The quality of water that people drink depends on where they live

Nitrate contamination of ground water occurs in predictable patterns, based on results of the U.S. Geological Survey’s (USGS)

National Water-Quality Assessment (NAWQA) Program. The NAWQA Program was begun in 1991 to describe the quality of the

Nation’s water resources, using nationally consistent methods. Soil-drainage characteristics and the amount of nitrogen contributed

by fertilizer, manure, and atmospheric sources were found to markedly influence the concentration of nitrate in ground water. These

two characteristics were compiled in a national map that shows patterns of risk for nitrate contamination of ground water (fig. 1).

Areas with well-drained soils and high nitrogen input (shown in grayish blue on the map) have the highest risk. Ground-water nitrate

data generally verified the risk patterns shown on the national map. The verification analysis is based on over 10,000 samples

collected through 1992.

EXPLANATION

Less than 6 tons nitrogen input per square mile and poorly drained soils

Less than 6 tons nitrogen input per square mile and well-drained soils

More than 6 tons nitrogen input per square mile and poorly drained soils

More than 6 tons nitrogen input per square mile and well-drained soils

INCREASING RISK OF


GROUND-WATER CONTAMINATION

Figure 1. Areas in the United States most vulnerable to nitrate contamination of ground water (shown in grayish

blue on the map) generally have well-drained soils and high nitrogen input from fertilizer, manure, and atmospheric

deposition. High-risk areas occur primarily in the western, midwestern, and southeastern portions of the Nation.

NITRATE IN GROUND WATERS OF THE UNITED STATES— 


ASSESSING THE RISK

Department of the Interior—U.S. Geological Survey

U S

G S
Risk of ground-water

contamination by nitrate

varies across the United

States

The risk of ground-water contamination by nitrate is not the same everywhere. Figure 1 shows four groups in

order of increasing risk: (1) poorly

drained soils with low nitrogen input

(white area on the map); (2) well-drained

soils with low nitrogen input (gray area);

(3) poorly drained soils with high nitrogen input (light blue area); and (4) welldrained soils with high nitrogen input

(grayish blue area). Figure 1 indicates

that the Midwest has a higher risk of contamination, but that parts of the western

and southeastern United States also are at

risk. The grayish blue areas generally

have well-drained soils and high amounts

of nitrogen applied to the land surface.

Well-drained soils can easily transmit

water and nitrate to ground water. In

contrast, the other three groups have a

lower risk of nitrate contamination

because of poorly drained soils and/or

low nitrogen input. Poorly drained soils

transmit water and chemicals at a slower

rate than well-drained soils (Mueller

and others, 1995). Drains and ditches

commonly are used to remove excess

water from poorly drained agricultural

fields, diverting nitrate to nearby streams.

Nitrate in ground water

generally follows the risk

map

Ground-water nitrate data from across

the Nation were analyzed to verify the

four risk groups shown on the national

map. Nitrate concentration in ground

water increases with higher nitrogen

inputs and more well-drained soils.

People who live in areas shown in grayish

blue on the national map and who get

their water from shallow wells are more

likely to drink high-nitrate ground water.

Figure 2 shows the median concentration

of nitrate in water from shallow wells in

agricultural areas and the percent of wells

with nitrate that exceeds the drinkingwater standard (10 mg/L) for each of the

four risk groups. The bars in figure 2

represent risk groups with the same color

shown on the national map. Median

nitrate concentration and percentage of

wells from which water exceeds the

drinking-water standard for nitrate are

Ground water provides drinking water

for more than one-half of the Nation’s

population (Solley and others, 1993).

In 1990, ground water accounted for

39 percent of water withdrawn for

public supply for cities and towns and

96 percent of water withdrawn by selfsupplied systems for domestic use.

Ground water is the sole source of drinking water for many rural communities and

some larger cities. A variety of chemicals, including nitrate, can pass through

the soil and potentially contaminate

ground water. Nitrate comes from

nitrogen, a plant nutrient supplied by

inorganic fertilizer and animal manure.

Additionally, airborne nitrogen compounds emitted by industry and automobiles are deposited on the land. Other

nonagricultural sources of nitrate, such as

septic systems, generally are less significant and are not shown on the national

map. Beneath agricultural lands, nitrate is

the primary form of nitrogen. It is soluble

in water and can easily pass through soil

to the water table. Nitrate can persist in

ground water for decades and accumulate

to high levels as more nitrogen is applied

to the land surface every year.

Although nitrate generally is not an

adult public-health threat, ingestion in

drinking water by infants can cause low

oxygen levels in the blood, a potentially

fatal condition (Spalding and Exner,

1993). For this reason, the U.S. Environmental Protection Agency (1995) has

established a drinking-water standard of

10 milligrams per liter (mg/L) for

nitrate. Nitrate concentrations in

natural ground waters are usually less

than 2 mg/L (Mueller and others, 1995).

Knowing where and what type of risks

to ground water exist can alert waterresource managers and private users of

the need to protect water supplies. By

targeting regions with the highest risk of

nitrate contamination, resources can be

directed to areas most likely to benefit

from pollution-prevention programs and

long-term monitoring. Use of risk guidelines to locate areas for prevention of

contamination might cost less than simply

identifying the most severely contaminated areas. Once ground water is

contaminated, it is expensive and in many

cases virtually impossible to clean up

(National Research Council, 1993).

INCREASING RISK OF GROUND-WATER CONTAMINATION

PERCENT OF WELLS EXCEEDING 10 MILLIGRAMS PER LITER NITRATE:

6% 8% 16% 26%

MEDIAN NITRATE CONCENTRATION (MILLIGRAMS PER LITER AS N)

POORLY DRAINED SOIL,


LOW NITROGEN INPUT

WELL-DRAINED SOIL,


LOW NITROGEN INPUT

POORLY DRAINED SOIL,


HIGH NITROGEN INPUT

WELL-DRAINED SOIL,


HIGH NITROGEN INPUT

0

1

2

3

4

5

Figure 2. Nitrate concentration in shallow ground water beneath

agricultural lands varies by soil permeability and the amount of nitrogen

applied to the land surface.
atmospheric sources are low. Septic

systems and cesspools in developed areas

of Long Island have been a major source

of nitrogen in ground water since the

1930’s (Porter, 1980). Residential

sources such as these are not yet included

on this national map but are likely to be

important for locations with well-drained

soils like Long Island.

Who is most likely to drink

high-nitrate ground water?

Where people live and their source of

water determine the quality of the water

they drink. In figure 3, three types of

wells are shown for the highest risk

(grayish blue) areas, which have high

nitrogen input and well-drained soils.

People who get their water from domestic

wells are more likely to drink water that

contains high concentrations of nitrate

than those who get their water from public-supply wells. Public-supply wells

typically are completed in deeper groundwater reservoirs where contamination is

less likely to occur (Mueller and others,

1995). In contrast, domestic wells usually

are shallower and located closer to potential sources of nitrate contamination.

Median nitrate concentration and percent

of wells in which water exceeds the drinking-water standard for nitrate are highest

for domestic wells in agricultural areas.

Summary

Widespread water-quality sampling

over large areas is expensive and timeconsuming. Benefits can be maximized

by understanding where ground water is

most at risk. By determining characteristics that influence ground-water contamination by nitrate, USGS scientists have

been able to map high- and low-risk areas

of the Nation. Areas with the highest risk

have high nitrogen input and well-drained

soils. Although local variations from

regional patterns of contamination risk

occur, data generally support the national

map. Continued development of this

predictive tool will help water-resource

managers direct scarce funds to where

they are most needed.

lowest in poorly drained soils with low

nitrogen input (white color) and highest in

well-drained soils with high nitrogen

input (grayish blue color).

Data for specific locations give examples of the difference in risk. Ground

water in the grayish blue area in southeastern Washington State has a median

nitrate concentration of 6.5 mg/L. In

contrast, ground water in the white and

gray areas in western New Mexico, where

nitrogen input is low, has a median nitrate

concentration of 0.3 mg/L. Poorly

drained soils can reduce the risk of

ground-water contamination, even in

areas with high nitrogen input. For example, ground water in light blue areas in

southern Indiana has a median nitrate

concentration of only 0.1 mg/L.

Although nitrogen input is high, most

soils in the area are poorly drained, which

restricts the movement of nitrate to the

water table (Mueller and others, 1995).

Additionally, drains and ditches carry soil

water off to streams rather than letting it

seep to ground water.

Data from two areas—the southeastern

United States and Long Island, New

York—did not reflect very well the risk of

ground-water contamination shown in

figure 1. Ground-water nitrate concentration generally is low in the Southeast,

despite high nitrogen input and, in some

areas, well-drained soils. For example,

grayish blue areas in southern Georgia

and northern Florida indicate higher risk,

yet the median nitrate concentration in

ground water is only 1.0 mg/L. Nitrate

levels in shallow ground waters of the

Southeast can be reduced by dilution,

denitrification, and uptake by plants

(Hubbard and Sheridan, 1989). Denitrification, a process that converts nitrate to

nitrogen gas, is fueled by organic matter

in water-logged soils (Weil and others,

1990). Other factors not shown on the

national map but that can affect nitrate

concentration in ground water include

land use, aquifer type, and rainfall and

irrigation amounts.

Nitrate concentration in ground water

on Long Island, New York (median of

9.4 mg/L), is higher than expected from

the map, even though nitrogen inputs

contributed by fertilizer, manure, and

INCREASING RISK OF GROUND-WATER CONTAMINATION

PERCENT OF WELLS EXCEEDING 10 MILLIGRAMS PER LITER NITRATE:

2% 7% 15%

MEDIAN NITRATE CONCENTRATION (MILLIGRAMS PER LITER AS N)

PUBLIC SUPPLY

DOMESTIC


(NONAGRICULTURAL)

DOMESTIC


(AGRICULTURAL)

5

4

3

2

1

0

Figure 3. Nitrate concentration is higher in domestic wells in agricultural

areas than in public-supply wells (all land-use areas) and domestic wells

in nonagricultural areas. (Data shown are for well-drained soils with high

nitrogen input.)
The National Water-Quality Assessment Program

The USGS began the NAWQA Program in 1991 to assess the status of and trends in the quality of the Nation’s streams and

ground waters. The program is designed to enhance understanding of natural and human factors that influence water quality, and

consists of studies in 60 major river basins and aquifers of the United States. Together, the 60 studies compose about one-half

of the land area of the United States and 60 to 70 percent of the water use and population served by public water supplies. The

similar design of each study and use of consistent methods allow comparisons at regional and national scales. This information is

being used to guide policy and to manage water resources at the national, State, and local levels.

Weil, R.R., Weismiller, R.A., and Turner,

R.S., 1990, Nitrate contamination of

groundwater under irrigated Coastal Plain

soils: Journal of Environmental Quality,

v. 19, p. 441–448.

Additional reading

More detail on these data is found in

the USGS report “Nutrients in Ground

Water and Surface Water of the United

States—An Analysis of Data through

1992,” by Mueller and others (1995). For

information about obtaining copies of this

and other USGS reports, contact :

Chief, NAWQA Program

U.S. Geological Survey

413 National Center

Reston, VA 20192

The E-mail address for the NAWQA

Program is: nawqa_whq@usgs.gov

References

Hubbard, R.K., and Sheridan, J.M., 1989,

Nitrate movement to groundwater in the

southeastern Coastal Plain: Journal of

Soil and Water Conservation, v. 44,

p. 20–27.

Mueller, D.K., Hamilton, P.A., Helsel,

D.R., Hitt, K.J., and Ruddy, B.C., 1995,

Nutrients in ground water and surface

water of the United States—An analysis

of data through 1992: U.S. Geological

Survey Water-Resources Investigations

Report 95-4031, 74 p.

National Research Council, 1993,

Ground water vulnerability assessment—

Contamination potential under conditions

of uncertainty: Washington, D.C.,

National Academy Press, 204 p.

Porter, K.S., 1980, An evaluation of

sources of nitrogen as causes of groundwater contamination in Nassau County,

Long Island: Ground Water, v. 18,

no. 6, p. 617–625.

Solley, W.B., Pierce, R.R., and Perlman,

H.A., 1993, Estimated use of water in the

United States in 1990: U.S. Geological

Survey Circular 1081, 76 p.

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

Occurrence of nitrate in groundwater—

A review: Journal of Environmental

Quality, v. 22, no. 3, p. 392–402.

U.S. Environmental Protection Agency,

1995, Drinking water regulations and

health advisories: Washington, D.C.,

Office of Water, U.S. Environmental

Protection Agency, 11 p.

—By Bernard T. Nolan and

Barbara C. Ruddy

May 1996 Fact Sheet FS–092–96

The data set on which this fact sheet is based and information on

the NAWQA Program are online at:

http://wwwrvares.er.usgs.gov/nawqa/nutrient.html