U.S. Department of the Interior

U.S. Geological Survey

 Fact Sheet 2008–3063

August 2008 Printed on recycled paper

Distribution of Elevated Nitrate Concentrations in Ground 

Water in Washington State

More than 60 percent of the population of Washington 

State uses ground water for their drinking and cooking 

needs. Nitrate concentrations in ground water are elevated 

in parts of the State as a result of various land-use practices, 

including fertilizer application, dairy operations and ranching, 

and septic-system use. Shallow wells generally are more 

vulnerable to nitrate contamination than deeper wells 

(Williamson and others, 1998; Ebbert and others, 2000). 

In order to protect public health, the Washington State 

Department of Health requires that public water systems 

regularly measure nitrate in their wells. Public water systems 

serving more than 25 people collect water samples at least 

annually; systems serving from 2 to 14 people collect water 

samples at least every 3 years. Private well owners serving 

one residence may be required to sample when the well is 

first drilled, but are unregulated after that. As a result, limited 

information is available to citizens and public health officials 

about potential exposure to elevated nitrate concentrations 

for people whose primary drinking-water sources are 

private wells. The U.S. Geological Survey and Washington 

State Department of Health collaborated to examine waterquality data from public water systems and develop models 

that calculate the probability of detecting elevated nitrate 

concentrations in ground water. Maps were then developed to 

estimate ground water vulnerability to nitrate in areas where 

limited data are available.

Probability that nitrate concentration will exceed 2 mg/L, in percent 

Vulnerability map of Washington for wells 145 feet deep
What is ground-water vulnerability?

Ground-water vulnerability is an estimate of the potential for contamination of 

ground water to a compound, such as nitrate, based on human-related and natural 

factors. Ground-water vulnerability maps are designed to estimate the potential 

for contamination of ground water in an area based on human-related and natural 

factors.

Where is ground water 

most vulnerable in 

Washington State?

Shallow wells in areas with a high 

percentage of agricultural land use 

and (or) high population density are 

most likely to have elevated nitrate 

concentrations in the water. Maps 

showing the estimated probability of 

elevated nitrate concentrations indicate 

that the agricultural regions are most 

at risk followed by urban areas. Areas 

of the State that are not affected by 

agricultural or urban activities had 

much lower probabilities of detecting 

elevated nitrate concentrations. 

Shallow wells (145 feet deep or 

less) in agricultural areas of eastern

Washington have a greater than 

90 percent probability of nitrate 

concentrations exceeding 2 milligrams 

per liter (mg/L) (areas of 100 feet on 

vulnerability map of Washington) and 

shallow wells in urban and agricultural 

areas of the Puget Sound area have a 

greater than 50 percent probability of 

nitrate concentrations exceeding 2 mg/L

(areas greater than 50 feet on map of 

Puget Sound). Nitrate concentrations at 

or greater than 2 mg/L generally are 

greater than naturally occurring levels 

and indicate human-related sources 

of nitrate such as fertilizer and sewage 

(Nolan and others, 1998).

wa19-0163_fig11B

Vulnerability map of Puget Sound for wells 145 feet deep

Probability that nitrate concentration will exceed 2 mg/L, in percent

0

>50

25
Why is vulnerability of ground water to 

nitrate of concern?

Nitrate reduces the ability of red blood cells to carry 

oxygen. In most adults and children, these red blood cells 

rapidly return to normal after exposure to nitrate. In infants, 

however, it can take much longer for the blood cells to return 

to normal. Infants who drink water with high levels of nitrate 

(or eat foods made with nitrate-contaminated water) may 

develop a serious health condition due to the lack of oxygen. 

This condition is called methemoglobinemia or “blue baby 

syndrome.” Some studies have determined that an increased 

risk of spontaneous abortion or certain birth defects occur if 

the pregnant mother drank water high in nitrate. Other 

health risks include bladder and ovarian cancer 

and non-Hodgkins lymphoma (Centers for 

Disease Control and Prevention, 1996; Ward 

and others, 1996; Weyer and others, 2001; 

Fewtrell, 2006). 

Once the concentration of nitrate 

in ground water increases, it can not be 

easily reduced. In many cases, water 

systems must install costly treatment 

systems, and resort to bottled water while 

a treatment system is being designed and 

installed. Nitrate in a drinking water supply 

may indicate that the well is vulnerable to 

other sorts of contamination. When vulnerable 

areas can be located, the process of protecting 

water supplies from contamination can begin. 

Can having a deep well 

increase the probability 

of obtaining low nitrate 

ground water?

Yes. In areas with a high 

probability of nitrate in ground water 

greater than 2 mg/L, wells drilled 

deeper can increase the probability 

of obtaining water with low nitrate 

concentrations. For a 90-percent 

chance of obtaining low nitrate ground 

water, wells installed in the agricultural 

areas of eastern Washington generally 

would need to be drilled 

to a depth of at 

least 750 feet. 

Likewise, wells 

in the urban 

and agricultural 

areas of the 

Puget Lowlands 

would need to be 

drilled to a depth 

of at least 400 feet.

How were the vulnerability maps 

created?

Using logistic regression, the occurrence of elevated 

nitrate concentrations (at or greater that 2 mg/L) in samples 

from public supply wells was related to natural and humancaused factors to assess ground-water vulnerability to nitrate. 

Data were from the Washington State Department of Health 

from 1995 though 2006. Significant factors were well depth, 

percentage of agricultural lands within a 4-kilometer radius 

of the well, population density near the well, average annual 

precipitation, and soil drainage class.

The maps were created using the logistic 

regression model and geographic information 

system (GIS) coverages of land use, soil

characteristics, population density, and 

precipitation. Maps can be created to 

depict the probability of elevated 

nitrate concentrations for wells 

of any depth or to depict the 

necessary well depths to obtain 

a certain probability of having 

low nitrate water.

Depth

Map showing well depth needed to obtain ground water 

with a likely nitrate concentration less that 2 mg/L

1,500 feet

0 feet
Author: Lonna Frans (253)-552-1694 lmfrans@usgs.gov

Contact: Washington Water Science Center

 934 Broadway, suite 300, Tacoma, WA 98498

 http://wa.water.usgs.gov/

Summary

This study was designed to estimate the probability of 

detecting elevated nitrate in ground water in Washington 

State. Elevated nitrate concentrations can cause health 

risks in certain individuals. Vulnerability maps showing 

the estimated probability of detecting elevated nitrate 

concentrations indicate that agricultural regions are at most 

risk followed by urban regions. Areas of the State that are 

not affected by agricultural or urban areas had much lower 

probabilities of detecting elevated nitrate concentrations. 

Maps also were generated that show that for a 90-percent 

probability of obtaining water with nitrate concentrations 

less than 2 mg/L, wells would have to exceed 750 feet deep 

in agricultural regions. Wells in urban areas generally would 

need to be drilled in excess of 400 feet deep.

How should these maps be used?

The vulnerability maps developed for this study illustrate 

the estimated probability of nitrate concentrations exceeding 

2 mg/L in ground water in Washington. The vulnerability 

maps do not show actual nitrate contamination of ground 

water. They are intended for regional scale use only and have 

limitations for use at the field-scale. Many unaccounted for 

field-scale complexities affect the concentration of nitrate in 

ground water and in a given well. For example, the models 

do not account for point sources of nitrate or preferential 

pathways in the soil. Although a well may be installed in a 

region with an estimated high probability of elevated nitrate 

concentration, the well may actually yield water with low 

nitrate concentrations due to complexities that cannot be 

represented in regional-scale models such as those developed 

in this study. 

References Cited

Centers for Disease Control and Prevention, 1996, 

Spontaneous abortions possibly related to ingestion of 

nitrate-contaminated well water, La Grange County, 

Indiana, 1991–1994: Morbidity and Mortality Weekly 

Report, v. 45, p. 569–572.

Ebbert, J.C., Embrey, S.S., Black, R.W., Tesoriero, A.J., and 

Haggland, A.L., 2000, Water quality in the Puget Sound 

Basin, Washington and British Columbia, 1996–98: U.S. 

Geological Survey Circular 1216, 31 p.

Fewtrell, Lorna, 2004, Drinking-water nitrate, 

methemoglobinemia, and global burden of disease—A 

discussion: Environmental Health Perspectives, v. 112, 

no. 14, p. 1371–1374.

Nolan, B.T., Ruddy, B.C., Hitt, K.J., and Helsel, D.R., 1998, 

A National look at nitrate contamination in ground water: 

Water Conditioning and Purification, v. 39, no. 12, p. 76–79.

Ward, M.H., Mark, S.D., Cantor, K.P., Weisenburger, D.D., 

Correa-Villase, Adolofo, and Zahm S.H., 1996, Drinking 

water nitrate and risk of non-Hodgkin’s lymphoma: 

Epidemiology v. 7, no. 5, p. 465–471.

Weyer, Peter, Cerhan, J.R., Kross, B.C., Hallberg, G.R., 

Kantamneni, Jiji, Breuer, George, Jones, M.P., Zheng, Wei, 

Lynch, C.F., 2001, Municipal drinking water nitrate level 

and cancer risk in older women: The Iowa Women’s Health 

Study, Epidemiology, v. 11, p. 327–338.

Williamson, A.K., Munn, M.D., Ryker, S.J., Wagner, R.J., 

Ebbert, J.C., and Vanderpool, A.M., 1998, Water quality 

in the central Columbia Plateau, Washington and Idaho, 

1992–1995: U.S. Geological Survey Circular 1144, 35 p.