting is characterized by steep
slopes on the sides of mountains, a thin soil cover and
highly fractured bedrock. Ground water is obtained
primarily from the fractures in the bedrock which may
be of sedimentary, metamorphic or igneous origin.
The fractures provide localized sources of ground
water and well yields are typically limited even
though the hydraulic conductivity is often high
because of the fractures. Due to the steep slopes, thin
soil and small storage capacity of the fractures, runoff
is significant. Thicker weathered zones (soils) may
develop locally particularly on talus slopes with local
perched zones common. These eastern facing slopes
are located in the rain shadow of the mountains and
only limited rainfall is derived from the moisture
laden prevailing westerly winds, thus ground-water
recharge rarely exceeds 1 inch/year. Ground-water
levels are extremely variable but are typically deep.
Most of these areas are water deficient on an annual
basis. The migration of pollutants introduced at the
surface will be dependent on the current climatic
conditions; pollutants will tend to infiltrate easier and
further during wet periods as opposed to dry periods.
Setting 1 Aa East Mountain Slopes
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Metamorphic/
Igneous
Thin or Absent
1 8+%
Metamorphic/
Igneous
1 00-300
Weight
5
4
3
2
1
5
3
General
Rating
1
1
3
10
1
4
2
Number
5
4
9
20
1
20
6
DRASTIC Index 65
Setting 1 Aa East Mountain Slopes
Feature
Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
100+
0-2
Metamorphic/
Igneous
Thin or Absent
18+%
Metamorphic/
Igneous
100-300
5
4
3
5
3
4
2
1
1
3
10
1
4
2
5
4
9
50
3
16
4
Agricultural
DRASTIC Index
91
41
-------
Western Mountain Ranges
(1A b) Mountain Slopes— West
This setting is similar to (1 Aa) Mountain Slopes—East
except that ground-water levels are typically more
shallow and precipitation greatly exceeds the amount
which falls on the eastern slopes. Even though
rainfall is more abundant, recharge is still low due to
the steepness of the slopes and density of the
underlying bedrock and may only exceed 2 inches/
year in places where precipitation is very high and soil
cover is unusually favorable. Due to increased
precipitation, pollutants may tend to migrate to the
water table more rapidly, but be more diluted, than on
the comparable eastern slopes.
Setting 1 Ab West Mountain Slopes
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Metamorphic/
Igneous
Thin or Absent
1 8+%
Metamorphic/
Igneous
1 00-300
Weight Rating
5 2
4 1
3 3
5 10
3 1
4 4
2 2
Agricultural
DRASTIC Index
Number
10
4
9
50
3
16
4
97
Setting 1 Ab West Mountain Slopes
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Metamorphic/
Igneous
Thin or Absent
18+%
Metamorphic/
Igneous
1 00-300
Weight
5
4
3
2
1
5
3
General
Rating
2
1
3
10
1
4
2
Number
10
4
9
20
1
20
6
DRASTIC Index 70
Western Mountain Ranges
(1Ba) Alluvial Mountain Valleys—East
This hydrogeologic setting of eastward facing interior
valleys is characterized by thin bouldery alluvium
which overlies fractured bedrock of sedimentary,
metamorphic or igneous origin. The alluvium, which
is derived from the surrounding steep slopes serves
as a localized source of water. Where soil cover
exists, it typically is gravel sized and offers little
protection from pollution. Water levels are typically
moderately deep because of the lack of precipitation
on the eastern slopes and the low net recharge.
Ground water is obtained from the coarser grained
deposits within the valley, but these deposits also
have a fraction of finer grained deposits which can
influence water movement. Ground water may also
be obtained from the fractures in the underlying
bedrock which are typically in direct hydraulic con-
nection with the overlying alluvium. Since these
valleys are usually structurally controlled, there is the
possibility that any pollutants introduced at the
surface may migrate into the fractures beneath the
alluvium and disperse rapidly from the site of
incidence.
42
-------
Setting 1 Ba East Alluvial Mtn. Valleys
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Gravel
2-6%
Sand and Gravel
1 00-300
Weight Rating
5 5
4 1
3 8
2 10
1 9
5 8
3 2
DRASTIC Index
Number
25
4
24
20
9
40
6
128
Setting 1 Ba East Alluvial Mtn. Valleys
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Gravel
2-6%
Sand and Gravel
100-300
Agricultural
Weight Rating
5 5
4 1
3 8
5 10
3 9
4 8
2 2
Agricultural
DRASTIC Index
Number
25
4
24
50
27
32
4
166
Setting 1 Bb West Alluvial Mtn. Valleys
General
Feature
Range
Weight Rating Number
Depth to Water
Table 15-30
Net Recharge 2-4
Aquifer Media Sand and Gravel
Soil Media Gravel
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
2-e
Sand and Gravel
100-300
5
4
3
2
1
7
3
8
10
9
35
12
24
20
9
40
DRASTIC Index 146
Western Mountain Ranges
(1Bb) Alluvial Mountain Valleys—West
This setting, which includes coastal valleys and
westward-sloping interior valleys, is similar to (1 Ba)
Narrow Alluvial Valleys—East. Water Levels are
typically shallower due to higher amounts of precipi-
tation and subsequently greater ground-water re-
charge. Soils tend to be deeper with better developed
soil profiles. Bedrock weathering is usually deeper,
with increased mass wasting due to freeze/thaw
cycles that may occur in the higher valleys of some
areas. The migration of pollutants introduced at the
surface will, in most cases, be predictably down-
gradient in the relatively short, straight, narrow, well-
defined valleys.
Setting 1 Bb West Alluvial Mtn. Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Gravel
2-6%
Sand and Gravel
1 00-300
Weight Rating
5 7
4 3
3 8
5 10
3 9
4 8
2 2
Agricultural
DRASTIC Index
Number
35
12
24
50
27
32
4
184
43
-------
Western Mountain Ranges
(1Ca) Mountain Flanks—East
This hydrogeologic setting is characterized by moder-
ate to steep topographic relief and dipping fractured
consolidated sedimentary rocks, which dip toward
and underlie the adjacent wide alluvial valleys. Soil
cover is usually thicker than on the upper mountain
slopes and typically has weathered to a sandy loam.
Alluvium and/or talus deposits are not included in
this setting. These sedimentary rocks, when frac-
tured, typically have hydraulic conductivities similar
to the fractured bedrock on the mountain slopes.
Depth to the water table varies, but is typically deep
dueto lack of precipitation and moderate topographic
relief, and net recharge is very low. Pollutants that
may be introduced at the surface will tend to migrate
most rapidly along dipping bedding planes, and
through fractures.
Setting 1 Ca East Mountain Flanks
Agricultural
Setting 1 Ca East Mountain Flanks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Thin Bedded SS,
LS.SH Sequences
Sandy Loam
12-18%
Bedded LS, SS,
SH
100-300
Weight
5
4
3
2
1
5
3
Rating
2
1
6
6
3
6
2
Number
10
4
18
12
3
30
6
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Thin Bedded SS,
LS.SH Sequences
Sandy Loam
12-18%
Bedded LS, SS,
SH
1 00-300
Weight Rating
5 2
4 1
3 6
5 6
3 3
4 6
2 2
Agricultural
DRASTIC Index
Number
10
4
18
30
9
24
4
99
Western Mountain Ranges
f 1 Cb) Mountain Flanks— West
This setting is similar to (1Ca) Mountain Flanks—
East. Ground-water levels, however, are typically not
quite as deep and ground-water recharge is greater
due to the greater amount of precipitation on the
western slopes. Soil depths are often greater, with
more developed soil profiles. These soils are charac-
terized by higher clay and loam content than those
that occur on the eastern slopes. Analagous to the
eastern flanks, any pollutants that are introduced will
tend to migrate along bedding planes and fractures.
DRASTIC Index 83
44
-------
Setting 1 Cb West Mountain Flanks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
LS.SH Sequences
Sandy Loam
12-18%
Bedded LS, SS,
SH
100-300
Weight
5
4
3
2
1
5
3
Rating
5
3
6
6
3
6
2
Number
25
12
18
12
3
30
6
DRASTIC Index 106
Setting 1 Cb West Mountain Flanks
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range Weight Rating Number
30-50
2-4
Thin Bedded SS,
LS.SH Sequences
Sandy Loam
12-18%
Bedded LS, SS,
SH
100-300
5 5
4 3
3 6
5 6
3 3
4 6
2 2
Agricultural
DRASTIC Index
25
12
18
30
9
24
4
122
Setting 1 D Glacial Mountain Valleys
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
4-7
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
GG nsra I
Rating
9
6
8
10
9
8
6
DRASTIC Index
Number
45
24
24
20
9
40
18
180
Setting 1 D Glacial Mountain Valleys
Western Mountain Ranges
(1D) Glaciated Mountain Valleys
This hydrogeologic setting is characterized by moder-
ate topographic relief, and very coarse grained
deposits associated with the near mountain glacial
features, such as cirques and paternoster lakes.
These deposits may serve as localized sources of
water. Water tables are typically shallow with coarse
grained deposits present at the surface. Mountain
glaciers may be present in some areas. Although
precipitation may not be great, recharge is relatively
high when compared to other settings in the region
because of the large volumes of water produced from
the glaciers during the summer melting cycle. These
recent glacial deposits are underlain by fractured
bedrock of igneous or metamorphic origin all of which
are in direct hydraulic connection with the overlying
deposits. The fractured bedrock may also serve as a
local source of ground water.
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Range
5-15
4-7
Sand and Gravel
Gravel
2-6%
Weight
5
4
3
5
3
Rating
9
6
8
10
9
Number
45
24
24
50
27
Impact Vadose
Zone Sand and Gravel
Hydraulic
Conductivity 700-1000
Agricultural
DRASTIC Index
32
12
214
45
-------
Western Mountain Ranges
(1Ea) Wide Alluvial Valleys (With External
Drainage)—East
This hydrogeologic setting is characterized by low
relief and moderately thick deposits of coarsegrained
alluvium deposited by water. It is similar to Narrow
Alluvial Valleys except that the valleys are better
developed and the streams which occupy their
channels have a shallower gradient. Typically the
alluvial deposits are finer grained and thicker than the
Narrow Alluvial Valleys. The alluvium in this setting
serves as the major source of ground water and is
often capable of supplying large quantities of water.
Surficial deposits are usually coarse grained and
water levels are relatively shallow even though
precipitation and net recharge are low. The alluvium
is underlain by layers of permeable sedimentary rock
which receive their primary source of recharge from
the adjacent mountain flanks. The sedimentary
sequence is underlain by fractured bedrock of igneous
or metamorphic origin. Ground water may also be
obtained from the permeable sedimentary rocks.
Setting 1 Ea East Wide Alluvial Valleys (External Drainage)
Agricultural
Setting 1 Ea East Wide Alluvial Valleys (External Drainage)
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
Rating
7
3
8
10
9
8
6
Number
35
12
24
20
9
40
18
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight Rating
5 7
4 3
3 8
5 10
3 9
4 8
2 6
Agricultural
DRASTIC Index
Number
35
12
24
50
27
32
12
192
Western Mountain Ranges
(1 Eb) Wide Alluvial Valleys (External
Drainage)—West
This setting is similar to (1 Ea) Wide Alluvial Valleys
(External Drainage)—East except that water levels
are typically shallow because of higher precipitation
and greater ground-water recharge. Soils tend to be
better developed and thicker in the areas bordering
the mountain flanks (1Cb), however, in the valley
lowlands, gravelly soils predominate. Pollutants
introduced at the surface in these wide alluvial
valleys tend to migrate rapidly in the coarser grained
deposits and travel into and along fracture planes.
DRASTIC Index 158
46
-------
Setting 1 Eb West Wide Alluvial Valleys (External Drainage)
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
4-7
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
Rating
9
6
8
10
9
8
6
Number
45
24
24
20
9
40
18
DRASTIC Index 180
Setting 1 Eb West Wide Alluvial Valleys (External Drainage)
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
4-7
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight Rating
5 9
4 6
3 8
5 10
3 9
4 8
2 6
Agricultural
DRASTIC Index
Number
45
24
24
50
27
32
12
214
Setting 1 F Coastal Beaches
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1 000
Weight
5
4
3
2
1
5
3
Rating
10
9
8
9
10
8
6
Number
50
36
24
18
10
40
18
DRASTIC Index 196
Western Mountain Ranges
(IF) Coastal Beaches
This hydrogeologic setting is characterized by low
topographic relief, near sea level elevation and sandy
surface soils. These areas have very high potential
infiltration rates. These areas are commonly ground
water discharge areas, which, when utilized for fresh
water supply, are quickly endangered by salt-water
intrusion. Due to their very permeable nature and thin
vadose zone, they are very vulnerable to pollution.
Under natural gradients, pollution of this zone is
usually discharged to the sea. However, with inland
pumping, flow is rapidly reversed to the pumping
center.
Setting 1 F Coastal Beaches
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1 000
Weight Rating
5 10
4 9
3 8
5 9
3 10
4 8
2 6
Agricultural
DRASTIC Index
Number
50
36
24
45
30
24
12
221
47
-------
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 1,025,000 km2 extending from the Puget
Sound-Willamette Valley area of Washington and
Oregon to west Texas. The region consists of an
irregular alternation of basins or valleys and mountain
ranges. From the standpoint of topography, it is useful
to contrast this region with the Western Mountain
Ranges. In the Western Mountain Ranges the high
areas, the mountains, are the dominant feature. In
the Alluvial Basins region the low areas, the basins
and valleys, are the dominant feature. The principal
exception to this generalization is the Coast Ranges of
southern California which, though included in this
region, 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 surf ace or subsurface flow leaves
this part of the region, and all water reaching it from
adjacent areas and from precipitation is returned to
the atmosphere by evaporation or by thetranspiration
of plants.
The basins and valleys are diverse in size, shape, and
altitude. They range in altitude from about 85 m
below sea level in Death Valley in California to 2,000
m above sea level in the San Luis Valley in Colorado.
The basins range in size from a few hundred meters in
width and a kilometer or two in length to, for the
Central Valley of California, as much as 80 km in
width and 650 km in length. The crests of the
mountains are commonly 1,000 to 1,500 m 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 open-
ings in the granitic and metamorphic rocks in the
mountainous area 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
meters) layer of generally unconsolidated alluvial
material that partially fills the basins. Except for the
part of the region in Washington and Oregon, the
material was derived from erosion of the adjacent
mountains and was transported down steep-gradient
streams into the basins where it was deposited as
alluvial fans. Generally, the coarsest material in an
alluvial fan occurs at its apex, adjacent to the
mountains; the material gets progressively finer
toward the center 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 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 Willamette Valley areas differ
geologically from the remainder of the region. The
48
-------
Puget Sound area is underlain by thick and very
permeable deposits of gravel and sand laid down by
streams of glacial meltwater derived from icetongues
that invaded the area from the north during the
Pleistocene. The gravel and sand are interbedded
with clay in parts of the area. The Willamette Valley is
mostly underlain by interbedded sand, silt and clay
deposited on floodplains by the Willamette 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 100 to 400
mm. 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 400 mm to
more than 800 mm. The region also receives runoff
from streams that originate in the mountains of the
Western Mountain Ranges region.
Because of the very thin cover of unconsolidated
material on the mountains in the Alluvial Basins
region, precipitation runs off rapidly down the valleys
and out onto the fans where it infiltrates into the
alluvium. 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 water that collects in
these areas, which are called playas, evaporates
relatively quickly, leaving both a thin deposit of clay
and other sediment transported by overland runoff
and a crust consisting of the soluble salts that were
dissolved in the water.
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." Water may movethrough permeable bedrock
from one basin to another, arriving, ultimately, at a
large playa referred to as a "sink" into the ground, as
the name might imply, but by evaporating, as in other
playas. In those parts of the Alluvial Basin region
drained by perennial streams, including the Puget
Sound-Willamette Valley area, the Central Valley of
California, and some of the valleys in Arizona and
New Mexico, 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 kilometers. 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.
Ground water is the major source of water in the
Alluvial Basins region. Many of the valleys in this
region have been developed for agriculture. Because
of the dry climate, agriculture requires intensive
irrigation. In the part of this region drained by the
Colorado River, ground water used for irrigation in
1975 amounted to about 6 billion cubic meters
(4,864,000 acre-feet). Most of the ground water is
obtained from wells drawing 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 signif-
icant, 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 9 m in one area, and in southern Arizona,
where subsidence of more than 4 m has been
observed.
In both the Alluvial Basins and the Colorado Plateau
regions, large volumes of water are transpired by
phreatophytes (water-loving plants) of small econom-
ic value that live along streams and in other wet
areas. In an effort to increase the amount of water
available for irrigation and other uses, numerous
studies have been made to determine the volumes of
water used by phreatophytes and to devise means to
control them. A few small control efforts have been
made, but none have proven economically effective.
Alluvial Basins
(2A) Mountain Slopes
This hydrogeologic setting is characterized by steep
slopes on the side of mountains, a thin soil cover and
highly fractured bedrock. Ground water is obtained
primarily from the fractures in the bedrock which may
be of sedimentary, metamorphic or igneous origin.
The fractures provide only localized sources of ground
water and well yields are typically limited even
though the hydraulic conductivity may be high
because of the fractures. Due to the steep slopes, thin
soil cover and small storage capacity of the fractures,
runoff is significant and ground-water recharge is
minimal. Ground-water levels are extremely variable,
but are typically deep.
49
-------
Setting 2 A Mountain Slopes
DRASTIC Index 74
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Metamorphic/
Igneous
Thin or Absent
12-18%
Metamorphic/
Igneous
1-100
Weight
5
4
3
5
3
4
2
Rating
3
1
3
10
3
4
1
Number
15
4
9
50
9
16
2
Alluvial Basins
(2B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by thin
bouldery alluvium which overlies fractured bedrock of
sedimentary, metamorphic or igneous origin. Slopes
in the valley typically range from 2-6%. The alluvium,
which is derived from the surrounding steep slopes
serves as a localized source of water. Water levels are
moderate in depth, but because of the low rainfall,
ground-water recharge is low. Ground water may
also be obtained from the fractures in the underlying
bedrock which are typically in direct hydraulic con-
nection with the overlying alluvium.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Metamorphic/
Igneous
Thin or Absent
12-18%
Metamorphic/
Igneous
1-100
Weight
5
4
3
2
1
5
3
General
Rating
3
1
3
10
3
4
1
Number
15
4
9
.>/-
1
"0
£>
'!-'
r<
»"*/—
20 «i4
3
20
3
Setting 2 B Alluvial Mountain Valleys
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
300-700
Weight
5
4
3
2
1
5
3
Rating
5
1
8
9
9
8
4
Number
25
4
24
18
9
40
12
DRASTIC Index 132
Agricultural
DRASTIC Index 105
50
-------
Setting 2 B Alluvial Mountain Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
300-700
Weight Rating
5 5
4 1
3 8
5 9
3 9
4 8
2 4
Agricultural
DRASTIC Index
Number
25
4
24
45
27
32
8
165
Alluvial Basins
(2C) Alluvial Fans
This hydrogeologic setting is characterized by gently
sloping alluvial deposits which are coarser near the
apex in the mountains and grade toward finer deposits
in the basins. Within the alluvial deposits are layers of
sand and gravel which extend into the central parts of
the adjacent basins. The alluvial fans serve as local
sources of water and also as the recharge area for the
deposits in the adjacent basin. The portion of the fan
extending farthest into the basin may function as a
discharge area, especially during seasons when the
upper portion of the fan is receiving substantial
recharge. Discharge zones are usually related to flow
along the top of stratified clay layers. Ground water
discharge zones are less vulnerable to pollution than
recharge zones. Where the discharge/recharge rela-
tionship is reversible the greater vulnerability of the
recharge condition must be evaluated. Ground-water
levels are extremely variable, and the quantity of
water available is limited because of the low precipi-
tation and low net recharge. Ground-water depth
varies from over 100 feet near the mountains to zero
in the discharge areas. The alluvial fans are underlain
by fractured bedrock of sedimentary, metamorphic or
igneous origin which are typically in direct hydraulic
connection with the overlying deposits. Limited
supplies of ground water are available from the
fractures in the bedrock.
Setting 2 C Alluvial Fans
Feature Range
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
50-75
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
300-700
Weight
5
4
3
2
1
5
3
General
Rating
3
1
8
9
9
8
4
Number
15
4
24
18
9
40
12
Setting 2 C Alluvial Fans
DRASTIC Index 122
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
300-700
Weight
5
4
3
5
3
4
2
Rating
3
1
8
9
9
8
4
Number
15
4
24
45
27
32
8
Agricultural
DRASTIC Index
155
51
-------
Alluvial Basins
(2D) Alluvial Basins (Internal Drainage)
This hydrogeologic setting is characterized by low
topographic relief and thick deposits of unconsoli-
dated alluvial material formed by coalescing alluvial
fans. The sand and gravel deposits within the alluvium
are the major source of water in the region. The sand
and gravel is interbedded with finer grained layers of
saturated clay and silt which serve as a source of
recharge to the sand and gravel when head differ-
ences are significant. The alluvium is underlain by
fractured igneous or metamorphic rocks and consoli-
dated sedimentary rocks. Although some of the
sedimentary rocks are permeable and water may be
obtained from fractures in the crystalline bedrock, the
abundance of water in the alluvium and the greater
depth of the bedrock serves to minimize use of these
sources. Since these basins have internal drainage,
natural gradients are low near the basin centers.
Thus, the primary direction of pollutant migration,
under normal conditions, would be downward, and
outward radially from the point of incidence.
Setting 2 D Alluvial Basins (Internal Drainage)
Agricultural
Setting 2 D Alluvial Basins (Internal Drainage)
General
Feature Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
0-2
Sand and Gravel
Sand
2-6%
S & G w/sig.
Silt and Clay
300-700
5
4
3
2
1
5
3
5
1
8
9
9
6
4
25
4
24
18
9
30
12
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Sand
2-6%
S & G w/sig
Silt and Clay
300-700
Weight Rating
5 5
4 1
3 8
5 9
3 9
4 6
2 4
Agricultural
DRASTIC Index
Number
25
4
24
45
27
24
8
157
Alluvial Basins
(2E) Playa Lakes
This hydrogeologic setting is characterized by very
low topographic relief and thin layers of clays and
other fine grained sediments which overlie alluvial
deposits. The playa areas serve as a catchment for
water during periods of significant runoff; when the
precipitation event is over, the water evaporates,
leaving a crust of soluble salts on the surface. Ground
water is obtained from the layers of sand which
underly the finer-grained deposits. Water levels are
extremely variable but are typically deep. The playa
beds are significant recharge areas due to the
ground-water "mounding" that occurs seasonally
beneath the playas. The rate of recharge, as compared
to evaporation, is largely a function of the permeability
of the materials forming the bed of the playa, and the
distribution, in time, of precipitation.
DRASTIC Index 122
52
-------
Setting 2 E Playa Lakes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Sand and Gravel
Shrink/ Agg. Clay
0-2%
S & G w/sig.
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
Rating
2
1
8
7
10
6
6
Number
10
4
24
14
10
30
18
DRASTIC Index 110
Setting 2 E Playa Lakes
Feature
Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
75-100
0-2
Sand and Gravel
Shrink/Agg. Clay
0-2%
S & G w/sig.
Silt and Clay
700-1000
5
4
3
5
3
2
1
8
7
10
10
4
24
35
30
24
12
Agricultural
DRASTIC Index
139
53
-------
3. Columbia Lava Plateau
(Thick sequence of lava flows irregularly interbedded with thin unconsolidated deposits and overlain by thin
soils)
The Columbia Lava Plateau occupies an area of
366,000 km2 in northeastern California, eastern
Washington and Oregon, southern Idaho, and
northern Nevada. As its name implies, it is basically a
plateau standing at an altitude generally between
500 and 1,800 m above sea level that is underlain by a
great thickness of lava flows irregularly interbedded
with silt, sand, and other unconsolidated deposits.
The plateau is bordered on the west by the Cascade
Range, on the north by the Okanogan Highlands, and
on the east by the Rocky Mountains. On the south it
grades into the Alluvial Basins regoin, as the area
occupied by lava flows decreases and the typical
"basin and range" topography of the Alluvial Basins
region gradually prevails. Most of the plateau in Idaho
is exceptionally flat over large areas, the principal
relief being low cinder (volcanic) cones and lava
domes. This area and much of the area in California,
southeastern Oregon, and Nevada is underlain by
much of the youngest lava, someof which is less than
1,000 years old. In Washington the flows are older,
some dating back to the Miocene Epoch. Altitudes in a
few of the mountainous areas in the plateau region
exceed 3,000 m.
The great sequence of lava flows, which ranges in
thickness from less than 50 m adjacent to the
bordering mountain ranges to more than 1,000 m in
south-central Washington and southern Idaho, is the
principal water-bearing unit in the region. 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 meters to more than 50 m and average about
15m. Most of the lava is basalt which reached the
surface both through extensive fissures and through
local eruption centers. Because basaltic lava is very
fluid when molten, it flows considerable distances
down surface depressions and over gently sloping
surfaces and forms, when it solidifies, a relatively flat
surface. Some flows are sheetlike and can be followed
visually for several kilometers along the walls of steep
canyons. Other flows, where the lava issuing from
eruption centers followed surface depressions, are
lobate, or tonguelike.
The volcanic rocks yield water mainly from permeable
zones that occur at or near the contacts between
some flow layers. The origin of these flow-contact or
interflow zones is complex but involves, among other
causes, the relatively rapid cooling of the top of flows,
which results in formation of a crust. As the molten
lava beneath continues to flow, the crust may be
broken into a rubble of angular fragments which in
places contain numerous holes where gas bubbles
formed and which give the rock the appearance of a
frozen froth. The slower cooling of the central and
lower parts of the thicker flows results in a dense,
flint-like rock which in the lower part contains rela-
tively widely spaced, irregular fractures and which
grade upward into a zone containing relatively closely
54
-------
spaced vertical fractures that break the rock into a
series of hexagonal columns (Newcomb, 1961).
Periods of time ranging from less than 100 years to
thousands of years elapsed between extrusion of
successive lava flows. As a result, parts of some flows
are separated by soil zones and, at places, by sand,
silt, and clay deposited by streams or in lakes that
existed on the land surface before being buried by
subsequent lava extrusions. These sedimentary
layers, where they occur between lava flows, are
commonly referred to as "interflow sediments."
Gravel, sand, silt, and clay, partly formed by the
present streams and partly of glacial origin, 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: (1) the
area in southeastern Washington, northeastern
Oregon, andtheLewiston area of Idaho, part of which
is underlain by volcanic rocks of the Columbia River
Group; and (2) the remainder of the area, 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 along faults form subsurface dams that affect
the movement of ground water. Water reaching the
interflow zones tends to move down the dip of the
flows from fold axes and to collect undip behind faults
that are transverse to the direction of movement
(Newcomb, 1961). As a result, the basalt in parts of
the area is divided into a series of barrier-controlled
reservoirs which are only poorly connected hydraul-
ically 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 exten-
sive, highly permeable interflow zones that are
relatively effectively interconnected through a dense
network of cooling fractures. Structural barriers to
ground-water movement, such as those of the
Columbia River Group, are of minor importance. This
is demonstrated by conditions in the 44,000-square-
kilometer area of the Snake River Plain east of Bliss,
Idaho, which Nace (1958) thought might be the
largest unified ground-water reservoir on the North
American continent. (It is probable that this distinction
is held by the Floridan aquifer, which underlies an
area of 212,000 km2 in Alabama, Florida, Georgia,
and South Carolina. See region 11).
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. Hydrol-
ogists estimate that the interflow zones, which range
in thickness from about 1 m to about 8 m, account for
about 10 percent of the basalt. MacNish and Barker
(1976) have estimated, on the basis of studies in the
Walla Walla River basin in Washington and Oregon,
that the hydraulic conductivity along the flow-contact
zones may be a billion times largerthan the hydraulic
conductivity across the dense zones. The lateral
extent of individual aquifers depends on the area
covered by the different lava flows, on the presence of
dikes and other igneous intrusions, and on faults and
folds that terminate the porous zones, especially in
the Columbia River Group.
The large differences in hydraulic conductivity be-
tween 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 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. The difference in
heads between different aquifers can result in the
movement of large volumes of water between
aquifers through the open-hole (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 200 to 1,200 mm of precipitation
annually. The areas that receive the least precipita-
tion include the plateau area immediately east of the
Cascades and the Snake River Plain. The areas that
receive the largest amounts of precipitation include
the east flank of the Cascades and the areas adjacent
to the Okanogan Highlands and the Rocky Mountains.
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
55
-------
the winter and thus coincides with the cooler, non-
growing season when conditions are most favorable
for recharge. Mundorff (Columbia-North Pacific
Technical Staff, 1970) estimates that recharge may
amount to 600 mm in areas underlain by highly
permeable young lavas that receive abundant precipi-
tation. Considerable recharge also occurs by infiltra-
tion 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 evapo-
transpiration in areas where the water table is at or
near the land surface. The famous Thousand Springs
and other springs along the Snake River canyon in
southern Idaho are, in fact, among the most spec-
tacular displays of ground-water discharge in the
world.
The Columbia Lava Plateau region is mantled by
mostly thin soils developed on alluvial and wind-laid
deposits that are well suited for agriculture. Because
of the arid and semiarid climate in most of the region,
many crops require intensive irrigation. In 1970, for
example, more than 15,000 km2 (3.75 million acres)
were being irrigated on the Snake River Plain. Water
for irrigation is obtained both by diversions from
streams and by wells that tap the lava interflow
zones. Much of the water applied for irrigation
percolates downward into the lava and then moves
through the ground-water system to the Columbia
and Snake Rivers and to other streams that have
deeply entrenched channels. The effect of this "return
flow" is graphically indicated by a long-term increase
in the flow of the Thousand Springs and other large
springs along the Snake River gorge between Milner
and King Hill—from about 110m3 sec"1 in 1902, prior
to significant irrigation, to more than 225 m3 sec"1 by
1 942, after decades of irrigation on adjacent and
upstream parts of the plateau. Prior to the start of
irrigation, the water represented by this increased
flow reached the Snake River below King Hill through
tributary streams and natural ground-water dis-
charge.
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
muchas30to60 m in several areas. In most of these
areas, the declines have been slowed or stopped
through regulatory restrictions or other changes that
have reduced withdrawals. Declines are still occur-
ring, at rates as much as a few meters per year, in a
few areas.
Columbia Lava Plateau
(3A) Mountain Slopes
This hydrogeologic setting is characterized by steep
slopes on the side of mountains bordering the plateau,
a thin soil cover and fractured bedrock. Steep slopes
also occur on cinder cones within the plateau. Ground
water is obtained primarily from the fractures in the
bedrock which may be sedimentary, metamorphic or
igneous origin. The fractures provide localized
sources of ground water and well yields are typically
limited. Due to the thin soil cover, topography, and
small storage capacity of the fractures, runoff is
significant. Ground-water levels are extremely vari-
able but are typically deep. Due to lack of rainfall, low
hydraulic conductivity, and steep topography, net
recharge is very low.
Setting 3 A Mountain Slopes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
12-18%
Metamorphic/
Igneous
1000-2000
Weight
5
4
3
2
1
5
3
Rating
2
3
3
10
3
4
8
Number
10
12
9
20
3
20
24
DRASTIC Index 98
56
-------
Setting 3 A Mountain Slopes
Agricultural
Setting 3 B Alluvial Mountain Valleys
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
12-18%
Metamorphic/
Igneous
1 000-2000
Weight
5
4
3
5
3
4
2
Rating
2
3
3
10
3
4
8
Number
10
12
9
50
9
16
16
Agricultural
DRASTIC Index 122
Columbia Lava Plateau
(3B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by thin
bouldery alluvium which overlies fractured bedrock of
sedimentary, metamorphic or igneous origin. The
alluvium, which is derived from the surrounding
steep slopes serves as a localized source of water.
Water levels are typically moderate and recharge to
the ground water may be of significance. Ground
water may also be obtained from the fractures in the
underlying bedrock which are typically in direct
hydraulic connection with the overlying alluvium.
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
2-4
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1 000
Weight
5
4
3
2
1
5
3
Rating
9
3
8
10
9
8
6
Number
45
12
24
20
9
40
18
DRASTIC Index 168
Setting 3 B Alluvial Mountain Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
2-4
Sand and Gravel
Gravel
2-6%
Sand and Gravel
700-1000
Weight Rating
5 9
4 3
3 8
5 10
3 9
4 8
2 6
Agricultural
DRASTIC Index
Number
45
12
24
50
27
32
12
202
Columbia Lava Plateau
I3C) Hydraulically Connected Lava Flows
This hydrogeologic setting is characterized by low
topographic relief, a thin sandy soil cover and a thick
sequence of successive lava flows which is irregularly
interbedded with thin unconsolidated deposits. The
lava beds are underlain by poorly-permeable bedrock
of igneous, sedimentary or metamorphic origin.
Ground water is obtained primarily from the interflow
zones comprised of sequential, thin, lava flows and
related sedimentary deposits, cooling fractures, lava
tubes, and minor structural features. Water levels are
extremely variable but are typically deep. Well yields
may vary from low to extremely high depending on
the characteristics of the underlying lava flows at a
particular site. Ground-water recharge may be ap-
preciable because the layers of lava are intercon-
nected hydraulically. This setting is characterized by
the deposits that occur in southwestern Idaho (Snake
River area), northern Nevada, southeastern Oregon,
and extreme northeastern California, which are of
Pliocene to Holocene age.
57
-------
Setting 3 C Hydraulically Connected Lava Flows
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
2-4
Basalt
Sand
2-6%
Basalt
2000+
Weight
5
4
3
2
1
5
3
Rating
3
3
9
9
9
9
10
Number
15
12
27
18
9
45
30
DRASTIC Index 156
Setting 3 C Hydraulically Connected Lava Flows
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
2-4
Basalt
Sand
2-6%
Basalt
2000+
Weight
5
4
3
5
3
4
2
Rating
3
3
9
9
9
9
10
Number
15
12
27
45
27
36
20
Agricultural
DRASTIC Index
182
Columbia Lava Plateau
(3D) Lava Flows Not Connected Hydraulically
This hydrogeologic setting is characterized by low
topographic relief, a thin cover of gravel, sand, silt,
and clay of stream and glacial origin and a sequence
of thick lava flows irregularly interbedded with
unconsolidated deposits, which have been deformed
into a series of folds and normal faults. The lava
sequence is underlain by poorly-permeable bedrock
of igneous, sedimentary, or metamorphic origin.
Ground water is obtained primarily from the interflow
zones of sedimentary deposits and cooling fractures
which occur between successive layers of lava.
Water levels are extremely variable, but are typically
deep. The presence of thick impermeable zones may
produce perched water-table conditions or disrupt
the hydraulic continuity of water bearing zones. The
flow of ground water is controlled by locally offset
normal faults which form a series of hydraulically
poorly-connected reservoirs. This setting is charac-
terized by deposits that occur in the Columbia River
area in southern Washington, northern Oregon, and
northern Idaho which are Miocene to Pliocene (?) in
age.
Setting 3 D Lava Flows Not Connected Hydraulically
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
2-4
Thin Bedded SS,
LS, SH Sequences
Sand
2-6%
Bedded LS, SS,
SH
1-100
Weight
5
4
3
2
1
5
3
Rating
3
3
6
9
9
6
1
Number
15
12
18
18
9
30
3
DRASTIC Index 105
58
-------
Setting 3 D Lava Flows Not Connected Hydraulically
Agricultural
Feature
Setting 3 E Alluvial Fans
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
50-75
2-4
Thin Bedded SS,
LS.SH Sequences
Sand
2-6%
5
4
3
5
3
3
3
6
9
9
15
12
18
45
27
Impact Vadose
Zone Bedded LS, SS,
SH
Hydraulic
Conductivity 1-100
4 6
2 1
Agricultural
DRASTIC Index
24
2
143
Columbia Lava Plateau
(3E) Alluvial Fans
This hydrogeologic setting is characterized by alluvial
sediments which are thickest near the mountain
slopes and thin toward the interior basin. Topography
is steep to moderate. Fan sediments range from
coarse, unsorted debris on the upper slopes grading
to well-sorted and stratified gravels, sands, and clays.
Recharge is a function of precipitation and evapora-
tion, sincethe permeability ofthesurface materials is
usually high. Ground-water movement is generally
unidirectional from the adjacent highlands toward
the basin. Depth to ground water is generally
moderate to deep. These fans may serve as local
sources of water and also as the recharge area for the
deposits in the adjacent basin and the lower extrem-
ities may serve as discharge areas to local streams.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Sand and Gravel
Sandy Loam
6-12%
S & G w/sig
Silt and Clay
300-700
Weight
5
4
3
2
1
5
3
General
Rating
2
3
8
6
5
6
4
Number
10
12
24
12
5
30
12
Setting 3 E Alluvial Fans
Feature
Range
DRASTIC Index 105
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
75-100
2-4
Sand and Gravel
Sandy Loam
6-12%
S & G w/sig
Silt and Clay
300-700
5
4
3
5
3
10
12
24
30
15
24
Agricultural
DRASTIC Index
123
59
-------
4. Colorado Plateau and Wyoming Basin
(Thin soils over consolidated sedimentary rocks)
The Colorado Plateau and Wyoming Basin region
occupies an area of 414,000 km2 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 2,500 to
3,500 m and underlain by essentially horizontal to
gently dipping layers of consolidated sedimentary
rocks. 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. The northern part of the region—the part
occupied by the Wyoming Basin—borders the Non-
glaciated Central region at the break in the Rocky
Mountains between the Laramie Range and the
Bighorn Mountains. The region contains small,
isolated mountain ranges, the most prominent being
the Henry Mountains and the La Sal Mountains in
southeastern Utah. It also contains, rather widely
scattered over the region, extinct volcanoes and lava
fields, the most prominent example being the San
Francisco Mountains in north-central Arizona.
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 Coloradothe
rock units include thick deposits of sodium- and
potassium-bearing minerals, principally halite (so-
dium chloride). The sandstones and shales are most
prevalent and most extensive in occurrence. The
sandstones are the principal sources of ground water
in the region and contain water in fractures developed
both along bedding planes and across the beds and in
interconnected pores. The most productive sand-
stones are those in which calcium carbonate or other
cementing material has been deposited only around
the point of contact of the sand grains. Thus, many of
the sandstones 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
ground water occur along parts of the valleys of major
streams, especially adjacent to the mountain ranges
in the northern and eastern parts of the region. These
deposits are partly of glacial origin. In most of the
remainder of the region there are large expanses of
exposed bedrock, and the soils, where present, are
thin and rocky.
Erosion has produced extensive lines of prominent
cliffs in the region. The tops of these cliffs are
generally underlain and protected by resistant sand-
stones. Erosion of the domes has produced a series of
concentric, steeply dipping ridges, also developed on
the more resistant sandstones.
Recharge of the sandstone aquifers occurs where
they are exposed above the cliffs and in the ridges.
60
-------
Average precipitation ranges from about 150 mm in
the lower areas to about 1,000 mm 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 condition described in the preceding paragraph,
whereby intermittent streams serve as sources of
ground-water recharge and perennial streams serve
as lines of ground-water discharge, is relatively
common in this region and in the Alluvial Basins
region to the south and west. Streams into which
ground water discharges are referred to as gaining
streams. Conversely, streams that recharge ground-
water systems are referred to as losing streams. The
gaining streams and the losing streams may be
different streams. However, in many areas the same
stream may be a gaining stream in its headwaters,
especially where these drain the wetter mountainous
areas, become a losing stream as it flows onto the
adjoining lower areas, and, ultimately, become a
gaining stream again in its lowermost reaches where
it serves as a regional drain.
The quantity of water available for recharge is small,
but so are the porosity and the transmissivity of most
of the sandstone aquifers. Because of the general
absence of a thick cover of unconsolidated rock in the
recharge areas, there is relatively I ittle opportunity for
such materials to serve as a storage reservoir for the
underlying bedrock. The water in the sandstone
aquifers is unconfined in the recharge areas and is
confined down-dip. Because most of the sandstones
are consolidated, the storage coefficient in the
confined parts of the aquifers is very small. This small
storage coefficient together with the small trans-
missivities, results in even smaller rates of with-
drawal causing extensive cones of depression around
pumping wells.
Springs exist at places near the base of the sandstone
aquifers where they crop out along the sides of
canyons. Discharge from the springs results in
dewatering the upper parts of the aquifers for some
distance back from the canyon walls.
The Colorado Plateau and Wyoming Basin is a dry,
sparsely populated region in which most water
supplies are obtained from the perennial streams that
flow across it from the bordering mountains. 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 pene-
trate to depths of a few hundred meters 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 rocks, mineralized (saline)
water—that is, water containing more than 1,000
mg/l of dissolved solids—is widespread in occur-
rence. Most of the shales and siltstones contain
mineralized water throughout the region and below
altitudes of about 2,000 m. Freshwater—water
containing less than 1,000 mg/l of dissolved solids—
occurs only in the most permeable sandstones and
limestones. Much of the mineralized water is due to
the solution of gypsum and halite by water circulating
through beds that contain these minerals. Although
the aquifers that contain mineralized water are
commonly overlain by aquifers containing freshwater,
this situation is reversed in a few places where
aquifers containing mineralized water are underlain
by more permeable aquifers containing freshwater.
Colorado Plateau and Wyoming Basin
(4A) Resistant Ridges
This hydrogeologic setting is characterized by moder-
ate to steep slopes, and a very thin soil cover which
overlies dipping fractured consolidated sedimentary
rocks. The resistant sandstones cap the cliffs and
ridges and form hogbacks. These same sandstone
units comprise the aquifers that are the principal
sources of ground water. The aquifers receive
recharge in the areas where the sandstone is exposed
at the surface. Recharge is low because of the
topography and the lack of precipitation in the area.
Water levels are extremely variable, but are typically
deep.
61
-------
Setting 4 A Resistant Ridges
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS, SS,
SH
1-100
Weight
5
4
3
2
1
5
3
Rating
2
1
6
10
3
6
1
Number
10
4
18
20
3
30
3
layer which commonly weathers to a sandy loam. The
sandstones serve as the principal source of ground
water. The water is obtained from fractures developed
along bedding planes and from within the pore
spaces. Water levels are typically deep and recharge
is low because of the lack of precipitation. Intermittent
streams often serve as sources of recharge, however,
the major source of recharge occurs in the resistant
ridges where the bedrock is exposed. The sandstones
may be also confined, with small storage values and
low yield wells.
DRASTIC Index 88
Setting 4 A Resistant Ridges
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS, SS,
SH
1-100
Weight Rating
5 2
4 1
3 6
5 10
3 3
4 6
2 1
Agricultural
DRASTIC Index
Number
10
4
18
50
9
24
2
117
Setting 4 B Consolidated Sedimentary Rocks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
6-12%
Bedded LS, SS,
SH
1-100
Weight
5
4
3
2
1
5
3
Rating
3
1
6
6
5
6
1
Number
15
4
18
12
5
30
3
DRASTIC Index 87
Colorado Plateau and Wyoming Basin
(4B) Consolidated Sedimentary Rocks
This hydrogeologic setting is characterized by alter-
nating layers of moderately-dipping, fractured, con-
solidated, sedimentary rocks covered by a sandy soil
62
-------
Setting 4 B Consolidated Sedimentary Rocks
Agricultural
Feature
Setting 4 C River Alluvium
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
50-75
0-2
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
6-12%
Bedded LS, SS,
SH
1-100
5 3
4 1
3 6
5 6
3 5
4 6
2 1
Agricultural
DRASTIC Index
15
4
18
30
15
24
2
108
Colorado Plateau and Wyoming Basin
(4 C) River A lluvium
This hydrogeologic setting is characterized by low
topography and deposits of alluvium along parts of
valleys of perennial and intermittent streams. The
alluvium yields small to moderate supplies of ground
water. Water is obtained from sand and gravel layers
which are interbedded with finer-grained alluvial
deposits; these are usually in direct hydraulic contact
with the perennial or intermittent stream. Water
levels are extremely variable but are commonly
moderately shallow. Although precipitation is low,
recharge is significant due to the low topography and
sandy loam soil cover. The alluvium is underlain by
consolidated sedimentary rocks which are often in
direct hydraulic connection with the overlying de-
posits.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig
Silt and Clay
700-1000
Weight
5
4
3
2
1
5
3
General
Rating
9
6
8
6
9
6
6
Number
45
24
24
12
9
30
18
DRASTIC Index 162
Setting 4 C River Alluvium
Feature Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
4-7
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig
Silt and Clay
700-1000
5 9
4 6
3 8
5 6
3 9
4 6
2 6
Agricultural
DRASTIC Index
45
24
24
30
27
24
12
186
Colorado Plateau and Wyoming Basin
(4D) Alluvium and Dune Sand
This hydrogeologic setting is characterized by moder-
ate topography derived from unconsolidated alluvial
sediments that have formed under various deposi-
tional environments. These alluvial deposits vary
from lacustrine deposits in the Wyoming Basin area
to dune sands in the Navajo area of northern Arizona
and northwestern New Mexico. Much of the entire
region is covered by thin alluvium. The hydraulic
conductivity of the alluvium is high throughout the
area, including the sand dunes portion. Recharge is
limited by low precipitation and evaporation. The
alluvium serves as moderate water supplies in some
areas; provides some discharge to streams, and acts
as storage for recharge to deeper aquifers.
63
-------
Setting 4 D Alluvium and Dune Sand
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Sand and Gravel
Sand
6-12%
S & G w/sig.
Silt and Clay
100-300
Weight
5
4
3
2
1
5
3
Rating
3
1
8
9
5
6
2
Number
15
4
24
18
5
30
6
DRASTIC Index 102
Setting 4 O Alluvium and Dune Sand
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Sand and Gravel
Sand
6-12%
S & G w/sig.
Silt and Clay
1 00-300
Weight
5
4
3
5
3
4
2
Rating
3
1
8
9
5
6
2
Number
15
4
24
45
15
24
4
• Agricultural
DRASTIC Index
131
64
-------
5. High Plains
(Thick alluvial deposits over fractured sedimentary rocks)
The High Plains region occupies an area of 450,000
km2 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. The plain originally extended from the
foot of the mountains to a terminous some hundreds
of kilometers east of its present edge. Erosion by
streams 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 2,000 m near the Rocky Mountains to
about 500 m 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. Some geologists
believe these depressions are due to solution of
soluble materials by percolating water and accom-
panying compaction of the alluvium. 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 intensively 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 meters to more than 200 m and
consists of poorly sorted and generally unconsolidated
clay, silt, sand, and gravel.
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 Quaternary deposits are composed
largely of material derived from the Ogallala and
consist of alluvial deposits of gravel, sand, silt, and
clay and extensive areas of sand dunes. The most
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 thus form a part of the High
Plains aquifer, includetheArikaree Group of Miocene
age and a small part of the underlying Brule Forma-
tion. The Arikaree Group underlies the Ogallala in
parts of western Nebraska, southwestern South
Dakota, southeastern Wyoming, and northeastern
65
-------
Colorado. It is predominantly a massive, very fine to
fine-grained sandstone that locally contains beds of
volcanic ash, siIty sand, and sandy clay. The maximum
thickness of the Arikaree is about 300 m, in western
Nebraska. The Brule Formation of Oligocene age
underliesthe 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
hydrauhcally 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, of Permian age, underlies parts of northeastern
Texas, western Oklahoma, and central Kansas and
contains layers of relatively soluble minerals includ-
ing gypsum, anhydrite, and halite (common salt)
which are dissolved by circulating ground water.
Thus, water from the rocks of Permian age is relatively
highly mineralized and not usable for irrigation and
other purposes that require freshwater. The older
formations in the remainder of the area contain
fractured sandstones and limestones interconnected
in parts of the area with the High Plains aquifer.
Although these formations yield freshwater, they are
not widely used as water sources.
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 400 mm along
the western boundary of the region to about 600 mm
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
also varies in a north-south direction. The average
annual rate of recharge has been determined to range
from about 5 mm in Texas and New Mexico to about
100 mm in the Sand Hills in Nebraska. This large
difference is explained by differences in evaporation
and transpiration and by differences in the permeabil-
ity of the surficial materials.
In some parts of the High Plains, especially in the
southern part, the near-surface layers of the Ogallala
have been cemented with lime (calcium carbonate) to
form a material of relatively low permeability called
caliche. Precipitation on areas underlain by caliche
soaks slowly into the ground. Much of this precipita-
tion collects in playas that are underlain by silt and
clay, which hamper infiltration, with the result that
most of the water is lost to evaporation. During years
of average or below average precipitation, all or
nearly all of the precipitation is returned to the
atmosphere by evapotranspiration. Thus, it is only
during years of excessive precipitation that significant
recharge occurs and this, as noted above, averages
only about 5 mm per year in the southern part of the
High Plains.
In the Sand Hills area, the lower evaporation and
transpiration and the permeable sandy soil results in
about 20 percent of the precipitation (or about 100
mm annually) reaching the water table as recharge.
The water table of the High Plains aquifer has a
general slope toward the east of about 2 to 3 m per km
(10 to 15 ft per mile). Gutentag and Weeks (1980)
estimate, on the basis of the average hydraulic
gradient and aquifer characteristics, that water moves
through the aquifer at a rate of about 0.3 m (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 meters 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. Gutentag and Weeks
(1980) estimate that in 1977 about 3.7 x 1010 m3
(30,000,000 acre-ft) of water was pumped from more
than 168,000 wells to irrigate about 65,600 km2
(16,210,000 acres). Most of this water is derived from
ground-water storage, resulting in a long-term con-
tinuing decline in ground-water levels in parts of the
region of as much as 1 m per year. The lowering of the
water table has resulted in a 10 to 50 percent
reduction in the saturated thickness of the High
Plains aquifer in an area of 130,000 km2 (12,000 mi2).
The largest reductions have occurred in the Texas
panhandle and in parts of Kansas and New Mexico.
The depletion of ground-water storage in the High
Plains, as reflected in the decline in the water table
and the reduction in the saturated thickness, 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 1 5
percent of the total volume of saturated material, that
the available (usable) storage in 1980 was about 4 x
1012 m3 (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.
66
-------
High Plains
(5A) Ogallala
This hydrogeologic setting is characterized by moder-
ately flat topography and thick deposits of poorly-
sorted, semi-consolidated, clay, silt, sand and gravel
that may be underlain by fractured sedimentary rock
which are in hydraulic connection with overlying
deposits. In some parts of the High Plains, especially
in the southern part, shallow zones of the unconsoli-
dated deposits have been cemented with calcium
carbonate. The permeability of this caliche layer
varies with the degree of cementation, fracturing, and
clay mineral content. Precipitation averages less than
20 inches per year and recharge is very low through-
out most of this water deficient area. The bedrock and
the overlying semi-consolidated deposits both serve
as extensive sources of ground water. Water levels
are typically deep, but extremely variable. The
Ogallala is underlain by bedded, unconsolidated
deposits of fractured sandstone, volcanic ash, silty
sand, sandy clay, and shales. These formations are
hydraulically connected to the Ogallala, and the
overlying alluvium, from which they derive their
recharge.
Setting 5 A Ogallala
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Sand and Gravel
Shrink/Agg. Clay
2-6%
S & G w/sig.
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
General
Rating
2
1
8
7
9
6
6
Number
10
4
24
14
9
30
18
Setting 5 A Ogallala
Feature Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
75-100
0-2
Sand and Gravel
Shrink/ Agg. Clay
2-6%
S & G w/sig.
Silt and Clay
700-1000
5 2
4 1
3 8
5 7
3 9
4 6
2 6
Agricultural
DRASTIC Index
10
4
24
35
27
24
12
136
High Plains
(SB) Alluvium
This hydrogeologic setting is characterized by low to
moderate relief, and is comprised of gravel, sand, silt,
and clay alluvial sediments. These deposits are
variable in thickness. They form, where saturated, a
portion of the High Plains aquifer, and locally all of it
where the Ogallala is missing. Water levels are
variable, but typically deep. Recharge is limited
throughout most of the area by low precipitation. The
shallow caliche layer of cemented, unconsolidated
deposits also develops in the alluvium in some
localities. Similar to the Ogallala, recharge to the
deeper sandstones is through the alluvial deposits.
DRASTIC Index 109
67
-------
Setting 5 B Alluvium
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Sand and Gravel
Sandy Loam
0-2%
S & G w/sig.
Silt and Clay
300-700
Weight
5
4
3
2
1
5
3
General
Rating
3
1
8
6
10
6
4
Number
15
4
24
12
10
30
12
Setting 5 B Alluvium
DRASTIC Index 107
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
50-75
0-2
Sand and Gravel
Sandy Loam
0-2%
S & G w/sig.
Silt and Clay
300-700
Weight Rating
5 3
4 1
3 8
5 6
3 10
4 6
2 4
Agricultural
DRASTIC Index
Number
15
4
24
30
30
24
8
135
Setting 5 C Sand Dunes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
2000+
Weight
5
4
3
2
1
5
3
Rating
5
1
8
9
9
8
10
Number
25
4
24
18
9
40
30
DRASTIC Index 150
High Plains
(5C) Sand Dunes
This hydrogeologic setting is characterized by hilly
topography comprised of sand dunes which overlie
thick poorly-sorted sand and gravel deposits. The
sand dunes are in direct hydraulic connection with
the underlying deposits. Because of their relatively
low water table, these dunes do not serve as sources
of ground water, but serve as local recharge areas. In
contrast to other areas of the High Plains, recharge
rates are higher due to lower evaporation and
permeable sandy soils, but are limited by available
precipitation.
Setting 5 C Sand Dunes
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Sand and Gravel
Sand
2-6%
Sand and Gravel
2000+
Weight Rating
5 5
4 1
3 8
5 9
3 9
4 8
2 10
Agricultural
DRASTIC Index
Number
25
4
24
45
27
32
20
177
High Plains
(5D) Play a Lakes
This hydrogeologic setting is characterized by low
topographic relief and thin layers of clays and other
fine grained sediments which overlie the alluvial
68
-------
deposits. The playa areas serve as a catchment for
water during periods of significant runoff. Ground
water is obtained from the layers of sand which
underlie the finer-grained deposits. Water levels are
extremely variable, but are typically deep. The playa
beds are significant recharge areas due to the rainfall
that collects in them. The rate of recharge, as
compared to evaporation, is largely a function of the
permeability of the materials forming the bed of the
playa, and the distribution, in time, of precipitation.
Setting 5 D Playa Lakes
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
0-2
Sand and Gravel
Shrmk/Agg. Clay
0-2%
S & G w/sig
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
General
Rating
2
1
8
7
10
6
6
Number
10
4
24
14
10
30
18
Setting S D Playa Lakes
Feature Range
DRASTIC Index 110
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
75-100
0-2
Sand and Gravel
Shrink/ Agg. Clay
0-2%
S & G w/sig.
Silt and Clay
700-1000
5 2
4 1
3 8
5 7
3 10
4 6
2 6
Agricultural
DRASTIC Index
10
4
24
35
30
24
12
139
High Plains
(5E) Braided River Deposits
This hydrogeologic setting is characterized by deposits
of alluvium which occur within the flood plain of
streams and rivers. The stream is characterized by a
low gradient, wide channel and a series of inter-
connected shallow channels which form a braided
pattern. Water levels are typically shallow, and some
streams may be intermittent. The river alluvium
sometimes serves as a significant source of ground
water but is most important as a source of recharge
since it overlies more productive semi-consolidated
deposits. The underlying deposits are in direct
hydraulic connection with the overlying alluvium, so
the potential for pollution of the aquifer is high.
Although precipitation, which averages less than 20
inches per year is a limiting factor, recharge may be
very high due to seasonal or perennial stream flow on
these very permeable deposits.
Setting 5 E Braided River Deposits
Feature Range
General
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
4-7
Sand and Gravel
Sand
0-2%
Sand and Gravel
1 000-2000
5
4
3
2
1
5
3
9
6
8
9
10
8
8
45
24
24
18
10
40
24
DRASTIC Index 185
69
-------
Setting 5 E Braided River Deposits
Agricultural
Feature Range Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
4-7
Sand and Gravel
Sand
0-2%
Sand and Gravel
1 000-2000
5
4
3
5
3
4
2
9
6
8
9
10
8
8
Agricultural
DRASTIC Index
45
24
24
45
30
32
16
216
70
-------
6. Nonglaciated Central Region
(Thin regolith over fractured sedimentary rocks)
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 theTriassic
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 a topo-
graphically complex area that ranges from the Valley
and Ridge section of the Appalachian Mountains on
the east westward across the Great Plains to the foot
of the Rocky Mountains. It includes, among other hilly
and mountainous areas, the Ozark Plateaus in
Missouri and Arkansas. Altitudes range from 150 m
above sea level in central Tennessee and Kentucky to
1,500 m along the western boundary of the region.
The region is also 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, carbonate rocks (lime-
stone and dolomite), and conglomerate. A small area
in Texas and western Oklahoma is underlain by
gypsum. Throughout most of the region the rock
layers are horizontal or gently dipping. Principal
exceptions are the Valley and Ridge section of 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. Around the Black Hills and along
the eastern side of the Rocky Mountains the rock
layers have been bent up sharply toward the moun-
tains 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. These basins were formed in
Triassic time when major faults in the crystalline
rocks of the Piedmont resulted in the formation of
structural depressions up to several thousand meters
deep and more than 25 km wide and 140 km long.
The nonglaciated Central region is an area of about
1,737,000 km2 extending from the Appalachian
Mountains on the east to the Rocky Mountains on the
The land surface in most of the region is underlain by
regolith formed by chemical and mechanical break-
down of the bedrock. In the western part of the Great
71
-------
Plains the residual soils are overlain by or intermixed
with eolian (wind-laid) deposits. The thickness and
composition of the regolith depend on the composi-
tion and structure of the parent rock and on the
climate, land cover, and topography. In areas under-
lain by relatively pure limestone, the regolith consists
mostly of clay and is generally only a few meters thick.
Where the limestones contain chert and in areas
underlain by shale and sandstone, the regolith is
thicker, up to 30 m 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.
Recharge of the ground-water system in this region
occurs primarily in the outcrop areas of the bedrock
aquifers in the uplands between streams. Precipita-
tion inthe region ranges from about 400 mm per year
in the western part to more than 1,200 mm in the
eastern part. This wide difference in precipitation is
reflected in recharge rates, which range from about 5
mm per year in west Texas and New Mexico to as
much as 500 mm 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 in areas where the
water table is within a few meters of land surface.
The principal water-bearing openings in the bedrock
are fractures along which the rocks have been broken
by stresses imposed on the Earth's crust at different
times since the rocks were consolidated. The fractures
generally occur in three sets. The first set, andtheone
that is probably of greatest importance from the
standpoint of ground water and well yields, consists
of fractures developed along the contact between
different rock layers, in other words, along bedding
planes. Where the sedimentary layers making up the
bedrock are essentially horizontal, the bedding-plane
fractures are more or less parallel to the land surface.
The two remaining sets of fractures 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. For example, in parts of the region one set
of vertical fractures is oriented in a northwest-
southeast direction and the other set in a northeast-
southwest direction. The vertical fractures facilitate
movement of water across the rock layers and thus
serve as the principal hydraulic connection between
the bedding-plane fractures.
In parts of the region in which the bedrock has been
folded or bent, the occurrence and orientation of
fractures are more complex. In these areas the dip of
the rock layers and the associated bedding-plane
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 a millimeter wide. The principal exception occurs
in limestones and dolomites, which are more soluble
in water than most other rocks. 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; one in this region is Big Spring, in Missouri,
which has an average discharge of 36.8 m3 sec"1.
The yield of wells depends on (1) 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 0.01 to 1 m3 min~1
(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 lime-
stone, 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. The
Edwards Plateau in Texas is bounded on the south by
the Balcones Fault Zone, in which limestone and
dolomite up to 1 50 m in thickness has been exten-
sively faulted. The faulting has facilitated the devel-
opment of solution openings which makes this zone
one of the most productive aquifers in the country.
Wells of the City of San Antonio are located in thrs
zone; individually, they have yields of more than 60
m3
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 Moun-
tains, and the Ridge and Valley section, the water in
the bedrock contains more than 1,000 mg/l of dis-
solved solids at depths less than 1 50 m. Most of the
salty water is believed to be connate— that is, it was
trapped in the rocks when they emerged from the sea
in which they were deposited. Other possible sources
include: (1 ) seawater that entered the rocks during a
later time when the land again was beneath the sea;
and (2) salty water derived from solution of salt beds
that underlie parts of the region.
72
-------
The presence of connate water at relatively shallow
depths is doubtless due to several factors, including,
in the western part of the area, a semiarid climate
and, consequently, a small rate of recharge. Other
factors probably include an extremely slow rate of
ground-water circulation at depths greater than a few
hundred meters.
Setting 6 A Mountain Slopes
General
Non-Glaciated Central
(6A) Mountain Slopes
This hydrogeologic setting is characterized by rela-
tively steep slopes on the side of mountains or hills, a
thin soil cover and fractured bedrock. Ground water is
obtained primarily from the fractures in the bedrock
which may be of sedimentary, metamorphic or
igneous origin but which are commonly alternating
sedimentary layers, and also from bedding planes
between the sedimentary layers. The fractures pro-
vide only localized sources of ground water and well
yields are typically limited. Although precipitation
may be significant in some areas, due to the steep
slopes, thin soil cover, and small storage capacity of
the fractures, runoff is significant and ground-water
recharge is low. Water levels are extremely variable
but are commonly moderately deep. Perched ground-
water zones are common. These sedimentary rocks
may range in attitude from nearly horizontal, as in
parts of the western Appalachian Plateau, to steeply
dipping, as seen in the Valley and Ridge province, the
Wichita, Arbuckle, and Ouachita Mountains, the
Black Hills, and on the eastern slopes of the Rockies.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
2
1
5
3
Rating
5
1
6
10
3
6
1
Number
25
4
18
20
3
30
3
DRASTIC Index 103
Setting 6 A Mountain Slopes
Feature Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS,
SS, SH
1-100
5 5
4 1
3 6
5 10
3 3
4 6
2 1
Agricultural
DRASTIC Index
25
4
18
50
9
24
2
132
Non-Glaciated Central
(6B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by thin
bouldery alluvium which overlies fractured bedrock of
sedimentary, metamorphic or igneous origin but
which is commonly comprised of alternating sedi-
mentary layers. The alluvium, which is derived from
the surrounding slopes, serves as a localized source
of water. Water is obtained from sand and gravel
layers which are interspersed between finer-grained
deposits. Surficial deposits have typically weathered
to a sandy loam. Water levels are relatively shallow
but may be extremely variable. Ground water may
also be obtained from the fractures in the underlying
bedrock which are typically in direct hydraulic con-
nection with the overlying alluvium.
73
-------
Non-Glaciated Central
(6C) Mountain Flanks
This hydrogeologic setting is characterized by moder-
ate topographic relief and moderately-dipping, frac-
tured, consolidated, sedimentary rocks. Soil cover is
usually thicker than on the mountain slopes and
typically has weathered to a sandy loam. Although
precipitation can be significant, ground-water re-
charge is only moderate due to the slope. Water levels
are typically moderately deep although they are
extremely variable. The mountain flanks serve as the
recharge area for aquifers which are confined in
adjacent areas. Ground water is obtained from the
permeable sedimentary rocks or from fractures in the
sedimentary rocks. The sedimentary rocks may be
underlain by fractured bedrock of igneous, meta-
morphic or sedimentary origin which yield little water.
Sedimentary beds may be either horizontal or dipping,
as indicated for the higher mountain slopes (6A), and
have a similar geographic distribution.
Setting 6 B Alluvial Mountain Valleys
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
General
Rating
7
6
8
6
9
6
6
Number
35
24
24
12
9
30
18
DRASTIC Index 152
Setting 6 B Alluvial Mountain Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
700-1 000
Weight
5
4
3
5
3
4
2
Rating
7
6
8
6
9
6
6
Number
35
24
24
30
27
24
12
Agricultural
DRASTIC Index
176
Setting 6 C Mountain Flanks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
6-12%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
2
1
5
3
Rating
5
3
6
6
5
6
1
Number
25
12
18
12
5
30
3
DRASTIC Index 105
74
-------
Setting 6 C Mountain Flanks
Agricultural
Setting 6 Da Alternating SS, LS, SH—Thin Soil
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
IS, SH Sequences
Sandy Loam
6-12%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
5
3
4
2
Rating
5
3
6
6
5
6
1
Number
25
12
18
30
15
24
2
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin Bedded SS,
LS, SH Sequences
Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
2
1
5
3
Rating
7
6
6
5
9
6
1
Number
35
24
18
10
9
30
3
Agricultural
DRASTIC Index
126
DRASTIC Index
129
Non-Glaciated Central
(6Da) Alternating SS, LS, and SH—Thin Soil
\
This hydrogeologic setting is characterized by low to
moderate topographic relief, relatively thin loamy
soils overlying horizontal or slightly dipping alter-
nating layers of fractured consolidated sedimentary
rocks. Ground water is obtained primarily from
fractures along bedding planes or intersecting vertical
fractures. Precipitation varies widely in the region,
but recharge is moderate where precipitation is
adequate. Water levels are extremely variable but on
the average moderately shallow. Shale or clayey
layers often form aquitards, and where sufficient
relief is present, perched ground-water zones of local
domestic importance are often developed.
Setting 6 Da Alternating SS, LS, SH—Thin Soil
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin Bedded SS,
LS.SH Sequences
Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
5
3
4
2
Rating
7
6
6
5
9
6
1
Number
35
24
18
25
27
24
2
Agricultural
DRASTIC Index
155
Non-Glaciated Central
(6Db) Alternating SS, LS, and SH—Deep Regolith
This hydrogeologic setting is identical to 6Da Alter-
nating SS, LS, and SH—Thin Soil except that the
surficial deposits typically have been weathered to
form clay loams which grade into weathered bedrock
which help retard the movement of pollutants through
the ground to the water table. These thick soil
deposits are usually in direct, hydraulic connection
with the underlying fractured sedimentary deposits.
75
-------
Setting 6 Db Alternating SS, LS, SH— Deep
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
2
1
5
3
Regolith
General
Rating
7
6
6
6
9
6
1
Number
35
24
18
12
9
30
3
DRASTIC Index 131
Setting 6 Db Alternating SS, LS, SH-
-Deep Regolith
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight Rating
5 7
4 6
3 6
5 6
3 9
4 6
2 1
Agricultural
DRASTIC Index
Number
35
24
18
30
27
24
2
160
Non-Glaciated Central
(6E) Solution Limestone
This hydrogeologic setting is characterized by moder-
ate, but variable, topographic relief and deposits of
limestone which have been partially dissolved along
bedding and fracture planes to form a network of
solution cavities and caves. Soil is usually thin or
absent, but where present is commonly a clayey
loam. Recharge is usually greater than 10 inches per
year because the region receives significant amounts
of rainfall which is easily recharged through the
solution channels. Runoff return through solution
channels into surface watercourses is sometimes
very high. Water levels are typically moderately deep.
The limestone serves as a significant source of
ground water because of the high hydraulic conduc-
tivity of the solution channels. Caves related to this
setting are widespread, but their greatest concentra-
tion occurs in a band 200-400 miles wide extending
from central Missouri through western Virginia.
Setting 6 E Solution Limestone
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
10+
Karst Limestone
Thin or Absent
6-12%
Karst Limestone
2000+
Weight
5
4
3
2
1
5
3
Rating
5
9
10
10
5
10
10
Number
25
36
30
20
5
50
30
DRASTIC Index 196
76
-------
Setting 6 E Solution Limestone
Feature Range
Agricultural
Setting 6 Fa River Alluvium With Overbank
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
10+
Karst Limestone
Thin or Absent
6-12%
Karst Limestone
2000+
5 5
4 9
3 10
5 10
3 5
4 10
2 10
Agricultural
DRASTIC Index
25
36
30
50
15
40
20
216
Non-Glaciated Central
(6Fa) River Alluvium With Overbank
This hydrogeologic setting is characterized by low
topography and deposits of alluvium along parts of
stream valleys. Water is obtained from sand and
gravel layers which are interbedded with finer grained
alluvial deposits. The floodplain is covered by varying
thicknesses of fine grained silt and clay called
overbank deposits. The overbank thickness is usually
thicker along major streams (commonly as much as
40 feet), and thinner along minor streams. Precipita-
tion varies widely over the region, but recharge is
somewhat reduced because of the impermeable
nature of the overbank deposits and subsequent
clayey loam soils which typically cover the surface.
There is usually substantial recharge, however, due
to infiltration from the associated stream. Water
levels are typically moderately shallow. The alluvium
is commonly in direct hydraulic connection with the
underlying sedimentary rocks.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1000-2000
Weight
5
4
3
2
1
5
3
General
Rating
7
8
8
3
10
1
8
Number
35
32
24
6
10
5
24
DRASTIC Index 136
Setting 6 Fa River Alluvium With Overbank
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1 000-2000
Weight
5
4
3
5
3
4
2
Rating
7
8
8
3
10
1
8
Number
35
32
24
15
30
4
16
Agricultural
DRASTIC Index
156
Non-Glaciated Central
(6Fb) River Alluvium Without Overbank
This setting is identical to 6Fa River Alluvium with
Overbank except that no significant fine grained
floodplain deposits occupy the stream valley. This
results in significantly higher recharge where precipi-
tation is adequate and sandy loam soils occur at the
surface. Water levels are typically closer to the
surface because the fine-grained overbank deposits
are not present.
77
-------
Setting 6 Fb River Alluvium Without Overbank
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
0-2%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
9
8
8
6
10
8
8
Number
45
32
24
12
10
40
24
DRASTIC Index 187
Setting 6 Fb River Alluvium Without Overbank
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
0-2%
Sand and Gravel
1 6*00-2000
Weight Rating
5 9
4 8
3 8
5 6
3 10
4 8
2 8
Agricultural
DRASTIC Index
Number
45
32
24
30
30
32
16
209
Setting 6 G Braided River Deposits
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
4-7
Sand and Gravel
Sand
0-2%
Sand and Gravel
1000-2000
Weight
5
4
3
2
1
5
3
General
Rating
10
6
8
9
10
8
8
Number
50
24
24
18
10
40
24
DRASTIC Index 190
Non-Glaciated Central
(6G) Braided River Deposits
This hydrogeologic setting is characterized by deposits
of alluvium which occur within the floodplain of
streams and rivers. The stream is characterized by a
low gradient, wide channel and series of intercon-
nected shallow channels which form a braided
pattern. Water levels are typically shallow. This
setting is found only in the western portion of this
ground-water region. The river alluvium does not
serve as a significant source of ground water where it
overlies more productive semi-consolidated deposits.
However, recharge from the river is substantial and
the underlying deposits are in direct hydraulic con-
nection with the overlying alluvium; therefore, the
potential for pollution of the aquifer is high. Although
precipitation commonly averages less than 20 inches
per year, recharge is relatively high due to the flat
topography and sandy surficial deposits.
Setting 6 G Braided River Deposits
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
4-7
Sand and Gravel
Sand
0-2%
Sand and Gravel
1000-2000
Weight Rating
5 10
4 6
3 8
5 9
3 10
4 8
2 8
Agricultural
DRASTIC Index
Number
50
24
24
45
30
32
16
221
Non-Glaciated Central
(6H) Triassic Basins
This hydrogeologic setting is characterized by moder-
ately dipping, highly faulted beds of sandstone, shale,
and silty limestone. Conglomeritic deposits occur in
78
-------
some areas. These basins tend to be bounded by high
angle faults, with the basins being elongate in the
NE-SW directions. The sedimentary beds may be cut
by narrow (dikes, etc.) igneous intrusions, and are
sometimes indurated by the intrusive activity. The
Triassic formations are often red in color due to high
iron concentrations, but green colors are also com-
mon. These deposits may serve as a localized source
of water and water levels are variable.
Setting 6 H Triassic Basins
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
4-7
Massive
Sandstone
Sandy Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
5
3
4
2
Rating
2
6
6
6
9
6.
1
Number
10
24
18
30
27
24
2
Agricultural
DRASTIC Index
135
Setting 6 H Triassic Basins
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
4-7
Massive
Sandstone
Sandy Loam
2-6%
Bedded LS,
SS, SH
1-100
Weight
5
4
3
2
1
5
3
Rating
2
6
6
6
9
6
1
Number
10
24
18
12
9
30
3
DRASTIC Index 106
79
-------
7. Glaciated Central Region
(Glacial deposits over fractured sedimentary rocks)
The Glaciated Central region occupies an area of
1,297,000 km2 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. The
part of the region in New York and Pennsylvania is
characterized by rolling hills and low, rounded
mountains that reach altitudes of 1,500 m. Westward
across Ohio to the western boundary of the region
along the Missouri River, the region is flat to gently
rolling. Among the more prominent topographic
features in this part of the region are low, relatively
continuous ridges (moraines) which were formetl at
the margins of ice sheets that moved southward
across the area one or more times during the
Pleistocene age.
The Glaciated Central region is underlain by relatively
flat-lying consolidated sedimentary rocks that range
in age from Paleozoic to Tertiary. They consist
primarily of sandstone, shale, limestone, and dolo-
mite. The bedrock is overlain by glacfal deposits
which, 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 meters thick, but
localized deposits as much as 30 m thick are common
on southerly slopes. In much of the central and
western parts of the region, the glacial deposits
exceed 100 m in thickness. The principal exception is
the "driftless" area in Wisconsin, Minnesota, Iowa,
and Illinois, where the ice, if it invaded the area, was
too thin to erode preexisting soils or to deposit a
significant thickness of till. Thus, the bedrock in this
area is overlain by thin soils derived primarily from
weathering of the rock. 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; thicknesses of 100 to 300 m occur in
the valleys of the Finger Lakes in New York. In most of
the region westward from Ohio to the Dakotas, the
thickness of the glacial deposits exceeds the relief on
the preglacial surface, with the result that the
locations of valleys and stream channels in the
preglacial surface are no longer discernible from the
land surface. The glacial deposits in 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. The dominant water-
80
-------
bearing fractures in the bedrock are along bedding
planes. Water also occurs in the bedrock in steeply
dipping fractures that cut across the beds and, in
some sandstones and conglomerates, in primary
pores that were not destroyed in the process of
cementation and consolidation.
Large parts of the region are underlain by limestones
and dolomites in which the fractures have been
enlarged by solution. Caves are relatively common in
the limestones where the ice sheets were relatively
thin, as near the southern boundary of the region and
in the "driftless" area. A few caves occur in other
parts of the region, notably in the Mohawk River
valley in central New York, where they were appar-
ently protected from glacial erosion by the configura-
tion of the bedrock surface over which the ice moved.
However, on the whole, caves and other large solution
openings, from which large springs emerge and
which yield large quantities of water to wells in parts
of the Nonglaciated Central region, 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 400 mm per year in the western part of the
region to about 1,000 mm in the eastern part.
Recharge also depends on the permeability of the
glacial deposits exposed at the land surface and on
the slope of the surface. 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 50 mm. In contrast, relatively flat areas
underlain by sand and gravel may receive as much as
300 mm of recharge annually in the eastern part of
the region. Recharge of the ground-water system in
the Glaciated Central region occurs primarily in the
fall, after plant growth has stopped and cool tempera-
tures have reduced evaporation, and again during the
spring thaw before plant growth begins. Of these
recharge periods, the spring thaw is usually dominant
except when fall rains are unusually heavy. Minor
amounts of rechargealso may occur during midwinter
thaws and during unusually wet summers.
Ground water in small to moderate amounts can be
obtained anyplace in the region, both from the glacial
deposits and from the bedrock. Large to very large
amounts are obtained from the sand and gravel
deposits and from some of the limestones, dolomites,
and sandstones in the North-Central States. The
shales are the least productive bedrock formations in
the region.
As is the case in the Nonglaciated Central region,
mineralized water occurs at relatively shallow depth
in the bedrock in large parts of this region. Because
the principal constituent in the mineralized water is
sodium chloride (common salt), the water is com-
monly referred to as saline or salty. 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 fresh-
water 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, both because of lower altitudes
and because of the upward movement of the saline
water to discharge. In those parts of the region
underlain by saline water, the concentration of
dissolved solids increases with depth. At depths of
500 to 1,000 m 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.
Because the Glaciated Central region resembles in
certain aspects both the Nonglaciated Central region
(region 6) to the south and the Northeast and
Superior Uplands region (region 9) to the north, it may
be useful to comment on the principal differences
among these three regions. First, and as is already
apparent, the bedrock in the Glaciated Central and
the Nonglaciated Central regions is similar in compo-
sition and structure. The difference in these two
regions is in the composition and other characteristics
of the overlying unconsolidated material. In the
Nonglaciated Central region this material consists of
a relatively thin layer that is derived from weathering
of the underlying bedrock and that in any particular
area is of relatively uniform composition. In the
Glaciated Central region, on the other hand, the
unconsolidated material consists of a layer, ranging
in thickness from a few meters to several hundred
meters, of diverse composition deposited either
directly from glacial ice (till) or by meltwater streams
(glaciofluvial deposits). From a hydrologic standpoint,
the unconsolidated material in the Nonglaciated
Central region is of minor importance both as a
source of water and as a reservoir for storage of water
for the bedrock. In contrast, the glacial deposits in the
Glaciated Central region serve both as a source of
ground water and as an important storage reservoir
for the bedrock.
The Glaciated Central region and the Northeast and
Superior Uplands region are similar in that the
unconsolidated material in both consists of glacial
deposits. However, the bedrock in the two regions is
different. The bedrock in the Glaciated Central region,
as we have already seen, consists of consolidated
sedimentary rocks that contain both steeply dipping
fractures and fractures along bedding planes. In the
Northeast and Superior Uplands, on the other hand.
81
-------
the bedrock is composed of intrusive igneous and
metamorphic rocks (nonbedded) in which most
water-bearing openings are steeply-dipping fractures.
Asa result of the differences in fractures, the bedrock
in the Glaciated Central region is, in general, a more
productive and more important source of ground
water than the bedrock in the Northeast and Superior
Uplands region.
The largest fresh-water supply in North America, the
Great Lakes, is located in this region. Bordering the
Great Lakes, there are abandoned beach ridges,
present-day beaches and sand dunes, all of which are
very sensitive environmental areas.
Setting 7 Aa Glacial Till Over Bedded Sedimentary Rock
General
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Thin Bedded SS,
LS, SH Sequences
Clay Loam
2-6%
Silt/Clay
100-300
5
4
3
2
1
5
3
5
6
6
3
9
1
2
25
24
18
6
9
5
6
DRASTIC Index 93
Glaciated Central
(7A a) Glacial Till Over Bedded Sedimentary Rocks
This hydrogeologic setting is characterized by low
topography and relatively flat-lying, fractured sedi-
mentary rocks consisting of sandstone, shale and
limestone which are covered by varying thicknesses
of glacial till. The till is chiefly unsorted deposits
which may be interbedded with loess or localized
deposits of sand and gravel. Although ground water
occurs in both the glacial deposits and in the inter-
secting bedrock fractures, the bedrock is the principal
aquifer. The glacial till serves as a source of recharge
to the underlying bedrock. Although precipitation is
abundant in most of the region, recharge is moderate
because of the glacial till and soils which are typically
clay loams. Depth to water is extremely variable
depending in part on the thickness of the glacial till,
but tends to average around 30 feet.
Setting 7 Aa Glacial Till Over Bedded Sedimentary Rock
Agricultural
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Thin Bedded SS,
LS, SH Sequences
Clay Loam
2-6%
Silt/Clay
1 00-300
5 5
4 6
3 6
5 3
3 9
4 1
2 2
Agricultural
DRASTIC Index
25
24
18
15
27
4
4
117
Glaciated Central
(7Ab) Glacial Till Over Outwash
This hydrogeologic setting is characterized by low
topography and outwash materials which are covered
by varying thicknesses of glacial till. The till is chiefly
unsorted deposits which may be interbedded with
loess or localized deposits of sand and gravel. Surf icial
deposits have usually weathered to a clay loam.
Although ground water occurs in both the glacial
deposits and in the underlying outwash, the outwash
serves as the principal aquifer because the fine-
grained deposits have been removed by glacial
meltwater. The outwash is in direct hydraulic con-
nection with the glacial till and glacial till serves as a
source of recharge for the underlying outwash. This
setting is similar to (7Aa) Glacial Till Over Bedded
Sedimentary Rock and (7Ac) Glacial Till Over Solution
Limestone in that although precipitation is abundant
in most of the region, recharge is moderate because
82
-------
of the relatively low permeability of the overlying
glacial till. Depth to water table is extremely variable
depending in part on the thickness of the glacial till,
but averages around 30 feet.
Glaciated Central
(7Ac) Glacial Till Over Solution Limestone
This hydrogeologic setting is characterized by low
topography and solution limestone which are covered
by varying thicknesses of glacial till. The till is chiefly
unsorted deposits which may be interbedded with
loess or localized deposits of sand and gravel. Surficial
deposits have usually weathered to a clay loam.
Although ground water occurs in both the glacial
deposits and in the underlying limestone, the lime-
stone, which typically contains solution cavities,
serves as the principal aquifer. The limestone is in
direct hydraulic connection with the glacial till and
the glacial till serves as a source of recharge for the
underlying limestone. This setting is similar to (7Aa)
Glacial Till Over Bedded Sedimentary Rock and (7Ab)
Glacial Till Over Outwash in that although precipita-
tion is abundant in most of the region, recharge is
moderate because of the relatively low permeability
of the overlying glacial till. Depth to water table is
extremely variable depending in part on the thickness
of the glacial till, but is typically moderately deep.
Setting 7 Ab Glacial Till Over Outwash
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Clay Loam
2-6%
Silt/Clay
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
7
6
8
3
9
1
8
Number
35
24
24
6
9
5
24
DRASTIC Index 127
Setting 7 Ab Qlacial Till Over Outwash
Feature
Agricultural
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
4-7
Sand and Gravel
Clay Loam
2-6%
Silt/Clay
1000-2000
5 7
4 6
3 8
5 3
3 9
4 1
2 8
Agricultural
DRASTIC Index
35
24
24
15
27
4
16
145
Setting 7 Ac Glacial Till Over Solution Limestone
General
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Karst Limestone
Clay Loam
2-6%
Silt/Clay
2000+
5
4
3
2
1
5
3
5
6
10
3
9
1
10
25
24
30
6
9
5
30
DRASTIC Index 129
83
-------
Setting 7 Ac Glacial Till Over Solution Limestone
Agricultural
Feature
Setting 7 Ad Glacial Till Over Sandstone
Range
We i g htR ating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Karst Limestone
Clay Loam
2-6%
Silt/Clay
2000+
5
4
3
5
3
4
2
5
6
10
3
9
1
10
25
24
30
15
27
4
20
Agricultural
DRASTIC Index
145
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
4-7
Massive
Sandstone
Clay Loam
2-6%
Silt/Clay
300-700
Weight
5
4
3
2
1
5
3
Rating
5
6
6
3
9
1
4
Number
25
24
18
6
9
5
12
DRASTIC Index
99
Setting 7 Ad Glacial Till Over Sandstone
Agricultural
Glaciated Central
(7A d) Glacial Till O ver Sandstone
This hydrogeologic setting is characterized by low
topography and relatively flat-lying fractured sand-
stones which are covered by varying thicknesses of
glacial till. The till is chiefly unsorted deposits which
maybeinterbeddedwith loess or localized deposits of
sand and gravel. Although ground water occurs in
both the glacial deposits and in the intersecting
bedrock fractures, the bedrock is the principal aquifer.
The glacial till serves as a source of recharge to the
underlying bedrock. Although precipitation is abun-
dant in most of the region, recharge is moderate
because of the glacial tills which typically weather to
clay loam. Depth to water table is extremely variable,
depending in part on the thickness of the glacial till,
but tends to average around 40 feet.
. Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
4-7
Massive
Sandstone
Clay Loam
2-6%
Silt/Clay
300-700
Weight
5
4
3
5
3
4
2
Rating
5
6
6
3
9
1
4
Number
25
24
18
15
27
4
8
Agricultural
DRASTIC Index
121
Glaciated Central
(7A e) Glacial Till O ver Shale
This hydrogeologic setting is similar to (7Ad) Glacial
Till Over Sandstone except that varying thickness of
till overlie fractured flat-lying shales. The till is chiefly
unsorted deposits with interbedded lenses of loess
and sand and gravel. Ground water is derived from
either localized sources in the overlying till or from
deeper, more permeable formations. The shale is
relatively impermeable and does not serve as a
source of ground water. Although precipitation is
abundant, recharge is minimal from the till to deeper
formations and occurs only by leakage of water
through the fractures.
84
-------
Setting 7 Ae Glacial Till Over Shale
Feature
General
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Massive Shale
Clay Loam
2-6%
Silt/Clay
1-100
5
4
3
2
1
5
3
5
6
2
3
9
1
1
25
24
6
6
9
5
3
Setting 7 Ae Glacial Till Over Shale
Feature
DRASTIC Index 78
Agricultural
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
4-7
Massive Shale
Clay Loam
2-6%
Silt/Clay
1-100
5 5
4 6
3 2
5 3
3 9
4 1
2 1
Agricultural
DRASTIC Index
25
24
6
15
27
4
2
103
Glaciated Central
(7Ba) Outwash
This hydrogeologic setting is characterized by moder-
ate to low topography and varying thicknesses of
outwash which overlie sequences of fractured sedi-
mentary rocks. The outwash consists of water-
washed deposits of sand and gravel which serve as
the principal aquifer in the area. The outwash also
serves as a source of recharge to the underlying
bedrock. Precipitation is abundant throughout most
of the area and recharge is moderate to high.
Recharge is somewhat restricted by the sandy loam
soil which typically develops in this setting. Water
levels are extremely variable but relatively shallow.
Outwash generally refers to water washed or ice
contact deposits, and can include a variety of morpho-
genic forms. Outwash plains are thick sequences of
sands and gravels that are laid down in sheet-like
deposits from sediment laden waters draining off,
and from within a glacier. These deposits are well
sorted and have relatively high permeabilities. Kames
and eskers are ice-contact deposits. A kame is an
isolated hill or mound of stratified sediments depos-
ited in an opening within or between ice blocks, or
between ice blocks and valley walls. An esker is a
sinuous or meandering ridge of well sorted sands and
gravels that are remnants of streams that existed
beneath and within the glaciers. These deposits may
be in direct hydraulic connection with underlying
fractured bedrock.
Setting 7 Ba Outwash
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
1000-2000
Weight
5
4
3
2
1
5
3
General
Rating
7
8
8
6
9
8
8
Number
35
32
24
12
9
40
24
DRASTIC Index 176
85
-------
Setting 7 Ba Outwash
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
1 000-2000
Weight
5
4
3
5
3
4
2
Rating
7
8
8
6
9
8
8
Agricultural
DRASTIC Index
Number
35
32
24
30
27
32
16
196
Setting 7 Bb Outwash Over Bedded Sedimentary
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
10+
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
2-6%
Sand and Gravel
100-300
Weight
5
4
3
2
1
5
3
Rating
7
9
6
6
9
8
2
Number
35
36
18
12
9
40
6
DRASTIC Index 156
Glaciated Central
(7Bb) Outwash Over Bedded Sedimentary
This hydrogeologic setting is characterized by moder-
ate to low topography and relatively flat-lying, frac-
tured sedimentary rocks consisting of sandstones,
shales, and limestone which are covered by varying
thicknesses of glacial outwash. The outwash consists
of a variety of water-washed deposits of sand and
gravel which serve as the principal aquifer in the
areas. The outwash also serves as a source of
recharge to the underlying bedrock. Precipitation is
abundant throughout most of the area and recharge
is moderate to high. Water levels are extremely
variable, but typically shallow.
Setting 7 Bb Outwash Over Bedded Sedimentary
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
10+
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
2-6%
Sand and Gravel
100-300
Weight
5
4
3
5
3
4
2
Rating
7
9
6
6
9
8
2
Number
35
36
18
30
27
32
4
Agricultural
DRASTIC Index
182
Glaciated Central
(7Be) Outwash Over Solution Limestone
This hydrogeologic setting is characterized by low
topography and solution limestone which is covered
by varying thicknesses of glacial outwash. The
outwash consists of varying types of water-washed
deposits that typically weather to sandy loam soils.
Both the outwash and the solution limestone serve as
principal aquifers in the area. The solution limestone
is in direct hydraulic connection with the glacial
outwash and the outwash serves as a source of
recharge for the underlying limestone. Water levels
are extremely variable and in part dependent on the
thickness of the overlying outwash.
86
-------
Setting 7 Be Outwash Over Solution Limestone
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
10+
Karst Limestone
Sandy Loam
2-6%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
7
g
10
6
9
8
8
Number
35
36
30
12
9
40
24
DRASTIC Index 186
Setting 7 Be Outwash Over Solution Limestone
Agricultural
Feature Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
10+
Karst Limestone
Sandy Loam
2-6%
Sand and Gravel
1000-2000
5 7
4 9
3 10
5 6
3 9
4 8
2 8
Agricultural
DRASTIC Index
35
36
30
30
27
32
16
206
Glaciated Central
(7C) Moraine
This hydrogeologic setting is characterized by moder-
ate to moderately steep topography and varying
thicknesses of mixed glacial deposits which overlie
sequences of relatively flat-lying fractured sedimen-
tary rocks. This setting is similar to (7Ba) Outwash in
that the sand and gravel within the morainal deposits
may be well-sorted and serve as the principal aquifer
in the area. These deposits also serve as a source of
recharge for the underlying bedrock. Moraines also
contain sediments that are typically unsorted and
unstratified; these deposits contain more fines than
outwash deposits, are less permeable and character-
istic of glacial till. Moraines are typically mounds or
ridges of till which were deposited along the margin
of a stagnant or retreating glacier. Surficial deposits
often weather to a sandy loam. Precipitation is
abundant throughout the region and ground-water
recharge is moderate. Water levels are extremely
variable, based in part on the thickness of the glacial
till, but are typically fairly shallow.
Setting 7 C Moraine
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Sandy Loam
6-12%
Silt/Clay
300-700
Weight
5
4
3
2
1
5
3
General
Rating
7
8
8
6
5
1
4
Number
35
32
24
12
5
5
12
DRASTIC Index 125
87
-------
Setting 7 C Moraine
Feature Range
Agricultural
Setting 7 D Buried Valley
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
7-10
Sand and Gravel
Sandy Loam
6-12%
Silt/Clay
300-700
5 7
4 8
3 8
5 6
3 5
4 1
2 4
Agricultural
DRASTIC Index
35
32
24
30
15
4
8
148
Glaciated Central
<7D) Buried Valley
This hydrogeologic setting is characterized by thick
deposits of sand and gravel that have been deposited
in a former topographic low (usually a pre-glacial river
valley) by glacial meltwaters. These deposits are
capable of yielding large quantities of ground water.
The deposits may or may not underlie a present-day
river and may or may not be in direct hydraulic
connection with a stream. Glacial till or recent
alluvium often overlies the buried valley. Usually the
deposits are several times more permeable than the
surrounding bedrock, with finer grained alluvium
covering the underlying sand and gravel. Soils are
typically a sandy loam. Recharge to the sand and
gravel is moderate and water levels are commonly
relatively shallow, although they may be quite
variable.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
7-10
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig
Silt and Clay
1 000-2000
Weight
5
4
3
2
1
5
3
General
Rating
5
8
8
6
9
6
8
Number
25
32
24
12
9
30
24
Setting 7 D Buried Valley
DRASTIC Index 156
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
7-10
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
1 000-2000
Weight
5
4
3
5
3
4
2
Rating
5
8
8
6
9
6
8
Number
25
32
24
30
27
24
16
Agricultural
DRASTIC Index
178
Glaciated Central
(7Ea) River Alluvium With Overbank Deposits
This hydrogeologic setting is characterized by low
topography and thin to moderately thick deposits of
flood deposited alluvium along portions of the river
valley. The alluvium is underlain by fractured bedrock
of sedimentary, metamorphic, or igneous origin.
Water is obtained from sand and gravel layers which
are interbedded with finer-grained alluvial deposits.
The floodplain is covered by varying thicknesses of
fine-grained silt and clay called overbank deposits.
The overbank thickness is usually greater along major
streams(as much as40feet) andthinner along minor
streams. Precipitation in the region varies, but
recharge is somewhat reduced because of the silty
and clayey overbank soils which typically cover the
surface. Water levels are moderately shallow. Ground
water is in direct hydraulic contact with the surface
stream. The alluvium may serve as a significant
source of water and may also be in direct hydraulic
contact with the underlying sedimentary rocks.
88
-------
Setting 7 Ea River Alluvium With Overbank Deposit
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Silty Loam
0-2%
Silt/Clay
700-1000
Weight
5
4
3
2
1
5
3
Rating
7
6
8
4
10
1
6
Number
35
24
24
8
10
5
18
DRASTIC Index 124
Setting 7 Ea River Alluvium With Overbank Deposit
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
4-7
Sand and Gravel
Silty Loam
0-2%
Silt/Clay
700-1000
Weight Rating
5 7
4 6
3 8
5 4
3 10
4 1
2 6
Agricultural
DRASTIC Index
Number
35
24
24
20
30
4
12
149
Glaciated Central
(7Eb) River Alluvium Without Overbank
This setting is identical to (6Fa) River Alluvium with
Overbank except that no significant fine-grained
floodplain deposits occupy the stream valley. This
results in significantly higher recharge where precipi-
tation is adequate and sandy soils occur at the
surface. Water levels are moderate to shallow in
depth. Hydraulic contact with the surface stream is
usually excellent, with alternating recharge/discharge
relationships varying with stream stage. These
deposits also serve as a good source of recharge to
the underlying fractured bedrock.
Setting 7 Eb River Alluvium Without Overbank Deposit
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
Rating
9
9
8
9
10
8
6
Number
45
36
24
18
10
40
18
DRASTIC Index 191
Setting 7 Eb River Alluvium Without Overbank Deposit
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1000
Weight Rating
5 9
4 9
3 8
5 9
3 10
4 8
2 6
Agricultural
DRASTIC Index
Number
45
36
24
45
30
32
12
224
89
-------
Glaciated Central
(7F) Glacial Lake Deposits
This hydrogeologic setting is characterized by flat
topography and varying thicknesses of fine-grained
sediments that overlie sequences of fractured sedi-
mentary rocks. The deposits are composed of fine-
grained silts and clays interlayered with fine sand
that settled out in glacial lakes and exhibit alternating
layers relating to seasonal fluctuations. As a conse-
quence of the thin alternating layers there is a
substantial difference between the vertical and
horizontal permeability with the horizontal commonly
two or more orders of magnitude greater than the
vertical. Due to their fine-grained nature, these
deposits typically weather to organic-rich sandy
loams with a range in permeabilities reflecting
variations in sand content. Underlying glacial deposits
or bedrock serve as the major source of ground water
in the region. Although precipitation is abundant,
recharge is controlled by the permeability of the
surface clays, however, in all instances recharge is
moderately high because of the impact of the low
topography. Water levels are variable, depending on
the thickness of the lake sediments and the underlying
materials.
Setting 7 F Glacial Lake Deposits
Feature Range Weight Rating Number
General
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
4-7
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
0-2%
S & G w/sig.
Silt and Clay
100-300
5
4
3
2
1
5
3
7
6
6
6
10
6
2
35
24
18
12
10
30
6
Setting 7 F Glacial Lake Deposits
Feature Range Weight Rating Number
Agricultural
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
4-7
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
0-2%
S & G w/sig.
Silt and Clay
100-300
5 7
4 6
3 6
5 6
3 10
4 6
2 2
Agricultural
DRASTIC Index
35
24
18
30
30
24
4
165
Glaciated Central
(7G) Thin Till Over Bedded Sedimentary
This hydrogeologic setting is characterized by moder-
ate to low topography and deposits of thin, patchy,
glacial till overlying alternating layers of fractured
consolidated sedimentary rocks. The till, where
present, is primarily unsorted deposits of clay, sand,
and gravel. Although ground water occurs in both the
till and in the intersecting fractures of the bedrock,
the bedrock is the principal aquifer. The glacial till
serves as a source of recharge to the underlying
bedrock. Although precipitation is abundant in most
of the region, recharge is moderate because of the
glacial tills and clayey soils. Water levels are ex-
tremely variable, but usually moderate.
DRASTIC Index 135
90
-------
Setting 7 G Thin Till Over Bedded Sedimentary
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Thin Bedded SS,
LS, SH Sequences
Clay Loam
2-6%
Silt/Clay
1 00-300
Weight
5
4
3
2
1
5
3
Rating
7
8
6
3
9
1
2
Number
35
32
18
6
9
5
6
DRASTIC Index 111
Setting 7 G Thin Till Over Bedded Sedimentary
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Thin Bedded SS,
LS, SH Sequences
Clay Loam
2-6%
Silt/Clay
100-300
Weight Rating
5 7
4 8
3 6
5 3
3 9
4 1
2 2
Agricultural
DRASTIC Index
Number
35
32
18
15
27
4
4
135
Setting 7 H Beaches, Beach Ridges and Sand Dunes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
10
9
8
9
10
8
8
Number
50
36
24
18
10
40
24
DRASTIC Index 202
Glaciated Central
(7H) Beaches, Beach Ridges, and Sand Dunes
This hydrogeologic setting is characterized by low
relief, sandy surface soil that is predominantly silica
sand, extremely high infiltration rates and low sorp-
tive capacity in the thin vadose zone. The water table
is very shallow beneath the beaches bordering the
Great Lakes. These beaches are commonly ground-
water discharge areas. The water table is slightly
deeper beneath the rolling dune topography and the
vestigial inland beach ridges. All of these areas serve
as recharge sources for the underlying sedimentary
bedrock aquifers, and they often serve as local
sources of water supply.
Setting 7 H Beaches. Beach Ridges and Sand Dunes
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
1 000-2000
Weight Rating
5 10
4 9
3 8
5 9
3 10
4 8
2 8
Agricultural
DRASTIC Index
Number
50
36
24
45
30
24
16
225
91
-------
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 247,000 km2 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
whose summits range from about a 100 meters above
sea level along its eastern boundary with the Coastal
Plain to 500 to 600 m along its boundary with the Blue
Ridge area to the west. The Blue Ridge is mountainous
and includes the highest peaks east of the Mississippi.
The mountains, some of which reach altitudes of
more than 2,000 m, have smooth-rounded outlines
and are bordered by well-graded streams flowing in
relatively narrow valleys.
The Piedmont and Blue Ridge region is underlain by
bedrock of Precambrian and Paleozoic age consisting
of igneous and metamorphosed igneous and sedi-
mentary rocks. These include granite, gneiss, schist,
quartzite, slate, marble, and phyllite. 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 10 to 20 m in thickness and
may be as much as 100 m 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. When the distinction between saprolite
and alluvium is not important, the term regolith is
used to refer to the layer of unconsolidated deposits.
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 (that is, breaks in the otherwise
"solid" rock). The hydraulic conductivities of the
regolith and the bedrock are similar and range from
aboutO.001 to 1 m day"1. The major difference in their
water-bearing characteristics is their porosities, that
of regolith being about 20 to 30 percent and that of
the bedrock aboutO.01 to 2 percent. Small supplies of
water adequate for domestic needs can be obtained
from the regolith through large-diameter 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,
as noted, 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 (or conduits) that
transmit water from recharge areas to discharge
areas. The yield of bedrock wells in the Piedmont and
Blue Ridge region depends on the number and size of
fractures penetrated by the open hole and on the
replenishment of the fractures by seepage into them
from the overlying regolith. Thus, the ground-water
system in this region can be viewed, from the stand-
point of ground-water development, as a terrane in
92
-------
which the reservoir and pipeline functions are effec-
tively separated. Because of its larger porosity, the
regolith functions as a reservoir which 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
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 in 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.
Because the yield of bedrock wells depends on the
number of fractures penetrated by the well, the key
element in selecting well sites is recognizing the
relation between the present surface topography and
the location of fractures in the bedrock. Most of the
valleys, draws, and other surface depressions indicate
the presence of more intensely fractured zones in the
bedrock which are more susceptible to weathering
and erosion than are the intervening areas. Because
fractures in the bedrock are the principal avenues
along which ground water moves, the best well sites
appear to be in draws on the sides of the valleys of
perennial streams where the bordering ridges are
underlain by substantial thicknesses of regolith.
Wells located at such sites seem to be most effective
in penetrating open water-bearing fractures and in
intercepting ground water draining from the regolith.
Chances of success seem to be somewhat less for
wells on the flood plains of perennial streams,
possibly because the alluvium obscures the topo-
graphic expression of bedrock fractures. The poorest
sites for wells are on the tops of ridges and mountains
where the regolith cover is thin or absent and the
bedrock is sparsely fractured.
As a general rule, fractures near the bedrock surface
are most numerous and have the largest openings, so
that the yield of most wells is not i ncreased by drilling
to depths greater than about 100 m. Exceptions to this
occur in Georgia, South Carolina, and North Carolina
and some other areas where water-bearing, low-
angle faults or fractured zones are present at depths
as great as 200 to 300 m.
The Piedmont and Blue Ridge region has long been
known as an area generally unfavorable for ground-
water development. This reputation seems to have
resulted both from the small reported yields of the
numerous domestic wells in use in the region that
were, generally, sited as a matter of convenience and
from a failure to apply existing technology to the
careful selection of well sites where moderate yields
are needed. As water needs in the region increase
and as reservoir sites on streams become increasingly
more difficult to obtain, it will be necessary to make
more intensive use of ground water.
Piedmont and Blue Ridge
(8A) Mountain Slopes
This hydrogeologic setting is characterized by steep
slopes on the side of mountains, a thin soil cover and
fractured bedrock. Ground water is obtained primarily
from the fractures in the bedrock which may be of
sedimentary, metamorphic, or igneous origin but
which is commonly metamorphic or igneous. The
fractures provide localized sources of ground water
and well yields are typically limited. Although precipi-
tation is abundant, due to the steep slopes, thin soil
cover and small storage capacity of the fractures,
runoff is significant and ground-water recharge is
only moderate. Water levels are extremely variable
but are commonly deep.
93
-------
Setting 8 A Mountain Slopes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
18+
Metamorphic/
Igneous
1-100
Weight
5
4
3
2
1
5
3
Rating
2
3
3
10
1
4
1
Number
10
12
9
20
1
20
3
DRASTir Indnx 75
Setting 8 A Mountain Slopes
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
18+
Metamorphic/
Igneous
1-100
Weight Rating Number
5 2
4 T
^ o
3 3
5 10
3 1
4 4
2 1
Agricultural
DRASTIC Index
10
1 ")
\ f.
9
50
3
16
2
102
Setting 8 B Alluvial Mountain Valleys
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Loam
2-6%
S & G w/sig.
Silt and Clay
300-700
Weight
5
4
3
2
1
5
3
DRAG
General
Rating
9
8
8
5
9
6
4
!TIP InHav
Number
45
32
24
10
9
30
12
1RO
Piedmont and Blue Ridge
(8B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by thin
bouldery alluvium which overlies fractured bedrock of
sedimentary, metamorphic, or igneous origin. The
alluvium, which includes both mass-wastage and
water-sorted debris, is derived from the surrounding
slopes, and serves as a localized source of water.
Water is obtained from sand and gravel layers which
are interspersed between finer-grained deposits.
Surficial deposits have typically weathered to a loam.
Water levels are usually relatively shallow but are
extremely variable. Ground water is also obtained
from the fractures in the underlying bedrock, which"
are typically in direct hydraulic connection with the
overlying alluvium.
Setting 8 B Alluvial Mountain Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Loam
2-6%
S & G w/sig.
Silt and Clay
300-700
Weight Rating
5 9
4 8
3 8
5 5
3 9
4 6
2 4
Agricultural
DRASTIC Index
Number
45
32
24
25
27
24
8
185
Piedmont and Blue Ridge
<8C) Mountain Flanks
This hydrogeologic setting is characterized by moder-
ate topographic relief and moderately-dipping, frac-
94
-------
tured, consolidated sedimentary rocks. Soil cover is
usually thicker than on the mountain slopes and
typically has weathered to a sandy loam or loam.
Although precipitation is abundant, ground-water
recharge is moderate due to the soil cover and slope.
Water levels are typically moderately-deep although
they are extremely variable. The mountain flanks
serve as the recharge area for aquifers which are
typically confined in adjacent valley areas.
Setting 8 C Mountain Flanks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
LS, SH Sequences
Loam
6-12%
Bedded LS,
SS, SH
100-300
Weight
5
4
3
2
1
5
3
Rating
5
3
6
5
5
6
2
Number
25
12
18
10
5
30
6
Setting 8 C Mountain Flanks
DRASTIC Index 106
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
LS, SH Sequences
Loam
6-12%
Bedded LS,
SS, SH
100-300
Weight
5
4
3
5
3
4
2
Rating
5
3
6
5
5
6
2
Number
25
12
18
25
15
24
4
Piedmont and Blue Ridge
(8D) Thick Regolith
This hydrogeologic setting is characterized by moder-
ate to low slopes covered by thick regolith and
underlain by fractured bedrock of igneous, sedimen-
tary, or metamorphic origin. The regolith is typically
clay-rich but may also serve as a source of ground
water for low-yield wells. This regolith functions as a
reservoir for ground-water recharge to the bedrock
which is in direct hydraulic connection with the
overlying regolith. The bedrock typically yields larger
amounts of ground water than the regolith when the
well intersects fractures in the bedrock.
Setting 8 D Thick Regolith
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
4-7
Weathered
Meta./lg.
Clay Loam
6-12%
Silt/Clay
1-100
Weight
5
4
3
2
1
5
3
Rating
9
6
4
3
5
1
1
Number
45
24
12
6
5
5
3
DRASTIC Index 100
Agricultural
DRASTIC Index 123
35
-------
Setting 8 D Thick Regolith
Agricultural
Setting 8 E River Alluvium
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
4-7
Weathered
Meta./lg.
Clay Loam
6-12%
Silt/Clay
1-100
Weight
5
4
3
5
3
4
2
Rating
9
6
4
3
5
1
1
Number
45
24
12
15
15
4
2
Agricultural
DRASTIC Index
117
Piedmont and Blue Ridge
(8E) River A lluvium
This hydrogeologic setting is characterized by low
topography and deposits of varying thickness of
alluvium along parts of stream valleys. The alluvium
is underlain by fractured igneous, metamorphic, or
consolidated sedimentary rocks. Water is obtained
from sand and gravel which is overlain and inter-
bedded with finer-grained alluvial deposits. Surficial
deposits usually weather to a sandy loam. The sand
and gravel within the alluvium serves as the principal
aquifer, but the alluvium also serves as the source of
ground-water recharge for the underlying aquifer.
Precipitation is abundant and recharge is moderately
high, limited only by the loamy surficial deposits.
Water levels are extremely variable, but are typically
moderately shallow.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
1000-2000
Weight
5
4
3
2
1
5
3
General
Rating
9
8
8
6
9
6
8
Number
45
32
24
12
9
30
24
Setting 8 E River Alluvium
DRASTIC Index 176
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
1000-2000
Weight
5
4
3
5
3
4
2
Rating
9
8
8
6
9
6
8
Number
45
32
24
30
27
24
16
Agricultural
DRASTIC Index
198
Piedmont and Blue Ridge
(8F) Mountain Crests
This hydrogeologic setting is characterized by moder-
ate to steep topography on the crests of mountains
with thin soil cover and exposed fractured bedrock.
Ground water is obtained primarily from the fractures
in the bedrock which may be of sedimentary, meta-
morphic, or igneous origin but which is commonly
metamorphic or igneous. The fractures provide
localized sources of ground water and well yields are
typically limited. Although precipitation is abundant,
due to the slopes, thin soil cover, and small storage
capacity of the fractures, runoff is significant and
ground-water recharge is low. Water levels are
extremely variable but commonly deep.
-------
Setting 8 F Mountain Crests
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Metamorphic/
Igneous
Thin or Absent
2-6%
Metamorphic/
Igneous
1-100
Weight Rating
5 1
4 1
3 3
2 10
1 9
5 4
3 1
DRASTIC Index
Number
5
4
9
20
9
20
3
70
Setting 8 F Mountain Crests
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Metamorphic/
Igneous
Thin or Absent
2-6%
Metamorphic/
Igneous
1-100
Agricultural
Weight Rating Number
5 1
4 1
3 3
5 10
3 9
4 4
2 1
Agricultural
DRASTIC Index
5
4
9
50
27
16
2
113
97
-------
3 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 415,000 km2.
The Northeast Upland encompasses the Adirondack
Mountains, the Lake Champlain valley, and nearly all
of New England. The parts of New England not
included are the Cape Cod area and nearby islands,
which are included in the Atlantic and Gulf Coastal
Plain region, and the Triassic lowland along the
Connecticut River in Connecticut and Massachusetts,
which is included in the Glaciated Central region. The
Superior Upland encompasses most of the northern
parts of Minnesota and Wisconsin adjacent to the
western end of Lake Superior. The Northeast and
Superior Uplands are characterized by rolling hills
and low mountains. Land-surface altitudes in the
Northeast Upland range from sea level to more than
1,500 m on some of the peaks in the Adirondacks and
White Mountains. In contrast to the mountainous
areas in the Northeast, the Superior Upland is in an
area of rolling hills whose summits reach altitudes of
only 300 to 600 m.
Bedrock in the region ranges in age from Precambrian
to Paleozoic and consists mostly of granite, syenite,
anorthosite, and other intrusive igneous rocks and
metamorphosed sedimentary rocks consisting of
gneiss, schist, quartzite, slate, and marble. Most of
the igneous and metamorphosed sedimentary rocks
have been intensely folded and cut by numerous
faults.
The bedrock is overlain by unconsolidated deposits
laid down by ice sheets that covered the areas one or
more times during the Pleistocene and by gravel,
sand, silt, and clay laid down by meltwater streams
and in lakes that formed during the melting of the ice.
The thickness of the glacial deposits ranges from a
few meters on the higher mountains, which also have
large expanses of barren rock, to more than 100 m in
some valleys. The most extensive glacial deposit is
till, which was laid down as a nearly continuous
blanket by the ice, both in valleys and on the uplands.
In most of the valleys and other low areas, the till is
covered by glacial outwash consisting of interlayered
sand and gravel, ranging in thickness from a few
meters to more than 20 m, that was deposited by
streams supplied by glacial meltwater. In several
areas, including parts of the Champlain valley and the
lowlands adjacent to Lake Superior, the unconsoli-
dated deposits consist of clay and silt deposited in
lakes that formed during the melting of the ice sheets.
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 60 m thick. Other sand and
gravel deposits, not thick or productive enough to be
included in the Alluvial Valleys region, occur locally in
most valley and lowland areas in the Northeast and
Superior Uplands region and serve* as important
sources of water.
Water occurs in the bedrock in fractures similar in
origin, occurrence, and hydraulic characteristics to
those in the Piedmont and Blue Ridge region. In fact.
98
-------
the primary difference in ground-water conditions
between the Piedmont and Blue Ridge region and the
Northeast and Superior Uplands region is related to
the materials that overlie the bedrock. In the Piedmont
and Blue Ridge, these consist of unconsolidated
material derived from weathering of the underlying
bedrock. In the Northeast and Superior Uplands the
overlying materials consist of glacial deposits which,
having been transported either by ice or by streams,
do not have a composition and structure controlled by
that of the underlying bedrock. These differences in
origin of the regolith between the Northeast and
Superior Uplands and the Piedmont and Blue Ridge
are an important consideration in the development of
water supplies, as is discussed in the following
paragraphs.
Recharge from precipitation generally begins in the
fall after plant growth stops. It continues intermit-
tently 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 1,200 mm per year, is twice
that on the Superior Upland, with the result that
recharge, both to the glacial deposits and to the
underlying bedrock, is largest in the Northeast. The
glacial deposits in the region serve as a storage
reservoir for the fractures in the underlying bedrock,
in the same way the saprolite functions in the
Piedmont and Blue Ridge region. The major difference
is that the glacial deposits on hills and other upland
areas are much thinner than the saprolite in similar
areas in the Piedmont and Blue Ridge and, therefore,
have a much smaller ground-water storage capacity.
Water supplies in the Northeast and Superior Uplands
region are obtained from open-hole drilled wells in
bedrock, from drilled and screened or open-end 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. Thus,
use of surface depressions in this region to select
sites of bedrock wells is not as satisfactory as in the
Piedmont and Blue Ridge.
Most of the rocks that underlie the Northeast and
Superior Uplands are relatively insoluble, and, con-
sequently, the ground water in both the glacial
deposits and the bedrock generally contains less than
500 mg/l of dissolved solids. Two of the most
significant water-quality problems confronting the
region, especially the Northeast Upland section, are
acid precipitation and pollution caused by salts used
to de-ice highways. Much of the precipitation now
falling on the Northeast (in 1982) has a pH in the
range of 4 to 6 units. Because of the low buffering
capacity of the soils derived from the 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 determined. The second problem—that of
de-icing salts—affects ground-water quality adjacent
to streets and roads maintained for winter travel.
Northeast and Superior Uplands
(9A) Mountain Slopes
This hydrogeologic setting is characterized by steep
slopes on the side of mountains, a thin soil cover and
fractured bedrock. Ground water is obtained primarily
from the fractures in the bedrock which may be of
sedimentary, metamorphic, or igneous origin but
which is commonly metamorphic or igneous. The
fractures provide localized sources of ground water,
and well yields are typically limited. Although precipi-
tation is abundant, due to the steep slopes, thin soil
cover and small storage capacity of the fractures,
runoff is significant and ground-water recharge is
moderate. Water levels are extremely variable but are
commonly deep.
99
-------
Setting 9 A Mountain Slopes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
18+
Metamorphic/
Igneous
1-100
Weight
5
4
3
2
1
5
3
Rating
2
3
3
10
1
4
1
Number
10
12
9
20
1
20
3
Setting 9 A Mountain Slopes
DRASTIC Index 75
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
Weight Rating Number
Setting 9 B Alluvial Mountain Valleys
75-100
2-4
Metamorphic/
Igneous
Thin or Absent
18+
Metamorphic/
Igneous
1-100
5 2
4 3
3 3
5 10
3 1
4 4
2 1
Agricultural
DRASTIC Index
10
12
9
50
3
16
2
102
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
700-1 000
Weight
5
4
3
2
1
5
3
General
Rating
9
8
8
6
9
8
6
Number
45
32
24
12
9
40
18
DRASTIC Index 180
Northeast and Superior Uplands
(9B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by thin,
bouldery alluvium which overlies fractured bedrock of
sedimentary, metamorphic, or igneous origin but
which are commonly alternating sedimentary layers.
The alluvium, which is derived from the surrounding
slopes serves as a localized source of water. Water is
obtained from sand and gravel layers which are
interspersed between fine-grained deposits. Surficial
deposits have typically weathered to a sandy loam.
Water levels are relatively shallow but may be
extremely variable. Ground water may also be
obtained from the fractures in the underlying bedrock
which are usually in direct hydraulic connection with
the overlying alluvium.
Setting 9 B Alluvial Mountain Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
7-10
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
700-1 000
Weight Rating
5 9
4 8
3 8
5 6
3 9
4 8
2 6
Agricultural
DRASTIC Index
Number
45
32
24
30
27
32
12
202
Northeast and Superior Uplands
(9C) Mountain Flanks
This hydrogeologic setting is characterized by moder-
ate topographic relief and moderately dipping, frac-
100
-------
tured, consolidated sedimentary rocks. Soil cover is
usually thicker than on the mountain slopes and
typically has weathered to a sandy loam. Although
precipitation can be significant, ground-water re-
charge is moderate due to the slope. Water levels are
typically moderately deep, although they are extreme-
ly variable. The mountain flanks serve as the recharge
area for aquifers which are confined in adjacent
lowland areas. Ground water is obtained from the
permeable sedimentary rocks or from fractures and
bedding planes in the sedimentary rocks. The sedi-
mentary rocks may be underlain by fractured bedrock
of igneous, metamorphic, or sedimentary origin
which yield little water.
Setting 9 C Mountain Flanks
Agricultural
Setting 9 C Mountain Flanks
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
LS, SH Sequences
Sandy Loam
12-18%
Bedded LS,
SS, SH
1 00-300
Weight
5
4
3
2
1
5
3
Rating
5
3
6
6
3
6
2
Number
25
12
18
12
3
30
6
DRASTIC Index 106
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
2-4
Thin Bedded SS,
US, SH Sequences
Sandy Loam
12-18%
Bedded LS,
SS, SH
1 00-300
Weight
5
4
3
5
3
4
2
Rating
5
3
6
6
3
6
2
Number
25
12
18
30
9
24
4
Agricultural
DRASTIC Index
122
Northeast and Superior Uplands
(9Da) Glacial Till Over Crystalline Bedrock
This hydrogeologic setting is characterized by moder-
ately low topographic relief and varying thicknesses
of glacial till overlying severely fractured, folded, and
faulted bedrock of igneous and metamorphic origin
with minor occurrences of bedded sedimentary rocks.
The till is chiefly unsorted deposits which may be
interbedded with localized deposits of sand and
gravel. Although ground water occurs in both the
glacial deposits and fractured bedrock, the bedrock is
the principal aquifer. The glacial till serves as a
recharge source. Although precipitation is abundant,
recharge is only moderately high because of the low
permeability of the glacial till and the surficial
deposits which typically weather to loam. Depth to
water is extremely variable depending in part on the
thickness of the glacial till, but is typically moderately
shallow.
101
-------
Setting 9 Da Glacial Till Over Crystalline Bedrock
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Metamorphic/
Igneous
Loam
2-6%
Silt/Clay
1-100
Weight
5
4
3
2
1
5
3
Rating
7
8
3
5
9
1
1
Number
35
32
9
10
9
5
3
DRASTIC Index 103
Setting 9 Da Glacial Till Over Crystalline Bedrock
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Range
15-30
7-10
Metamorphic/
Igneous
Loam
2-6%
Weight
5
4
3
5
3
Rating
7
8
3
5
9
Number
35
32
9
25
27
Impact Vadose
Zone Silt/Clay
Hydraulic
Conductivity 1-100
4
2
Agricultural
DRASTIC Index
4
2
134
Setting 9 Db Glacial Till Over Outwash
Feature
General
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
30-50
7-10
Sand and Gravel
Loam
2-6%
Silt/Clay
1 000-2000
5
4
3
2
1
5
3
5
8
8
5
9
1
8
25
32
24
10
9
5
24
DRASTIC Index 129
Setting 9 Db Glacial Till Over Outwash
Agricultural
Northeast and Superior Uplands
(9Db) Glacial Till Over Outwash
This hydrogeologic setting is characterized by low
topography and outwash materials which are covered
by varying thicknesses of glacial till. The till is chiefly
unsorted deposits which may be interbedded with
localized deposits of sand and gravel. Surficial
deposits have usually weathered to a loam. Although
ground water occurs in both the glacial till and in the
underlying outwash, the outwash serves as the
principal aquifer because the fine-grained deposits
have been removed by glacial meltwater. The out-
wash is in direct hydraulic connection with the glacial
till and the glacial till serves as a source of recharge
for the underlying outwash. Precipitation is abundant
in the region but recharge is moderate because of the
relatively low permeability of the overlying glacial till.
Depth to water table is extremely variable depending
in part on the thickness of the glacial till, but averages
around 30 feet.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
30-50
7-10
Sand and Gravel
Loam
2-6%
Silt/Clay
1000-2000
Weight Rating
5 5
4 8
3 8
5 5
3 9
4 1
2 8
Agricultural
DRASTIC Index
Number
25
32
24
25
27
4
16
153
Northeast and Superior Uplands
(9E) Outwash
This hydrogeologic setting is characterized by moder-
ate topographic relief and varying thickness of out-
wash which overlie fractured bedrock of sedimentary.
102
-------
metamorphic, or igneous origin. The outwash consists
of water-washed deposits of sand and gravel which
often serve as the principal aquifers in the area, and
which typically have a sandy loam surficial layer. The
outwash also serves as a source of recharge to the
underlying bedrock. Recharge is abundant and
ground-water recharge is high. Water levels are
extremely variable, but are relatively shallow.
Setting 9 E Outwash
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
9
9
8
6
9
8
8
Number
45
36
24
12
9
40
24
Northeast and Superior Uplands
(9F) Moraine
This hydrogeologic setting is characterized by moder-
ate topography and varying thicknesses of mixed
glacial deposits which overlie fractured bedrock of
sedimentary, igneous, or metamorphic origin. This
setting issimilarto(9E)Outwash in that the sand and
gravel within the morainal deposits is well-sorted and
serves as the principal aquifer in the area. These
deposits also serve as a source of rec'harge for the
underlying bedrock. Moraines also contain sediments
that are typically unsorted and unstratified; these
deposits contain more fines than outwash deposits,
are less permeable and characteristically more like
glacial till. Moraines are typically mounds or ridges of
till which were deposited along the margin of a
stagnant or retreating glacier. Surf icial deposits often
weather to a sandy loam. Precipitation is abundant
throughout the region and ground-water recharge is
moderately high. Water levels are extremely variable,
based in part on the thickness of the glacial till, but are
typically fairly shallow.
DRASTIC Index 190
Setting 9 E Outwash
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
1 000-2000
Weight Rating
5 9
4 9
3 8
5 6
3 9
4 8
2 8
Agricultural
DRASTIC Index
Number
45
36
24
30
27
32
16
210
Setting 9 F Moraine
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Sandy Loam
6-12%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
Rating
7
8
8
6
5
8
6
Number
35
32
24
12
5
40
18
DRASTIC Index 166
103
-------
Setting 9 F Moraine
Agricultural
Setting 9 Ga River Alluvium With Overbank
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Sandy Loam
6-12%
Sand and Gravel
700-1000
Weight Rating
5 7
4 8
3 8
5 6
3 5
4 8
2 6
Agricultural
DRASTIC Index
Number
35
32
24
30
15
32
12
180
Northeast and Superior Uplands
(9Ga) River Alluvium With Overbank
This hydrogeologic setting is characterized by low
topography and thin to moderately thick deposits of
alluvium along parts of river valleys. The alluvium is
underlain by fractured bedrock of sedimentary,
metamorphic, or igneous origin. Water is obtained
from sand and gravel layers which are interbedded
with finer-grained alluvial deposits. The flood plain is
covered by varying thicknesses of fine-grained silt
and clay, called overbank deposits. The overbank
thickness is usually greater along major streams (as
much as 40 feet) and thinner along minor streams.
Precipitation is abundant, but recharge is somewhat
reduced because of the silty overbank deposits and
subsequent clayey loam soils which typically cover
the surface. Water levels are typically moderately
shallow and may be hydraulically connected to the
stream or river. The alluvium may serve as a signifi-
cant source of water and is also usually in direct
hydraulic connection with the underlying bedrock.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1 000-2000
Weight
5
4
3
2
1
5
3
General
Rating
7
8
8
3
10
1
8
Number
35
32
24
6
10
5
24
DRASTIC Index 136
Setting 9 Ga River Alluvium With Overbank
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1 000-2000
Weight Rating
5 7
4 8
3 8
5 3
3 10
4 1
2 8
Agricultural
DRASTIC Index
Number
35
32
24
15
30
4
16
156
Northeast and Superior Uplands
(9Gb) River Alluvium Without Overbank
This hydrogeologic setting is identical to (9Ga) River
Alluvium With Overbank except that no significant
fine-grained flood plain deposits occupy the stream
valley. This results in significantly higher recharge
where precipitation is adequate and sandy soils occur
at the surface. Water levels are moderate to shallow
in depth. Hydraulic contact with the surface stream is
usually excellent, with alternating recharge/discharge
relationships varying with stream stage. These
deposits serve as a good source of recharge to the
underlying fractured bedrock.
104
-------
Setting 9 Gb River Alluvium Without Overbank
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadosa
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
0-2%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
9
9
8
6
10
8
8
Number
45
36
24
12
10
40
24
DRASTIC Index 191
Setting 9 Gb River Alluvium Without Overbank
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
0-2%
Sand and Gravel
1 000-2000
Weight Rating
5 9
4 9
3 8
5 6
3 10
4 8
2 8
Agricultural
DRASTIC Index
Number
45
36
24
30
30
32
16
213
705
-------
10. A tlantic and Gulf Coastal Plain
(Complexly interbedded sand, silt, and clay)
The Atlantic and Gulf Coastal Plain region is an area
of about 844,000 km2 extending from Cape Cod,
Massachusetts, on the north to the Rio Grande in
Texas on the south. This region does not include
Florida and parts of the adjacent States; although
those areas are a part of the Atlantic and Gulf Coastal
Plain physiographic province, they together form a
separate ground-water region. (See region 11,
"Southeast Coastal Plain.")
The Atlantic and Gulf Coastal Plain region ranges in
width from a few kilometers near its northern end to
nearly a thousand kilometers in the vicinity of the
Mississippi River. The great width near the Missis-
sippi reflects the effect of a major downwarped zone
in the Earth's crust that extends from the Gulf of
Mexico to about the confluence of the Mississippi and
Ohio Rivers. This area is referred to as the Mississippi
embayment.
The topography of the region ranges from extensive,
flat, coastal swamps and marshes 1 to 2 m above
sea level to rolling uplands, 100 to 250 m above sea
level, along the inner margin ofthe region.
The region is underlain by unconsolidated sediments
that consist principally of sand, silt, and clay trans-
ported by streams from the adjoining uplands. These
sediments, which range in age from Jurassic to the
present, range in thickness from less than a meter
near the inner edge of the region to more than 12,000
m in southern Louisiana. The greatest thicknesses
are along the seaward edge of the region and along
the axis of the Mississippi embayment. The sediments
were deposited on floodplains and as deltas where
streams reached the coast and, during different
invasions of the region by the sea, were reworked by
waves and ocean currents. Thus, the sediments are
complexly interbedded to the extent that most of the
named geologic units into which they have been
divided contain layers of the different types of
sediment that underlie the region. These named
geologic units (or formations) dip toward the coast or
toward the axis of the Mississippi embayment, with
the result that those that crop out at the surface form
a series of bands roughly parallel to the coast or to the
axis of the embayment. The oldest formations crop
out along the inner margin of the region, and the
youngest crop out in the coastal area.
Within any formation the coarsest grained materials
(sand, at places interbedded with thin gravel layers)
tendtobe most abundant near source areas. Clay and
silt layers become thicker and more numerous
downdip.
Although sand, silt, and clay, as noted above, are the
principal types of material underlying the Atlantic and
Gulf Coastal Plain, there are also a small amount of
gravel interbedded with the sand, a few beds com-
posed of mollusk shells, and a small amount 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 26,000 km2 in eastern North Carolina, is more
than 200 m 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
106
-------
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.
Movement of water from recharge areas to discharge
areas is controlled, as in all ground-water systems, by
hydraulic gradients, but in this region the pattern of
movement is complicated by down-dip thickening of
clay which hampers upward discharge. As a result,
movement down the dip of the permeable layers
becomes increasingly slow with increasing distance
from the outcrop areas. This causes many flow lines
to converge on the discharge areas located on major
streams near the downdip part of outcrop areas.
These areas of concentrated ground-water discharge
are referred to as "artesian-water gaps" by LeGrand
and Pettyjohn (1981).
Wells that yield moderate to large quantities of water
can be constructed almost anywhere in the region.
Because most of the aquifers consist of unconsoli-
dated sand, wells require screens; wh'ere the sand is
fine-grained and well sorted, the common practice is
to 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.
The reduction of storage in an aquifer in the vicinity of
a pumping well is reflected in a decline in ground-
water levels and is necessary in order to establish a
hydraulic gradient toward the well. If withdrawals are
continued for long periods in areas underlain by thick
sequences of unconsolidated deposits, such as the
Atlantic and Gulf Coastal Plain, 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 9 m as of 1978. Subsidence near
pumping centers in the Atlantic Coastal Plain has not
yet been confirmed but is believed to be occurring,
though at a slower rate than along the Texas Gulf
Coast.
The depletion of storage in confining beds is perma-
nent, and subsidence of the land surface that results
from such depletion is also permanent. On the other
hand, depletion of storage in aquifers may not be fully
permanent, depending on the availability of recharge.
In arid and semiarid regions, recharge rates are
extremely small, and depletion of aquifer storage is,
for practical purposes, permanent. Depletion of
storage in aquifers in these regions is referred to as
mining. In humid regions, recharge is sufficient to
replace aquifer storage rather quickly, once with-
drawals are stopped, so that depletion of aquifer
storage in these areas is not considered to be mining.
The important point is that depletion of storage in the
confining layers of silt and clay in both arid and humid
regions is permanent but is not normally considered
to be ground-water mining. The term "mining" is
applied by most ground-water hydrologists only to
areas in which aquifer storage is being permanently
depleted.
Depletion of storage in the aquifers underlying large
areasoftheAtlanticandGulf 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.
This is a water-management problem that will
become more important as rates of withdrawal and
the lowering of water levels increase. Solutions to
this problem include (1) concentrating withdrawals
as close as possible to outcrop (recharge) areas, (2)
dispersing withdrawals in regions remote from the
outcrop areas over the widest possible area, and (3)
increasing withdrawals from surficial aquifers to the
maximum possible extent.
Another problem that affects ground-water develop-
ment in the region concerns the presence of saline
water in the deeper parts of most aquifers. The
occurrence of saline water is controlled by the
circulation of freshwater which, as noted previously,
becomes increasingly slow down the dip of the
aquifers. Thus, in some of the deeper aquifers, the
interface between freshwater and saltwater is in-
shore, but in parts of the region, including parts of
Long Island, New Jersey, and Mississippi, the inter-
face in the most intensively developed aquifers is a
significant distance offshore. Pumping near the
interfaces has resulted in problems of saltwater
encroachment locally.
Another significant feature of the ground-water
system in this region is the presence of "geo-
pressured" zones at depths of 1,800 to 6,100 m 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 than 4,000 m above land surface. Because the
elevated temperature, natural gas, and high pres-
107
-------
sures are all potential energy sources, these zones
are under intensive investigation.
Atlantic and Gulf Coastal Plain
(10A a) Confined Regional A quifers
This hydrogeologic setting is characterized by moder-
ately low topographic relief and gently dipping,
complexly interbedded unconsolidated and semi-
consolidated deposits which consist primarily of
sand, silt, and clay. Outcrops of these deposits form a
series of bands roughly parallel to the coast or to the
axis of the Mississippi Embayment. The outcrop areas
and overlying semi-permeable beds are the principal
sources of recharge to the formations which serve as
regional aquifers. Precipitation is abundant and
recharge is moderately high in the outcrop areas but
low regionally to deep zones. Surficial deposits
typically weather to a sandy loam. Large quantities of
water are obtained from the sand and gravel and sand
deposits within the aquifer. Water levels are ex-
tremely variable and typically are shallower toward
the shoreline. When ground water is heavily pumped
near the shoreline, these aquifers are very susceptible
to salt-water intrusion. Since the shallow aquifers
are very vulnerable to pollution due to their permeable
nature, and the deeper aquifers are recharged from
the shallow ones, the entire system is somewhat
susceptible to ground-water pollution. The degree of
vulnerability varies according to the nature of the
deposits and the amount of recharge.
Setting 10 Aa Confined Regional Aquifers
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Sand and Gravel
*
«
Silt and Clay
300-700
Weight
5
4
3
2
1
5
3
Rating
1
1
8
1
1
1
4
Number
5
4
24
2
1
5
12
DRASTIC Index 53
Setting 10 Aa Confined Regional Aquifers
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Sand and Gravel
#
•*
Silt and Clay
300-700
Weight
5
4
3
5
3
4
2
Rating
1
1
8
1
1
1
4
Number
5
4
24
5
3
4
8
Agricultural
DRASTIC Index
53
Atlantic and Gulf Coastal Plain
(WAb) Unconsolidated & Semi-Consolidated
Shallow Surficial Aquifer
This setting is very similar to (10Aa) Confined
Regional Aquifers except that the principal aquifer is
the shallow surficial deposits which serve as a local
source of water and typically provide recharge for the
regional aquifer. Water is obtained from the surficial
sand and gravel which may be separated from the
underlying regional aquifer by a confining layer. This
confining layer typically leaks providing recharge to
the deeper zones. Surficial deposits are sandy loams.
Water levels tend to be quite shallow, especially near
the coast. Precipitation is abundant and recharge to
the ground water is high. These deposits are very
vulnerable to ground-water pollution due to their
permeable nature.
705
-------
Setting 10 Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer
General
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
1O
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
700-1 000
5
4
3
2
1
5
3
9
9
8
6
9
8
6
45
36
24
12
9
40
18
DRASTIC Index 184
Setting 10 Ab Unconsolidated & Semi-Consolidated Shallow
Surficial Aquifer
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
5
3
4
2
Rating
9
9
8
6
9
8
6
Number
45
36
24
30
27
32
12
Agricultural
DRASTIC Index
206
Atlantic and Gulf Coastal Plain
(10B a) River A lluvium With O verbank
This hydrogeologic setting is characterized by low
topography and thin to moderately thick deposits of
alluvium along parts of river valleys. The alluvium is
underlain by consolidated and semi-consolidated
sedimentary rocks. Water is obtained from sand and
gravel layers which are interbedded with finer-
grained alluvial deposits. The floodplain is covered by
varying thicknesses of fine-grained, silty deposits
called overbank deposits. The overbank thickness is
usually greater along major streams (as much as 40
feet) and thinner along minor streams. Precipitation
in the region is abundant, but recharge is somewhat
reduced because of the silty overbank deposits and
subsequent silty soils which typically cover the
surface. Water levels are typically moderately shal-
low. The alluvium may serve as a significant source of
water and may be in direct hydraulic connection with
the underlying sedimentary rocks. The alluvium may
also serve as a source of recharge to the underlying
bedrock. Many streams in this setting provide only
fine-grained deposits (silts and clays) and as such do
not form good aquifers. They still, however, provide a
good source of recharge.
Setting 10 Ba River Alluvium With Overbank Deposit
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Silty Loam
0-2%
Silt/Clay
700-1000
Weight
5
4
3
2
1
5
3
Rating
7
8
8
4
10
1
6
Number
35
32
24
8
10
5
18
DRASTIC Index 132
109
-------
Setting 10 Ba River Alluvium With Overbank Deposit
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Silty Loam
0-2%
Silt/Clay
700-1000
Weight Rating
5 7
4 8
3 8
5 4
3 10
4 1
2 6
Agricultural
DRASTIC Index
Number
35
32
24
20
30
4
12
157
Atlantic and Gulf Coastal Plain
(lOBb) River Alluvium Without Overbank
This setting is identical to (10Ba) River Alluvium With
Overbank except that no significant fine-grained
floodplain deposits occupy the stream valley. This
results in significantly higher recharge and sandy
soils at the surface. Water levels are typically closer to
the surface because banks of fine-grained deposits
are not present. Throughout much of this region there
is an abundance of coarse-grained material, which
limits this setting for water supply. These materials,
however, provide a good source of recharge to the
underlying consolidated and semi-consolidated bed-
rock.
Setting 10 Bb River Alluvium Without Overbank Deposit
General
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
10+
Sand and Gravel
Sand
0-2%
S & G w/sig.
Silt and Clay
1000-2000
5
4
3
2
1
5
3
9
9
8
9
10
6
8
45
36
24
18
10
30
24
DRASTIC Index 187
Setting 10 Bb River Alluvium Without Overbank Deposit
Agricultural
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
10+
Sand and Gravel
Sand
0-2%
S & G w/sig.
Silt and Clay
1 000-2000
5 9
4 9
3 8
5 9
3 10
4 6
2 8
Agricultural
DRASTIC Index
45
36
24
45
30
24
16
220
Atlantic and Gulf Coastal Plain
(IOC) Swamp
This hydrogeologic setting is characterized by low
topographic relief and deposits of sand, and sand and
gravel, which overlie consolidated and semi-consoli-
dated sedimentary rocks. Surficial deposits are
typically sand mixed with organic material. The
surficial sands are usually in hydraulic connection
with the underlying aquifers and serve as a source of
recharge. Precipitation is abundant and potential
recharge is high. Water levels are typically at or near
the surface during the majority of the year. While a
swamp is frequently a ground water discharge zone,
and as such not especially vulnerable to pollution, it is
surficially an environmentally sensitive area. It
should also be noted that a slight reversal in gradient
would easily convert the swamp into a ground water
recharge zone. Thus, it is potentially highly vulnerable
to ground-water pollution.
110
-------
Setting 10 C Swamp
Feature Range
General
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
0-5
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
1000-2000
10
9
8
9
10
50
36
24
18
10
40
24
DRASTIC Index 202
Setting IOC Swamp
Feature Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
0-5
10+
Sand and Gravel
Sand
0-2%
Topography
Impact Vadose
Zone Sand and Gravel
Hydraulic
Conductivity 1000-2000
10
9
8
9
10
50
36
24
45
30
32
16
Agricultural
DRASTIC Index 233
-------
7 7. Southeast Coastal Plain
(Thick layers of sand and clay over semi-consolidated carbonate rocks)
The Southeast Coastal Plain is an area of about
212,000 km2 in Alabama, Florida, Georgia, and South
Carolina. It is a relatively flat, low-lying area in which
altitudes range from sea level at the coast to about
100 m down the center of the Florida peninsula and
as much as 200 m on hills in Georgia near the interior
boundary of the region. Much of the area, including
the Everglades in southern Florida, is a nearly flat
plain less than 10m 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. From the coast up to altitudes of nearly
100 m, the surficial deposits are associated with
marine terraces formed when the Coastal Plain was
inundated at different times by the sea. 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. The most
extensive Miocene deposit is the Hawthorn Forma-
tion. The formations of middle to late Miocene age,
and where those formations are absent, the surficial
deposits overlie semi-consolidated limestones and
dolomites that are as much as 1,500 m thick. These
carbonate rocks range in age from early Miocene to
Paleocene and are generally referred to collectively
as Tertiary limestones.
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 meters
thick composed largely of loose aggregations of shells
of foraminifers and fragments of echinoids and other
marine organisms interbedded with much thinner
layers of cemented and cherty limestone. The
Floridan, one of the most productive aquifers in the
world, is the principal source of ground-water sup-
plies in the southeast Coastal Plain region.
In southern Florida, south of Lake Okeechobee, and in
a belt about 30 km 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
which underlies the southeastern part of Florida and
which in the Miami area consists of 30 to 100 m of
cavernous limestone and sand 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 (conservation areas) developed
112
-------
in the Everglades. It is the principal source of water
for municipal, industrial, and irrigation uses and can
yield as much as 5 m3 min~1 (1,300 gal min~1) to
small-diameter wells less than 25 m deep finished
with open holes only 1 to 2 m in length.
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
throughout the region and are the primary sources of
ground water where the water intheFloridan aquifer
contains more than about 259 mg/l of chloride.
The Floridan aquifer, as noted above, 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. 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. The principal recharge
areas include a broad area along the west side of
Florida extending from the central part of the penin-
sula 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 120 mm yr~1 (5 in. yr~1).
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. 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 meters to
several kilometers. The larger depressions, which
represent a more advanced stage of solution of the
limestone and collapse of the overlying material, 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 potenti-
ometric surface of the aquifer stands at a higher
altitude than the water table. In most of these areas,
which include the southern third of the Florida
peninsula, the east coast area and major stream
valleys of Florida, and the coastal zone and major
stream vaMeys of Georgia and South Carolina, wells
open to the aquifer will flow at the land surface. Such
wells are called "flowing artesian wells." The most
spectacular discharge from the Floridan aquifer is
through sinkholes exposed along streams and off-
shore. Florida has 27 springs of the first magnitude at
which the average discharge exceeds 2.83 m3 sec"1
(100 ft3 sec'1). The largest is Silver Springs, which
has an average discharge of 23.2 m3 sec~1 (530
million gallons per day) and reached a maximum
discharge of 36.5 m3 sec~1 on September 28, 1960.
Heath and Conover(1 981 (estimate that the combined
discharge from Florida's springs is 357 m3 sec~1 (8
billion gallons per day).
The marked difference in ground-water conditions
between the Southeast Coastal Plain and the Atlantic
and Gulf Coastal Plain regions is apparent in the
response of ground-water levels to withdrawals. In
the Atlantic and Gulf Coastal Plain region most large
withdrawals are accompanied by a pronounced
continuing decline in ground-water levels. In the
Southeast Coastal Plain, on the other hand, large
withdrawals have significantly lowered ground-water
levels in only a few areas.
Southeast Coastal Plain
(11 A) Solution Limestone
This hydrogeologic setting is characterized by low to
moderate topographic relief and deposits of limestone
which have been partiallydissolvedtoform a network
of solution cavities and caves. Surficial deposits
typically consist of sands. Precipitation is abundant
and recharge is high. Water levels are variable but are
usually moderate in the limestone and shallow in the
overlying surficial sands. These sands serve as an
important source of recharge for the limestones. Due
to the presence of a shallow water table and direct
recharge to the limestone these surficial sands are
very vulnerable to pollution. Near the coast, these
aquifers are very susceptible to salt-water intrusion.
113
-------
Setting 11 A Solution Limestone
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Karst Limestone
Sand
2-6%
Karst Limestone
2000+
Weight
5
4
3
7
1
5
3
Rating
9
9
10
9
9
10
10
Number
45
36
30
18
9
50
30
DRASTIC Index 218
Setting 11 A Solution Limestone
Feature Range Weight Rating Number
Agricultural
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
10+
Karst Limestone
Sand
2-6%
Karst Limestone
2000+
5 9
4 9
3 10
5 9
3 9
4 10
2 10
Agricultural
DRASTIC Index
45
36
30
45
27
40
20
243
Setting 11 B Coastal Deposits
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
General
Rating
9
9
8
9
10
8
6
Number
45
36
24
18
10
40
18
DRASTIC Index 191
Southeast Coastal Plain
(11 B) Coastal Deposits
This hydrogeologic setting is characterized by flat
topography and unconsolidated deposits of carbonate,
sand, gravel, clay, and shell beds which overlie semi-
consolidated carbonate rocks. The surficial deposits
serve as direct use sources of ground water and also
serve as recharge for the underlying carbonate rocks
where the gradient is downward toward the car-
bonates. The carbonates serve as a source of ground
water but may contain saline water in some areas.
Precipitation is abundant and recharge is high. Water
levels may vary, but are typically close to the surface.
Setting 11 B Coastal Deposits
Feature Range
Agricultural
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
5-15
10+
Sand and Gravel
Sand
0-2%
Sand and Gravel
700-1 000
5
4
3
5
3
4
2
9
9
8
9
10
8
6
45
36
24
45
30
32
12
Agricultural
DRASTIC Index
224
Southeast Coastal Plain
(11C) Swamp
This hydrogeologic setting is characterized by flat
topographic relief, very high water levels, and deposits
of limestone which have partially been dissolved to
114
-------
form a network of solution cavities and caves. Soils
are typically sand and recharge is high due to the
abundant precipitation. The limestone typically serves
as the major regional aquifer. Water levels are
typically at or above the surface during the majority of
the year. These swamps are typically discharge areas,
but due to their environmental vulnerability, and
possible gradient reversal, they should be regarded as
areas of maximum (potential) recharge.
Southeast Coastal Plain
(1 ID) Beaches and Bars
This hydrogeologic setting is characterized by moder-
ate to flat topographic relief and unconsolidated
deposits of water-washed sands. These sands are
well-sorted and very permeable, and may serve as
localized sources of ground water. These deposits
also serve as a source of recharge to the underlying
unconsolidated coastal deposits. Precipitation is
abundant and recharge is high. Water levels may
vary, but are typically shallow. These arears are highly
susceptible to pollution due to their high permeabil-
ities.
Setting 11 C Swamp
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Karst Limestone
Sand
0-2%
Karst Limestone
2000+
Weight
5
4
3
2
1
5
3
General
Rating
10
9
10
9
10
10
10
Number
50
36
30
18
10
50
30
Setting 11 D Beaches and Bars
Setting 11 C Swamp
DRASTIC Index 224
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sand
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
2
1
5
3
General
Rating
9
9
8
9
9
8
6
Number
45
36
24
18
9
40
18
Setting 11 D Beaches and Bars
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Karst Limestone
Sand
0-2%
Karst Limestone
2000+
Weight Rating
5 10
4 9
3 10
5 9
3 10
4 10
2 10
Agricultural
DRASTIC Index
Number
50
36
30
45
30
40
20
251
DRASTIC Index 190
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sand
2-6%
Sand and Gravel
700-1000
Weight
5
4
3
5
3
4
2
Rating
9
9
8
9
9
8
6
Number
45
36
24
45
27
36
12
Agricultural
DRASTIC Index
225
115
-------
12. Hawaiian Islands
(Lava flows segmented in part by dikes, interbedded with ash deposits, and partly overlain by alluvium)
I Off
40*
40°
'Off
VI
HAWAIIAN
--. ISLAND
/ F I C
O C E A F<
The Hawaiian Islands region encompasses the State
of Hawaii and consists of eight major islands occu-
pying an area of 16,707 km2 in the Pacific Ocean
3,700 km southeast of California. The islands are the
tops of volcanoes that rise from the ocean floor and
stand at altitudes ranging from a few meters to more
than 4,000 m above sea level. Each island was
formed by lava that issued from one or more eruption
centers. The islands have a hilly to mountainous
appearance resulting from erosion that has carved
valleys into the volcanoes and built relatively narrow
plains along parts of the coastal areas.
Each of the Hawaiian Islands is underlain by hundreds
of distinct and separate lava flows, most of which are
composed of basalt. The lavas issued in repeated
outpourings from narrow zones of fissures, first
below sea level, then above it. The lavas that extruded
below the sea are relatively impermeable. Those
formed above sea level tend to be highly permeable,
with interconnected openings that formed as the lava
cooled, cavities and openings that were not filled by
the overlying flow, and lava tubes (tunnels). The
central parts of the thicker flows tend to be more
massive and less permeable; the most common
water-bearing openings are joints and faults that
formed after the lava solidified. Thin layers of ash and
weathered volcanic rock occur irregularly between
some of the flows that formed above sea level. The
lava flows in valleys and parts of the coastal plains are
covered by a thin layer of alluvium consisting of coral
(limestone) fragments, sand-size fragments of basalt,
and clay.
The fissures through which the lava erupted tend to
cluster near eruption centers. Flows from the fissures
moved down depressions on the adjacent slopes to
form layers of lavathatdipat angles of4to 10degrees
toward the margins of the volcanoes. The result, prior
to modification by erosion, is a broad, roughly circular,
gently convex mountain similar in shape to a warrior's
shield. Thus, volcanoes of the Hawaiian type are
referred to as shield volcanoes. When eruption along
a fissure ceases, the lava remaining in the fissure
solidifies to form a dike.
All of the islands have sunk, to some extent, as a
result of a downward flexing of the Earth's crust
caused by the weight of the volcanoes. This has
resulted in flows that formed above sea level being
depressed below sea level. The upper parts of these
flows contain freshwater that serves as an important
source of water.
In mineral composition and nature of the water-
bearing openings, the lavas that form the Hawaiian
Islands are very similar to those in the Columbia
Plateau region. Thus, from these two standpoints,
116
-------
these regions could be combined into one. There is,
however, one important difference that justifies their
treatment as separate regions. This difference relates
to the presence of seawater around and beneath the
islands, which significantly affects the occurrence
and development of water supplies.
From the standpoint both of description and of
development, it is useful to divide the ground-water
system of the Hawaiian Islands into three parts. The
first part consists of the higher areas of the islands in
the vicinity of the eruption centers. The rocks in these
areas are formed into a complex series of vertical
compartments surrounded by dikes developed along
eruption fissures. The ground water in these com-
partments is referred to as dike-impounded water.
The second, and by far the more important, part of the
system consists of the lava flows that flank the
eruption centers and that contain fresh ground water
floating on saline ground water. These flank flows are
partially isolated hydraulically from the vertical
compartments developed by the dikes that surround
the eruption centers. The fresh ground water in these
flows is referred to as basal ground water. In parts of
the coastal areas the basal water is confined by the
overlying alluvium. The third part of the system
consists of fresh water perched, primarily in lava
flows, on soils, ash, or thick impermeable lava flows
above basal ground water.
The ground-water system is recharged by precipita-
tion which ranges annually from about 160 mm to
more than 11,000 mm. This wide range in precipita-
tion reflects the effect of the islands on the moist
northeast trade winds. As the moisture-laden winds
are deflected upward by the mountains, precipitation
falls on the higher elevations. Precipitation is heaviest
on mountains below 1,000 m and lightest in the
coastal areas on the leeward side of the islands and at
elevations above 1,000 m on the islands of Maui and
Hawaii. The average annual precipitation on the
islands is estimated to be about 1,800 mm. Because
of the highly permeable nature of the volcanic soils, it
is estimated that about 30 percent of the precipitation
recharges the ground-water system.
Some discharge of dike-impounded ground water
doubtless occurs through fractures in the dikes into
the flanking lava flows. This movement must be
small, however, because water stands in the com-
partments at levels hundreds of meters above sea
level and the principal discharge occurs as springs on
the sides and at the heads of valleys where erosion
has removed parts of the dikes. Both the basal ground
water and the perched ground water in the lava flows
surrounding the dike-bounded compartments is
recharged by precipitation and by streams leaving the
dike-bounded area. Discharge is to streams and to
springs and seeps along the coast.
The basal water is the principal source of ground
water on the islands. Because the freshwater is
lighter (less dense) than seawater, it floats as a lens-
shaped body on the underlying seawater. The thick-
ness of the freshwater zone below sea level essenti-
ally depends on the height of the freshwater head
above sea level. Near the coast the zone is thin, but
several kilometers inland from the coast on the larger
islands it reaches thicknesses of at least a few
hundred meters. In parts of the coastal zone, and
especially on the leeward side of the islands, the
basal ground water is brackish.
Forty-six percent of the water used in Hawaii in 1 975,
or 3.1 x 106 m3 day"1, was ground water. It is obtained
through horizontal tunnels and through both vertical
and inclined wells. Tunnels are used to obtain
supplies of basal water near the coast where the
freshwater zone is thin. Tunnels are also used to tap
dike-impounded water. These tunnels encounter
large flows of water when the principal impounding
dike is penetrated and it is necessary to drain most of
the water in the saturated zone above the tunnel
before construction can be completed. Thereafter, the
yield of the tunnel reflects the rate of recharge to the
compartment tapped by the tunnel. To avoid a large
initial waste of water and to preserve as much storage
as possible, the Honolulu Board of Water Supply has
begun to construct inclined wells to obtain dike-
impounded water. Vertical wells are used to obtain
basal water and perched ground water in inland areas
where the thickness of the freshwater zone permits
the use of such wells.
Hawaii
(12A) Mountain Slopes
This hydrogeologic setting is characterized by steep
slopes composed of volcanic lava flows, breccia, and
related extrusive magmatic rocks. Soils are thin, but
highly permeable where present. Rubble alluvial
deposits are common. Because of the steep topog-
raphy and elevation the water table tends to be deep.
Water occurs in the fractures and vesicular zones of
the basaltic lava flows, and along the relatively
horizontal inter-flow zones. Overall, hydraulic conduc-
tivity is moderately high, due to the density of fracture
zones. Perched water table zones are common, where
water in an inter-flow zone between successive lava
flows is delayed from moving downward by a dense
layer of clayey material, or basalt. The dense layer
acts as an aquitard. Rainfall is high, and with perme-
able surface material recharge is also high.
117
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Hawaii
(12B) Alluvial Mountain Valleys
This hydrogeologic setting is characterized by narrow,
steep-walled valleys, with moderate to steep seaward
slope. The valleys contain alluvial material varying
typically from zero to a few tens of feet in thickness.
Waterfalls and related features are common near the
ocean. The alluvium consists of basaltic debris and
the weathered products thereof. Soils are moderately
developed, thin, and quite permeable. Rainfall is high,
infiltration, or recharge, is high, and vegetation is
lush. The alluvium below stream grade is generally
saturated at a shallow level, and is sometimes
hydraulically connected to the permanent water
table, sometimes to perched zones, and sometimes,
particularly in the upper reaches, leaks into the
vadose zone. Hydraulic conductivity of both the
alluvium and underlying aquifers is high.
Setting 12 A Mountain Slopes
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
10+
Basalt
Thin or Absent
18+%
Basalt
2000+
Weight
5
4
3
2
1
5
3
Rating
1
g
9
10
1
g
10
Number
5
36
27 ,
20
1
45
30
Setting 12 A Mountain Slopes
DRASTIC Index 1 64
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range Weight
Rating Number
Setting 12 B Alluvial Mountain Valleys
100+
10+
Basalt
Thin or Absent
18+%
Basalt
2000+
5
4
3
5
3
4
2
1
g
g
10
1
g
10
Agricultural
DRASTIC Index
5
36
27
50
3
36
20
177
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
12-18%
1000-2000
Weight
5
4
3
2
1
5
3
General
Rating
g
g
8
6
3
8
8
Number
45
36
24
12
3
40
24
DRASTIC Index 184
118
-------
Setting 12 B Alluvial Mountain Valleys
Agricultural
Setting 12 C Volcanic Uplands
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
10+
Sand and Gravel
Sandy Loam
12-18%
Sand and Gravel
1000-2000
Weight
5
4
3
5
3
4
2
Rating
9
9
8
6
3
8
8
Number
45
36
24
30
9
32
16
Agricultural
DRASTIC Index
192
Hawaii
(12C) Volcanic Uplands
This hydrogeologic setting is characterized by moder-
ately rolling topography, at medium elevations, and
rich, dark, soils developed from the basaltic bedrock.
The soils are permeable, rainfall is high, and recharge
is high. Bedrock is composed primarily of alternating
extrusive basaltic lava flows and interlayered weath-
ered zones formed between flows. Ground water
occurs at moderate to deep depths, and aquifer yield
is controlled by fracture zones, vesicular zones (both
primarily cooling features) and the inter-flow weath-
ered zones. Hydraulic conductivity is high. As with
other settings in Hawaii, heavy pumping stresses
often result in salt-water intrusion. This is a reflection
of the fact that each island is surrounded by and
underlain by salt water, with the fresh water occur-
ring in a lenticular body that floats on the salt water.
Ground-water yield is therefore limited quite specifi-
cally to the amount of water recharged annually.
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
10+
Basalt
Sandy Loam
6-12%
Basalt
2000+
Weight
5
4
3
2
1
5
3
Rating
2
9
9
6
5
9
10
Number
10
36
27
12
5
45
30
Setting 12 C Volcanic Uplands
DRASTIC Index 165
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
75-100
10+
Basalt
Sandy Loam
6-1 2%
Basalt
2000+
Weight
5
4
3
5
3
4
2
Rating
2
9
9
6
5
9
10
Number
10
36
27
30
15
36
20
Agricultural
DRASTIC Index 174
Hawaii
(12D) Coastal Beaches
This hydrogeologic setting is characterized by low to
moderate topography, near sea level, with sandy
materials at the surface. The sandy soils are very
permeable, and direct recharge from rainfall is high
where ground-water levels permit. Because of their
location these settings are often discharge areas
where ground water is lost into the ocean. Manage-
ment of this area is essential to the maximum
utilization of the ground-water resources of the
islands. It should be noted that all discharge areas are
potential recharge areas, and as such potentially
vulnerable to pollution.
119
-------
Setting 12 D Coastal Beaches
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
2-6%
Sand and Gravel
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
10
9
8
9
9
8
8
Number
50
36
24
18
9
40
24
DRASTIC Index 201
Setting 12 D Coastal Beaches
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
0-5
10+
Sand and Gravel
Sand
2-6%
Sand and Gravel
1000-2000
Weight Rating
5 10
4 9
3 8
5 9
3 9
4 8
2 8
Agricultural
DRASTIC Index
Number
50
36
24
45
27
32
16
230
720
-------
13. Alaska
(Glacial and alluvial deposits, occupied in part by permafrost, and overlying crystalline,
metamorphic, and sedimentary rocks)
The Alaska region encompasses the State of Alaska,
which occupies an area of 1,519,000 km2 at the
northwest corner of North America. Physiographical-
ly, Alaska can be divided into four divisions—from
south to north, the Pacific Mountain System, the
Intermontane Plateaus, the Rocky Mountain System,
and the Arctic Coastal Plain. The Pacific Mountain
System is the Alaskan equivalent of the Coast Range,
Puget Sound Lowland, and Cascade provinces of the
Washington-Oregon area. The Intermontane Plateaus
is a lowland area of plains, plateaus, and low
mountains comparable to the area between the
Cascades-Sierra Nevada and the Rocky Mountains.
The Rocky Mountain System is a continuation of the
Rocky Mountains of the United States and Canada,
andthe Arctic Coastal Plain is the geologic equivalent
of the Great Plains of the United States and Canada.
The coastal areas and lowlands range in altitude from
sea level to about 300 m, and the higher mountains
reach altitudes of 1,500 to 3,000 m. Mt. McKinley in
the Pacific Mountain System is the highest peak in
North America, with an altitude of about 6,300 m.
As would be expected of any area its size, Alaska is
underlain by a diverse assemblage of rocks. The
principal mountain ranges have cores of igneous and
metamorphic rocks ranging in age from Precambrian
to Mesozoic. These are overlain and flanked by
younger sedimentary and volcanic rocks. The sedi-
mentary rocks include carbonates, sandstones, and
shales. In much of the region the bedrock is overlain
by unconsolidated deposits of gravel, sand, silt, clay,
and glacial till.
Climate has a dominant effect on hydrologic condi-
tions in Alaska. Mean annual air temperatures range
from -12°C in the Rocky Mountain System and the
Arctic Coastal Plain to about 5°C in the coastal zone
adjacent to the Gulf of Alaska. The present climate
and the colder climates that existed intermittently in
the past have resulted in the formation of permafrost,
or perennially frozen ground. Permafrost is present
throughout the State except in a narrow strip along
the southern and southeastern coasts. In the northern
part of the Seward Peninsula, in the western and
northern parts of the Rocky Mountain System, and in
the Arctic Coastal Plain, the permafrost extends to
depths as great as 600 m and is continuous except
beneath deep lakes and in the alluvium beneath the
deeper parts of the channels of streams. South of this
area and north of the coastal strip, the permafrost is
discontinuous and depends on exposure, slope.
121
-------
vegetation, and other factors. The permafrost is
highly variable in thickness in this zone but is
generally less than 100 m thick.
Much of the water in Alaska is frozen for at least a part
of each year: that on the surf ace as ice in streams and
lakes or as snow or glacier ice and that below the
surface as winter frost and permafrost. Approximately
half of Alaska, including the mountain ranges and
adjacent parts of the lowlands, was covered by
glaciers during the Pleistocene age. About 73,000
km2, or one-twentieth of the region, is still occupied
by glaciers, most of which are in the mountain ranges
that border the Gulf of Alaska. Precipitation, which
ranges from about 130 mm yr~1 in the Rocky Mountain
System and the Arctic Coastal Plain to about 7,600
mm yr~1 along the southeast coast, falls as snow for 6
to 9 months of the year and even year-round in the
high mountain regions. The snow remains on the
surface until thawing conditions begin, in May in
southern and central Alaska and in June in the arctic
zone. During the period of subfreezing temperatures,
there is no overland runoff, and many streams and
shallow lakes not receiving substantial ground-water
discharge are frozen solid.
From the standpoint of ground-water availability and
well yields, Alaska is divided into three zones. In the
zone of continuous permafrost, ground water occurs
beneath the permafrost and also in small, isolated,
thawed zones that penetrate the permafrost beneath
large lakes and deep holes in the channels of streams.
In the zone of discontinuous permafrost, ground
water occurs below the permafrost and in sand and
gravel deposits that underlie the channels and flood-
plains of major streams. In the zone of discontinuous
permafrost, water contained in silt, clay, glacial till,
and other fine-grained deposits usually is frozen.
Thus, in this zone the occurrence of ground water is
largely controlled by hydraulic conductivity. In the
zone not affected by permafrost, which includes the
Aleutian Islands, the western part of the Alaska
Peninsula, and the southern and southeastern coastal
areas, ground water occurs both in the bedrock and in
the relatively continuous layer of unconsolidated
deposits that mantle the bedrock.
Relatively little is known about the occurrence and
availability of ground water in the bedrock. Perma-
frost extends into the bedrock in both the zones of
continuous and discontinuous permafrost, but springs
that issue from carbonate rocks in the Rocky Mountain
System indicate the presence of productive water-
bearing openings. Small supplies of ground water
have also been developed from sandstones, from
volcanic rocks, and from faults and fractures in the
igneous and metamorphic rocks.
Recharge of the aquifers in the Alaska region occurs
when the ground is thawed in the areas not underlain
by permafrost. This period generally lasts only from
June through September. Because the ground, even
in nonpermafrost areas, is still frozen when most
snowmelt runoff occurs, relatively little recharge
occurs in interstream areas by infiltration of water
across the unsaturated zone. Instead, most recharge
occurs through the channels of streams where they
flow across the alluvial fans that fringe the moun-
tainous areas and in alluvial deposits for some
distance downstream. Because of the large hydraulic
conductivity of the sand and gravel in these areas, the
rate of infiltration is large. Seepage investigations
along Ship Creek near Anchorage indicate channel
losses of 0.07 m3 sec"1 km , which gives an
infiltration rate through the wetted perimeter of about
0.4 m day"1
Discharge from aquifers occurs in the downstream
reaches of streams and through seeps and springs
along the coast. The winter flow of most Alaskan
streams is sustained by ground-water discharge. In
the interior and northern regions, this discharge is
evidenced by the buildup of ice (referred to locally as
"icings") in the channels of streams and on the
adjacent floodplains.
Unlikethe 12 regions which comprise the contiguous
United States, both Alaska and Hawaii are political
subdivisions, not discrete ground-water regions.
Hawaii can betreated as a single region because of its
smaller size and relative geologic simplicity. Alaska,
however, due to its size and complexity includes
several major ground-water regions. For purposes of
this document, these regions are considered hydro-
geologic settings.
Alaska
(13A) Alluvium
This hydrogeologic setting includes floodplains, ter-
races, and alluvial fans of both major valleys and
upland and mountain valleys. Braided streams are
present in the major valley floodplains. Heavy silt/
rock flour loading in streams results in subtantial silt
and clay deposition along with the alluvial sands and
gravels. Ground-water levels are usually shallow
near the streams, into which the ground water
discharges, and considerably deeper along the higher
terraces. Recharge to the ground water is seasonal,
following snowmelt and thawing of frozen areas.
Except for the south coastal area, precipitation is light
to moderate and usually in the form of snow.
Topography is moderate, with a unidirectional down-
stream ground-water movement. Hydraulic conduc-
tivities are moderate to very high in the cleaner
portions of the sand and gravel aquifers.
122
-------
Setting 13 A Alluvium
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
General
Rating
7
3
8
6
9
6
6
Number
35
12
24
12
9
30
18
Setting 13 A Alluvium
DRASTIC Index 140
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig.
Silt and Clay
700-1 000
Weight Rating
5 7
4 3
3 8
5 6
1 9
4 6
2 6
Agricultural
DRASTIC Index
Number
35
12
24
30
27
24
12
164
Alaska
(13B) Glacial and Glaciolacustrine Deposits of
the Interior Valleys
This hydrogeologic setting is characterized by tills and
associated outwash deposits, as well as glacier-
related lake deposits of interbedded sand, silt, and
clay. Ground-water levels are relatively shallow.
Surface soils are typically organic sandy loams with
moderate conductivity. Recharge is moderate to low,
primarily limited by the period of thaw and annual
precipitation. Topography is moderate, and the hy-
draulic conductivity of the outwash aquifers is
generally high.
Setting 138 Glacial & Glaciolacustraine Deposits: Interior
Valleys
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
2-4
Sand and Gravel
Sandy Loam
2-6%
Silt/Clay
1 000-2000
Weight
5
4
3
2
1
5
3
Rating
9
3
8
6
9
1
8
Number
45
12
24
12
9
5
24
DRASTIC Index 131
Setting 13 B Glacial & Glaciolacustraine Deposits: Interior
Valleys
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
5-15
2-4
Sand and Gravel
Sandy Loam
2-6%
Silt/Clay
1 000-2000
Weight
5
4
3
5
3
4
2
Rating
9
3
8
6
9
1
8
Number
45
12
24
30
27
4
16
Agricultural
DRASTIC Index
158
123
-------
Alaska
(13C) Coastal-Lowland Deposits
This hydrogeologic setting includes coastal plains,
deltaic deposits of major streams, beaches and near-
shore bars and spits, and deposits of deep alluvial
coastal basins and valleys. Permafrost severely
affects the northernmost portions of this setting,
which is within the permanent permafrost zone.
Where not permanently frozen, recharge rates are
high seasonally, particularly along streams which are
hydraulically connected to the ground water. Ground-
water depths are at or near the elevation of the
surface streams, and topographic slopes are low to
moderate. The primary aquifers in this setting are the
alluvial sands and gravels that are interbedded with
silts and clays. Thick sequences of all types of
materials are common.
Setting 13 C Coastal Lowland Deposits
Agricultural
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig
Silt and Clay
700-1000
Weight Rating
5 7
4 3
3 8
5 6
3 9
4 6
2 6
Agricultural
DRASTIC Index
Number
35
12
24
30
27
24
12
164
Alaska
(13D) Bedrock of the Uplands and Mountains
This hydrogeologic setting is characterized by deposits
of carbonate rocks, limestone, sandstone, volcanics,
and other igneous and metamorphic rocks. These
formations underlie a thin veneer of alluvium beneath
a large portion of the state. Water levels within this
setting are variable, but generally deep. Exceptions to
this are discharge zones along the flanks of many
mountains. The most notable example of this are
springs discharging from carbonate rocks along the
flanks of mountains. Recharge is limited by precipita-
tion, topography, and predominant permafrost. Soils
are generally thin and poorly developed. Aquifer
conductivities vary from low in some of the fractured
metamorphics to very high in the solution-dissolved
carbonates.
Setting 13 C Coastal Lowland Deposits
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
2-4
Sand and Gravel
Sandy Loam
2-6%
S & G w/sig
Silt and Clay
700-1 000
Weight
5
4
3
2
1
5
3
Rating
7
3
8
6
9
6
6
Number
35
12
24
12
9
30
18
DRASTIC Index 140
124
-------
Setting 13 D Bedrock of the Uplands and Mountains
General
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
100+
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS,
SS, SH
300-700
Weight
5
4
3
2
1
5
3
Rating
1
1
6
10
3
6
4
Number
5
4
18
20
3
30
12
DRASTIC Index 92
Setting 13 D Bedrock of the Uplands and Mountains
Agricultural
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
100+
0-2
Thin Bedded SS,
LS, SH Sequences
Thin or Absent
12-18%
Bedded LS,
SS, SH
300-700
1
1
6
10
3
6
4
5
4
18
50
9
24
8
Agricultural
DRASTIC Index
118
125
-------
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Appendix A
Processes and Properties Affecting Contaminant Fate and Transport
Most potential ground-water contaminants are re-
leased at or slightly above the water table as a result
of various industrial, agricultural, and other human
activities.
The attenuation of contaminants as they travel
through the unsaturated zone and ground-water
system is affected by a variety of naturally occurring
physical processes and chemical reactions that often
cause the contaminant to change its physical state or
chemical form. This change may result in removal of
the contaminant from the ground-water system. The
extent of these reactions is dependent on hydrogeo-
chemical conditions present in the ground water such
as pH, redox-potential and solid surface area. How-
ever, the chemical processes within dynamic ground-
water systems are complex, and are highly dependent
on site-specific aquifer and soil characteristics as
well as the effects of individual contaminants in the
system (Cherry et al., 1984). Therefore, although the
importance of these chemical reactions in attenuation
of contaminants is widely recognized, prediction of
the amount of attenuation of a contaminant in any
environment is still very difficult.
Attenuation includes those mechanisms that lessen
the severity or amounts of contaminants. The com-
ponents which affect attenuation arethephysical and
chemical processes and properties including density,
solubility, sorption, biodegradation, oxidation-reduc-
tion, dilution, hydrolysis, dispersion, viscosity,
mechanical filtration, ion exchange, volatilization,
and buffering or neutralization. The degree of attenu-
ation that occurs is dependent on: (1 ) the time that the
contaminant is in contact with the material through
which it passes; (2) the grain size, and physical and
chemical characteristics of the material through
which it passes; and (3) the distance which the
contaminant has traveled. For most materials, the
longer the time, the greater the surface area and the
greater the distance of travel, the greater the degree
of attenuation. Movement of ground water is slower
in rocks with high surface areas, such as found in a
fine-grained porous medium, than in rocks where
water movement is primarily through fault and
fracture channels or solution openings. Additionally,
flow velocity decreases with lower gradients and
increasing depth; subsequently, ground water is in
prolonged contact with rock materials (Matthess and
Harvey, 1982).
Another factor affecting attenuation includes surface
area in the aquifer media. The greater the surface
area of the material through which the contaminant
passes, the greater the potential for sorption of the
contaminant. Likewise, the greater the reactivity of
the material through which the contaminant passes,
the greater the potential for attenuation.
The many physical processes and chemical reactions
present in a ground-water system may work individu-
ally or in combination to provide varying degrees of
attenuation depending on the hydrogeochemical
conditions and the particular contaminant. The
following discussion addresses each physical and
chemical process and describes the respective impact
on contaminants.
Density
The density of any substance is defined as mass per
unit volume. The movement of a contaminant in an
aquifer is directly affected by the density of the fluid
with respect to the density of the ground water. Low
density contaminants tend to float on top of the water
table; high density contaminants tend to sink to the
bottom of the aquifer.
Once a contaminant has entered an aquifer, it will be
transported as a function of density in the direction of
ground-water flow at a rate between a fraction of an
inch a day to a few feet per day. Under the ideal
condition of a homogeneous aquifer media, the
contaminant will begin to disperse forming an
elliptical plume (Pye et al., 1983; Todd, 1980). Move-
ment and dispersion of the plume is affected by the
density of the contaminant, the character of the
geologic formation through which the contaminant
passes andthe reactive nature of the contaminant. In
a uniform geologic formation, the more dense the
contaminant, the greater will be the downward
migration of that contaminant and the slower the
contaminant will travel in relation to the velocity of
ground-water flow.
Solubility
As a contaminant is introduced into an aquifer, the
contaminant is generally partially dissolved in water,
forming either miscible or immiscible solutions. A
potential contaminant may also remain insoluble,
131
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depending on the chemical characteristics of the
contaminant. The solubility of a substance is defined
as the mass of a substance that will dissolve in a unit
volume of solute under specified chemical conditions
(Freeze and Cherry, 1979). The solubility of a consti-
tuent in water is dependent on variations in tempera-
ture, pressure, pH, redox potential (Eh), and the
relative concentrations of other substances in solu-
tion. The interactions of these chemical parameters
make it difficult to predict the solubilities of many
substances in ground water (Davis and DeWiest,
1 966; Snoeyink and Jenkins, 1980).
Substances are dissolved in water, or become soluble,
because the water molecule exhibits a charge which
tends to attract other molecules in solution. When the
attractive forces that hold a substance together are
less than or equal to the attractive force of the water
molecule, the substance will dissolve. Conversely,
those substances that are held together by attractive
forces stronger than the attraction of the water
molecule do not dissolve to any appreciable degree in
water, thus forming immiscible liquids or solids. A
good example is oil and water; the two substances do
not mix because the oil is only slightly soluble in
water. Substances that have been dissolved may be
reprecipitated as a consequence of equilibria shifts
and deposited in the void spaces of the aquifer. In
addition, immiscible fluids may be transformed
through similar changes in solubility.
The chemical reaction which transforms a dissolved
substance to a solid form is precipitation. The
precipitation of a dissolved substance may be initiated
by changes in pressure, temperature, pH, concentra-
tion, or oxidation-reduction. In addition, the introduc-
tion of another substance that changes the equilibrium
concentrations in the solution, or which reacts
chemically with the dissolved substance may cause
precipitation. The resultant solid is deposited in the
void spaces of the aquifer, thereby reducing the space
available for transport of the ground water.
Several types of contaminants can be effectively
removed from the ground water through precipitation.
Calcium salt solutions have been shown to effectively
precipitate free fluorides (Tolman et al., 1978). Alkalis
and/or sulfides may precipitate heavy metals. Stover
and Kincannon (1983) have conducted successful
experiments with regulated pH conditions, demon-
strating the precipitation of metals using lime. Since
oxidation-reduction reactions may change the chem-
ical state of a substance by rendering it insoluble, this
reaction has proven effective in changing dissolved
chromium to a less soluble state thereby removing it
from the ground water (Tolman et al., 1978; Fuller
and Artiola, 1 978). The FMC Corporation (1983) has
conducted extensive studies using hydrogen peroxide
to oxidize various sulfide compounds and initiate
precipitation. Vapors escaping from a contaminated
site may cause heavy metals to be transported and
re-deposited. Each of these chemical reactions
provides a method of changing the solubility of a
substance, thereby removing the contaminant from
the ground water and precipitating it in the void
spaces of the aquifer. Even though the contaminant
has changed form, the precipitate may be re-dissolved
and the process repeated. When a precipitate re-
dissolves, the contaminant may not be in its original
form and may form a different solute which may or
may not be harmless.
Sorption
Sorption is a combination of two processes, adsorp-
tion and absorption. Adsorption occurs when mole-
cules or ions are attached to the surface of charged
particles by weak Van der Waals or covalent bonds.
Adsorption differs from absorption in that the latter
involves penetration by the absorbed substance
(Keenan and Wood, 1971; Matthess and Harvey,
1982). Sorption occurs on all surfaces where bonding
conditions are present. Sorption increases with in-
creasing surface area, which is usually a function of
decreasing grain size. Colloidal particles range in
diameter size from 10"3 to 10~6 mm. These particles
tend to have a large charge relative to their surface
area (Freeze and Cherry, 1979). Porous geologic
materials that are composed of an appreciable
amount of colloidal-sized particles exhibit a higher
capability to sorb constituents onto the particle
surfaces.
The subsurface materials that exhibit sorptive
properties include clay minerals, hydrous iron, man-
ganese, aluminum oxides, organic substances (par-
ticularly humus), glauconites and the rock-forming
minerals mica, feldspar, aluminous augite and horn-
blende (Matthess and Harvey, 1982; Freeze and
Cherry, 1979; Davis and DeWiest, 1966). These
minerals are commonly present in colloidal form and
contain especially large surface areas available for
sorption.
The surface charge on a mineral in the saturated or
unsaturated zone creates an attractive force. This
charge may be due to (1) imperfections or substitu-
tions in the crystal lattice of the particle or (2)
chemical reactions at the surface of the particle
involving weak hydrogen bonding, due to the presence
of water. The pH of the water and the crystal structure
of the mineral have a direct affect on the charge of the
particle surface; waters with a high pH and highly
crystalline materials typically produce net negative
charges on the particle surface thus favoring the
sorption of positive constituents or cations (Matthess
and Harvey, 1982). There is a direct relationship
between the quantity of a substance sorbed on a
particle surface and the quantity of the substance
suspended in solution. In general, an increase in the
132
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concentration of the substance in solution will
increase the quantity sorbed.
The presence of organic materials in porous materials
appears to be an important factor in the sorption of
non-ionic organic substances. Those organic sub-
stances that are nonpolar (not attracted to water) and
relatively insoluble tend to be readily sorbed by soils
and sediments containing clays and organic carbon.
The sorption of nonpolar aromatic and chlorinated
hydrocarbons has been shown to increase with
decreasing particle size and subsequently increasing
organic carbon content (Karickhoff et al., 1979).
Sorption of polar organics primarily occurs through
weak hydrogen bonds to mineral particles (Cherry et
al., 1 984; Brown et al., 1983). Studies by Haque et al.
(1974) and Griffen et al. (1978) indicate that sorption
of PCBs was enhanced in materials with greater
surface area and higher organic content. The sorptive
capabilities of clays and soils appear promising for
attentuation of some contaminants, however, further
experimentation is necessary due to the complexity of
chemical reactions that occur in the sorption process.
Ion Exchange
The process of ion exchange is similar to sorption,
however, stronger ionic bonding occurs on the particle
surfaces. Ion-exchange processes are virtually limited
to colloidal size particles because these particles have
large electrical charges with respect to their surface
areas. Colloidal particles range in diameter size from
1CTJto 10~emm.
Ion exchange occurs when there is a surface charge
imbalance. These surface charges are a result of (1)
imperfections or ionic substitutions within the crystal
or particle, or (2) chemical dissociation reactions at
the particle surface. Upon exposure to water the
charged molecules attract hydroxyl groups (OH") to
the surface. When these hydroxyl groups break down,
the resulting charge imbalance attracts oppositely
charged particles (Freeze and Cherry, 1979). Ionic
substitutions within particle surfaces also cause a
charge imbalance that attracts oppositely charged
ions. These ions comprise an adsorbed layer that is
interchangeable; thus the process is reversible. An
example of ionic substitutions occurs within silicate
minerals. Aluminum ions tend to substitute for the
silica ions, forming an unbalanced charge on the
mineral surface. The nature of the surface charge
that develops is dependent on pH; positively charged
surfaces develop at low pH and a negatively charged
surface prevails with a high pH. Clay minerals are the
primary geologic materials of colloidal size that exhibit
surface charges as a result of ionic substitutions.
Organic materials such as humus and plant roots in
soils and recent sediments also exhibit high ion-
exchange capacities (Davis and DeWeist, 1966;
Matthess and Harvey, 1982).
The most common ion exchange involves the transfer
of cations on charged surfaces. Cation exchange
capacity is the capability of a charged surface layer to
attract positive ions in the zone adjacent to that
charged surface (Freeze and Cherry, 1979). The
affinity for attraction of cations varies with the
valence, or charge, of the ion andthe ionic size. Other
things being equal, the affinity for ion exchange is
greater when the ion has a higher valence. For ions of
the same valence, the affinity for exchange increases
with atomic number and decreases with increasing
hydrate radius (Matthess and Harvey, 1982).
Other colloidal particles that exhibit ion-exchange
capacities include hydrated oxides of iron and man-
ganese. Hydrated oxides of iron selectively sorb zinc,
copper, lead, mercury, chromium, molybdenum,
tungsten, and vanadium through ion exchange.
Similarly, hydrated oxides of manganese will bond to
copper, nickel, cobalt, chromium, molybdenum, and
tungsten (Matthess and Harvey, 1 982). Clay minerals
tend to preferentially bond zinc, copper, lead, mercury,
and radioactive elements such as rubidium, cesium,
arid strontium. Certain organic dyes are firmly bonded
to clays by strong electrostatic bonds (Matthess and
Harvey, 1982). For cationic organic substances,
increasing valence will tend to increase the capacity
for bonding to clay surfaces, and vice versa for anionic
organic constitutents (Brown et al., 1983).
Ion exchange can provide a means for attenuation of
heavy metals and certain organic substances if the
bonding is sufficiently strong to prevent reversal of
the chemical reaction and release of the contaminant
back into the ground-water system.
Oxidation-Reduction
Oxidation and reduction (redox) are geochemically
important processes because together with pH, they
control the solubility, and thus the presence of many
substances in water. These reactions involve the
transfer of electrons between dissolved, gaseous,
and solid substances in the water. As a result of this
electron transfer, there is a change in the oxidation
state of the substance. A redox reaction consists of
two parts or half reactions. In the oxidation reaction,
the substance loses, or donates electrons; in the
reduction reaction, the substance accepts, or gains
electrons. Oxidation and reduction reactions are
always coupled; no free electrons can exist in solution
and electrons must be conserved (Snoeyink and
Jenkins, 1980).
Deposits above the water table contain voids which
are usually filled with atmospheric gases containing
oxygen. Percolating water carries dissolved atmos-
733
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pheric oxygen to the water table where the processes
of diffusion and dispersion can carry it to deeper
water levels (Matthess and Harvey, 1982). The
presence or absence of dissolved oxygen in the
ground water is one factor which controls whether
oxidizing or reducing conditions will predominate.
Oxidation may be initiated in ground water by the
presence or introduction of an oxidizing agent, such
as potassium permanganate, or a change in valence
state of ions such as Fe+3 and Mn+3. In general,
oxidation processes are increased in warm climates,
and are more complete in humid and humid/arid
climates than in arid climates.
Microorganisms are responsible for a large proportion
of redox reactions which occur in ground water. The
principal microorganisms involved in redox processes
are bacteria which contain enzymes. Bacteria and
their enzymes utilize redox processes to provide
energy for cell synthesis and maintenance (Freeze
and Cherry, 1979). Bacteria that require oxygen are
known as aerobic bacteria, while anaerobic bacteria
cannot tolerate dissolved oxygen in the water.
In many contaminated ground-water systems, dis-
persion exerts a strong influence on the redox state of
the ground water. Dispersion causes a continuous
mixing of waters that are different in chemical
composition and redox potential. The mixing of these
waters by dispersion affect the redox and pH condi-
tions and may instigate other chemical reactions
within the system.
The use of oxidation-reduction reactions for the
attenuation of contaminants has proven effective for
both inorganic and organic substances. The introduc-
tion of oxidizing agents into ground water is the most
important mechanism of oxidation after microorga-
nisms. Detoxification through oxidation of cyanides
(Farb, 1978) and organic cyanides (Harsh, 1975) has
been accomplished through the application of sodium
hypochloride in conjunction with pH adjustments to
produce substances that are insoluble. Dohnalek and
Fitzpatrick (1983) documented removal of hydrogen
sulf ide from ground water in laboratory studies using
oxidants. The FMC Corporation (1983) has conducted
extensive experimentation using hydrogen peroxide
to oxidize various sulfides and organic sulfides
thereby rendering them insoluble. Certain organic
compounds such as phenols, aldehydes, hydroqui-
nine, as well as chlorine compounds and cyanides
can also be ozidized by hydrogen peroxide (FMC,
1983). Matthess (1981) achieved treatment of
arsenic-contaminated ground water by accelerating
the natural precipitation process through the injection
of theoxidant potassium permanganate. The soluble
arsenic species was oxidized to the less soluble
arsenate state and precipitated as iron and manga-
nese arsenates and hydroxides, thus removing the
arsenic from the ground water and eliminating the
contamination problem. Injection of oxygenated
water into an aquifer has also been shown to improve
water quality by stimulating iron and manganese
bacteria. The bacteria then provided the adsorption-
oxidation mechanism that precipitated the iron and
manganese hydroxides (Rott et al., 1981). Other
chemicals susceptible to oxidation include phenols,
aromatic amines, and dienes (Cherry et al., 1984).
The application of a strong reducing agent has also
proven effective in changing the oxidation state of
chromium causing the formation of an insoluble
chromium product.
The mechanisms of oxidation and reduction provide a
means for reducing the solubility and causing subse-
quent precipitation through several reactions. Those
most effective reactions for reducing solubility include
a change in oxidation state, the formation of new
compounds, and the enhancement of naturally
occurring bacterial processes.
Biodegradation
Biodegradation results from the enzyme-catalyzed
transformation of organic compounds by microbes,
principally bacteria, fungi, actinomycetes, algae, and
yeasts. Biological treatment can eliminate hazardous
organic wastes by transforming them into innocuous
forms, degrading them by mineralization to carbon
dioxide and water, or by anaerobically decomposing
them to carbon dioxide and methane (Kobayashi and
Rittmann, 1982). Bacteria and other microbes require
nutrients to produce the necessary enzymes that use
or attack the organic compounds. Most microbes
require oxygen, water, and nutrients such as carbon,
nitrogen, phosphorus, and trace metals. Aerobic
bacteria require the presence of free oxygen; anaer-
obic bacteria require the absence of dissolved oxygen.
The metabolic processes of both types of bacteria are
energy efficient and tend to enhance certain critical
reactions. Reactions such as reductive dehalogena-
tion, nitroreduction and reduction of sulfoxides are
catalyzed by anaerobic bacteria (Kobayashi and
Rittmann, 1982).
Biodegradation of a broad range of organic com-
pounds particularly those that are man-made, have
been demonstrated in laboratory studies. It is difficult
to predict the exact transformations that may occur,
due to the complexity of chemical reactions present in
natural systems of mixed microbes and organic
compounds (Cherry et al., 1984; Kobayashi and
Rittmann, 1982). Biodegradation is dependent on
interactions in a natural environment such as redox
potential, dissolved oxygen, pH, temperature, pre-
sence of other compounds, salinity, other competing
organisms, and the concentrations of compounds and
organisms. Organic compounds need to be fairly
soluble in water in order to be utilized by microbes.
134
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Biodegradation can be limited if there are antagonistic
interactions between two types of microbes, such as
bacteria and fungi (Kobayashi and Rittmann, 1982).
In addition, very low compound concentrations in a
substrate may pose problems; certain organisms
require minimal threshold values for survival and/or
production of necessary enzymes.
Certain man-made organic compounds are refractory
or resistant to biodegradation. This resistance is
generally due to the presence of chemical substitu-
ents such as nitrogroups, chlorines, and amines, that
are attached to the parent compound. Generally,
larger molecules are less degradable than smaller
ones (Kobayashi and Rittmann, 1982). Other impor-
tant refractory compounds are halogenated organics
which are very resistant to biodegradation (Brown et
al., 1983). These halogenated organics include pesti-
cides, plasticizers, solvents, and trihalomethanes.
Chlorinated compounds such as DDT and other
pesticides have been the most frequently studied
compounds. The first step in degradation of halo-
genated organics involves dehalogenation by several
biological mechanisms. Anaerobic reductive dehalo-
genation is an important mechanism in degradation
of pesticides and certain halogenated aliphatic com-
pounds.
Kobayashi and Rittmann (1982) and Tabak et al.
(1980), indicate that most man-made organic com-
pounds will undergo biodegradation to some extent.
Actinomycetes and fungi are known to attack a wide
variety of complex organic compounds. These mi-
crobes can grow under low nutrient conditions, wide
temperature ranges and wide pH ranges. Actinomy-
cetes break compounds down into groups that can be
utilized by other organisms. Certain types of fungi are
able to degrade complex hydrocarbons including the
degradation of DDT. Fungi are believed to be capable
of degrading PCBs more efficiently than bacteria
(Gibson, 1978). Fungal metabolism is generally
incomplete and requires other microbes for complete
degradation. Bacteria have been found to degrade a
wide variety of compounds under aerobic conditions.
Bacteria are the major agents in the degradation of
hydrocarbons and heterocyclic compounds (Kobayashi
and Rittmann, 1982; Jhaveri and Mazzacca, 1983;
Weldon, 1979; Tabak et al., 1980; Liu et al., 1981;
Claus and Walker, 1964; Cherry et al., 1984).
Anaerobic bacteria degrade organic compounds to
carbon dioxide and methane under oxygen-deficient
conditions. Although little is known about these
bacteria, four groups that utilize each of the metabolic
products are responsible for degradation of the other
groups. These bacteria are capable of dehalogenation,
nitrosamine degradation, reduction of epoxide groups,
reduction of nitro groups and the breakdown of
aromatic structures (Kobayashi and Rittmann, 1982;
Tabak et al., 1980). In a study conducted by Ehrlich et
al. (1982) an aquifer contaminated by phenols and
polynuclear aromatic hydrocarbons such as naph-
thelene showed significant reductions in these
contaminants within 1000 m of the contamination
source. Contaminant attenuation has been attributed
to anaerobic degradation of the hydrocarbons by
bacteria. Laboratory studies indicate that anaerobic
bacteria are capable of degrading certain halogenated
1 - and 2-carbon organic compounds such as trihalo-
methanes, chloroform, and trichloroethylenes (Bouwer
etal., 1981).
Hydrolysis
The breakdown of substances under the influence of
H+ and OH" ions in the water is known as hydrolysis.
The breakdown of minerals by hydrolysis is an
important reaction that occurs in the ground water,
causing relatively insoluble minerals to form new
minerals while releasing ions into solution. The
hydrolysis process is dependent on pH, which controls
the amounts of hT and OH" ions in solution, in
addition to the oxidation-reduction potential (Matthess
and Harvey, 1982). Hydrolysis is most effective at
high temperatures, low pH, and low redox potential.
Hydrolysis is the basic reaction in the weathering
processes which acts upon rocks and aids in the
production of clays and soils.
Hydrolysis of an organic compound involves the
introduction of a hydroxyl group (-OH) into the
chemical structure, usually with the loss of a chemical
group (X). The rate of hydrolysis of organic compounds
is dependent on pH conditions and the presence of
metal ions. A common hydrolysis reaction involves
the replacement of halogens (X) by a hydroxyl group
(Cherry et al., 1984). The occurrence of hydrolysis
may aid in contamination attenuation. Certain organic
compounds may be broken down by hydrolysis into
simpler compounds that may then be easily assimi-
lated through other processes. An example would be
the hydrolysis of esters into a simple alcohol and acid
that would comprise less harmful constituents in the
ground water.
Hydrolysis is an important process in the attenuation
of pesticides. It may be used to help predict the rate of
decay of pesticides in the soil. (Cohen et al., 1984;
Cherry et al., 1984). Hydrolysis of atrazine and other
pesticide derivatives has been shown to operate
faster when humic material is present. Hydrolysis
rates for breakdown of pesticides have been deter-
mined for certain organic groups (Cherry et al., 1984;
Callahan et al., 1979; Cohen, 1984).
Volatilization
Volatilization is defined as the loss of a compound to
the atmosphere. This process provides an attenuation
735
-------
mechanism for those compounds that are resistant to
degradation and/or weakly absorbed, and to those
that exhibit low solubilities and high vapor pressures
(Callahan et al., 1979; Brown et al., 1983). Organic
constituents with high vapor pressures are more
easily volatilized from the soil. Compounds that are
not soluble tend to be available for volatilization
longer because they are not readily removed by
water. Persistent organic constituents that are not
easily removed by other processes may tend to
volatilize after a period of time. Organic compounds
tend to volatilize more easily if they are less strongly
sorbed by the soil.
Factors that affect volatilization include vapor pres-
sure, water solubility, soil moisture, adsorption, wind
speed, turbulence, temperature, depth below land
surface and time (Brown et al., 1983; Callahan et al.,
1 979). Studies indicate that the highest volatilization
of organics occurs within minutes of application and
decreases substantially within one hour (Wetherold
etal., 1981).
Volatilization of organics is generally restricted to the
purgable or volatile organic compounds. These com-
pounds include hydrocarbons, compounds with sim-
ple function groups such as alcohols, halides, and
sulfur-containing compounds, and compounds con-
taining unsaturated functional groups such as alde-
hydes, ketones, and esters. Increasing air humidity,
soil temperature, and soil moisture have been shown
to increase volatilization rates (Wetherhold et al.,
1981).
A buffer solution is comprised of a weak acid or base
plus a salt of that acid or base. A solution of this type
will maintain a relatively constant pH even though a
strong acid or base is added. A common example of
this is the acetic acid-sodium acetate solution which
will maintain a relatively constant pH when HCI is
added, due to the H+ions from the HCI combining with
the acetate ions, as follows:
Cl'+
HCI
Therefore, no change occurs in the hydrogen ion
concentration.
Carbonate systems provide very effective buffering
systems in natural waters and waste waters (Snoeyink
and Jenkins, 1 980). The system is essentially based
on a weak acid, carbonic, and sodium bicarbonate. As
a consequence of the natural equilibria established
between these parameters a relatively constant, near
neutral pH is maintained for most ground waters,
making many important biological processes possible.
The precipitation of chromium from water is directly
controlled by variable pH values by providing suitable
electron donors to change the chromium to a less
soluble oxidation state (Tolman et al., 1978; Fuller
and Artiola, 1978). The use of variable pH levels
enables the detoxification of cyanide through oxida-
tion and subsequent precipitation of insoluble cyanide
compounds (Farb, 1978).
Buffering and Neutralization
Buffering and neutralization are chemical reactions
which are similar. Neutralization is achieved by
balancing the pH or activity of the hydrogen ion
concentration so that a neutral solution is produced.
Buffering refers to the ability of a substance to
maintain a constant pH over a wide range of concen-
trations. The neutralization of an acid or base
produces water and neutral salts. Lime is effective in
neutralization of acidic wastes.
Many biological processes rely on maintaining neutral
pH levels to enhance biodegradation of organic
constituents (Brown et al., 1983). Neutral pH levels
are maintained in soils by their natural buffering
capacity. Aluminum ions in the surface of clay colloids
maintain an equilibrium of hydroxide ions in the soil
solution. The actual pH range of a soil may vary
according to the predominant clay constituent present
(Brown et al., 1 983). Neutralization of contaminants
through pH adjustment is generally achieved by the
addition of an acid or base, precipitation, and oxidation
reduction.
Dilution
The dilution of ground-water contaminants occurs
through the addition of water by precipitation or other
sources, introduced into the ground-water system.
Dilution is an integral mechanism of dispersion
occurring on a microscopic and macroscopic scale
(Todd, 1980). These mixing mechanisms produce
longitudinal and transverse dispersion of the con-
taminant such that the concentration decreases with
the distance from the point of introduction. According
to Todd (1980), dilution may be the most important
mechanism for attenuation after the pollutant enters
the ground-water system.
Dispersion
A porous medium is composed of particles of varying
sizes, shapes, and orientations. As water flows
through a porous medium, the velocity varies across
pore space and around particles. As a result, when a
contaminant is introduced into a ground-water
system, it tends to spread, or disperse, so as to
gradually occupy an increasing volume of that flow
system. Thus, dispersion constitutes a non-steady,
136
-------
irreversible mixing process by which the contaminant
disperses within the surrounding ground water (Todd,
1980).
Dispersion has two components, longitudinal and
transverse. Longitudinal dispersion occurs in the
direction of flow and is caused by differences in
macroscopic velocities as the water moves across
pore spaces and around particles winding a tortuous
path through the media in the direction of flow.
Transverse dispersion occurs in two dimensions
normal to ground-water flow and results from
repeated division and deflection of the water flow by
the particles (Todd, 1980; Bouwer, 1978). Figure A-1
illustrates transverse and longitudinal dispersion in a
saturated porous medium.
Figure A-1 Schematic of pathlines showing longitudinal and
transverse dispersion (Bouwer, 1978).
Longitudmal dispersion occurs
when a contaminant enters
at A or B.
Longitudinal and transverse
dispersion occurs when a
contaminant enters at C
Dispersion is a phenomenon that is caused by a
combination of two processes, molecular diffusion
and mechanical dispersion that occurs with laminar
flow in a porous medium (Todd, 1 980; Wilson et al.,
1976). The result of these processes produces a
contaminant plume with distinctly different character-
istics dependent on the way the contaminant is
introduced into the system. Figure A-2(a) illustrates
the configuration of a plume that forms from the
continuous input of contaminant, whereas Figure A-
2(b) represents input of a contaminant in pulses. The
contaminant plume developes an expanding elliptical
shape with declining concentration per unit mass of
aquifer because of the process of dispersion (Freeze
and Cherry, 1979).
The relative rates of dispersion and the subsequent
configuration of the contaminant plume are depend-
ent on the homogeneity of the aquifer. Most laboratory
testing of dispersion has been restricted to homoge-
neous, sandy mediums. Heterogeneous aquifer media
present a complex dispersion pattern related to the
respective hydraulic conductivities of the individual
stratigraphic units. High conductivity units dominate
the flow of contaminants in the ground-water system
as well as provide zones of migration where contam-
inants would move more quickly than through
adjacent units of low conductivity (Freeze and Cherry,
1979). The predomination of heterogenous geologic
units that serve as aquifers has necessitated the
quantification of contaminant transport through
mathematical models (Freeze and Cherry, 1979;
Bouwer, 1978; Roberts, 1981; Anderson, 1984).
These models have been extended to include mole-
cular diffusion, the adsorption of solutes by the media
and the decay of radioactive materials. The primary
emphasis of these models is to provide an effective
means of predicting the extent of contaminant
dispersion, contaminant flow velocities, and concen-
trations at various points within the plume. Most
modeling efforts are constrained by the lack of
adequate control data.
Viscosity
The viscosity of a fluid is the property of resistance to
relative motion and shear deformation during flow.
The more viscous the fluid, the greater the shear
stress, and thus, the resistance to flow. Viscosity is
affected by temperature; the higher the temperature,
the lower the viscosity, and the easier it will be for a
fluid to move through the pores in a media. Viscosity
of water has a direct affect on hydraulic conductivity
that can bequantifiedasan inverse linear relationship
(Bouwer, 1978). Reducing the viscosity by half will
double the hydraulic conductivity.
Thus, the viscosity of a contaminant will partially
control the rate of migration. More viscous contam-
inants will not move as easily through porous media.
Consideration of contaminant viscosity if it differs
significantly from water viscosity, in conjunction with
other applicable chemical reactions, may be neces-
sary for prediction of contaminant migration.
Mechanical Filtration
Mechanical filtration removes contaminants which
are larger than the pore spaces of the host medium.
This process is most effective in finer-grained mate-
rials such as clay or soil, but can occur in coarse-
grained media depending on the paniculate sizes
being filtered. The effects of mechanical filtration
increase with decreasing pore and/or channel size
within the media. Retention of larger particles may
effectively reduce the permeability of the media.
Chemical reactions such as precipitation may form
larger, insoluble particles that are retained by the
media, thereby affecting porosity and permeability.
The effectiveness of mechanical filtration for removal
of contaminants is thus dependent on grain size and
sorting of the media, and hydraulic conditions within
the media, andtheparticulatesizeofthecontaminant
being transported through the medium.
137
-------
Figure A-2 Plume configuration based on contaminant input (Freeze and Cherry, 1979).
Continuous
Point Source
Of Tracer
(a)
-»• Uniform Flow
Instantaneous
Point Source
3<7ti
3o>
(b)
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139
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Appendix B
Characteristics of Ground-Water Contaminants
Contaminants have been divided into inorganic
compounds and organic compounds. For purposes of
this discussion, inorganic compounds are subdivided
into metals and nonmetals, while organic compounds
are separated into groups bearing similar molecular
structures which influence those processes affect ing
the fate and transport of ground-water contaminants.
Inorganic Metals
The mobility and attenuation of metals in any
hydrogeologic setting is of function of the hydro-
chemical ground-water environment. Metals of pri-
mary importance include cadmium, chromium, cop-
per, lead, mercury, manganese, silver, zinc, and iron
for which maximum Federal Drinking Water Stand-
ards have been established. With the exception of
iron, metals typically occur naturally in the environ-
ment in concentrations below 1 mg/l. Concentrations
are low due to the processes of adsorption, hydrolysis,
precipitation, and oxidation-reduction.
Metals tend to be hydrolized by water and exist as one
or more ionic species. These metals combine readily
with ligands to form ionic or neutral aqueous
complexes. These ligands may be inorganic ions such
as HCOa, CDs,2 SO;,2 Cf, F", and NOi (Freeze and
Cherry et al., 1979). Any dissolved organic constit-
uents that are present may also cause complexation
orchelation. Increases in the concentrations of these
anions increase the concentrations of the complexes
that are formed. The occurrence of a complexed
species is dependent on the pH and the equilibrium of
a particular complex in the aqueous solution.
The oxidation-reduction potential of the ground water
directly affects the oxidation state of the metal and
may also affect the nonmetallic anions with which it
forms complexes. Changes in the oxidation state of a
metal may control the relative solubility or insolubility
in water. The mechanism of sorption of trace metals
is dependent on redox potential and pH.
Sorption of trace metals is an important process
which may maintain metal concentrations far below
that provided through solubility constraints. Trace-
metal sorption occurs due to the presence of colloidal
size clay particles, organic matter, and iron and
manganese hydroxides. In most oxidizing environ-
ments, the iron and manganese oxides occur as
surface coatings on grains thereby increasing their
ability to sorbtrace metals(Freeze and Cherry, 1979).
This is particularly effective for Co, Ni, Cn, and Zn in
soils and freshwater sediments.
The hydrochemical environment of a ground-water
system exhibits many effects on trace metals making
the prediction of transport and migration difficult and
complex. In general, the processes of sorption and
precipitation cause the trace metals to migrate very
slowly with respect to ground water flow velocities.
Thus, the occurrence of generally localized contam-
ination by trace metals is common.
Cadmium (Cd)
Cadmium-contaminated wastes are generated as
byproducts of cadmium-nickel battery production,
pigments for plastics, enamels and paints, fumicides,
and in electroplating and metal coatings. The solu-
bility and sorption of cadmium are controlled by pH.
Under acidic or low pH conditions, cadmium solubility
increases while sorption by colloids decreases (Brown
et al., 1983). Precipitation of cadmium carbonates
and cadmium phosphates may reduce cadmium
concentrations at low pH values. Precipitation of
cadmium sulfides occurs in reducing environments.
The primary mechanism for cadmium attenuation is
through sorption to organic matter in soils as organic-
metallic complexes. The contaminant level as estab-
lished in the Federal Primary Drinking Water
Standards is 0.01 mg/l.
Chromium (Cr)
Chromium is present in waste streams as a con-
sequence of its use as a corrosion inhibitor, produc-
tion of refractory bricks to line metallurgical furnaces,
plating operations, topical antiseptics and astrin-
gents, and the tanning and dye industries (Brown et
al., 1983). The oxidation-state of the chromium ion
directly affects its toxicity; chromium is the most toxic
and mobile at an oxidation state of +6. This is the most
common form of chromium in industrial wastes, thus
making chromium a concern. The soluble salts of
chromium, such assulfateand nitrate, are more toxic
than the insoluble salts of oxides and phosphates.
Under acid conditions, the presence of either oxygen
or an oxidizing agent can change the oxidation state
of chromium from +6to+3(Tolman et al., 1978; Fuller
and Artiola, 1978). The +3 chromium is less toxic and
140
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generally immobile in ground water because it will
readily precipitate with carbonates, hydroxides, and
sulfides to form insoluble compounds. The maximum
contaminant level as established in the Federal
Primary Drinking Water Standards is 0.05 mg/l.
Copper (Cu)
Industrial wastes from textile mills, cosmetics manu-
facturing, and hardboard production contain signif-
icant amounts of copper. The sorption of copper onto
colloids is a function of pH; sorption increases at
higher pH values. Organic matter present in soils
forms very stable complexes with copper (Brown et
al., 1983). These include complexation with carboxyl
and phenolic groups where sorption is high when iron
and manganese oxide concentrations are low. Exper-
iments indicate that copper is sorbed appreciably by
quartz and even more strongly by clays. Copper is also
beneficial because low concentrations are necessary
for the metabolic processes of decomposition by
bacteria. The maximum contaminant level as estab-
lished in the Federal Secondary Drinking Water
Standards is 1.0 mg/l.
Lead(Pb)
Lead is found in wastes from the manufacture of
lead-acidic storage batteries, gasoline additives,
ammunition, pigments, paints, herbicides, and insec-
ticides. Lead may precipitate as sulfates, hydroxides,
and carbonates. The presence of free lead ions
depends on the stability of the lead complex at varying
pH values. At high pH levels, lead is less soluble and
preferentially sorbed onto clay surfaces. Under
reduced conditions, lead becomes mobile and may
form insoluble complexes with organic compounds
(Brown et al., 1 983). The maximum contaminant level
as established in the Federal Primary Drinking Water
Standards is 0.05 mg/l.
Mercury (Hg)
Mercury is present in a wide variety of industrial
wastes such as electrical apparatus manufacturing,
production of chlorine and caustic soda, pharma-
ceuticals, paints, plastics, paper products, and mer-
cury batteries. Many pesticides have metals as part of
their composition. Of these pesticides, over 40
percent use mercury as the major metal component
(Brown et al., 1 983). Mercury in the +2 oxidation state
is rapidly and strongly complexed by covalent bonding
to sulfur-containing organic compounds and inor-
ganic soils. Colloidal particles of clay, iron and
manganese oxides, fine sands, and organic matter
readily absorb mercury. Sorption by clay particles is
most effective at high pH values. The solubility of
various mercury ionic complexes can be affected by
changes in pH and/or oxidation-reduction. Insoluble
precipitates of mercury, sulfates, hydroxides, and
nitrates form at high pH conditions. Insoluble mer-
cury sulfide occurs in reducing conditions, whereas,
insoluble mercury chlorides favor oxidizing condi-
tions.
Organic mercury compounds such as phenyl, alkyl,
and methoxyethyl mercury used as fungicides may be
degraded by certain bacteria. However, other bacter-
ial forms tend to produce toxic mercury compounds
with organic matter (Brown et al., 1983). The most
toxic form of mercury occurs as methyl mercury and
poses a contamination problem for the aquatic food
chain. The maximum contaminant level as estab-
lished in the Federal Primary Drinking Water Stand-
ards is 0.002 mg/l.
Manganese (Mn)
The major source of manganese-contaminated waste
waters are from the iron and steel industries and from
the manufacture of paints, disinfectants, and fertil-
izers. The manganese ion commonly occurs as Mn +2,
which is soluble and mobile, and Mn +4, which is
insoluble and thus non-mobile. Under reduced condi-
tions, Mn +2 is strongly sorbed to clay minerals and
organic matter, but becomes less soluble as pH
increases. Under oxidizing conditions several stable
manganese compounds will form (Brown et al., 1983).
Manganese is considered a secondary constituent
under Federal Drinking Water Standards; maximum
contaminant levels are set at 0.05 mg/l.
Silver (Ag)
Silver is found in the waste streams of a variety of
industrial processes including photographic, mirror,
and electroplating manufacturing (Brown et al.,
1983). Silver tends to be sorbed through ion ex-
change by colloidal particles and precipitated with
common inorganic anions such as carbonate sulfates
and chlorides. The maximum contaminant level as
established in the Federal Primary Drinking Water
Standards is 0.05 mg/l.
Zinc (Zn)
Industrial wastes containing zinc are a byproduct of
brass and bronze alloy production, galvanized metals
for pipes, utensils, insecticides, glues, rubber, inks,
and glass (Page, 1974). Zinc can be attenuated
through precipitation, absorption, and ionic substitu-
tion (Brown et al., 1983). Zinc may be ionically
substituted for aluminum, iron, or magnesium in
many clay minerals. Zinc is primarily sorbed onto
organic colloids which are very soluble and mobile.
Zinc may be sorbed onto the particle surfaces of alloys
and is generally immobile. The solubility of zinc
precipitates is dependent on the stability of the
complexthat forms under variablepH conditions.The
only insoluble zinc precipitate is zinc sulfate. All other
141
-------
precipitates of zinc are soluble. Zinc is rendered
insoluble in soils and water with a pH greater than
6.5. The maximum contaminant level as established
in the Federal Secondary Drinking Water Standards is
5.0 mg/l.
Iron (Fe)
Iron, under oxidizing conditions in ground water,
forms hydrous oxides which provide a major attenua-
tion mechanism for the sorption of trace metals such
as cobalt, nickel, copper, and zinc in soils and
freshwater sediments. When this oxide occurs as
coatings on grains of a media, it can greatly increase
the sorptive capacity of that medium. Iron compound
stabilities are dependent on pH and oxidation-reduc-
tion potential. Iron in reduced form is soluble and
remains in solution. However, either very small-scale
variations in the pH/Eh relationship or in bacterial
activity can result in precipitation of iron in the
hydrous oxide form. The maximum contaminant level
as established in the Federal Secondary Drinking
Water Standards is 0.3 mg/l. Unlike most other
limits, the level for iron was not set because of
associated health risks, but rather for water quality
problems associated with staining and color. Iron
oxides precipitate and stain due to their relative
insolubility.
Inorganic Nonmetals
The chemical behavior of non-metallic substances in
water has a significant effect on ground-water
quality. Most non-metals tend to be fairly mobile in
the ground-water system as ionic species. The type
and amount of each species present is a function of
temperature, pressure, pH, redox potential, dissolved
concentrations, reactivity, and microbial activity.
The following discussion focuses on the major non-
metallic chemicals occurring in ground water. For
each chemical, information is presented on the
source(s) of the chemical and its water chemistry
characteristics. These nonmetallic chemicals can
occur either naturally in ground water or as a result of
human activities.
Nitrogen
The most common inorganic contaminant is dissolved
nitrogen in the form of nitrate (NOa-). Dissolved
nitrogen also occurs in the form of ammonium (NH4+),
ammonia (NH3-), nitrite (N02-), nitrogen (N2), and
nitrous oxide (N20). Common sources of nitrate in
ground water are from the burial of nitrogen-rich
wastes, application of fertilizers, and disposal of
sewage. When nitrogen-rich compounds are added to
the environment, nitrogen is converted to different
forms. The processes of nitrification (conversion of
NH4+ to NOa- by oxidation) and ammonification
(conversion of organic N to NH4-> generally occur
above the water table where oxygen and organic
matter are abundant (Freeze and Cherry, 1979).
Concentrations of NOa- are not limited by solubility.
Thus, this anionic form is very mobile and stable
under oxidizing conditions. N03- is not easily retarded
or transformed by chemical processes. The presence
of reducing conditions may initiate denitrification, a
process where NOa- is converted to N2 or N20. These
resulting forms are of less concern from a ground-
water pollution standpoint because they pose no
health risk. A maximum contaminant level of 10 mg/l
N has been established for nitrates because of health
concerns in infants when this level is exceeded.
Phosphorous
Phosphorous is not generally considered to be an
intrinsically harmful constituent in ground water in
normal concentrations, but its presence can cause
significant environmental problems by decreasing
available oxygen through accelerated algae and
aquatic vegetative growth. The most common source
of phosphorous contamination is by agricultural
activity and decomposition of organic wastes. Dis-
solved phosphorous can occur in many forms de-
pending on the pH of the water. Hydrolysis and
mineralization can convert insoluble forms of phos-
phates to the soluble phosphate ion for use by plants
and organisms (Brown et al., 1983). Degradation and
mobilization of phosphorous by microbes accounts
for a portion of its attenuation. Under certain condi-
tions, phosphorouswill precipitate as iron, aluminum,
or calcium phosphate or be sorbed by iron and
aluminum oxides and hydroxides.
Boron
Boron is released during the decomposition of organic
materials. Partial sorption of boron may occur on iron
and aluminum hydroxyl compounds and clays. The
sorption of boron to these materials is pH dependent;
sorption will not occur at high pH levels. The amount
of boron that will be sorbed is dependent on surface
area and appears to be irreversible due to the
formation of covalent bonds. No drinking water
standards for human consumption of boron have
been set.
Sulfur
Sulfur is moderately abundant in the earth's soils and
is an important plant nutrient. Sulfur, in some form, is
widespread in industrial waste from processes such
as Kraft mills, sugar refining, petroleum refining, and
copper and coal extraction (Brown et al., 1983). Sulfur
is commonly found in two forms; as sulfate (SCu 2) in
142
-------
oxidizing conditions, and as sulfide (HS ) or (H2S)
under reducing conditions. Sulfides are toxic and
produce an odor in water. The FMC Corporation
(1983) has conducted extensive laboratory testing
using hydrogen peroxide to oxidize sulfides to sulfur
and water. Hydrogen peroxide has been shown to be
effective in neutralizing other sulfur compounds that
are common industrial waste effluents. These include
polysulfides, sulfites, thiosulfates, polythionates,
dithionites, and dithionates. Siulfates are relatively
mobile in the ground-water system as anions. Some
clays have the capability to sorb sulfate onto their
particle surfaces (Brown et al., 1983). Sulfates also
tend to form inorganic ligands and complex with
metal ions increasing their solubility. A maximum
contaminant level for sulfates is established in the
Federal Secondary Drinking Water Standards at 250
mg/l.
Fluoride
The mobility of fluoride depends on the types and
quantities of cations present in the water that have
formed salts with the fluoride ion. Sodium salts of
fluoride (NaF) have high solubilities as opposed to
calcium salts (CaF2) which have low solubilities.
Fluoride may be a natural constituent of ground water
produced from the dissolution of fluoride-bearing
rocks or from industrial wastes such as the production
of phosphatic fertilizers, hydrogen fluoride, and
fluorinated hydrocarbons (Brown et al., 1983). Fluo-
ride may also tend to complex with metallic ions. Soils
with high cation exchange capacities are capable of
retaining fluoride. The limit for concentration of
fluoride has been established at 1.4 to 2.4 mg/l in the
Federal Primary Drinking Water Standards.
Chloride
Chloride is very soluble and thus highly mobile in
ground water. Chloride in ground water results from
the dissolution of chloride-bearing rocks such as
halite, and is a common product or byproduct (e.g.
chlorinated hydrocarbon wastes) in most industrial
wastes. Another source of chloride is from spillage or
leakage of brines that are produced in oil and gas
drilling operations. The maximum contaminant level
as established in the Federal Secondary Drinking
Water Standards is 250 mg/l.
Arsenic
Arsenic is contained in wastes from the production of
herbicides, pesticides, pigments, and wood preserv-
atives (Freeze and Cherry, 1979; Brown et al., 1983).
Arsenic in natural ground water occurs in four
oxidation states which exist as many different species
under variable conditions. In general, most forms of
arsenic tend to become soluble under oxidizing
conditions. Solubility is controlled by pH and redox
potential.
The movement of arsenic in the environment is
affected by sorption to soils and volatilization. Sorp-
tion and/or precipitation by soil colloids is an
important attenuation mechanism. These colloids
include iron and aluminum hydroxides or clays.
Sorption increases with increasing pH, clay, and
hydroxide content. Levels of arsenic as low as 10
mg/l have been shown to be toxic (Brown et al.,
1983). The maximum contaminant level as estab-
lished in the Federal Primary Drinking Water Stand-
ards is 0.05 mg/l.
Selenium
Sources of selenium which can cause ground-water
contamination include glass, electronics, steel, rub-
ber, and photographic industries. Selenium has
properties which are similar to sulfur. Selenium has
three oxidation states. These typically form selenities
and selanates of sodium and calcium, and soluble
selenium salts. Selenium anions form selanates with
mercury, copper, and cadmium which are very
insoluble (Brown et al., 1983). Selenium in ground
water is least soluble under acid conditions. Mechan-
isms for selenium attenuation include sorption onto
hydrous iron oxides and precipitation to the insoluble
ferric oxide selenite. The maximum contaminant level
as established in the Primary Federal Drinking
Water Standard is 0.01 mg/l.
Organic Compounds
The contamination of ground-water resources by
organic compounds has resulted in the initiation of
studies on their occurrence and behavior in the
ground-water system. Many organic compounds of
environmental concern are at trace levels, parts per
billion or parts per million. However, even these
minute levels may exhibit toxic effects on aquatic and
mammalian life forms. The United States Environ-
mental Protection Agency (EPA) has developed a list
of what are considered to be the 129 priority
pollutants and the relative frequency of these mater-
ials in industrial waste waters (Keith and Telliard,
1979) (Table B-1).
There are several chemical and biochemical reactions
that are recognized as having a potential to signif-
icantly control contamination migration or attenua-
tion in ground-water systems. These mechanisms
include sorption, hydrolysis, oxidation-reduction, and
biodegradation. A discussion of these processes is
contained in Section 6, Processes and Properties
Affecting Contaminant Fate and Transport.
The solubility of organic compounds may be divided
into two broad groups; polar and nonpolar. Polar
143
-------
Table B-1. EPA List of 129 Priority Pollutants and the Relative Frequency of These Materials in Industrial Wastewaters
(Keith and Telliard, 1979)
Percent
of
samples8
Number of
Industrial
categories'"
Percent Number of
of Industrial
samples8 categories'"
31 are purgeable organics
1.2
2.7
29.1
29.3
16.7
7.7
5.0
6.5
10.2
1.4
7.7
1.9
4.2
0.4
1.5
40.2
5
10
25
28
24
14
10
16
25
8
17
12
13
2
1
28
Acrolein
Acrylonitrile
Benzene
Toluene
Ethylbenzene
Carbon Tetrachloride
Chlorobenzene
1,2-Dichloroethane
1,1,1 -Trichloroethane
1,1-Dichloroethane
1,1-Dichloroethylene
1 , 1 ,2-Trichloroethane
1 , 1 ,2-Tetrachloroethane
Chloroethane
2-Chloroethyl vinyl ether
Chloroform
2.1
1.0
34.2
1.9
0.1
1.9
4.3
6.8
0.3
2.5
10.2
10.5
0.2
7.7
0.1
5
5
25
6
1
12
17
11
4
15
19
21
2
18
2
1,2-Dichloropropane
1,3-Dichloropropane
Methylene chloride
Methyl chloride
Methyl bromide
Bromoform
Dichlorobromomethane
Trichlorofluoromethane
Dichlorodifluoromethane
Chlorodibromomethane
Tetrachloroethylene
Trichloroethylene
Vinyl chloride
1 ,2-trans-Dichloroethylene
bis(Chloromethyl) ether
46 are base/neutral extractable organic compounds
6.0
0.5
0.2
1.1
1.0
0.4
10.6
0.9
1.5
1.8
1.1
1.5
0.04
41.9
6.4
5.8
7.6
18.9
4.5
4.2
8.5
9
5
1
7
6
3
18
9
13
9
3
9
1
29
12
15
20
23
12
14
13
!1 ,2-Dichlorobenzene
1 ,3-Dichlorobenzene
1 ,4-Dichlorobenzene
Hexachloroethane
Hexachlorobutadiene
Hexachlorobenzene
1 ,2,4-Tnchlorobenzene
bis(2-Chloroethoxy) methane
Naphthalene
2-Chloronaphthalene
Isophorone
Nitrobenzene
2,4-Dinitrotoluene
2,6-Dinitrotoluene
4-Bromophenyl phenyl ether
bis(2-Ethylhexyl) phthalate
Di-n-octyl phthalate
Dimethyl phthalate
Diethyl phthalate
Di-n-butyl phthalate
Acenaphthylene
Acenaphthene
Buty benzyl phthalate
5.7
7.2
5.1
7.8
m R
1 U.U
2.3
1.6
1.8
3.2
0.8
0.2
0.6
0.1
0
0.2
1.1
0.8
0.1
1.2
0.1
0.1
1.4
11
12
9
14
16
6
6
6
8
4
4
7
2
0
4
4
7
1
5
1
2
6
Fluorene
Fluoranthene
Chrysene
Pyrene
( Phenanthrene
I Anthracene
Benzola (anthracene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzolalpyrene
lndeno(1,2,3-c,d)pyrene
Dibenzo(a,h)anthracene
Benzo(g,h,i)perylene
4-Chlorophenyl phenyl ether
3,3'-Dichlorobenzidine
Benzidine
bis(2-Chloroethyl) ether
1 ,2-Diphenylhydrazine
Hexachlorocyclopentadiene
N-Nitrosodiphenylamine
N-Nitrosodimethylamine
N-Nitrosodi-n-propylamine
bis(2-Chloroisopropyl) ether
1 1 are acid extractable organic compounds
26.1
2.3
2.2
1.6
1.1
6.9
25
11
9
6
6
18
Phenol
2-Nitrophenol
4-Nitrophenol
2,4-Dinitrophenol
4,6-Dinitro-o-cresol
Pentachlorophenol
1.9
2.3
3.3
4.6
5.2
8
10
12
12
15
p-Chloro-m-cresol
2-Chloropenol
2,4-Dichlorophenol
2,4,6-Trichlorophenol
2,4-Dimethylphenol
26 are pesticides/ PCB's
0.3
0.4
0.2
0.6
0.8
0.2
0.5
0.5
0.1
0.04
0.1
0.2
0.2
0.2
3
4
2
4
6
4
3
5
3
1
2
2
3
2
a-Endosulfan
0-Endosulfan
Endosulfan sulfate
a-BHC
/3-BHC
5-BHC
7-BHC
Aldrin
Dieldrin
4,4'-DDE
4,4'-DDD
4,4'-DDT
Endrin
Endrin aldehyde
0.3
0.1
0.2
0.2
0.6
0.5
0.9
0.8
0.6
0.6
0.5
—
3
1
4
2
2
1
2
3
2
3
1
—
Heptachlor
Heptachlor epoxide
Chlordane
Toxaphene
Aroclor 1016
Aroclor 1221
Aroclor 1232
Aroclor 1242
Aroclor 1248
Aroclor 1254
Aroclor 1260
2,3,7,8-Tetrachlorodibenzo-p-dioxin
(TCDD)
144
-------
Table B-1. (Continued)
Percent
of
samples8
18.1
19.9
14.1
30.7
53.7
55.5
43.8
Number of
Industrial
categories'1
20
19
18
25
28
28
27
Antimony
Arsenic
Beryllium
Cadmium
Chromium
Copper
Lead
Percent
of
samples3
13 are metals
16.5
34.7
18.9
22.9
19.2
54.6
Number of
Industrial
categories'"
20
27
21
25
19
28
Mercury
Nickel
Selenium
Silver
Thallium
Zinc
33.4
Miscellaneous
19
Total cyanides
Not available
Not available
Asbestos (fibrous)
Total phenols
aThe percent of samples represents the times this compound was found in all samples in which it was analyzed for divided by the total as of 31
August 1978. Numbers of samples ranged from 2,532 to 2,998 with the average being 2,617.
bA total of 32 industrial categories and subcategories were analyzed for organics and 28 for metals as of 31 August 1978.
organics exhibit an affinity for water; and therefore do
not bond or sorb to particle surfaces. Non-polar
organics are not attracted to water and therefore tend
to be easily sorbed. The solubility of an organic
substance also affects its ability to be oxidized,
hydrolyzed, and biodegraded. These properties differ
between the organic groups and those interactions
are often strongly dependent on the hydrogeochem-
ical environmental factors including the pH, redox
potential, and other constituent concentrations in the
water.
The study of organic compounds, known as organic
chemistry, deals with the compounds of carbon
(Sawyer and McCarty, 1978). All organic compounds
contain carbon in combination with one or more
elements, most commonly, hydrogen, oxygen, nitro-
gen, phosphorous, and sulfur. Organic compounds
generally exhibit several properties that make them
different from inorganic substances. Organic com-
pounds are generally combustible, less soluble in
water, and have lower boiling and melting points.
Reactions of organic compounds are generally molec-
ular so they tend to be slower than most other
chemical reactions. All organic compounds are either
natural, synthetic, or fermentative in origin. Organic
wastes are often produced from the processing of
natural and synthetic organic materials and fermenta-
tion at industrial facilities.
The basis of an organic compound is the element
carbon. Carbon is diverse because it maintains four
covalent bonds in addition to the ability of the carbon
atoms to link together by covalent bonding in a wide
variety of ways (Sawyer and McCarty, 1978). These
bonds may occur as a continuous chain, a branched
chain, a cyclic ring, or as chains or rings containing
other elements (Figure B-1). These structures serve
as the basis for classification of organic compounds.
For example, aliphatic compounds contain chains or
branched chains of carbon atoms and aromatic
compounds have carbon atoms linked in a six-mem-
ber carbon ring which contains three double bonds
that give them stability. Each of these compounds can
be subdivided into groupings or homologous series
where each member in the series differs from other
members by the addition of an extra carbon group.
The naming of organic compounds is complex. The
details of nomenclature may be found in a standard
chemistry text or the CRC Handbook of Physics and
Chemistry (Weast, 1983). The Office of Technology
Assessment (1984) provides a comprehensive list of
organic compounds that are known to occur in ground
water, their ranges of detected concentrations,
Figure B-1 Covalent bonding arrangements of carbon atoms (Lippencott et al., 1978).
I I I I I
c—c—c—c—c-
I I I I I
Straight Chain
I I I I
•c-c—c-c
I I I I
—c-
Branched Chain
c-c:
c:
X
Ring
^
Aromatic Ring
145
-------
examples of uses and quantitative estimates of
carcinogenic potency, and noncarcinogenic toxicity
(Table B-2). This list has been subdivided according to
characteristic organic classes; aromatic hydrocar-
bons, oxygenated hydrocarbons, hydrocarbons with
specific elements (N,S,P,CI,l,F,Br) and "others." The
"others" group generally corresponds to the aliphatic
hydrocarbons which includes many petroleum prod-
ucts. The following discussions use this classification
for simplicity, but expands upon the groups found
within these classes.
Aliphatic Compound..
A hydrocarbon is a basic organic compound of carbon
and hydrogen that may be of two types; saturated and
unsaturated. A saturated hydrocarbon has adjacent
carbon atoms joined by singlecovalent bonds with all
other bonds to hydrogen atoms. Unsaturated hydro-
carbons have at least two carbon atoms joined by
more than one covalent bond with all other bonds
satisfied by hydrogen (Sawyer and McCarty, 1978;
Lippencott et al., 1978).
Saturated compounds range from a compound with
one carbon atom, to those with each successive
compound containing an additional carbon atom.
These compounds are known as the alkanes or the
methane series and are relatively inactive. The
principal source of alkanes is petroleum. Mixtures of
these compounds comprise gasoline and diesel fuel.
Some other alkanes include ethane and propane.
Methane is the simplest hydrocarbon (CH4) and is a
major end product of anaerobic treatment processes
as well as a constituent of natural gas.
In the alkane series, butane has two isomers. An
isomer is a compound that has the same molecular
formula, but different structural formulas (Lippencott
et al., 1973). Many organic compounds exhibit this
property. Compounds containing rings of saturated
carbon atoms are known as cycloalkanes; they are
more reactive due to the strained structure of the
small ring. These are commonly known as the
napthenes and have cyclo-prefixes.
The unsaturated hydrocarbons can lose hydrogen to
bond with other elements or compounds. The alkene
or ethylene series of compounds all have one double
bond between two adjacent carbon atoms. The
compounds are commonly called olefins and are
formed in large quantities during the processing of
petroleum products. The most important reaction of
thealkenes is polymerization, where small molecules
unite to form giant molecules or polymers. The most
common reaction is the polymerization of ethylene to
form polyethylenes. The alkadienes or alkapolyenes
contain more than two carbon-carbon double bonds.
Those hydrocarbons containing triple bonds between
carbon atoms are known as the alkyne or acetylene
series. These compounds represent starting sub-
stances for many synthetic fibers.
Oxygenated Hydrocarbons
Oxygenated hydrocarbons refer to any organic com-
pound that contains an (OH) group, an oxygen group,
or responds as an acid in a solution. These may
include both aromatic and aliphatic hydrocarbon
groups.
Alcohols or hydroxy alkyl compounds are considered
to be a step in the primary oxidation product of
hydrocarbons. The alcohols are classified into three
groups: primary, secondary, and tertiary, depending
on the location of the (OH) group. The common
alcohols are methyl, ethyl, isopropyl, and n-butyl.
Methyl alcohol is used in the synthesis of organic
compounds and in antifreeze. Ethyl alcohol is used in
the production of beverages, synthesis of organic
compounds, and in medicines. Isopropyl alcohol is
used extensively in organic synthesis as is n-butyl
alcohol. Short chain alcohols are soluble in water and
may be volatized and biodegraded (Brown et al.,
1983). Polyhydroxyl alcohols contain two hydroxyl
groups per molecule and are known as glycols. These
are commonly used as radiator anti-freeze com-
pounds and are very toxic. Glycerol is a trihydroxy
alcohol used extensively in soaps, foods, cosmetics,
and medicines. Most alcohols are easily oxidized by
oxidizing agents and many microorganisms. The
aromatic alcohols compose a homologous series with
the pre-word phenyl, for example phenyl methyl.
These alcohols are also subject to chemical and
biological oxidation.
Primary alcohols are oxidized to aldehydes, while
secondary alcohols oxidize to ketones. Common
aldehydes include formaldehyde and acetaldehyde.
Formaldehyde is used extensively in organic syn-
thesis and is toxic to microorganisms, however,
under dilute concentrations it can be used as food by
microorganisms and oxidized to carbon dioxide and
water. The chemical names of all aldehydes end in
-al. Many of the aromatic aldehydes exhibit fragrant
odors, such as coumarin and vanillan. The ketones
are used as industrial solvents and in the synthesis of
organic products. The most common ketone is
acetone. Both aromatic aliphatic ketones are easily
oxidized by microorganisms.
Organic acids represent the highest oxidation state
possible in an organic compound; further oxidation
produces carbon dioxide and water (Sawyer and
McCarty, 1978). All organic acids contain a carboxyl
group. Thus, acids with one carboxyl group are known
as monocarboxylic acids and so on. A wide variety of
saturated and unsaturated acids occur in nature as
constituents of waxes, fats, and oils. These are known
as fatty acids which are typically straight chain
146
-------
Table B 2.
Contaminant
Substances Known to Occur in Ground Water, Ranges of Detected Concentrations, Exceeded Standards, Examples
of Uses, and Quantitative Estimates of Carcinogenic Potency and Noncarcinogenic Toxicity (Ota, 1984)
Concentration Standard
Examples of uses
Carcino-
genic
potency
Noncar-
cinogenic
toxicity
Aromatic hydrocarbons
Acetanilide
Alkyl benzene sulfonates
Aniline
Anthracene
Benzene
Benzidine
Benzyl alcohol
Butoxymethylbenzene
Chrysene
Creosote mixture
Dibenzla.h.lanthra-
cene
Di-butyl-p-benzoquinone
Dihydrotrimethylquinoline
4,4-Dinitrosodiphenyl-
amine
Ethylbenzene
Fluoranthene
Fluorene
Fluorescein
Isopropyl benzene
4,4'-Methylene-bis-2-
chloroaniline (MOCA)
Methylthiobenzothiazole
Napthalene
o-Nitroaniline
Nitrobenzene
4-Nitrophenol
n-Nitrosodiphenylamine
Phenanthrene
n-Propylbenzene
Pyrene
Styrene (vinyl benzene)
Toluene
1,2,4-Trimethylbenzene
Xylenes (m,o,p)
Oxygenated hydrocarbons
Acetic acid
Acetone
Benzophenone
Butyl acetate
N-Butyl-benzylphthalate
Di-n-butyl phthalate
Diethyl ether
Diethyl phthalate
Diisopropyl ether
2,4-Dimethyl-3-hexanol
(parts per billion)
18
0.6-20,230
10
0.9-4,000
31
290
6.7-82
18-471
48
0.1-6,400
0.07-300
10-3,000
10-38
470
20-34
Intermediate manufacturing, Pharmaceuticals, dyestuffs
Detergents
Dyestuffs, intermediate, photographic chemicals,
Pharmaceuticals, herbicides, fungicides, petroleum
refining, explosives
Dyestuffs, intermediate, semiconductor research
Detergents, intermediate, solvents, antiknock gasoline
Dyestuffs, reagent, stiffening agent in rubber
compounding
Solvent, perfumes and flavors, photographic developer
inks, dyestuffs, intermediate
NA
Organic synthesis
Wood preservatives, disinfectants
NA
NA
Rubber antioxidant
NA
Intermediate, solvent
NA
Resinous products, dyestuffs, insecticides
Dyestuffs
Solvent, chemical manufacturing
Curing agent for polyurethanes and epoxy resins
Solvent, lubricant, explosives, preservatives,
intermediate, fungicide, moth repellant
Dyestuffs, intermediate, interior paint pigments,
chemical manufacturing
Solvent, polishes, chemical manufacturing
Chemical manufacturing
Pesticides, retarder of vulcanization of rubber
Dyestuffs, explosives, synthesis of drugs, biochemical
research
Dyestuffs, solvent
Biochemical research
Plastics, resins, protective coatings, intermediate
Adhesive solvent in plastics, solvent, aviation and high
octane blending stock, dilutent and thinner,
chemicals, explosives, detergents
Manufacture of dyestuffs, Pharmaceuticals, chemical
manufacturing
Aviation gasoline, protective coatings, solvent,
synthesis of organic chemicals
Food additives, plastics, dyestuffs, Pharmaceuticals,
photographic chemicals, insecticides
Dyestuffs, solvent, chemical manufacturing, cleaning
and drying of precision equipment
Organic synthesis, odor fixative, flavoring,
Pharmaceuticals
Solvent
Plastics, intermediate
Plasticizer, solvent, adhesives, insecticides, safety
glass, inks, paper coatings
Chemical manufacturing, solvent, analytical
chemistry, anesthetic, perfumes
Plastics, explosives, solvent, insecticides, perfumes
Solvent, rubber cements, paint and varnish removers
Intermediate, solvent, lubricant
Low
High
Low
Low
Low
Moderate
Low
Low
Low
Low
Low
Low
Low
147
-------
Table B-2. (Continued)
Contaminant
Concentration Standard
Examples of uses
Carcino- Noncar-
genic cinogenic
potency toxicity
2,4-Dimethyl phenol
Di-n-octyl phthalate 23
1,4-Dioxane 2,100
Ethyl acrylate —
Formic acid —
Methanol (methyl —
alcohol)
Methylcyclohexanone —
Methyl ethyl ketone —
Methylphenyl acetamide —
Phenols (e.g., p-Tert- 10-234,000
butylphenol)
Phthalic acid —
2-Propanol —
2-Propyl-1-heptanol —
Tetrahydrofuran —
Varsol —
Hydrocarbons with specific elements
(e.g., with N,P,S,CI,Br,l,F)
Acetyl chloride —
Alachlor (Lasso) 190-1,700
Aldicarb (sulfoxide and 36-405
sulfone; Temik)
Aldrin -
Atrazine —
Benzoyl chloride —
Bromacil 72-110
Bromobenzene 1.9-5.8
Bromochloromethane —
Bromodichloromethane 1.4-110
Bromoform 2.4-110
Carbofuran 4-160
Carbon tetrachloride 0.3-18,700
Chlordane —
Chlorobenzene 2.7-41
Chloroform 1.4-1,890
Chlorohexane —
Chloromethane (methyl 44
chloride)
Chloromethyl sulfide —
2-Chloronaphthalene 83
Chlorpyrifos —
Chlorthal-methyl (DCPA, —
or Dacthal)
o-Chlorotoluene 2.4
p-Chlorotoluene —
Dibromochloromethane 2.1-55
Dibromochloropropane 1.-137
(DBCP)
Dibromodichloroethylene —
Solvent
*
*
*
*
NA
Pharmaceuticals, plastics, disinfectants, solvent,
dyestuffs, insecticides, fungicides, additives to
lubricants and gasolines
Plasticizer for polyvinyl chloride and other vinyls
Solvent, lacquers, paints, varnishes, cleaning and
detergent preparations, fumigants, paint and varnish
removers, wetting agent, cosmetics
Polymers, acrylic paints, intermediate
Dyeing and finishing, chemicals, manufacture of
fumigants, insecticides, solvents, plastics, refrigerants
Chemical manufacturing, solvents, automotive
antifreeze, fuels
Solvent, lacquers
Solvent, paint removers, cements and adhesives,
cleaning fluids, printing, acrylic coatings
NA
Resins, solvent, Pharmaceuticals, reagent,
dyestuffs and indicators, germicidal paints
Dyestuffs, medicine, perfumes, reagent
Chemical manufacturing, solvent, deicing agent,
Pharmaceuticals, perfumes, lacquers, dehydrating
agent, preservatives
Solvent
Paint and varnish thinner
Dyestuffs, Pharmaceuticals, organic preparations
Herbicides
Insecticide, nematocide
Insecticides
Herbicides, plant growth regulator, weed control agent
Medicine, intermediate
Herbicides
Solvent, motor oils, organic synthesis
Fire extinguishers, organic synthesis
Solvent, fire extinguisher fluid, mineral and salt
separations
Solvent, intermediate
Insecticide, nematocide
Degreasers, refrigerants and propellents, fumigants,
chemical manufacturing
Insecticides, oil emulsions
Solvent, pesticides, chemical manufacturing
Plastics, fumigants, insecticides, refrigerants and
propel lants
NA
Refrigerants, medicine, propellants, herbicide,
organic synthesis
Oil: plasticizer, solvent for dyestuffs, varnish gums and
resins, waxes
Wax: moisture-, flame-, acid-, and insect-proofing of
fibrous materials; moisture- and flame-proofing of
electrical cable; solvent (see oil)
NA
Herbicide
Solvent, intermediate
Solvent, intermediate
Organic synthesis
Fumigant, nematocide
NA
High
High
Moderate
Moderate
High
Moderate
Moderate
Moderate
Low
Moderate
Moderate
Moderate
Low
148
-------
Table B-2. (Continued)
Contaminant
Dibromoethane (ethylene
dibromide, EDB)
Dibromomethane
Dichlofenthion (DCFT)
o-Dichlorobenzene
p-Dichlorobenzene
Dichlorobenzidine
Dichlorocyclooctadiene
Dichlorodiphenyldichloro-
ethane (ODD, TDE)
Dichlorodiphenyldichloro-
ethylene (DDE)
Dichlorodiphenyltrichloro-
ethane (DDT)
1,1-Dichloroethane
1 ,2-Dichloroethane
1 , 1 - Dichloroethy lene
(vinylidiene chloride)
1 ,2-Dichloroethylene
(cis and trans)
Dichloroethyl ether
Dichloroiodomethane
Dichloroisopropylether
( = bis-2-
chloroisopropylether)
Dichloromethane
(methylene chloride)
Dichloropentadiene
2,4-Dichlorophenol
2,4-Dichlorophenoxy-
acetic acid (2,4-D)
1 ,2-Dichloropropane
Concentration
35-300
44.9
—
2.7
0.6-0.7
—
—
—
0.01-0.8
0.05-0.22
0.5-11,330
250-847
1.2-4,000
0.2-323
1,100
2.8-4.1
—
4-8,400
0.36
—
1-85,000
46-60
Standard Examples of uses
•* Fumigant, nematocide, solvent, waterproofing
preparations, organic synthesis
Organic synthesis, solvent
Pesticides
• Solvent, fumigants, dyestuffs, insecticides, degreasers.
polishes, industrial odor control
it Insecticides, moth repellent, germicide, space
odorant, intermediate, fumigants
Intermediate, curing agent for resins
Pesticides
Insecticides
Degradation product of DDT, found as an impurity
in DDT residues
• Pesticides
Solvent, fumigants, medicine
* Solvent, degreasers, soaps and scouring compounds,
organic synthesis, additive in antiknock gasoline,
paint and finish removers
* Saran (used in screens, upholstery, fabrics, carpets,
etc.), adhesives, synthetic fibers
* Solvent, perfumes, lacquers, thermoplastics, dye
extraction, organic synthesis, medicine
Solvent, organic synthesis, paints, varnishes, lacquers,
finish removers, drycleaning, fumigants
NA
Solvent, paint and varnish removers, cleaning solutions
it Solvent, plastics, paint removers, propellants.
blowing agent in foams
NA
• Organic synthesis
it Herbicides
* Solvent, intermediate, scouring compounds, fumigant,
Carcino- Noncar-
genic cinogenic
potency toxicity
Moderate
Moderate
Moderate
Low
High
Low
Low
Moderate
Moderate
Dieldrin —
Diiodomethane 2.0
Diisopropylmethyl —
phosphonate (DIMP)
Dimethyl disulfide —
Dimethylformamide —
2,4-Dinotrophenol (Dino- 124-400
seb, DNBP)
Dioxins (e.g., TCDD) —
Dodecyl mercaptan —
(lauryl mercaptan)
Endosulfan 0.8
Endrin —
Ethyl chloride —
Bis-2-ethylhexylphthalate 12-170
Di-2-ethylhexylphthalate —
Fluorobenzene 67
Fluoroform 3.5
Heptachlor —
Heptachlorepoxide —
Hexachlorobicyclohepta- 2.2
diene
Hexachlorobutadiene 2.53
a-Hexachlorocyclohexane 6
(= Benzenehexachlor-
ide, or
a-BHC)
nematocide, additive for antiknock fluids
Insecticides High
Organic synthesis
NA
NA
Solvent, organic synthesis
Herbicides
Impurity in the herbicide, 2, 4, 5-T High
Manufacture of synthetic rubber and plastics,
Pharmaceuticals, insecticides, fungicides
Insecticides
Insecticides
Chemical manufacturing, anesthetic, solvent,
refrigerants, insecticides
Plastics Low
Plasticizers
Insecticide and larvicide intermediate
Refrigerants, intermediate, blowing agent for foams
Insecticides Moderate
Degradation product of heptachtor, also acts as
an insecticide
NA
Solvent transformer and hydraulic fluid, heat-transfer
liquid
Insecticides
149
Moderate
High
High
-------
Table B-2. (Continued)
Contaminant
Concentration Standard
Examples of uses
Carcino- Noncar-
genic cinogenic
potency toxicity
/3-Hexachlorocyclo- 3.8
hexane (/3-BHC)
0-BHC)
7-Hexachlorocyclohexane 0.5-43
(7-BHC, or Lindane)
Hexachlorocyclopenta- —
diene
Hexachloroethane 4.6
Hexachloronorbomadiene —
Kepone —
Malathion —
Methoxychlor —
Methyl bromide 7.4
Methyl parathion 4.6
Parathion —
Pentachlorophenol (PCP) —
Phorate (Disulfoton) —
Polybrominated biphenyls —
(PBBs)
Polychlorinated biphenyls 8-40
(PCBs)
Prometon —
RDX (Cyclonite) 3,400
Simazine —
Tetrachlorobenzene 5,000
Tetrachloroethanes 4
(1,1,1,2 e 1,1,2,2)
Tetrachloroethylene (or 717-2,405
perchloroethylene,
PCE)
Toxaphene 1-570
Triazine 2
1,2,4-Trichlorobenzene 37
Trichloroethanes (1,1,1 0.2-26,000
and 1,1,2)
1,1,2-Trichloroethylene 210-37,000
(TCE)
Trichlorofluoromethane 26
(Freon 11)
2,4,6-Trichlorophenol —
2,4,5-Trichlorophenoxy- —
acetic acid (2,4,5-T)
2,4,5-Trichlorophenoxy- —
propionic acid
(2,4,5-TP or Silvex)
Trichlorotrifluoro- 35-135
ethane
Trinitrotoluene (TNT) 620-12,600
Tris-(2,3-dibromopropyl) —
phosphate
Vinyl chloride 50-740
Other hydrocarbons
Alkyl sulfonates —
Cyclonexane 540
1,3,5,7-Cyclooctatetraene —
Dicyclopentadiene —
(DCPD)
2,3-Dimethylhexane —
Insecticides
* Insecticides Moderate
Intermediate for resins, dyestuffs, pesticides, fungi-
cides, Pharmaceuticals
Solvent, pyrotechnics and smoke devices. Low
explosives, organic synthesis
NA
• Pesticides High
• Insecticides
• Insecticides Moderate
Fumigants, pesticides, organic synthesis
* Insecticides
• Insecticides High
• Insecticides, fungicides, bactericides, algicides, Moderate
herbicides, wood preservative
• Insecticides
Flame retardant for plastics, paper, and textiles Low
* Heat-exchange and insulating fluids in closed Moderate
systems
Herbicides
* Explosives
• Herbicides Moderate
* NA
* Degreasers, paint removers, varnishes, lacquers. Moderate
photographic film, organic synthesis, solvent,
insecticides, fumigants, weed killer
* Degreasers, do/cleaning, solvent, drying agent, Low
chemical manufacturing, heat-transfer medium,
vermifuge
* Insecticides Moderate
Herbicides
Solvent, dyestuffs, insecticides, lubricants,
heat-transfer medium (e.g., coolant)
* Pesticides, degreasers, solvent Low
* Degreasers, paints, drycleaning, dyestuffs, textiles. Low
solvent, refrigerant and heat exchange liquid,
fumigant, intermediate, aerospace operations
Solvent, refrigerants, fire extinguishers, intermediate Moderate
Fungicides, herbicides, defoliant Low
• Herbicides, defoliant Moderate
• Herbicides and plant growth regulator High
Drycleaning, fire extinguishers, refrigerants,
intermediate drying agent
Explosives, intermediate in dyestuffs and
photographic chemicals
Flame retardant
Organic synthesis, polyvinyl chloride and copolymers,
adhesives Low
Detergents
Organic synthesis, solvent, oil extraction
Organic research
Intermediate for insecticides, paints and varnishes,
flame retardants
NA
150
-------
Table B-2. (Continued)
Contaminant
Concentration Standard
Examples of uses
Carcino- Noncar-
genic cinogenic
potency toxicity
Fuel oil
Gasoline
Jet fuels
Kerosene
Lignin
Methylene blue activated
_
2,000-9,000
—
243,000
7,500i
11
Fuel, heating
* Fuel
Fuel
Fuel, heating, solvent, insecticides
Newsprint, ceramic binder, dyestuffs, drilling fuel
additive, plastics
• Dyestuffs, analytical chemistry
substances (MBAs)
Propane
Tannin
4,6,8-Trimethyl-1 -nonene
Undecane
Metals and cations
Aluminum
Antimony
Arsenic
Barium
Beryllium
Cadmium
Calcium
Chromium
Cobalt
Copper
Iron
Lead
Lithium
Magnesium
Manganese
Mercury
Molybdenum
Nickel
Palladium
Potassium
Selenium
Silver
Sodium
Thallium
Titanium
Vanadium
Zinc
— Fuel, solvent, refrigerants, propellents, organic
synthesis
7,500i Chemical manufacturing, tanning, textiles,
electroplating, inks, Pharmaceuticals, photography,
paper
NA
— Petroleum research, organic synthesis
(parts per million)
0.1-1,200 * Alloys, foundry, paints, protective coatings, electrical High
industry, packaging, building and construction,
machinery and equipment
— Hardening alloys, solders, sheet and pipe, pyrotechnics Moderate
0.01-2,100 * Alloys, dyestuffs, medicine, solders, electronic devices, High
insecticides, rodenticides, herbicides, preservative
2.8-3.8 * Alloys, lubricant High
less than 0.01 it Structural material in space technology, inertial Moderate
guidance systems, additive to rocket fuels,
moderator and reflector of neutrons in
nuclear reactors
0.01-180 * Alloys, coatings, batteries, electrical equipment, fire High
protection systems, paints, fungicides, photography
0.5-225 Alloys, fertilizers, reducing agent
0.06-2,740 * Alloys, protective coatings, paints, nuclear and High
high-temperature research
0.01-0.18 * Alloys, ceramics, drugs, paints, glass, printing. High
catalyst, electroplating, lamp filaments
0.01-2.8 * Alloys, paints, electrical wiring, machinery, Moderate
construction materials, electroplating, piping,
insecticides
0.04-6,200 * Alloys, machinery, magnets
0.01-5.6 * Alloys, batteries, gasoline additive, sheet and pipe. High
paints, radiation shielding
— • Alloys, Pharmaceuticals, coolant, batteries, solders, Low
propellents
0.2-70 Alloys, batteries, pyrotechnics, precision instruments,
optical mirrors
0.1-110 * Alloys, purifying agent High
0.003-0.01 * Alloys, electrical apparatus, instruments, fungicides, High
bactericides, mildew-proofing, paper,
Pharmaceuticals
0.4-40 * Alloys, pigments, lubricant
0.05-0.5 * Alloys, ceramics, batteries, electroplating, catalyst Moderate High
— Alloys, catalyst, jewelry, protective coatings, electrical Low
equipment
0.5-2.4 Alloys, catalyst
0.6-20 * Alloys, electronics, ceramics, catalyst High
9-330 * Alloys, photography, chemical manufacturing, mirrors, High
electronic equipment, jewelry, equipment, catalyst,
Pharmaceuticals
3.1-211 * Chemical manufacturing, catalyst, coolant, non-glare
lighting for highways, laboratory reagent
— Alloys, glass pesticides, photoelectric applications High
— Alloys, structural materials, abrasives, coatings Low
243 * Alloys, catalysts, target material for x-rays High,
moderate
0.1-240 * Alloys, electronics, automotive parts, fungicides, Moderate
roofing, cable wrappings, nutrition
151
-------
Table B-2. (Continued)
Contaminant
Concentration Standard
Examples of uses
Carcino- Noncar-
genic cinogenic
potency toxicity
Nonmetals and anions
Ammonia
Boron
Chlorides
Cyanides
Fluorides
Nitrates
Nitrites
Phosphates
Sulfates
Sulfites
Micro-organisms
Bacteria (coliform)
Viruses
Radionuclides
Cesium 137
Chromium 51
Cobalt 60
Iodine 131
Iron 59
Lead 210
Phosphorous 32
Plutonium 238,243
Radium 226
Radium 228
Radon 222
Ruthenium 106
Scandium 46
Strontium 90
Thorium 270
Tritium
Uranium 238
Zinc 65
Zirconium 95
1-900
1.0-49,500
1.05-14
0.1-250
1.4-433
0.4-33
0.2-32,318
(parts per billion)
(picocuries
per milliliter)
6.4
*
*
*
0.8-25
12.5
0.817
150-353
10-500
Fertilizers, chemical manufacturing, refrigerants,
synthetic fibers, fuels, dyestuffs
Alloys, fibers and filaments, semi-conductors,
propellents
Chemical manufacturing, water purification,
shrink-proofing, flame-retardants, food processing
Polymer production (heavy duty tires), coatings,
metallurgy, pesticides
Toothpastes and other dentrifices, additive to drinking
water
Fertilizers, food preservatives
Fertilizers, food perservatives
Detergents, fertilizers, food additives
Fertilizers, pesticides
Pulp production and processing food preservatives
High
Moderate
Gamma radiation source for certain foods
Diagnosis of blood volume, blood cell life, cardiac
output, etc.
Radiation therapy, irradiation, radiographic testing,
research
Medical diagnosis, therapy, leak detection, tracers
(e.g., to study efficiency of mixing pulp fibers,
chemical reactions, and thermal stability of additives
to food products), measuring film thicknesses
Medicine, tracer
NA
Tracer, medical treatment, industrial measurements
(e.g., tire tread wear and thickness of films and ink)
Energy source, weaponry
Medical treatment, radiography
NA
Medicine, leak detection, radiography, flow rate
measurement
Catalyst
Tracer studies, leak detection, semi-conductors
Medicine, industrial applications (e.g., measuring
thicknesses, density control)
NA
Tracer, luminous instrument dials
Nuclear reactors
Industrial tracer (e.g., to study wear in alloys,
galvanizing, body metabolism, function of oil
additives in lubricating oils)
NA
structures. The lower members of the saturated acid
series are liquids, ranging in order from the sharp
odors of formic and acetic acid to the unpleasant
odors of butyric and valeric acids. Butyric acid gives
rancid butter its disagreeable odor. Industrial wastes
from the dairy industry must be treated to prevent
formation of these acids. These acids range from
being completely soluble in water to relatively
insoluble. The saturated acids include acrylic, oleic,
and linoleic acids which are the general constituents
of the glycerides of most fats and oils (Sawyer and
McCarty, 1978; Lippencott et al., 1978). Organic
acids are utilized by microorganisms through oxida-
tion processes and are converted to carbon dioxide
and water. Biodegradation of higher acids may be
limited by their solubility in water. A wide variety of
aromatic carboxylic acids are known such as benzole
acid, a preservative, salycilic acid, a constituent of
aspirin, and phthalic acid, an important constituent in
the manufacture of organic compounds. These acids
752
-------
are also subject to biodegradation by microorganisms
to carbon dioxide and water.
The phenols are an important aromatic hydrocarbon.
They consist of a basic ring hydrocarbon or benzene
with an attached (OH) group. Phenols are generally
known as carbolic acid which is widely used as a
disinfectant, and in concentrated solutions is toxic to
bacteria. Phenols occur as natural constituents of
industrial wastes from coal and petroleum process-
ing. Until recently, phenol was thought to be toxic to
bacteria for biodegradation applications. However,
current studies suggest that bacteria may be able to
degrade low concentrations (Erlich et al., 1982; Tabak
et al., 1980). Studies by the FMC Corporation (1 983)
indicate that hydrogen peroxide is capable of oxidizing
phenols in the presence of a catalyst to produce
carbon dioxide and water.
The next higher group of phenols are creosols. They
are found in coal tar and exhibit even higher
germicidal propertiesthan phenols, but are lesstoxic.
Creosols are commonly found in spray disinfectants
such as lysol and in creosote, used in wood preser-
vation. Phenols with more than one (OH) group are
termed polyhydric. Three industrially important iso-
mers of polyhydric phenols include the catechols,
resorcinals, and hydroquinone. These isomers are
readily oxidized by microorganisms (Sawyer and
McCarty, 1978).
The degradation of aliphatic hydrocarbons by micro-
organisms depends on molecular weight, water
solubility, number of double bonds, degree of branch-
ing, and whether the compound is an open chain or
cyclic compound. Thus, the simplest compounds such
as a straight chain hydrocarbon will be the most
easily degraded as opposed to a more complex cyclic
compound (Brown et al., 1 983). The degradation rate
decreases with either a decreasing number of double
bonds or with the number and size of alkyl groups.
Sediments containing aliphatic hydrocarbons are
generally deficient in nitrogen and phosphorous.
Addition of these fertilizers greatly enhances bio-
degradation rates (Brown et al., 1 983). Volatilization
of low molecular weight hydrocarbons is a mechan-
ism that occurs with increasing temperature and soil
moisture content.
Aromatic Compounds
The aromatic compounds contain stable ring struc-
tures, or cyclic groups, with special alternating single
and double covalent bonds. These bonds are not
normal. Thus, aromatic compounds do not bond to
substances by addition, but rather by substitution of a
hydrogen atom for an element or compound. The
simplest aromatic ring is made up of a six-ringed
carbon atoms bonded to six hydrogen atoms and is
known as benzene or the benzene ring. Substitutions
may occur at one or more hydrogen atom sites. The
benzene series constitutes a single ring with alkyl
substitutions; these include toluene and xylene and
their respective isomers. These products are found in
coal tar and crude petroleum and are used primarily
as solvents.
A polyring aromatic hydrocarbon consists of one or
more cyclic rings that are bonded through shared
carbon atoms; these carbon atoms do not have
attached hydrogen atoms. The polyring aromatic
compounds include naphthelene and anthracene
used in the manufacture of dyes, and phenanthra-
cene, an important constituent of alkaloids such as
morphine and vitamin D. Halogenated and nitro-
genous aromatics will be discussed in the next
section.
Aromatic compounds are usually present in wastes
generated by petroleum refineries, organic chemical
plants, rubber industries, and waste streams asso-
ciated with combustion processes (Brown et al.,
1 983). Most aromatic hydrocarbons are toxic and/or
carcinogenic and fairly resistant to degradation. The
decomposition rate of aromatic hydrocarbons is
basically substance dependent, however, simple
compounds typically degrade more easily. In addition,
the more soluble compounds are more easily de-
graded by microorganisms (Tabak et al., 1980).
Hydrocarbons with Specific Elements
The final group of hydrocarbons may be either
aliphatic or aromatic, but has one or more additional
groups with specific elements as substituents, name-
ly nitrogen, sulfur, and phosphorous and the halo-
gens, chlorine, fluorine, iodine and bromine. The
halogenated organics have received the most atten-
tion as ground-water contaminants. These com-
pounds are refractory, or very resistant to degrada-
tion. This is thought to be due to the presence of a
halogen; its location and type determine the relative
persistence of the compound (Kobayashi and Ritt-
mann, 1982). These compounds range from simple
alkyl halides to polyhalogen compounds to complex
halogenated hydrocarbons such as DDT. Common
halogenated compounds include methyl chloride,
ethyl chloride, ethylene bromide, chloroform, carbon
tetrachloride, chlorobenzene, freon (dichlorodif luoro-
methane) and trihalomethanes. Methyl and ethyl
chloride were once used as refrigerants; ethyl
chloride is used in the manufacture oftetraethyl lead,
an antiknock gasoline additive. Chloroform has been
found in drinking water due to the reaction of chlorine
with natural organic substances in water. Freon is an
extensively used refrigerant due to its non-toxic and
non-flammable properties (Sawyer and McCarty,
1978).
753
-------
Chlorinated hydrocarbons were formerly used ex-
tensively as pesticides and herbicides, many of which
are very resistant to degradation. These include
dioxin, DDT, DDE, Aldrin, Dieldrin, Endrin, Lindane,
Chlordane, Toxaphene, 2,4-D and 2,4,5 TP Silvex
which have been banned from usage or greatly
restricted because of their toxicity and carcinogenic
potentials (Solomons, 1978; Brown et al., 1983;
Abrams et al., 1975). These products were used
extensively for agricultural and defoliant purposes.
Other pesticides and herbicides have been studied to
determine their potential for attenuation through
hydrolysis, reductive dehalogenation, and biodegrada-
tion.
Hydrolysis involves the introduction of an (OH) group
that commonly replaces the halide. Hydrolysis rates
are dependent on pH, the presence of humic mater-
ials, and individual compounds (Cherry et al., 1984;
Cohen et al., 1984). Reductive dehalogenation in-
volves the removal of the halogen through oxidation-
reduction reactions in low redox state ground water
(Cherry et al., 1984) and by certain microorganisms
(Kobayashi and Rittmann, 1982). Biodegradation of
halogenated hydrocarbons has been documented
under both aerobic and anaerobic conditions (Koba-
yashi and Rittmann, 1982; Cherry et al., 1984;
Bouwer et al., 1981; Tabak et al., 1980; Brown et al.,
1983).
Other types of pesticides include organic phosphor-
ous and carbomate pesticides. Organic phosphorous
pesticides include parathion, which is very toxic, and
malathion which has low toxicity for mammals.
Phosphorous pesticides tend to hydrolyze quickly at
or above a neutral pH, thus losing their toxic
properties. The carbamates have generally low
toxicities to mammals. These include IPC, a herbicide,
captan, a fungicide, and ferbam and sevin as insect-
icides. These pesticides are highly susceptible to
degradation (Sawyer and McCarty, 1978).
As a rule, chlorinated aromatics are less degradable
and less soluble than their aliphatic counterparts.
This has proven true for the chlorinated benzenes
including hexachlorobenzene (HCB) and its deriv-
atives. These are found as by-products of industrial
processes, and in chlorinated solvents and pesticides.
The rates of degradation of these compounds are
slow; they may persist in the soil and water for several
years without significant degradation. Certain plants
such as lettuce, carrots, grasses, and potatoes tend to
absorb HCBs from the soil (Brown et al., 1983). Rates
of degradation are variable depending on the degree
of chlorination; the less chlorinated, the more de-
gradabale (Tabak et al., 1980).
Another widely publicized group of halogenated
organics are the polychlorinated biphenyls (PCBs).
These are biphenyl molecules with the presence of
one or more chlorine atoms at several locations on
the phenyl structures. These mixtures have been
commercially produced since 1 929 with a total of 210
possible compounds. PCBs are classified according to
chlorine content with most industrial mixtures con-
taining 40 to 60 percent chlorine (Solomons, 1980).
PCBs had many uses including heat exchangers in
transformers, in capacitors and thermostats, plasti-
cizers in food bags and polystyrene cups, in printing
inks, and in waxes. Because the PCBs are highly
persistent and fat soluble, they tend to collect in the
tissues of many animals and humans. The EPA
bannedthe manufacture, processing, and distribution
of PCBs in 1 979 (Solomons, 1980).
Degradation of PCBs has been found to be affected by
the nature of the chlorine (Brown et al., 1983).
Degradation tends to increase as the amount of
chlorine substitution decreases; the relative position
of the chlorine also affects rates of degradation. In
general, the lower chlorinated compounds were
found to be degradable in mixed microbial populations
(Kobayashi and Rittmann, 1982).
Other hydrocarbons with specific elements have the
nitrogen group as substituents. These include the
amines, amides, anilines, and nitriles. The amines
are alkyl derivatives of ammonia and may be primary,
secondary, or tertiary depending on the number of
hydrogen ammonia atoms that are replaced. The
amines are found in industrial wastes from fish and
beet-sugar industries, and little is known about their
susceptibility to biodegradation. The amides are
derived from organic acids and ammonia under
special conditions. The nitriles are organic cyanides
that are extensively used in the manufacture of
synthetic fibers (Sawyer and McCarty, 1978). The
most commonly used nitriles include acrylonitrile and
acetonitrile. Attenuation of nitriles occur through
oxidation reactions at specified pH values (Harsh,
1975). The primary form of amines are known as
analines and are important compounds for organic
synthesis and in dyes. The amines were shown to
range in ease of biodegradability depending on the
individual compound (Tabak et al., 1980; Kobayashi
and Rittmann, 1982).
Mercaptans or thiols are aliphatic compounds that
contain sulfur and they have a structure similar to
alcohols. Mercaptans are known to have disagree-
able odors and are typically byproducts of kraft
pulping and petroleum processing. The FMC Corpor-
ation (1983) has shown that thiols are readily oxidized
under acid conditions to insoluble products.
References
Abrams, E. F., D. Derkics, C. V. Fong, D. K. Guinan,
and K. M. Slimak, 1975. Identification of organic
154
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compounds in effluents from industrial sources;
NTISPB-241641,211 pp.
Bouwer, E. J., B. E. Rittmann, and P. L. McCarty,
1 981. Anaerobic degradation of halogenated 1 -and
2-carbon organic compounds; Environmental Sci-
ence & Technology, Vol. 15, No. 5, pp. 596-599.
Brown, K. W., G. B. Evans, Jr., and B. D. Frentrop,
eds., 1983. Hazardous waste land treatment;
Butterworth Publishers, 692 pp.
Cherry, J. A., R. W. Gillham, and J. F. Barker, 1984.
Contaminants in groundwater: chemical processes;
Groundwater Contamination, National Academy
Press, pp. 46-66.
Cohen, S. Z., S. M. Creeger, R. F. Carsel, and C. G.
Enfield, 1984. Potential for pesticide contamination
of ground water resulting from agricultural uses;
American Chemical Society Symposium Series
#259, Treatment Disposal of Pesticide Wastes,
Krueger and Seiber, eds., Washington, D.C.
Erlich, G. G., D. F. Goerlitz, E. M. Godsy, and M. F.
Hult, 1982. Degradation of phenolic contaminants
in ground water by anaerobic bacteria: St. Louis,
Minnesota; Ground Water, Vol. 20, No. 6, pp.
703-710.
FMC Corporation, 1983. Industrial waste treatment
with hydrogen peroxide; Industrial Chemical Group,
Philadelphia, Pennsylvania, 23 pp.
Freeze, R. A. and J. A. Cherry, 1979. Groundwater;
Prentice-Hall, 604 pp.
Fuller, W. H. and J. Artiola, 1978. Use of limestone to
limit contaminant movement from landfills; Pro-
ceedings of the 4th Annual Research Symposium
on Land Disposal of Hazardous Wastes, U. S. EPA-
600/9-78-016, pp. 282-298.
Harsh, K., 1975. In situ neutralization of an acrylo-
nitrile spill; Ohio Environmental Protection Agency,
Dayton, Ohio, pp. 187-189.
Keith, L. A. and W. A. Telliard, 1979. Priority
pollutants, I-A perspective view; Environmental
Science & Technology, Vol. 13, No. 4, pp. 416-423.
Kobayashi, H. and B. E. Rittmann, 1982. Microbial
removal of hazardous organic compounds; Environ-
mental Science & Technology, Vol. 16, No. 3, pp.
170A-183A.
Lippencott, W. T., A. B. Garrett, and F. H. Verhoek,
1978. Chemistry; John Wiley & Sons, pp. 646-697.
Office of Technology Assessment, 1984. Protecting
the nation's groundwater from contamination. Vol.
I, II; U.S. Congress, Washington, D.C., 503 pp.
Page, A. L., 1974. Fate and effects of trace elements
in sewage sludge when applied to agricultural
lands; U.S. EPA-670/2-74-005.
Sawyer, C. N. and P. L. McCarty, 1978. Chemistry for
environmental engineering; McGraw-Hill, pp. 94-
163.
Solomons, T. W., 1980. Organic chemistry; John
Wiley & Sons, pp. 634-639.
Tabak, H. H., S. A. Quave, C. I. Mashni, and E. F. Barth,
1980. Biodegradability studies with priority pol-
lutant organic compounds; Staff Report, Waste-
water Research Division, U.S. EPA Research
Center, Cincinnati, Ohio.
Tolman, A., A. Ballestero, W. Beck, and G. Emrich,
1978. Guidance manual for minimizing pollution
from waste disposal sites; U.S. EPA-600/2-78-
142, pp. 328-331.
Weast, R. C., ed., 1983. CRC handbook of chemistry
and physics; CRC Press, Inc.
755
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Appendix C
Sources of Ground-Water Contamination
The quality of the ground water may be altered by a
wide variety of human activities and naturally occur-
ring phenomena. The innumerable waste materials
and byproducts of man's activities provide potential
for ground-water contamination through a variety of
mechanisms.
Ground water quality problems that are attributed to
man's influence are commonly related to: (1) water-
soluble products that are placed on the land surface
and in streams or surface impoundments, (2) sub-
stances that are deposited in the ground above the
water table, and (3) disposal, storage, or extraction of
materials below the water table (Lehr et al., 1976).
Sources of ground-water pollution are associated
with a broad range of industrial, agricultural, com-
mercial, and domestic act ivites. Many of the problems
that arise from wastes as a result of these activities
are not well understood, due to their complexity.
Technical solutions are available for many ground
water quality problems through planning, manage-
ment, and/or prevention practices.
The application of a rating system designed to
estimate potential for ground-water contamination is
of concern with regard to individual contamination
situations. Because ground-water contamination
may occur from a variety of sources, it may be
necessary to consider and possibly reevaluate the
importance of a rating factor as the scale of the area
being evaluated changes.
Soil attenuation characteristics such as sorptive
capabilities, microorganisms, degradation capacities,
and textures are of major importance when consider-
ing the placement of wastes on the land's surface
(e.g. stockpiles, sludge, wastewaters) and the sub-
sequent potential for ground-water pollution. How-
ever, the effect of soil is relatively unimportant for
situations where the soil has been removed, such as
at a landfill, or where contaminants are buried
beneath the soil surface (e.g. storage tanks). Thus,
engineering and other practical considerations of an
area can obviate the application of DRASTIC param-
eters.
Dry contamination sources that are emplaced on the
land surface, such as stockpiles, fertilizers, and
pesticides are dissolved and disseminated by rainfall
resulting in the generation of ground-water pollution.
Evaluation of the DRASTIC parameters suggests that
the most important parameters with regard to this
category of activities are: Depth to Water, which
controls contact time of the pollutant with the
unsaturated zone; Net Recharge, which limits the
quantity of leachate generated; Soil Media, which
affects both organic and inorganic attenuation mech-
anisms; and the Vadose Zone, which also directly
affects attenuation properties. Parameters of lesser
impact for this category of activities include: Aquifer
Media and Hydraulic Conductivity, since these are
impacted less by surface-applied pollutants. Topo-
graphy may be important for surface storage facilities,
but most agricultural activities are confined to
relatively flat terrain.
Wet contamination sources emplaced on the land
surface include waste waters, irrigation waters, and
spills. In this situation, the most important param-
eters are Depth to Water, Soil Media, Impact of the
Vadose Zone, and Topography which will affect the
attenuation and infiltration rates of the liquid contam-
inants. Again, because the source is on the surface.
Hydraulic Conductivity and Aquifer Media are less
important. Net recharge has a less negative effect
since the contaminant is already liquid. High net
recharge may result in dilution.
This type of rationale can also be applied to either
liquid or dry sources emplaced below the surface
which may or may not intersect the water table. The
potential for liquid sources below the water table to
cause contamination, such as leaking underground
storage tanks or drainage wells, is affected primarily
by the Depth to Water, the Impact of the VadoSeZone,
Hydraulic Conductivity, and the Aquifer Media. These
factors are directly related to attenuation and migra-
tion rates of the contaminant. Surface characteristics
such as Topography, Net Recharge, and Soil Media
would subsequently be of lesser importance for
potential pollution evaluations.
Lastly, for dry contaminant sources emplaced below
the surface (e.g. landfills, quarries) it is necessary to
consider Net Recharge in terms of volumes of
leachate generated; the Hydraulic Conductivity in
relation to migration rates; and Aquifer Media for
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possible attenuation of contaminants, dispersion,
dilution, and routing. Again, surface characteristics
are of lesser importance; Topography, Soil Media, and
Impact of Vadose Zone.
Thus, man's activities and the intensity of these
activities present many potential contamination
problems. The impact of these activities is discussed
in Section 5, Impact—Risk Factors. Activities are not
directly involved in the determination of the DRASTIC
Index, but their impact is always of serious concern.
These activities may be categorized according to their
relative position with respect to the ground water;
Table 11 represents a comprehensive list of activities
that are potential sources of contamination and their
respective modes of emplacement. Each of these
sources will be discussed individually in relation to
their effects and potential for ground-water contam-
ination.
Ground Water Quality Problems that Orig-
inate on the Land Surface
Land Disposal
One of the major causes of ground-water pollution is
the disposal of solid or liquid waste materials directly
onto the land surface in either individual deposits or
spread over the land. Any soluble products present in
the waste can be transported into the ground water
either with the liquid portion of the wastes or as a
consequence of precipitation. Land disposal practices
include the application of sewage sludge, manure,
garbage, industrial wastes, waste tailings, and spoil
piles. These activities are capable of producing a wide
variety of contaminants, including organic chemicals,
inorganic chemicals, and reactive ions.
Stockpiles and Mine Tailings
The presence of material stockpiles, and mine tailings
and spoils, pose a potential source of ground-water
contamination. An estimated 20 percent of total
production materials are stored in stockpiles of
varying sizes (OTA, 1984). Materials that are com-
monly stockpiled that may affect ground-water quality
include salt, coal, various metallic ores (e.g. copper,
uranium, titanium, vanadium, silver, lead, zinc),
phosphates, and gypsum. These stockpiles are usual-
ly exposed to precipitation such that the precipitation
will dissolve or react with other constituents to
produce leachate that can percolate into the ground-
water system. The stockpiling of salt in snow-belt
states provides a prime example of how stockpiling
can have a dramatic impact on ground-water quality.
The salt, if uncovered, is easily dissolved by precipita-
tion and either infiltrates to the subsurface or runs off
into streams (Lehr, et al., 1976).
The stockpiling of coal and associated mining proc-
esses can also degrade the quality of the ground
water. Most coal that is mined is associated with
pyrite, an iron sulfide. Contact with water tends to
break the pyrite down into iron hydroxide and sulfuric
acid. These constituents increase the iron and sulfate
levels if they reach the ground water (Bouwer, 1 978).
Spoil piles are generally disturbed soils and over-
burden from surface mining, or waste rock from
underground mining. Tailings are the solid wastes
from the on-site operations of cleaning and extracting
the ores. Both types of waste are usually stored on the
land surface. Water moving through the waste piles
will mobilize many hazardous constituents, depend-
ing on the materials. Because of their quantity,
distribution, and nature of their contents, spoil piles
and tailings are major potential sources of ground-
water contamination. Hazardous constituents from
this source can include lead, zinc, copper, and other
heavy metals, arsenic, sulfuric acid, and radioactive
elements such as uranium and thorium (Bouwer,
1978; Todd, 1980). The impact of spoil piles and
tailings is dependent on the location, size, and
composition of the tailings and spoils, the climate
(amounts of precipitation), the hydrogeologic set-
ting, and type of control and containment of the
wastes.
Disposal of Sewage and Water Treatment Plant
Sludge
The land application of treated waste waters and
sewage sludge from municipal and industrial sources
is often used as an alternative to more expensive
disposal processes. The waste waters are typically
applied as spray irrigation by various land treatment
systems (Bouwer, 1 978; Todd, 1980). Land treatment
of waste waters is generally capable of removing
nitrogen, phosphorous, organic waste matter, bacter-
ia, and viruses. Sludge wastes may be applied as
compost to agricultural and forested land, disposed of
in landfills or applied in land reclamation projects
(OTA, 1984). Municipal sludges typically contain
nitrogen, phosphorous, organic material, bacteria,
viruses, and metals. The presence of undesirable
constituents may limit the land application of sludges
and waste waters. Some hazardous materials may be
preferentially sorbed by plants or infiltrate into
ground-water resources. However, studies have
shown that high-rate land treatment systems are
capable of removing infectious bacteria and viruses
through the mechanisms of sorption, mechanical
filtration, or die-off (Freeze and Cherry, 1979; Bouwer,
1978). The rate and duration of sludge and waste
water application is dependent on the soil and waste
characteristics, the length of the application, the
nutrient uptake of the cover crop, and the climate.
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Salt Spreading
The increased usage of de-icing salts rather than
sand and other abrasives in snow belt states provides
an actual and a potential source of ground-water
contamination. The salts consist of commercial rock
and marine salt, with ferric ferrocyanide and sodium
ferrocyanide added to reduce caking (Bouwer, 1 978).
The sodium ferrocyanide is soluble and can increase
the concentrations of cyanide present in ground
water above safe drinking water levels. In addition,
chromate and phosphate may be added to decrease
the corrosiveness of the salt; these also affect
ground-water quality. Many cases of ground-water
contamination have been cited from road salt appli-
cations (Bouwer, 1978). Contamination may be
minimized by designing roads to reduce de-icing
requirements, better collection and disposal of salty
run-off, protective salt storage, and use of alternate
de-icing materials.
Animal Feed Lots
Accumulations of animal wastes at feed lots can
contaminate ground water by infiltration of leachate
or surface runoff. The primary contaminant of the
leachate is organic, or ammonium, nitrogen which is
converted to nitrate in the vadose zone. Additional
hazardous constituents include bacteria, viruses, and
phosphates (Bouwer, 1978; OTA, 1984). Animal
feedlots are most concentrated in the corn belt and
High Plains regions. Ground-water contamination
tends to be more severe at feed lots underlain by
coarse-textured soils with shallow water tables than
by fine-textured soils with deeper water tables. These
wastes may be controlled by applying animal waste
slurries intermittently to maintain aerobic conditions
in the upper soil. The nitrogen will be converted to
nitrate in the upper soil layer and then be denitrified,
or converted to nitrogen gas and water in the deeper
anaerobic soil layers.
Fertilizers and Pesticides
Modern agricultural practices employ the extensive
use of fertilizers and pesticides to obtain high crop
yields. The primary fertilizers are compounds of
nitrogen, phosphorous, and potassium. Excess potas-
sium and phosphorous are readily sorbed onto soil
particles and seldom constitute a contamination
problem. Crop uptake of fertilizer nitrogen varies from
about 40 to 80 percent of the amount applied
(Bouwer, 1980). The remainder is either volatized and
returned to the atmosphere through denitrification,
or percolates downward as nitrate to contaminate
ground water. Efficient application of fertilizer, par-
ticularly at peak requirements during growth cycles,
minimizes nitrate contamination (OTA, 1984). Unfor-
tunately, most farmers apply enough fertilizer for the
entire growing season prior to planting, thus in-
creasing the potential for nitrate contamination.
Pesticides are chemicals used for the control of
insects, fungi, or other undesirable organisms and
weeds. Agricultural activities account for between 69
and 72 percent of pesticide use; government agencies
and industries use 21 percent; home and garden uses
constitutethe remainder. Agricultural applications of
pesticides have resulted in ground-water contamina-
tion in at least 18 states involving a variety of
pesticides (OTA, 1984; Cohen et al., 1984). Contam-
ination can occur from common-use practices, spills,
disposal of excess pesticides, and leakage from
storage containers. Applications of pesticides by
airplanes pose a significant problem due to the large
quantities applied and indiscriminant application.
The movement and attenuation of pesticides in the
soil and ground water is dependent on several
parameters including water solubility, vapor pressure,
speciation, hydrolysis half-life, photolysis half-life,
soil and water sorption properties, and climate (OTA,
1984; Cohen, et al., 1984). The primary parameters
that account for pesticide attenuation are sorption,
volatilization, biodegradation, and solubility. Those
pesticides that are relatively refractory tend to be the
least soluble and most resistant to biodegradation
and hydrolysis (Tabak et al., 1984; Cohen et al., 1984).
The most toxic and refractory pesticides are the
chlorinated hydrocarbons such as DDT, lindane,
endrin, DDE, and chlordane. The organic phosphate
pesticides such as malathion and parathion are less
toxic than the chlorinated hydrocarbons and more
easily attenuated. The least toxic pesticides are the
carbamates such as sevin and captan; these are
easily biodegraded by common soil organisms.
Accidental Spills of Hazardous Materials
A wide variety of hazardous materials are transported
throughout the country by truck, rail, and aircraft and
transferred at handling facilities such as airports and
loading docks. Accidental spills of hazardous mater-
ials provide a definite potential for ground-water
contamination. The National Academy of Sciences
(NAS) estimated that approximately 16,000 spills
occur annually, involving a variety of materials such
as hydrocarbons (i.e. gasoline, jet fuel), pain products,
flammable compounds, various acids, and anhydrous
ammonia (OTA, 1984).
There are presently few methods available to quickly
and adequately clean up an accidental hazardous
waste spill. To make matters worse, a common
practice is to spray the spill area with water in order to
flush the compounds from the road. The hazardous
materials are then washed into drainage ditches or
streams from which they have the potential to
infiltrate and contaminate the ground water.
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The attenuation of hazardous materials is dependent
on the site-specific hydrogeochemical characteristics,
the chemical(s) spilled, and any remedial actions
performed by authorities on the surface materials at
the time of the spill. An example of a successful
hazardous material spill clean-up is given by Harsh
(1975). An acrylonitrile spill (cyanide compound)
resulting from a train derailment was quickly remed-
iated by the Ohio Environmental Protection Agency
through the application of oxidants that neutralized
the hazardous substance.
Particulate Matter from Airborne Sources
A relatively minor, but potential source of ground-
water contamination arises from the fallout of
paniculate matter from the atmosphere. These
materials fall to the surface of the earth and are
carried as soluble or insoluble products by water to
the subsurface. The primary source of atmospheric
pollution is automobile emissions and various indus-
trial processes. The major contaminants from these
emissions include sulfur and nitrogen compounds,
asbestos, and heavy metals (OTA, 1984). The distrib-
ution of particulates in the atmosphere and on the
surface depends on their size when released, weather
patterns, and climate. The attenuation of these
pollutants is dependent on the site-specific hydro-
geochemical characteristics, the location of pollutant
fallout, and the chemical nature of the pollutant itself.
The infiltration of airborne contaminants tends to be
higher in heavily industrialized areas (Lehr et al.,
1976). Perhaps the major environmental concern
today related to airborne contaminants is the effect of
acid rain on the surface and subsurface water quality.
Several contamination occurrences from airborne
pollutants have been cited by EPA, particularly
increases in heavy metal concentrations in drinking
water due to industrial emissions.
Ground Water Quality Problems that
Originate in the Ground Above the
Water Table
Leaching Tile Fields, Cesspools, and Privies
A widely distributed source of ground-water pollution
is septic tanks and cesspools. These sources dis-
charge large volumes of waste water into the
subsurface. A septic tank system consists of a buried
tank and leaching tile field designed to collect water
suspended wastes, remove settleable solids from
liquids by gravity separation, and permit infiltration of
the effluent into the soil for general degradation by
soil microorganisms. According to OTA (1984), in the
mid-1970's there were an estimated 19.5 million
domestic on-site disposal systems present in the
United States and unknown numbers of commercial
and industrial systems.
Septic tank systems are the most frequently reported
source of contamination on a local and regional basis
(OTA, 1984). The contaminants are primarily derived
from human wastes and household cleaners; these
include nitrates, chloride, coliform bacteria, viruses,
and a variety of organic and inorganic chemicals. The
major factors that affect the potential for contam-
ination of ground water by septic systems are the
density of systems per area, the hydrogeologic
conditions, and the attenuation capacity of the soil
through which the effluent percolates. Problems tend
to arise from systems that are old and deteriorated
and from those systems that are poorly constructed.
Several parameters must be considered in the design
and installation of a septic system; these include
ground water flow velocity and direction, the storage
and carrying capacities of the receiving soil, the
sorptive qualities of the soil, the resident micro-
organism community and the depth to the water
table.
Holding Ponds and Lagoons
Holding ponds and lagoons, because of their relative
numbers and size, present a significant potential for
ground-water contamination. Surface impoundments
are used by industries and municipalities for the
treatment, retention, and/or disposal of non-hazard-
ous and hazardous liquid wastes. These ponds or
lagoons vary in size from two to three feet deep, to
more than 30 feet deep, with areas covering from a
fraction of an acre to thousands of acres (OTA, 1984).
Agricultural, municipal, industrial, and oil and gas
production lagoons are typically less than five acres in
size. However, industrial impoundments are generally
1000 acres or larger. The size of a mining impound-
ment depends on the ore and type of mining. The
wastewater in industrial surface impoundments may
consist of suspended and dissolved solids, pathogenic
organisms, oil and grease, detergents, heavy metals,
and toxic organic chemicals (Pye et al., 1983).
Holding ponds are supposedly "liquid tight," however,
the majority leak large quantities of material, thereby
contributing to ground-water pollution. The installa-
tion of a double-liner system of plastic and clay has
recently become a requirement for these facilities
under new RCRA regulations. The presence of a liner
combined with a leachate collection drain system will
lower the potential of contamination by constituents
in the holding pond.
The potential for contamination by a holding pond is
dependent on head conditions in the pond, soil
permeability, depth to the water table, rates of
evaporation and precipitation, geochemical charac-
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teristics of the soil (ion exchange and sorption), the
chemical composition and amounts of waste, andthe
capacity for degradation by soil organisms (Pye et al.,
1983).
Sanitary Landfills
Sanitary landfills are constructed excavations that
are filled with hazardous and non-hazardous waste
materials and covered daily by soils. Daily coverage by
soils preventsthe production of odors, smoke, andthe
presence of vermin and insects. Precipitation and
surface runoff can infiltrate into these landfills
producing leachate that can contain soluble hazard-
ous materials. Some hazardous constituents may be
attenuated as the leachate filters through the soil
zone. However, leachate may still reach the ground
water. Landfills are commonly covered with clay
and/or soil combinations to minimize the infiltration
of water and production of leachate. However, these
coverings are not totally impermeable and some
moisture penetrates the protective covers.
Approximately 40 percent of the solid hazardous
industrial wastes are disposed of in some type of
landfill in addition to the large quantities of non-
hazardous wastes that are also deposited in these
facilities (OTA, 1984). Problems arise with older
landfills that are improperly constructed, have failing
liners, have accepted unknown types and quantities
of wastes, are deteriorating, or have been abandoned.
These older and abandoned landfills pose a definite
threat to ground water because geologic and hydro-
geologic characteristics were not considered in the
initial site selection or design.
Ground-water contamination from landfills may be
minimized by proper design, construction, operation,
and maintenance of a facility (Wilson et al., 1976).
Provisions must be made for controlling traffic,
unloading and handling different types of wastes,
placement of cover materials, testing of wastes for
acceptance, and adequate ground-water monitoring
to prevent contamination problems.
Waste Disposal in Excavations
The removal of materials such as clay, limestone, and
sand and gravels commonly produces pits or excava-
tions that may have been eventually abandoned and
used as unregulated waste disposal sites. A wide
variety of materials have been emplaced in excava-
tions in unknown quantities. These include garbage,
junk automobiles, and liquid wastes such as oil-field
brines and spent acids from steel mills. Disposal of
unregulated waste poses the threat of ground-water
contamination. Often these pits contain some amount
of water, evidence that they are hydraulically con-
nected to the water table and possibly deeper
aquifers. Leachate from these wastes may therefore
have direct access to the ground-water system. This
contamination may be avoided through preventative
regulations and correct disposal of wastes at a
properly designed sanitary landfill or other facility.
Leakage from Underground Storage Tanks
Underground storage tanks are used by industries,
commercial establishments, and individuals for stor-
age and treatment of products or raw materials
(Wilson et al., 1976). The primary storage materials
are hydrocarbon fuels, but many other substances
are commonly stored in underground tanks, such as
acids, metals, industrial solvents, chemicals, and
chemical wastes. The most numerous underground
storage tanks are those used for gasoline and fuel oil.
Gasoline leakage from leaking tanks has caused
severe pollution problems throughout the nation.
Because gasoline is less dense than water, it floats on
the surface of the ground water and may leak into
basements, sewers, wells, and springs. In confined
areas, vapors from these leaks, disseminated by
ground water, can cause a serious explosion hazard.
Most tanks are composed of either steel or fiberglas.
Unprotected steel is subject to rusting or corrosion by
various materials. Fiberglas may crack or be slowly
dissolved by alcohol blends. Tank age is a principal
factor in ground-water contamination; studies have
shown that older tanks exhibit a tendency to leak
through deterioration of the tank (OTA, 1984).
Ground-water pollution is minimized by proper tank
design and emplacement, properly placed monitoring
wells, frequent inspections, and fluid level monitoring.
Leakage from Underground Pipelines
Pipelines are used to transport, collect, and/or
distribute waste and non-waste products. Leaks that
occur in pipelines are often difficult to detect and
locate. Pipeline leakage occurs due to ruptures,
external and internal corrosion, incorrect operation,
and defective welds or pipes (Todd, 1980). Wastes
transported in pipelines include municipal sewage,
petroleum products, brines, ammonia, sulfur, and
coal. Interstate pipelines and all spills are regulated
by the Department of Transportation. Other collection
and distribution pipelines are not regulated other
than during initial installation and are not required to
report leaks and spills.
Artificial Recharge
Artificial recharge includes a variety of techniques
used to increase the amount of water infiltrating into
an aquifer. This is achieved through the construction
of pits, ponds or wells, or direct land application so
that the water will seep into the ground or flow or be
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pumped directly into the aquifer. Water that is used
for artificial recharge commonly consists of storm
runoff, irrigation return flows, stream water, cooling
water, and treated sewage effluent. The infiltration of
this water may directly affect ground-water quality,
resulting in subsequent contamination. Recharge is
becoming increasingly popular due to the growing
demands for water and the need for recycling.
Sumps and Dry Wells
Sumps and dry wells are typically installed to provide
drainage in problem areas by collecting runoff or
spilled liquids. These liquids are usually channeled
into the subsurface where they are transmitted to the
ground water. These facilities are especially prone to
cause problems if they are located near streams,
lakes or ponds, or estuaries where ground-water
levels are naturally high.
Graveyards
Leachate from graveyards may cause ground-water
pollution, especially if non leak-proof or no caskets
were used. The potential for ground-water contam-
ination by graveyards is dependent on several factors,
including the characteristics of the soil and depth to
the water table (Lehr et al., 1976). Areas that receive
high amounts of precipitation and have high water
tables may be especially prone to contamination
problems. Graveyards also pose a potential contam-
ination problem in areas of hard rock and limestone
terrain where soil covers are thin, and in glaciated
areas where the glacial sand and gravel lenses are
hydraulically interconnected. Few actual cases have
been documented and any contamination problems
would be highly localized.
Ground Water Quality Problems that Orig-
inate in the Ground Below the Water Table
Waste Disposal in Wet Excavations
The removal and mining of various materials often
produces excavations and pits that are commonly
abandoned after activities cease. These pits often
extend below the water table and thereby contain
quantities of water. These excavations were used in
the past as repositories for both solid and liquid
wastes and presently have the potential to produce
large quantities of leachate. The leachate produced is
in direct hydraulic connection with the shallow
ground water and possibly with other deeper aquifers.
drainage wells and canals may be constructed. A
drainage well is a vertical cased hole in the ground or
the bottom of a pond, for example, that allows water
to drain into deeper, more permeable materials.
Pollution of the recharged surface water may thus
contaminate the aquifer.
The construction of extensive channels and channel
deepening may have the potential to affect ground-
water quality. Deepened channels may affect the
ground-water gradient and allow infiltration of sur-
face water or the intrusion of salt or brackish water,
thereby causing contamination. Extensive channels
in coastal areas have allowed tidal ocean waters to
flow inland and affect ground-water quality.
Abandoned or Improperly Constructed Wells
The presence of abandoned wells provides a signif-
icant means for ground-water contamination. Wells
that have been abandoned commonly provide con-
duits for the migration of contaminated water. The
water may be runoff or surface water that enters
through the open conduit, or may be the migration of
fluid between an aquifer and a zone of undesirable
water quality. When a well is abandoned, the casing
is often pulled or is allowed to deteriorate so that
casing leaks develop. This readily permits fluids
under pressure to migrate, upward or downward, and
contaminate the aquifers. Another problem is the
migration of saline water into freshwater aquifers. In
areas where the hydraulic gradient is toward the
freshwater aquifer, abandoned or improperly con-
structed wells provide a conduit resulting in salt
water contamination and intrusion. The migration of
contaminated fluids may also occur in areas of deep
well disposal injection operations. Contaminated
liquids under pressure are able to migrate into other
aquifers via uncased wells and deteriorated casings
in wells.
Exploratory Wells
Exploratory wells and test holes are frequently drilled
to determine the presence of underground mineral
resources such as coal, salt, oil, or gas, and as seismic
shot holes where an explosive is discharged to
produce shock waves for seismic sensors. Where
these open holes penetrate more than one aquifer,
they permit interaquifer leakage. Thus, brackish
water from a saline aquifer could migrate upward or
downward to contaminate a fresh water aquifer.
These open holes also allow surface waters to enter
the hole and migrate into other aquifers.
Drainage Wells and Canals
In areas that consist of surficial clays and flat-lying
land with ponds and marshes that are poorly drained,
Water Supply Wells
Improperly constructed water supply wells have the
potential to contaminate ground water and produce
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polluted water. Large diameter dug wells are partic-
ularly prone to contamination from surface runoff if
not properly protected during and following construc-
tion. Improper grouting of the annular space around
the casing may allow surface contaminants to enter
the aquifer. Drainage from animal feedlots, barn-
yards, septic tanks, or cesspool effluent may also
contaminate improperly constructed wells. Proper
training and licensing of well drillers and the imple-
mentation of water well construction standards
minimizes pollution problems of this nature.
Waste Disposal Wells
Wells are used to inject or discharge either hazardous
or non-hazardous waste into a permeable geologic
unit beneath the surface. Industrial injection wells
may range in depth from a few tens of feet to several
thousand feet. The EPA estimates that at least 21,000
wells in the United States require some type of
corrective action (OTA, 1984). Contamination from
injection wells can occur in a number of ways,
including: (1) faulty well construction, (2) the migra-
tion of pressurized fluids into nearby wells and
aquifers, (3) the forcing upward of pressurized fluids
into faults or fractures in confining beds, (4) injection
into or above usable aquifers, (5) the migration of
contaminated fluids into other hydraulically connect-
ed aquifers used as drinking water supplies, and (6)
faulty well sealing that allows the entrance of surface
runoff into the well. Since the implementation of the
Federal Underground Injection Control Act (UIC), the
installation and operation of underground injection
wells is regulated.
Mines
Excavation and operation of both surface and under-
ground mines can disrupt the natural continuity of
aquifers and introduce fractures and pressure chang-
es which affect the ground-water flow. This allows
water to migrate through the fractured overburden
into other aquifers and/or to mix with mine spoils and
tailings. Often mines will intersect the water table, or
various aquifers, necessitating dewatering of the
mine by pumping large quantities of water to the
surface. This water may be contaminated and pollute
aquifers, streams, or surface bodies of water into
which it infiltrates or drains. Dewatering of mines
may also cause salt water intrusion by lowering the
fresh water table. Underground mining tends to
introduce water and oxygen into the subsurface
which results in the oxidation of pyrite and the
subsequent formation of acidic mine drainage (Pyeet
al., 1983). Acid mine drainage is a mixture of iron
salts, other salts, and sulfuric acid, and may sub-
stantially contribute to ground-water contamination.
In order to protect the surface and ground-water
resources, mining activities must be carefully de-
signed, operated, and regulated from their planning
phase to final reclamation of the land.
Salt Water Intrusion
Withdrawals of ground water in excess of recharge
capabilities often results in contamination of an
aquifer by salt water intrusion. This problem is
especially prevalent in densely populated coastal
areas where large quantities of fresh water are
pumped daily. Overdrafting disrupts the natural
hydrologic processes and affects subsequent impacts
on aquifers and ground-water quality. Increased
pumpage of fresh water will tend to lower the water
table in the vicinity of the well, causing changes in the
hydraulic gradient. This allows intrusion of saltwater
along the hydraulic slope into a pumping well (Freeze
and Cherry, 1979). Land subsidence may also occur
with increased pumping and reduction of the water
table. In some coastal areas, injection of freshwater
into aquifers is used to prevent salt-water intrusion.
References
Bouwer, Herman, 1978. Groundwater hydrology;
McGraw-Hill, 480 pp.
Cohen, S. Z., S. M. Creeger, R. F. Carsel, and C. G.
Enf ield, 1984. Potential for pesticide contamination
of ground water resulting from agricultural uses;
American Chemical Society Symposium Series
#259, Treatment Disposal of Pesticide Wastes,
Krueger and Seiber, eds., Washington, D.C.
Freeze, R. A. and J. A. Cherry, 1979. Groundwater;
Prentice-Hall, 604 pp.
Harsh, K., 1975. In situ neutralization of an acrylo-
nitrile spiN; Ohio Environmental Protection Agency,
Dayton, Ohio, pp. 187-189.
Lehr, Jay H., Wayne A. Pettyjohn, Truman Bennett,
James R. Hanson, and Laurence E. Sturtz, 1976. A
manual of laws, regulations and institutions for
control of ground water pollution; U.S. EPA-440/9-
76-006.
Office of Technology Assessment, 1984. Protecting
the nation's groundwater from contamination. Vol.
I, II; U.S. Congress, Washington, D.C., 503 pp.
Pye, V. I., R. Patrick, and J. Quarles, 1983. Ground-
water Contamination in the United States; Uni-
versity of Pennsylvania Press, 314 pp.
Tabak, H. H., S. A. Quave, C. I. Mashni, and E. F. Barth,
1980. Biodegradability studies with priority pol-
lutant organic compounds; Staff Report, Waste-
water Research Division, U.S. EPA Research
Center, Cincinnati, Ohio.
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Todd, D. K., 1980. Groundwater hydrology; John
Wiley & Sons, 535 pp.
Wilson, J. L, R. L. Lenton, and J. Porras, 1976.
Groundwater pollution: technology, economics and
management; Department of Civil Engineering,
Massachusetts Institute of Technology, Report No.
TR208, pp. 17-21.
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