U.S. Department of the Interior USGS Fact She

U.S. Geological Survey 

Modified from p

Improvements to the DRASTIC Ground-Water 

Vulnerability Mapping Method

INTRODUCTION 

Ground-water vulnerability maps are designed to show 

areas of greatest potential for ground-water contamination on 

the basis of hydrogeologic and anthropogenic (human) factors. 

The maps are developed by using computer mapping hardware and software called a geographic information system 

(GIS) to combine data layers such as land use, soils, and 

depth to water. Usually, ground-water vulnerability is determined by assigning point ratings to the individual data layers 

and then adding the point ratings together when those layers 

are combined into a vulnerability map. 

Probably the most widely used ground-water vulnerability mapping method is DRASTIC, named for the seven factors considered in the method: Depth to water, ne

Aquifer media, Soil media, Topography, Impact o

zone media, and hydraulic Conductivity of the aq

and others, 1985, p. iv). The DRASTIC method h

used to develop ground-water vulnerability maps 

parts of the Nation; however, the effectiveness of 

has met with mixed success (Koterba and others, 

513; U.S. Environmental Protection Agency, 199

and Resek, 1996; Rupert, 1997). DRASTIC maps 

not calibrated to measured contaminant concentra

The DRASTIC ground-water vulnerability m

method was improved by calibrating the point rat

to measured nitrite plus nitrate as nitrogen (NO2+

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM – NAWQA
2

concentrations in ground water on the basis of statistical correlations between NO2 + NO3–N concentrations and land use, 

soils, and depth to water (Rupert, 1997). This report describes 

the calibration method developed by Rupert and summarizes 

the improvements in results of this method over those of the 

uncalibrated DRASTIC method applied by Rupert and others 

(1991) in the eastern Snake River Plain, Idaho. 

DESCRIPTION OF STUDY AREA

The 35,800-square-mile upper Snake River Basin extends from western Wyoming to south-central Idaho (fig. 1). 

About 50 percent of the upper Snake River Basin is forest and 

rangeland, about 30 percent is irrigated agricultural land, and 

the remaining area is barren. Most of the 2.3 million acres of 

irrigated land is near the Snake River and near the mouths of 

tributary basins. Most cities and industrial centers are adjacent to the Snake River. Major dams and lakes in the basin 

store about 4.4 million acre-feet of water for irrigation of 

more than 1 million acres annually. 

The 10,800-square-mile eastern Snake River Plain (fig. 1) 

is about 60 miles wide and 170 miles long. The plain is underlain by a series of highly vesicular and fractured olivine basalt flows of Quaternary age, each averaging 20 to 25 feet in 

thickness; total thickness is as much as 5,000 feet (Whitehead, 

1992). These basalt flows 

are highly transmissive to 

ground water and supply 

most of the drinking water 

in the study area. Sedimentary rocks of Paleozoic age 

and volcanic rocks of Tertiary age predominate north, 

east, and south of the plain. 

Sedimentary rocks of Quaternary and Tertiary ages 

and, to a lesser extent, volcanic rocks of Quaternary 

and Tertiary ages predominate in the mountain valleys. 

DEVELOPMENT OF THE 

UNCALIBRATED 

DRASTIC 

VULNERABILITY MAP IN 

IDAHO

The first published map 

of ground-water vulnerability in Idaho was developed 

by Rupert and others (1991), 

who used a modified form 

of the DRASTIC method. 

Three of the seven DRASTIC factors—depth to water, 

net recharge (land use), and 

soil media—were used because they were believed to 

be the most important factors with respect to grou

vulnerability, and because they were the most rea

ble data. Land use was used as a surrogate for net r

because irrigated agricultural areas provide the lar

of re-charge in southern Idaho. The point ratings 

ent from those used in the DRASTIC method, but 

determined in the same manner; a committee used 

professional judgment to determine the point ratin

resultant map was termed "relative ground-water 

ity" because the vulnerability ratings (low, medium

very high) were determined relative to each other 

not based on measured ground-water quality data 

others, 1991). 

DEVELOPMENT OF THE NEW CALIBRATED 

PROBABILITY MAP

The vulnerability map developed using the u

DRASTIC method (Rupert and others, 1991) was 

by calibrating the point rating scheme on the basi

tions of NO2+NO3–N concentrations in ground w

land-use, soils, and depth-to-water data; nonparam

tics and a GIS were used to quantify the relations

basis of the relations, a point rating scheme was d

that classifies areas according to their potential fo

water contamination by NO2+NO3–N. That poin

scheme then was entered into the GIS, and the pro

map was produced. 

The Wilcoxon rank-sum nonparametric stati

(Ott, 1993, p. 279, 793) was used to correlate NO

concentrations with land-use, soils, and depth-to-

This test determines whether differences in NO2+

concentrations are statistically significant between 

ous data groups, for example, whether NO2+NO3

trations in ground water in irrigated agricultural a

tistically different from concentrations in rangelan

The test calculates a p-value; if the resulting p-va

than 0.05, then the data sets are significantly diffe

95-percent confidence level. If the p-value is less 

then the data sets are significantly different at the 

confidence level. For this report, the 95-percent c

level was used as the cutoff value for determining 

differences between data sets were statistically sig

Ground-Water Quality Data Used for Calibra

The NO2+NO3–N ground-water quality data 

calibration of the probability map were collected fo

Statewide Ground Water Monitoring Program (IS

which is a cooperative program between the U.S. 

Survey (USGS) and the Idaho Department of Wate

(Idaho Department of Water Resources, 1991; Ne

Crockett, 1992; Neely, 1994; Crockett, 1995). Th

data were ideal for calibration of the map because 

selected in a random manner, all wells were samp

the same techniques, all samples were analyzed b

laboratory, and all data were stored consistently in 

Table 1. Rating scheme for 

probability of ground-water 

contamination by dissolved 

nitrite plus nitrate as nitrogen, 

eastern Snake River Plain 

Rating

Land use (points)

Urban . . . . . . . . . . . 3

Irrigated 

agriculture . . . . . 2

Rangeland . . . . . . . 1

Dryland 

agriculture . . . . . 1

Forest . . . . . . . . . . . 1

Rating

Soil drainage (points)

Excessive . . . . . . . . 4

Well . . . . . . . . . . . . 3

Moderate . . . . . . . . 2

Poor . . . . . . . . . . . . 1

Rating

Depth to water (points)

0 to 300 feet . . . . . . 2

301 to 900 feet . . . . 1

Resultant

probability 

Contamination rating

probability (points)

Very high . . . . . . . . 8

High . . . . . . . . . . . . 7

Medium . . . . . . . . . 6

Low . . . . . . . . . . . . 4 to 5
3

21

90th percentile

75th percentile

Median

25th percentile

10th percentile

Number of samples

EXPLANATION FOR BOXPLOTS

(Figures 2, 3, 4, 5, 6, 7) 

15

10

5

0

LAND USE

16

493

102

51

Rangeland Dryland

agriculture

p = 0.01

p = 0.00

p = 0.00

p = 0.05 p = 0.00 p = 0.3

Number of samples

Urban Irrigated

agriculture

p = 0.41

p = 0.00 p = 0.17

p = 0.01

Figure 2. Correlations between concentrations of 

nitrite plus nitrate as nitrogen and land use, upper 

River Basin, 1991–94. DISSOLVED NITRITE PLUS NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 30

25

20

15

10

5

0

SOIL DRAINAGE

21

131

536

Well Exc

p = 0.00

p = 0.00

p = 0.00

Number of samples

Poor Moderate

p = 0.00

p = 0.00

p = 0.01

DISSOLVED NITRITE PLUS NITRATE AS

NITROGEN, IN MILLIGRAMS PER LITER

Figure 3. Correlations between concentrations of 

nitrite plus nitrate as nitrogen in ground water and 

soil drainage categories, upper Snake River Basin

[STATSGO, State Soil Geographic Data Base (U.S

ment of Agriculture, 1991)]

data base. Data collected from 726 wells during calendar 

years 1991–94 were used for calibration. 

Land-Use Probability Ratings

Land-use data for Idaho were obtained from the Idaho 

Department of Water Resources, which combined the data 

from three sources: a map denoting vegetation types; a map 

differentiating between sprinkler- and gravity-fed irrigation 

methods; and a map differentiating dryland agriculture from 

irrigated agriculture (Rupert and others, 1991, p. 12). Landuse data were not available for Wyoming, Utah, or Nevada. 

NO2+NO3–N concentrations in ground water in irrigated agriculture and urban areas were significantly higher 

than in rangeland, dryland agriculture, and forest areas (fig. 2). 

In addition, NO2+NO3–N concentrations in urban areas were 

significantly higher than in irrigated agriculture areas, but the 

small number of wells sampled in urban areas might have 

biased this result. These results are similar to those reported 

by Rupert (1994, p. 29). 

Urban areas were assigned a probability rating of 3 

(table 1) because NO2+NO3–N concentrations in ground 

water in urban areas were higher than in all other land-use 

areas. Irrigated agriculture areas were assigned a probability 

rating of 2 because NO2+NO3–N concentrations in irrigated 

areas were significantly higher than in other land-use areas 

except urban. Rangeland, dryland agriculture, and forest areas 

were combined and assigned a probability rating of 1 because 

NO2+NO3–N concentrations were lowest in those areas and 

differences in concentrations were not statistically significant. 

Soil Probability Ratings

Soil data were obtained from the State Soil Geographic 

Data Base (STATSGO), developed by the U.S. Natural Resource Conservation Service (U.S. Department of Agriculture, 1991). Soil criteria considered were clay content, drainage, hydrologic group, percentage of organic matter, permeability, and Unified Soil Classification ratings. STATSGO 

soil data were aggregated from many large-scale soil surveys 

(1:12,000 to 1:62,500) into one large data base at 

mate scale of 1:250,000 (U.S. Department of Agr

1991, p. 1, 2).

NO2+NO3–N concentrations and soil draina

most strongly correlated of the soil criteria exami

Although the relations are not illustrated, NO2+N

centrations also increased significantly with decre

amounts of clay and decreasing amounts of organ

Concentrations increased with increasing amount

Lines above boxplots in the following figures indicate results of 

individual Wilcoxon rank-sum tests between each category. 

Resulting p-values less than or equal to 0.05 (concentrations are 

different at a 95-percent or greater confidence level) are labeled 

on solid red lines. Result-ing p-values greater than 0.05 

(concentrations are not different at a 95-percent confidence 

level) are labeled on dashed black lines. The 10th and 90th 

percentiles are not shown on boxplots if fewer than 10 wells 

were sampled.
4

8

7

6

5

4

3

2

1

0

Figure 4. Correlations between concentrations of dissolved

nitrite plus nitrate as nitrogen and depth to first-encountered

ground water, eastern Snake River Plain, 1991–94. DISSOLVED NITRITE PLUS NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

DEPTH TO WATER, IN FEET

190

109

34

9

0–100 101–300 301–600 601–900

p = 0.03

p = 0.00

p = 0.01

p = 0.11 p = 0.00 p = 0.15

Number of samples

and gravel, as represented by the Unified Soil Classification 

ratings. Surprisingly, soil permeability and hydrologic group 

were not correlated with NO2+NO3–N concentrations. Soil 

permeability is an estimate of how much water can pass 

through a saturated soil and commonly is measured on disturbed soil samples in a laboratory environment. The lack of 

correlation between NO2+NO3–N concentrations and soil 

permeability is probably because secondary soil features such 

as desiccation cracks and root pores are not included in the 

permeability rating, and important criteria such as clay content and organic matter content also are not accounted for. 

Soil drainage denotes the frequency and duration of wet periods of the soil (U.S. Department of Agriculture, 1993, p. 98). 

Soils with poor drainage typically are saturated with water 

and can have chemically reducing (anaerobic) conditions. 

Reducing conditions can lead to denitrification that can, in 

turn, minimize NO2+NO3–N concentrations. Correlations 

with soil drainage probably reflect these processes. 

Differences in NO2+NO3–N concentrations between all 

soil drainage categories were statistically significant, so each 

soil drainage category was assigned a different point rating. 

Excessively drained soils were assigned a probability rating 

of 4, well-drained soils were assigned a rating of 3, moderately drained soils were assigned a rating of 2, and poorly 

drained soils were assigned a rating of 1 (table 1). 

Depth-to-Water Probability Ratings

Depth to first-encountered ground water in the eastern 

Snake River Plain was mapped by Maupin (1992), who divided 

depth to water into five categories: 0 to 100 feet, 101 to 300 

feet, 301 to 600 feet, 601 to 900 feet, and greater than 900 feet. 

The differences in NO2+NO3–N concentrations in groundwater samples from depths of 0 to 300 feet and from 301 to 

900 feet were statistically significant (p<0.05, fig. 4). However, the differences in concentrations in first-enc

water from depths of 0 to 100 feet and 101 to 300 

from depths of 301 to 600 feet and 601 to 900 fee

significant (p>0.05). Because differences in NO2+

concentrations were not significant, depth to first-

tered water categories of 0 to 100 feet and 101 to 

were combined and assigned a probability rating o

Depth categories of 301 to 600 feet and 601 to 90

combined and assigned a probability rating of 1. 

GROUND-WATER PROBABILITY MAP

A map showing the potential for elevated NO

concentrations was developed (fig. 1) using the rat

shown in table 1. The map is termed a probability m

of a vulnerability map because (1) the probability c

based on the results of statistical comparisons of N

in ground water, and (2) the term probability more c

what the maps portray; whether an area has a hig

relative probability for ground-water contaminati

NO2+NO3–N.

Many different combinations of point rating 

depth to first-encountered ground water, land use

drainage were evaluated. For example, a rating of 

instead of 2 points was tried for depth to first-enc

ground water of 0 to 300 feet. The point rating sche

that produced the lowest p-values was used in the 

bility map. The final map produced good correlat

NO2+NO3–N; the largest p-value was 0.01, and f

p-values were less than 0.01, which suggests stati

nificant differences (greater than 99-percent confi

of NO2+NO3–N between the probability categor

25

20

15

10

5

0

PROBABILITY OF GROUND-WAT

CONTAMINATION BY NITRIT

PLUS NITRATE AS NITROGEN

64

152

264

High Ve

p = 0.00

p = 0.00

p = 0.00

Number of samples

Low Medium

p = 0.01

p = 0.00

p = 0.00

DISSOLVED NITRITE PLUS NITRATE AS

NITROGEN, IN MILLIGRAMS PER LITER

Figure 5. Correlations between concentrations of 

nitrite plus nitrate as nitrogen collected for the Idah

Ground Water Monitoring Program and contaminati

ity, eastern Snake River Plain, 1991–94.
5

20

15

10

5

0

122

106

220

p = 0.13

p = 0.01

p = 0.23 p = 0.00

Number of samples

168

p = 0.06

Figure 6. Correlations between concentrations of dissolved

nitrite plus nitrate as nitrogen from an independent data set

and relative ground-water vulnerability ratings of Rupert and

others (1991), eastern Snake River Plain, 1980–91. DISSOLVED NITRITE PLUS NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

RELATIVE GROUND-WATER

VULNERABILITY

Low Medium High Very high

p = 0.00

COMPARISON OF NEW PROBABILITY MAP WITH OLD 

VULNERABILITY MAP

The effectiveness of the ground-water probability map 

to predict elevated NO2+NO3–N concentrations in ground 

water was compared with the effectiveness of the relative 

ground-water vulnerability map produced using the uncalibrated DRASTIC method (Rupert and others, 1991). Comparisons were made by correlating both maps with an independent set of NO2+NO3–N data, which were retrieved from the 

USGS data base in Boise for calendar years 1980–91. All 

data collected by the ISGWMP were excluded from this data 

set. A direct comparison was possible because the same landuse, soil, and depth-to-water data layers were used to develop 

both maps. 

The relative ground-water vulnerability map produced by 

Rupert and others (1991) correlated poorly with NO2+NO3–N 

concentrations in ground water (fig. 6). The differences in 

NO2+NO3–N concentrations in the low and medium, low 

and very high, and high and very high relative ground-water 

vulnerability categories were not statistically significant 

(p>0.05). Mean and median NO2+NO3–N concentrations in 

the very high vulnerability category were lower than concentrations in the high category. The median NO2+NO3–N concentration in the medium vulnerability category was lower 

than in the low category. Results of this correlation show that 

the relative vulnerability map produced with the uncalibrated 

DRASTIC method is not effective in predicting elevated 

NO2+NO3–N concentrations in ground water. 

The probability map produced by Rupert (1997) correlated well with NO2+NO3–N concentrations in ground water 

(fig. 7). The mean and median NO2+NO3–N concentrations 

increased in all categories as the probability rating increased. 

Differences in NO2+NO3–N concentrations betw

ability categories except between low and medium 

tistically significant (p<0.05). Even though the di

concentrations between the low and medium prob

gories was not significant, the mean and median N

concentrations were higher in the medium catego

the low category. Results of this correlation dem

that the probability map is effective in predicting 

NO2+NO3–N concentrations in ground water. 

CONCLUSIONS

The effectiveness of vulnerability and proba

can be improved by calibrating the point ratings o

of the results of statistical correlations between gr

quality and hydrogeologic and anthropogenic fact

mining vulnerability point ratings on the basis of 

sional judgment, as generally is done with the unc

DRASTIC method, and was done in the Idaho are

and others (1991), is not as effective. 

The most significant weakness of the relativ

ity map developed using the uncalibrated DRAST

(Rupert and others, 1991) is that soil permeability 

primary soil factor, similar to other uncalibrated D

maps. There was no correlation between NO2+N

soil permeability, but there was a strong correlatio

drainage types, presumably because soil drainage 

indicator of nitrate leaching conditions. Calibration 

ability maps with ground-water quality is the mos

way to determine which hydrogeologic and anthro

factors are related to the chemical compound of in

The probability map was calibrated only for N

and may not be effective for other compounds. Ca

40

30

20

10

0

171 124

266

p = 0.00

p = 0.00

p = 0.45 p = 0.00

Number of samples

8

p = 0.00

Figure 7. Correlations between concentrations of 

nitrite plus nitrate as nitrogen from an independen

and contamination probability, eastern Snake Rive

1980–91. DISSOLVED NITRITE PLUS NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER

PROBABILITY OF GROUND-WA

CONTAMINATION BY NITRI

PLUS NITRATE AS NITROGE

Low Medium High Very 

p = 0.00
6

probability maps developed using the logistic regression statistical method (Rupert, 1998) demonstrated that land use, 

precipitation, soil hydrologic group, and well depth were significantly correlated with atrazine/desethyl-atrazine detections in ground water. In contrast, depth to water, land use, 

and soil drainage likely were significantly correlated with elevated NO2+NO3–N concentrations. The differences between 

atrazine/desethyl-atrazine and NO2+NO3–N relations likely 

were due to differences in chemical behavior of these compounds in the environment and possibly to differences in the 

extent of use and rates of their application. 

USES OF GROUND-WATER PROBABILITY MAPS

Ground-water probability maps can be used by resource 

protection agencies to focus pollution prevention programs on 

areas of greatest concern and to focus ground-water sampling 

programs on areas of greatest potential for contamination. 

Organizations and programs that might benefit from such 

maps include the agri-chemical industry; agricultural producers; Native American tribes; county and city governments; 

planning and zoning commissions; education programs for 

applicators; and State programs related to Wellhead Protection, Drinking Water, Home-A-Syst, and Best Management 

Plans (BMPs). 

THE NATIONAL WATER-QUALITY ASSESSMENT 

PROGRAM

Funding and resources for this report were provided by 

the USGS National Water-Quality Assessment Program. 

Long-term goals of this program are to describe the status and 

trends of the quality of a large, representative part of the 

Nation’s surface- and ground-water resources and to provide 

a sound, scientific understanding of the primary natural and 

human factors affecting the quality of these resources. In 

meeting these goals, the program will produce a wealth of 

water-quality information that will be useful to policy makers and managers at national, State, and local levels. The 

upper Snake River Basin was among the first 20 study units 

that began this water-quality assessment. 

—Michael G. Rupert

REFERENCES CITED

Aller, Linda, Bennett, Truman, Lehr, J.H., and Petty, R.J., 1985, 

DRASTIC—a standardized system for evaluating ground water 

pollution potential using hydrogeologic settings: U.S. Environmental Protection Agency, Robert S. Kerr Environmental 

Research Laboratory, Office of Research and Development, 

EPA/600/2–85/018, 163 p. 

Barbash, J.E., and Resek, E.A., 1996, Pesticides in ground water—

distribution, trends, and governing factors: Chelsea, Mich., Ann 

Arbor Press, Inc., 588 p. 

Crockett, J.K., 1995, Idaho Statewide Ground Water Quality Monitoring Program—summary of results, 1991 through 1993: Idaho 

Department of Water Resources Water Information Bulletin 

no. 50, pt. 2, 67 p.

Idaho Department of Water Resources, 1991, Idaho’s Stat

Water Quality Monitoring Program—the first six 

beyond: Boise, Idaho Department of Water Resou

Koterba, M.T., Banks, W.S.L., and Shedlock, R.J., 1993, 

shallow groundwater in the Delmarva Peninsula: J

Environmental Quality, v. 22, no. 3, p. 500–518. 

Maupin, M.A., 1992, Depth to water in the eastern Snake 

and surrounding tributary valleys, southeastern Ida

lated using water levels from 1980 to 1988: U.S. G

Survey Water-Resources Investigations Report 90

1 sheet, scale 1:750,000. 

Neely, K.W., 1994, Idaho Statewide Ground Water Quali

Program network design: Boise, Idaho Departmen

Resources Water Information Bulletin no. 50, pt. 1

Neely, K.W., and Crockett, J.K., 1992, Idaho’s Statewide 

Quality Monitoring Program—status report 1991: 

Department of Water Resources, 22 p.

Ott, R.L., 1993, An introduction to statistical methods and 

sis: Belmont, Calif., Duxbury Press, Wadsworth P

Company, 1051 p. 

Rupert, M.G., 1994, Analysis of data on nutrients and org

pounds in ground water in the upper Snake River B

and western Wyoming, 1980–91: U.S. Geological 

Water-Resources Investigations Report 94–4135, 

———1997, Nitrate (NO2+NO3–N) in ground water of 

Snake River Basin, Idaho and western Wyoming, 

U.S. Geological Survey Water-Resources Investig

Report 97–4174, 47 p. 

———1998, Probability of detecting atrazine/desethyl-at

vated concentrations of nitrate (NO2+NO3–N) in 

in the Idaho part of the upper Snake River Basin: U

cal Survey Water-Resources Investigations Report 

32 p. 

Rupert, M.G., Dace, Tana, Maupin, M.A., and Wicherski, 

Ground-water vulnerability assessment, Snake Riv

southern Idaho: Boise, Idaho Department of Healt

fare, Division of Environmental Quality, 25 p.

U.S. Department of Agriculture, 1991, State Soil Geograp

(STATSGO): U.S. Department of Agriculture, Soi

tion Service, Miscellaneous Publication no. 1492, 

———1993, Soil survey manual: U.S. Department of Ag

Handbook no. 18, 437 p.

U.S. Environmental Protection Agency, 1993, A review o

assessing aquifer sensitivity and ground water vuln

pesticide contamination: U.S. Environmental Prote

EPA/813/R–93/002, 147 p. 

Whitehead, R.L., 1992, Geohydrologic framework of the 

Plain regional aquifer system, Idaho and eastern O

Geological Survey Professional Paper 1408–B, 32 

For more information contact:

District Chief

U.S. Geological Survey, WRD

230 Collins Road

Boise, Idaho 83702-4520

Additional information on the National Water-Quality Assessm

and other U.S. Geological Survey programs can be accessed on 

Wide Web at

http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.

Data and information compiled for Upper Snake River Basin st

ties can be accessed on the World Wide Web at

http://wwwidaho.wr.usgs.gov/nawqa/usnk_home.h