United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-87/035 Sept. 1987
Project Summary
DRASTIC: A Standardized
System for Evaluating Ground
Water Pollution Potential Using
Hydrogeologic Settings
Linda Aller, Truman Bennett, Jay H. Lehr, Rebecca Petty, and Glen Hackett
A methodology is described that will
allow the pollution potential of any
hydrogeologic setting to be systemat-
ically 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 form the
basis of the system and incorporate the'
major hydrogeologic factors which
affect and control ground-water move-
ment including depth to water, net
recharge, aquifer media, soil media,
topography, impact of the vadose zone
media and hydraulic conductivity of the
aquifer. These factors, which form the
acronym DRASTIC, are incorporated
into a relative ranking scheme that uses
a combination of weights and ratings
to produce a numerical value called the
DRASTIC Index.
Hydrogeologic settings are combined
with DRASTIC Indexes to create units
which can be graphically displayed on
a map. The application of the system
to 10 hydrogeologically variable coun-
ties resulted in maps with symbols and
colors which illustrate areas of ground-
water contamination vulnerability. The
system optimizes the use of existing
data to rank areas with respect to
pollution potential to help direct inves-
tigations and resource expenditures
and to prioritize protection, monitoring
and clean-up efforts.
The full report was submitted in
partial fulfillment of Contract No. CR-
810715-01 by the National Water Well
Association under sponsorship of the
U.S. Environmental Protection
Agency. This report covers a period
from October 1983 to March 1987,
and work was completed as of April
1987.
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
information 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 methodology has been
incorporated into a standardized system
that can be readily displayed on maps
using existing information. The concepts
inherent to the system were developed
assuming a contaminant with the mobil-
ity of water, introduced at the surface
and flushed into the ground water by
precipitation. The methodology is
designed to evaluate ground-water
pollution potential from a regional
perspective and should be applied to
areas 100 acres or larger in size.
-------
Results
The system that has been developed
has two major parts, the designation of
mappable units, termed hydrogeologic
settings; and the superposition of a
relative ranking system, called DRASTIC,
which helps the user evaluate the
relative ground-water pollution potential
of any hydrogeologic setting.
The standardization system for evalu-
ating ground-water pollution potential
has been developed within the frame-
work of an existing classification system
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 deve-
loped within each region. A hydrogeo-
logic 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 mappab
unit with common hydrogeologic cha
acteristics, and as a consequenci
common vulnerability to contaminatioi
These hydrogeologic settings form tti
basis of the system and permit furth<
delineation of the factors that affei
pollution potential (Figure 2).
Inherent in each hydrogeologic settin
are the physical characteristics th<
affect the pollution potential of grour
water. After evaluating a number i
factors, the most important mappab
factors that control ground-water polli
tion 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 th
Aquifer
2. Alluvial Basins
9. Northeast and
, Superior Uplands
7. Glaciated
Central
region
6. Nonglaciated
Central
region
9. Northeast and
Superior Uplands
1. Western Mountain
Ranges
16. Nonglaciated
Central regio
v
4. Colorado
Plateau
and
Wyoming
Basin
O^^yV
6. Nonglaciated
Central .region
6. Nonglaciated
Central'region
. Coastal Pl^
0 500 Af//es
0 flOO Kilometers
Figure 1. Ground-water regions of the United States (After Heath. 1984).
2
-------
Hawaii
(12C) Volcanic Uplands
This hydrogeologic setting is characterized by moderately 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 weathered 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 weathered 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 occurring in a lenticular body that floats on the salt water.
Ground water yield is therefore limited quite specifically to the amount of water recharged
annually
Figure 2. Format of hydrogeological setting.
The DRASTIC factors represent meas-
urable parameters for which data are
generally available from a variety of
sources without detailed reconnais-
sance. Sources of this information are
listed in Table 1.
Each DRASTIC factor has been eval-
uated with respect to each 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), with the most
significant factors having a weight of 5
and the least significant a weight of 1.
These weights are constant and may not
be changed.
A special case for the DRASTIC Index
was developed for agricultural areas
where the application of pesticides are
a concern. Pesticide DRASTIC was
created to address the important pro-
cesses which affect the fate and trans-
port of pesticides into the subsurface.
The weights assigned for each factor
have been modified to reflect the poten-
tial impacts of pesticide application on
ground water (Table 3).
Each DRASTIC factor has been divided
into either ranges or significant media
types which have an impact on pollution
potential. Each range or media has then
been evaluated with respect to each
other to determine the relative signifi-
cance of each range or media with
respect to pollution potential. These
ranges and media have been assigned
a rating of from 1 to 10, with the most
significant range or media having a rating
of 10, and the least significant a rating
of 1 (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
pollution potential. A DRASTIC Index
calculated for one setting may then be
compared to values obtained in other
settings m the same region or in a
different region.
Hydrogeologic settings are combined
with DRASTIC Indexes to create mappa-
ble units which present a "picture" of
the geologic and hydrogeologic condi-
tions of an area. To fully demonstrate and
test the system, DRASTIC was applied
to 10 counties across the United States
representing a diversity of hydrogeologic
conditions including:
1. Cumberland County, Maine;
2. Finney County, Kansas;
3. Gillespie County, Texas;
4. Greenville County, South Carolina;
5. Lake County, Florida;
6. Minidoka County, Idaho;
7. New Castle County, Delaware;
8. Pierce County, Washington;
9. Portage County, Wisconsin; and
10. Yolo County, California.
These counties were chosen to repre-
sent both rural and urban areas and to
exemplify both an abundance and scar-
city of available hydrogeologic data.
Pollution potential maps showing hydro-
geologic settings and DRASTIC Indexes
were manually drawn for each county.
To assist in map readability, each dem-
onstration map was color-coded using
the DRASTIC Index. A national color code
scheme was developed based on a
simplified statistical evaluation of fre-
quency of Index occurrence. The colors
of the spectrum were chosen to show
the levels of relative vulnerability to
pollution. The warm colors indicate areas
with potentially greater problems and
cool colors indicate areas with lower
susceptibility to ground-water pollution.
Figure 3 illustrates the superposition of
the national color code on a portion of
the pollution potential map for Yolo
County, California. Various screens have
been chosen to simulate the color
variations on the map.
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
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
-------
Table 1. Sources of Hydrogeologic Information
Depth to
Source Water
U.S. Geological Survey X
State Geological Surveys X
State Department of Natural/ Water
Resources X
U.S. Department of Agriculture-Soil
Conservation Service
State Department of Environmental
Protection X
Clean Water Act "208" and other
Regional Planning A uthorities X
County and Regional Water Supply
Agencies and Companies (private
water suppliers) X
Private Consulting Firms
Ihydrogeologic, engineering) X
Related Industry Studies (mining, well
drilling, quarrying, etc.) X
Professional Associations
(Geological Society of America,
National Water Well Association.
American Geophysical Union) X
Local Colleges and Universities
(Departments of Geology. Earth
Sciences, Civil Engineering) X
Other Federal/ State Agencies
(Army Corps of Engineers. National
Oceanic and Atmospheric
Administration X
greater the depth of understanding, the
more useful the system becomes.
DRASTIC produces mappable results
that provide a basis for the comparative
ownli la t inn nf aroflQ with rfl*»nflPt to thfi
c vdi ua LIUI i ui a i GOO win i i c9|joL>i \.\j n iw
potential for ground-water pollution.
DRASTIC should not be used for site
specific investigations, but is best applied
on a regional basis to areas greater than
100 acres in size. DRASTIC is designed
to assist individuals with resource
allocation and prioritization of many
types of ground-water related activities
as well as to provide a practical educa-
tional tool.
Net Aquifer Soil
Recharge Media Media
X X
X X
X X
X X
X X
X X
X
X
X
X X
X X
X X
Impact of the Hydraulic
Conductivi
Topography Vadose Media of the Aquifer
X X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Table2. Assigned Weights for DRASTIC Table 3. Assigned Weights for Pesticide
Features
Feature Weight
Depth to Water 5
Net Recharge 4
Aquifer Media 3
Soil Media 2
Topography 1
Impact of the Vadose Zone
A4~w:« K
DRASTIC Features
Feature
Depth to Water
Net Recharge
Aquifer Media
Soil Media
Topography
Impact of the Vadose Zone
A4<*w;».
Pesticide
Weight
5
4
3
5
3
A
Hydraulic Conductivity of the
Aquifer
Hydraulic Conductivity of the
Aquifer
-------
Table 4. Ranges and Ratings for Depth
to Water
Depth to Water
{reel/
Range Rating
0-5 10
5-15 9
15-30 7
30-50 5
50-75 3
75-100 2
700+ 7
Weight: 5 Pesticide Weight: 5
Table 5. Ranges and Ratings for Net
Recharge
Net Recharge
(Inches)
Range Rating
0-2 1
2-4 3
4-7 6
7-10 8
70+ 9
Weight: 4 Pesticide Weight: 4
Table 6. Ranges and Ratings for Aquifer Media
Aquifer Media
Range
Massive Shale
Metamorphic/ Igneous
Weathered Metamorphic/ Igneous
Glacial Till
Bedded Sandstone, Limestone and Shale
Sequences
Massive Sandstone
Massive Limestone
Sand and Gravel
Basalt
Karst Limestone
Weight: 3
Table 7. Ranges and Ratings for Soil Table
Media
Soil Media
Range Rating
Thin P" Absent in
Gravel 10
Sand 9
Peat 8
Shrinking and/or
Aggregated Clay 7
Sandy Loam 6
Loam 5
Silty Loam 4
Clay Loam 3
Muck 2
Nonshrinking and
Nonaggregated Clay 1
Rating Typical Rating
1-3 2
2-5 3
3-5 4
4-6 5
5-9 6
4-9 6
4-9 6
4-9 8
2-10 9
9-10 10
Pesticide Weight: 3
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
Weight: 1 Pesticide Weight: 3
Weight: 2
Pesticide Weight: 5
-------
Table 9. Ranges and Ratings for Impact of the Vadose Zone Media
Impact of the Vadose Zone Media
Range
Confining Layer
Silt/Clay
Shale
Limestone
Sandstone
Bedded Limestone. Sandstone. Shale
Sand and Gravel with Significant Silt and Clay
Metamorphic/ Igneous
Sand and Gravel
Basalt
Karst Limestone
Weight: 5
Rating
1
2-6
2-5
2-7
4-8
4-8
4-8
2-8
6-9
2-10
8-10
Pesticide Weight: 4
Table 10. Ranges and Ratings for
Hydraulic Conductivity
T . . _ . Hydraulic Conductivity
Typical Rating W«2'
Range Rating
3 1-100 1
3 100-300 2
6 300-700 4
6 700-1000 6
6 1 000-2000 8
6 20OO+ 10
4 Weight: 3 Pesticide Weight: 2
8
9
10
-------
Sea/e //i
I f/gure 3. Pollution potential map for a portion of Yolo County. California, showing the superposition of the national color code.
-------
Linda Alter, Truman Bennett. Jay H. Lehr. Rebecca Petty, and Glen Hackett
are with National Water Well Association. Dublin, OH 43017.
Jerry 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 87-213 914/AS; Cost: $48.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-87/035
------- |