National Water-Quality Assessment Program

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

The major sources of nutrients to streams and ground water are precipitation, dissolution of natural minerals from soil or geologic formations, fertilizer application, and effluent from sewage-treatment plants. The first three of these are nonpoint sources. Treatment-plant effluent is a point source.

Precipitation
Minerals
Fertilizer
Sewage Effluent

Precipitation

The Earth's atmosphere is about 78 percent nitrogen and contains about three-fourths of the nitrogen available in the environment. Most of this nitrogen is in the form of elemental nitrogen gas, but compounds of nitrogen and oxygen also are present. Some of these compounds are produced by chemical reactions in the atmosphere, and a substantial amount are released into the atmosphere from the combustion of fossil fuel, such as coal and gasoline. Nitrogen compounds in the atmosphere undergo transformations that eventually leave the nitrogen in the form of nitrate. (This process also contributes to the formation of "acid rain.") Nitrate can dissolve in rainwater or snow and then can reach streams or ground water in runoff or seepage. More than 3.2 million tons of nitrogen are deposited in the United States each year from the atmosphere.

The largest reservoir of phosphorus in the environment is not the atmosphere but minerals in rocks, sediment, and soil. Where natural deposits of phosphorus minerals are mined, such as in Florida and Idaho, runoff and seepage may be a source of phosphorus to streams. In general, however, phosphorus compounds are much less soluble than nitrogen compounds and do not readily move in runoff or seepage.

Fertilizer

A major human influence on nitrogen and phosphorus in the environment is the use of fertilizers in agricultural and urban areas. Commercial nitrogen fertilizers are applied either as ammonia or nitrate, but ammonia is rapidly converted to nitrate in the soil. Excess nitrate, not taken up by plants, can enter streams or seep down to ground water. Animal manure is also used as a nitrogen fertilizer. Organic nitrogen and urea in the manure are converted to ammonia and, ultimately, to nitrate in the soil. In the Southeast, manure is the single largest source of applied nitrogen, whereas commercial fertilizers are the predominant sources of nitrogen in the Midwest and West. Phosphorus fertilizer generally is applied as a compound of phosphate. Phosphate is not very mobile in soil; it tends to remain attached to solid particles rather than dissolving in water. However, soil erosion can carry a considerable amount of particulate phosphate to streams. About 11 million tons of nitrogen and 2 million tons of phosphorus are applied annually in commercial fertilizer. Another 6.5 million tons of nitrogen and 2 million tons of phosphorus are applied in manure.

Rates and timing of applications differ for irrigated row crops, such as corn (in Gooding County, Idaho; photograph by Michael Rupert),
(36K GIF)

and

dryland grains, such as wheat (near Palouse, Washington; photograph by Jim Ebbert).
(34K GIF)

(5K GIF)

Annual sales of commercial nitrogen and phosphorus fertilizer in the United States. Between 1945 and 1985, the use of nitrogen fertilizer increased twentyfold. The use of phosphorus fertilizer increased about fourfold between 1945 and 1980. During 1989-93, the annual use of both fertilizers remained fairly constant.

(16K GIFs)

Organic nitrogen, ammonia, and organic phosphorus are present in sewage and in sewage-treatment-plant effluents. During 1978-81, sewage-treatment plants discharged about 1.3 million tons of nitrogen per year to the Nation's waters, and other industrial point sources discharged an additional 0.3 million tons per year. Phosphate also occurs in sewage as a component of detergents and other cleaning products. About 0.3 million tons of phosphorus per year was discharged during 1978-81. Between 1980 and 1990, the Nation's population grew by about 10 percent, so sewage discharges also may have increased.

Nutrients in sewage effluent have been among the primary targets of pollution-control legislation, beginning with the Clean Water Act in 1972. The organic forms have largely been controlled through upgrading treatment plants. Advanced treatment processes have been used to decrease ammonia discharge in some areas. But these processes result in an increase in nitrate discharge, so the total nitrogen discharge does not change. Phosphate is expensive to remove from effluent, so it has been controlled primarily by limitations or "bans" on phosphate in detergents.

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/h7.html