Abstract

The U.S. Environmental Protection

Agency’s (USEPA) Regional

Vulnerability Assessment (ReVA)

Program has developed a set of statistical tools to support regional-scale, integrated ecological risk-assessment studies. One of these tools, developed by

the U.S. Geological Survey (USGS), is

used with available water-quality data

obtained from USGS National WaterQuality Assessment (NAWQA) and

other studies in association with land

cover, geology, soils, and other geographic data to develop logistic-regression equations that predict the vulnerability of ground water to nitrate concentrations exceeding specified thresholds in the Mid-Atlantic Region. The

models were developed and applied to

produce spatial probability maps showing the likelihood of elevated concentrations of nitrate in the region. These

maps can be used to identify areas that

currently are at risk and help identify

areas where ground water has been

affected by human activities. This

information can be used by regional

and local water managers to protect

water supplies and identify land-use

planning solutions and monitoring programs in these vulnerable areas.

Introduction 

The USEPA has recognized that

regional, State, and local water

managers in the Mid-Atlantic Region

need to have an estimate of groundwater vulnerability at the regional scale

for both environmental (ecological)

and human-health (public drinkingwater consumption) purposes. During

the last 3 years, the USEPA and the

USGS have been conducting regional

assessments of ground-water quality to

develop estimates of regional groundwater vulnerability in the Mid-Atlantic

Region (Greene and others, 2005).

This research addresses questions of

overall water-quality conditions in the

region, and was designed to assess

ground-water vulnerability to contamination.

Greene and others (2005) describe

the statistical methodology (logistic

regression) that can be used to assess

the risk of nonpoint-source contamination of ground water in areas of the

Mid-Atlantic Region where limited data

are available. Spatial probability maps

showing the likelihood of elevated concentrations of nitrate were developed

to identify areas that currently are at

risk at multiple management thresholds. These maps help identify areas

where ground water has been affected

by human activities, and can help

regional and local water managers protect water supplies. In addition, validation and the uncertainty (confidence

interval) of the model-predicted probabilities were developed and applied to

all spatial probability maps (Greene

and others, 2005).

Regional Vulnerability

Assessment (ReVA) Program

The USEPA’s ReVA Program is

developing and demonstrating

approaches and tools for quantifying,

assessing, and communicating ecological vulnerabilities at regional scales so

that risk-management activities can be

targeted and prioritized. ReVA’s focus

is to develop a set of methods applicable to assess the distribution and condition of environmental resources and

provide information to facilitate decision-making at the regional, watershed,

and local scales. 

Shallow Ground Water

Ground water is that part of precipitation that does not evaporate or

runoff to streams and rivers and infiltrates through the soil to the saturated

zone beneath the water table. The

unsaturated material above the water

table contains air and water in spaces

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS 2004-3067

2005

1 U.S. Geological Survey, Baltimore, MD

2 Former graduate student at University of Maryland,

Baltimore County; now with PricewaterhouseCoopers,

Washington, D.C.

3 U.S. Environmental Protection Agency, Office of

Research and Development, Research Triangle Park, NC
between the soil and rock particles and

supports vegetation. In the saturated

zone below the water table, ground

water fills the spaces between the rock

particles and within bedrock fractures

(fig. 1). Recharge is the process by

which precipitation is transmitted

downward through the soil and ground

water is replenished. The amount of

water that is infiltrated and the location

of the infiltrated water depends partly

on land use and land cover. Areas that

are undeveloped (natural or agricultural

areas) with permeable soils and relatively flat slopes usually have greater

recharge than urban areas covered

with impervious surfaces (such as pavement).

The quality of ground water is

determined by the mineral composition

of the rocks themselves and the chemical composition of substances dissolved in the water. In addition, the

chemical composition of the ground

water may be affected by what is in

the precipitation, and by biological and

chemical reactions occurring on the

land surface and in the soil zone.

Human-related activities on the land

surface may also affect the quality of

ground water. These activities may

increase the possibility of ground-water

contamination through the disposal of

wastes, the application of fertilizers, the

development of septic systems in rural

areas, and mining, among others. This

potential contamination may find its

way into the shallow ground water

through recharge (fig. 1).

Nitrate in Ground Water 

Many contaminants in ground

water occur naturally, however, elevated nitrate concentrations in ground

water typically are caused by anthropogenic (human-related) activities that

include crop fertilization and domestic

septic systems. Spalding and Exner

(1993) suggested that nitrate may be

the most widespread contaminant in

ground water. Because of the extensive presence of this contaminant in

ground water and its potential health

effects, areas of elevated nitrate concentrations may help identify environments that are susceptible to contamination (U.S. Environmental Protection

2 Ground-Water Vulnerability to Nitrate Contamination in the Mid-Atlantic Region
Agency, 1996a). 

Spatial probability maps of groundwater vulnerability were developed

using nitrate because it has been suggested that it can be used as an indicator of overall ground-water quality (U.S.

Environmental Protection Agency,

1996a). In addition, nitrate is a USEPAregulated contaminant and its presence

in high concentrations is a potential

health risk. The USEPA has set a maximum contaminant level for nitrate in

drinking water of 10 mg/L (milligrams

per liter) as nitrogen (U.S.

Environmental Protection Agency,

1996b). 

Data and Variables

Water-quality data collected from

October 1985 through September

1996 as part of the NAWQA Program

(Gilliom and others, 1995) and other

USGS projects were compiled in 1997

for a regional analysis of pesticides and

nitrate in surface and ground water of

the Mid-Atlantic Region (Ator, 1998;

Ferrari and others, 1997; Ator and

Ferrari, 1997). These ground-waterquality data consisted of 1,551 samples

taken from 937 different sites. Ten

sites were eliminated because of either

missing or incomplete data, thereby

reducing the data set to 927 sites (fig.

2).

Land cover and geology type have

been related previously to the occurrence of elevated nitrate concentrations in ground water throughout the

Mid-Atlantic Region (Ator and Ferrari,

1997; Nolan 2001). The other variables considered in the statistical

model were nitrogen sources consisting

of manure and/or fertilizer application,

and atmospheric deposition.

Population density and soils data consisting of hydrologic groups, organic

matter, depth of soil layer, depth to

water table, percent sand, silt, and clay

also were explored with the statistical

model to determine whether they were

significant explanatory variables.

Logit Model

A logistic-regression (logit) statistical model was developed by relating

statistically significant geographic variables and predicting the occurrence of

nitrate in ground water above a predetermined management or environmenGround-Water Vulnerability to Nitrate Contamination in the Mid-Atlantic Region 3
tal concentration threshold. After

developing the best-fit logistic-regression model for each threshold, the

entire Mid-Atlantic Region was evaluated in order to produce spatial probability maps depicting the likelihood of elevated nitrate concentrations exceeding

a specified threshold. 

Ground-Water Vulnerabilities at

Multiple Management

Concentration Thresholds

Spatial probability maps showing

the likelihood of elevated concentrations of nitrate can be used to identify

areas that currently are at risk. These

maps help identify areas where ground

water has been affected by human

activities, and can help regional and

local water managers protect water

supplies or target areas for further

monitoring. 

3 Milligrams per Liter 

A map depicting the probability of

nitrate exceeding 3 mg/L as nitrogen

may be used for evaluating the relative

risk of overall nitrate concentrations to

ecological health of the region.

Ground water in agricultural areas

where manure is applied as fertilizer, or

areas overlying carbonate rocks or

coarse sand deposits may have a

greater than 50-percent probability of

nitrate concentrations exceeding 3

mg/L (fig. 3) (Greene and others,

2005).

10 Milligrams per Liter

Nitrate is a USEPA-regulated contaminant and its presence in high concentrations (above the regulatory limit

of 10 mg/L as nitrogen) is a potential

health risk (U.S. Environmental

Protection Agency, 1996a). A map

depicting the probability of nitrate concentrations exceeding 10 mg/L (fig. 4)

could be used for evaluating the relative risk of high concentrations of

nitrate to human health in the MidAtlantic Region. Ground water in agricultural areas where manure is applied

as fertilizer and in carbonate rocks may

have a greater than 50-percent probability of nitrate concentrations exceeding 10 mg/L (Greene and others,

2005).

References Cited

Ator, S.W., 1998, Nitrate and pesticide

data for waters of the Mid-Atlantic

Region: U.S. Geological Survey

Open-File Report 98-158, 5 p.

Ator, S.W., and Ferrari, M.J., 1997,

Nitrate and selected pesticides in

ground water of the Mid-Atlantic

Region: U.S. Geological Survey

Water-Resources Investigations

Report 97-4139, 8 p.

Ferrari, M.J., Ator, S.W., Blomquist,

J.D., and Dysart, J.E., 1997,

Pesticides in surface water of the

Mid-Atlantic Region: U.S.

Geological Survey WaterResources Investigations Report

97-4280, 12 p.

Gilliom, R.J., Alley, W.M., and Gurtz,

M.E., 1995, Design of the

National Water Quality

Assessment Program—

Occurrence and distribution of

water-quality conditions: U.S.

Geological Survey Circular 1112,

33 p.

Greene, E.A., LaMotte, A.E., and

Cullinan, K.A., 2005, Groundwater vulnerability to nitrate contamination at multiple thresholds

in the Mid-Atlantic Region using

spatial probability models: U.S.

Geological Survey Scientific

Investigations Report 2004-5118,

24 p.

Nolan, B.T. 2001, Relating nitrogen

sources and aquifer susceptibility

to nitrate in shallow ground waters

of the United States: Ground

Water, v. 39, no. 2, p. 290-299.

Spalding, R.F., and Exner, M.E., 1993,

Occurrence of nitrate in groundwater—A review: Journal of

Environmental Quality, v. 22, p.

392-402.

U.S. Environmental Protection

Agency, 1996a, Environmental

indicators of water quality in the

United States: Washington, D.C.,

U.S. Environmental Protection

Agency, Office of Water, EPA 841-

R-96-002, 25 p. 

______ , 1996b, Drinking water regulations and health advisories:

Washington, D.C., U.S.

Environmental Protection Agency,

Office of Water, EPA 822-B-96-

002, 11 p.

For further information, please contact:

District Chief, MD-DE-DC District

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, Maryland 21237

or visit the Maryland-Delaware-District

of Columbia District Homepage on the

World Wide Web at:

http://md.water.usgs.gov

For information about the USEPA

Regional Vulnerability Assessment

(ReVA) Program, visit:

http://www.epa.gov/reva

The U.S. Environmental Protection Agency,

through its Office of Research and

Development, partially funded and collaborated in the research described in this Fact

Sheet under Interagency Agreement number DW 14939071 to the U.S. Geological

Survey. It has been subjected to agency

review and approved for publication.

Additional Information

FS 2004-3067

Editor: Valerie M. Gaine

Graphics and design: Timothy W. Auer