&EFA
United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-85/018 Aug. 1985
Project Summary
DRASTIC: A Standardized
System for Evaluating
Groundwater Pollution Potential
Using Hydrogeologic Settings
-•^
.s
A methodology is described that will
allow the pollution potential of any
hydrogeologic setting to be systemati-
cally evaluated anywhere in the United
States. The system has two major por-
tions: the designation of mappable
units, termed hydrogeologic settings,
and the superposition of a relative rat-
ing system called DRASTIC.
Hydrogeologic settings are described
for different regions in the United
States. These settings incorporate the
major hydrogeologic factors that 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 are used to infer
the potential for contaminants to enter
ground water. These settings form the
basis for the entire system, and create
units that 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 priorit-
ize 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 appropriate
areas.
This Project Summary was devel-
oped by EPA's Robert S. Kerr Environ-
mental Research Laboratory, Ada. OK.
to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back}.
Introduction
This research project was designed to
create a methodology that will permit the
ground-water pollution potential of any
hydrogeologic setting to be systemati-
cally evaluated anywhere in the United
States. The project design also included
the development of a standardized sys-
tem that can be displayed on maps.
Therefore, a set of demonstration maps
will be prepared to show how the system
could display the information on maps.
Inherent in this demonstration project is
the idea that the standardized system
cannot be finalized until it has been
extensively tested in a wide variety of
representative settings. Thus, this sys-
tem and the setting descriptions will be
continually evolving until the demonstra-
tion project is complete.
Results
The system that has been developed
has two major parts: the designation of
mappable units, termed hydrogeologic
settings; and the superposition of a rela-
tive ranking system, called DRASTIC,
which helps the user evaluate the rela-
tive ground-water pollution potential of
any hydrogeologic setting.
A hydrogeologic setting is a composite
description of all the major geologic and
hydrologic factors which affect and con-
trol ground-water movement into,
through and out of an area. It is defined as
a mappable unit with common hydrogeo-
logic characteristics, and as a conse-
quence, common vulnerability to
contamination.
The standardization system for eval-
uating ground-water pollution potential
has been developed within the frame-
work of an existing classification system
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2. Alluvial Basins
' Nortneast
Superior Uplands
. Nonglaciated Central
9. Northeast and
Superior Uplands
Region
V1. Western Mountain
Ranges
i 6. Nonglaciated
\ Central
Region
. Colorado/if^
Plateau
and
Wyoming
Basin
6. Nonglaciated
Central Region
6. Nonglaciated
Central Region
c
4
7.
Glaciated
Central
Region
'6. Nonglaciated
Central
Region
500 miles
1 .'. .' . 1 .' . J
I i i i i I I i I
0 800 kilometers
Figure 1. Ground- water regions of the United States (After Heath, USGS, Water Supply Paper No. 2242, 1984).
of ground-water regions of the United
States (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
For the purpose of the present system,
Region 12 (Alluvial Valleys) has been
incorporated into each of the other
regions and Region 15 (Puerto Rico and
Virgin Islands) has been omitted.
Because pollution potential cannot be
determined on a regional scale smaller
"hydrogeologic settings" were devel-
oped within each region. These hydro-
geologic settings create units which are
mappable and, at the same time, permit
further delineation of the factors that
affect pollution potential (Figure 2).
Inherent in each hydrogeologic setting
are the physical characteristics that
affecj the pollution potential of ground
water. After evaluating a number of fac-
tors, 'the most important mappable fac-
tors that control ground-water pollution
potential were determined to be:
D - Depth to Water Table
R -(Net) Recharge
A -Aquifer Media
S -Soil.Media
T -Topography (Slope)
I - Impact of Vadose Zone
C -Conductivity (Hydraulic) of the
Aquifer
The DRASTIC factors represent meas-
urable parameters for which data are
generally available from a variety of sour-
ces without detailed reconnaissance.
Sources of this information are listed in
Table 1.
Each DRASTIC factor has been evalu-
ated with respect to each other to deter-
mine the relative importance of each
factor. Each DRASTIC factor has been
assigned a relative weight ranging from 1
to 5 (Table 2), with the most significant
factors having a weight of 5 and the least
significant a weight of 1.
Figure 2. Format of hydrogeologic setting.
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Table 1. Sources of Hydrogeologic Information
Depth to
Water
Source Table
U.S. Geological Survey
State Geological Surveys
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 (hydrogeotogk. 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
X
Net
Recharge .
X
X
X
X
X
X
X
X
X
Aquifer
Media
X
X
X
X
X
X
X
X
X
X
X
Impact
of the
Soil Vadose
Media Topography Zone
X X
X
X
X X
X
X
X
X
X
X
X
X
Hydraulic
Conductivity
of the
Aquifer
X
X
X
X
X
X
X
X
X
A special case for the DRASTIC INDEX
was developed for agricultural areas
where herbicides and pesticides are ap-
plied. The weights assigned for each agri-
cultural use are shown in Table 3.
Each DRASTIC factor has been divided
into either ranges or significant media
types which have an impact on pollution
potential and each range has been evalu-
ated with respect to each other to deter-
mine the relative significance of each
range with respect to pollution potential
(Tables 4-10).
Once the 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 pol-
lution potential.
Conclusion
The system presents a simple and
easy-to-use approach to assess the
ground-water pollution potential of any
area. Although the final system appears
Table 2. Assigned Weights for
Drastic Features
Feature Weight
Depth to Water Table 5
Net Recharge 4
Aquifer Media 3
Soil Media 2
Topography 1
Impact of the Vadose Zone 5
Hydraulic Conductivity
of the Aquifer 3
simplistic, the system actually includes
many complex concepts and relation-
ships. Before an attempt is made to make
full use of this system, the user must
develop an appreciation for the complex-
ity of evaluating ground-water pollution
potential. It is not necessary to under-
stand every concept in detail, but the
greater the depth of understanding, the
more useful the system becomes.
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.
DRASTIC provides mappable results
that can be used to provide a quick refer-
ence of relative pollution potential of dif-
ferent areas. DRASTIC is designed to be
used as a planning or screening tool.
DRASTIC arid associated maps cannot be
used exclusively in site specific evalua-
tions because of local complexities in
geologic conditions.
Table 3. Assigned Weights for
Agricultural Drastic
Features.
Feature
Agricultural
Weight
Depth to Water Table 5
Net Recharge 4
Aquifer Media 3
Soil Media 5
Topography 3
Impact of the Vadose Zone 4
Hydraulic Conductivity
of the Aquifer 2_
Table 4. Ranges and Ratings for
Depth to Water
Depth to Water
(Feet)
Range
0-5
5-10
15-30
30-50
50-75
75-100
7.00+
Weight. 5
Rating
to
9
7
5
3
2
1
Agricultural Weight: 5
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Table 5. Ranges and Ratings for
Net Recharge
Net Recharge
(Inches)
Range Rating
0-2 1
2-4 3
4-7 6
7- JO 8
70+ 9
Weight: 4 Agricultural Weight: 4
Table 6. Ranges and Ratings for Aquifer Media
Aquifer Media
Range
Massive Shale
Metamorphic/ Igneous
Weathered Metamorphic/ Igneous
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
Agricultural
Typical Rating
2
3
4
6
6
6
8
9
10
Weight: 3
4
Table 7. Ranges and Ratings for Soil Media
Soil Media
Range
Thin or Absent
Gravel
Sand
Shrinking and/or Aggregated Clay
Sandy Loam
Loam
Silty Loam
Clay Loam
Nonshrinking and Nonaggregated Clay
Weight: 2
Table 8. Ranges and Ratings for Topography
Topography
(Percent Slope)
Range Rating
0-2 10
2-6 9
6-12 5
12-18 3
18+ 1
Rating
10
10
9
7
6
5
4
3
1
Agricultural Weight: 5
Weight: 1
Agricultural Weight: 3
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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
Rating
1-2
2-5
2-7
4-8
4-8
Typical Rating
1
3
6
6
6
Sand and Gravel with
significant Silt and Clay
Metamorphic/lgneous
Sand and Gravel
Basalt
Karst Limestone
Weight: 5
4-8 6
2-8 4
6-9 8
2-10 9
8-10 10
Agricultural Weight: 4
Table 10. Ranges and Ratings for
Hydraulic Conductivity
Hydraulic Conductivity
Igpd/ft')
Range
1-100
100-300
300-700
700-1000
1000-2000
2000+
Weight: 3
Rating
1
2
4
6
8
10
Agricultural Weight: 2
This Project Summary was prepared by staff of Robert S. Kerr Environmental
Research Laboratory, U.S. Environmental Protection Agency, Ada, OK 74820.
Jerry T. Thornhill is the EPA Project Officer (see below).
The complete report, entitled "DRASTIC: A Standardized System for Evaluating
Ground Water Pollution Potential Using Hydrogeologic Settings," (Order No. PB
85-228 146/AS; Cost: $17.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
* U.e.QOVEmMCNTMVNTWQOFFICC:1MI 559-111/20645
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