National Water-Quality Assessment Program

Nutrients in the Nation's Waters--Too Much of a Good Thing?

Ground Water
Surface Water

Ground Water

Ground water provides the drinking-water supply to more than half the population of the United States. Public-supply wells are usually large wells providing water to hundreds of people within towns and cities. Domestic-supply wells usually provide water to a single family on whose property the well is located. Nitrate concentrations for these two types of wells are commonly quite different.

On the basis of samples available within the NAWQA study units, drinking water from public-supply wells and domestic-supply wells outside of agricultural areas is not likely to have high levels of nitrate. Public-supply wells are subjected to regular testing, and if concentrations exceed the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) of 10 mg/L for drinking water, the well is likely to be abandoned.

The median nitrate concentration in samples from public-supply wells was less than 0.2 mg/L, well below the MCL for drinking water. Concentrations in only 1 percent of the samples from public-supply wells exceeded the MCL. The highest concentrations were in samples from domestic-supply wells in agricultural areas, where potential sources of nitrate contamination are more prevalent. The median concentration in samples from these wells was greater than "background" levels (2 mg/L), and 12 percent exceeded the MCL. These values are based on data from about 1,100 public-supply wells and about 3,200 domestic-supply wells, two-thirds of which were in agricultural areas. However, data were not available for all wells within each NAWQA study unit, so the percentages may not be identical to those for the entire NAWQA area or the Nation.

Surface Water

For drinking-water supplies that come from surface-water sources (streams and reservoirs), it is difficult to estimate how often nitrate concentrations can be expected to be above the MCL. Water utilities perform regular analyses of their own supplies and report to State health agencies the date on which a standard was exceeded (an "exceedance"). But State records of exceedances are not always accessible by computer and are not collected or summarized nationally. Concentrations that do not exceed a standard are not reported. Therefore, variations in nitrate concentration are not available as public record.

Where nonpoint sources of nitrate, such as fertilizer applications to fields or lawns, are prevalent upstream from a water-supply intake, nitrate concentrations in the water often are highest when the fertilizer is washed off during storms. Where point sources, such as sewage-treatment plants, are located upstream from a water-supply intake, nitrate concentrations often are highest during low flows when little additional water is available to dilute the nitrate in effluents. However, most water-supply intakes are purposely located upstream from nearby point sources rather than downstream.

Standard water-treatment practices remove very little of the nitrate in drinking water. Water utilities commonly have more than one source of supply, which can be blended to ensure that the concentration of nitrate in water delivered to the public meets drinking-water standards.

U.S. Department of the Interior, U.S. Geological Survey Maintainer: Kerie Hitt (kjhitt@usgs.gov) Last update: 17:47:47 Mon 09 Jul 2001 Privacy Statement || Disclaimer || Accessibility URL: http://water.usgs.gov/nawqa/circ-1136/h4.html