"DRASTIC" Methodology

 
 
 One of the tools created for the purpose of groundwater
protection in the United States is a methodology referred to as
"DRASTIC," developed as the result of a cooperative agreement
between the National Water Well Association (NWWA) and the U.S.
Environmental Protection Agency (EPA). The procedure was designed
to provide for systematic evaluation of groundwater-pollution
potential in any hydrogeologic setting.
 
 DRASTIC consists of several components, the first of which is
the designation of mappable hydrogeologic parameters (Aller et al.,
1987). The seven variables from which the name of the model is
derived, include Depth to water, Recharge, Aquifer media, Soil
media, Topography, Impact of the vadose zone, and Conductivity
(hydraulic). These mappable parameters are generally available for
all of the U.S. or can be derived from various sources.
 
 The numerical ranking system, another DRASTIC component, is
used to assess the groundwater-pollution potential for each
hydrogeologic variable. The system contains three parts: 1)
weights; 2) ranges; and 3) ratings (Aller et al., 1987). Each
DRASTIC parameter has been assigned a relative weight between 1 and
5, with 5 being considered most significant in regard to
contamination potential and 1 being considered least significant
(Table 1). In turn, each of the variables is "sub-divided" into
either numerical ranges (e.g., depth to water in feet) or media
types (e.g., materials making up a soil) which impact pollution
potential. Table 2 illustrates the ranges and ratings for soil
media. Finally, the ratings are used to quantify the ranges/media
with regard to likelihood of groundwater pollution.
 
 The final result for each hydrogeologic setting (i.e.,
geographic area) is a numerical value obtained using the following
simple equation:
 
 DRASTIC INDEX=
 DrDw+RrRw+ArAw+SrSW+TrTw+IrIw+CrCw (1)
 Where: R = rating
 W = weight
 
 A high numerical index resulting from Equation (1) is assumed
to be indicative of a geographic area that is likely to be
susceptible to groundwater pollution. But, as Aller et al. (1987)
warn, "the DRASTIC Index provides only a relative evaluation tool
and is not designed to provide absolute answers." Thus, one must 
 
 
 Table 1
 Assigned weights for hydrogeologic settings
 
SETTING WEIGHT
 
Depth to Water  5
Net Recharge 4
Aquifer Media 3
Soil Media 2
Topography 1
Impact of the Vadose Zone Media 5
Hydraulic Conductivity of the Aquifer 3
 
Source: Aller et al., 1987
 
 
 
 
 Table 2
 Ranges and ratings for soil media
 
RANGE RATING
 
Thin or absent 10
Gravel 10
Sand 9
Peat 8
Shrinking and/or Aggregated Clay 7
Sandy Loam 4
Loam  5
Silty Loam 4
Clay Loam 3
Muck 2
Nonshrinking and Nonaggregated Clay 1
 
Source: Aller et al., 1987
 
 
understand that DRASTIC was intended as a reconnaissance tool, but
has proven its value as an indicator of areas deserving a detailed
hydrogeologic evaluation.
 
 Placed within a spatial context, the methodology is based upon
a series of seven maps, one for each hydrogeologic parameter. 
While the DRASTIC methodology can be implemented manually by means
of visual map comparison and overlay, the time requirements and
error potential associated with printed maps at different scales
and plotted on various map projections are well known by resource
managers (e.g., Marble, 1987). Therefore the DRASTIC model was
automated with the framework of a digital geographic information
system. Such an approach is consistent with the notion that
modelling is an important key to furthering research in GIS (Smith,
1988) and that approaches which improve the process of spatial
inquiry are useful for resource evaluation.
 
STATEWIDE IMPLEMENTATION
 
 CALMIT and DEC undertook a cooperative project in 1988 aimed
at production of DRASTIC maps at 1:250,000 for all of Nebraska. 
The procedures employed in the statewide project, which were again
based upon use of the Erdas-GIS software, are summarized below.
 
Preprocessing Steps
 Prior to dealing with each of the seven layers in the model,
certain "housekeeping" or preprocessing steps were executed. Most
importantly, we identified and recorded the corner coordinates of
each 1:250,000 scale USGS quadrangle map comprising the study site. 
For our statewide product, we processed data on the basis of those
maps, with the spatial extent of each expressed in UTM (Universal
Transverse Mercator) coordinates. These were later concatenated to
provide statewide coverage.
 
Parameter #1: Depth to Water
 Depth to water was computed using two datasets: 1) land-
surface topography, available on magnetic tape from the USGS as
Digital Elevation Models (DEM) at 1:250,000 and 2) water-table-
surface topography, available from the state geological survey. 
The water-table contours were digitized and georeferenced (using
map corner coordinates) to the appropriate USGS quadrangle map. 
This map was then rasterized and smoothed (using a low-pass filter)
to produce a digital file that was in the same format as the DEM
used to describe the land surface. Water-table elevations were
simply subtracted from land-surface elevation on a pixel-by-pixel
basis to compute depth to water.
 
Parameter #2,3 and 6: Recharge, Auquifer Media, and Impact of the
Vadose Zone
 Since neither digital datasets nor paper maps existed for
recharge, aquifer media, and impact of the vadose zone, the first
step was to employ a hydrogeologist to map these three parameters
using well-long data, climatic maps, and topographic-regions maps. 
Aller et al. (1987) provide detail on the computation of such
parameters. Each prepared map was then digitized and registered to
the appropriate quadrangle coordinates. The data was rasterized
and converted into a format consistent with the depth-to-water map
produced in the previous step, and DRASTIC indices assigned.
 
Soils
 Digitized "STATSGO" soil-association data for Nebraska were
acquired from the Soil Conservation Service. STATSGO information
is produced at 1:250,000, and available on magnetic tape. The
digital soils data were rectified, rasterized, and classified into
the appropriate DRASTIC index values (Aller et al., 1987).
 
Topography
 Percent slope of the land surface was calculated using the
USGS DEM's. The Erdas software allowed for (%) slope computation
and recoding to a DRASTIC index value. The topography variable was
the easiest and most straightforward of the seven DRASTIC variables
to calculate.
 
Hydraulic Conductivity
 Maps of hydraulic conductivity for specific area of Nebraska
are generally not available; however, maps of the two components
of conductivity - transmissivity and saturated thickness - are more
frequently available or can be generated by a hydrogeologist from
well logs. Our procedure involved digitizing contour lines of
transmissivity and saturated thickness as derived from
hydrogeologic interpretation, creating a "DEM-like" surface image
of each, and then dividing transmissivities by saturated
thicknesses on pixel-by-pixel basis. The result was a map of
hydraulic conductivity which was then classified into DRASTIC index
values according to Aller et al. (1987).
 
 Care must be used when interpreting DRASTIC maps to not read
more into them than the method is designed to produce. This is
especially true with regard to the spatial resolution, which by the
nature of the model, is extremely coarse.
 
Potential for Groundwater Contamination Using the DRASTIC
Methodology
 
 This data portrays the relative potential for contamination of
groundwater based on a standardized methodology referred to as
DRASTIC, which was devloped by the United States Environmental
Protection Agency and the National Water Well Association (Aller
and others, 1987). The method incorporates weighted factors
affecting contaminant transport to groundwater. The scale at which
this analysis was performed (1:250,000) restricts the usefulness of
this data to relative evaluations on a regional basis and does not
allow site-specific applications.
 
 DRASTIC was developed to provide a systematic evaluation of
the potential for groundwater contamination that is consistent on
a national basis. Each letter of the name DRASTIC refers to one of
the hydrogeologic factors used in evaluation. They are:
 
 D - Depth to water
 R - Recharge to the aquifer (net)
 A - Aquifer media
 S - Soil media
 T - Topography (slope)
 I - Impact of vadose (unsaturated) zone
 C - Conductivity (hydraulic) of the aquifer
 
  Each factor (or parameter) was mapped according to a standard
ranking of various values. Parameter maps at a scale of 1:250,000
were then superimposed according to a weighting based on the
relative importance of each to potential groundwater contamination. 
Each parameter map was related to the other parameter maps by
overlaying cells 640 meters by 640 meters in size. The final data
represents the combination of the seven parameters.
 
 DRASTIC makes four major assumptions:
 1. The contaminant is introduced at the ground surface;
 2. The contaminant is flushed into the ground water by 
 precipitation; 
 3. The contaminant has the mobility f water; and
 4. The area evaluated is 100 acres or larger.
 
  Land-use practices such as application of irrigation water or
agricultural chemicals were not considered. These factors are
known to be important in the occurrence of nonpoint-source
contamination. The data was produced by the Center for Advanced
Land Management Information Technologies, Conservation and Survey
Division, Institute of Agriculture and Natural Resources,
University of Nebraska-Lincoln, under contract to the Nebraska
Department of Environmental Control. For more information the
reader is referred to Rundquist and other (1989) and Rundquist and
others (1991).
 
 References
 
 Aller, L., Bennett, T., Lehr, J. H., Petty, R.J., and Hackett
G., 1987, DRASTIC: A standardized system for evaluating ground
water pollution potential using hydrogeologic settings: NWWA/EPA
Series, EPA-600/2-87-035.
 Rundquist, D. C., Li, R. Y., Di, L., Liu, M., 1989, Applying
concepts of geographic systems and expert systems to the "DRASTIC"
ground water-vulnerability model: Water Resources Center,
University of Nebraska-Lincoln.
 Rundquist, D. C., Rodekohr, D. A., Di, L., Peters, A. J.,
Ehrman, R. L., Murray, G., 1991,, Statewide groundwater-
vulnerability assessment in Nebraska using the DRASTIC/GIS model.