NAWQA Circular 1136

National Water-Quality Assessment Program

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

U.S. Geological Survey Circular 1136

WHAT ARE MAJOR INFLUENCES ON NUTRIENT CONCENTRATIONS IN WATER?

Four major influences on nutrient concentrations in ground water and streams are land use, soil drainage, geology, and depth to ground water.

Land Use
Soil Drainage
Geology
Depth to Ground Water

Land Use

Activities on the land surface may have a considerable effect on both ground water and streams. Nationally, these activities have been grouped into categories of "land use," including forest land, rangeland, agricultural land, urban land, and wetland. Water-quality sampling locations can be assigned to these categories on the basis of land use in the watersheds upstream from surface-water sampling sites or in the vicinity of ground-water wells.

In general, nitrate concentrations in shallow ground water were higher in agricultural areas than in urban, forest, or rangeland areas. Nitrate also was elevated in surface water downstream from agricultural areas, but was not as high as in ground water. Nitrate concentrations similar to those found downstream from agriculture were found downstream from urban areas.

(140K GIF)
Urban runoff such as this combined sewer overflow entering Fall Creek in Indianapolis, Indiana, can bring high concentrations of ammonia, nitrate, and phosphate into streams (photograph by Charles Crawford).

Ammonia and phosphorus concentrations were higher downstream from urban areas than at sites downstream from any other land uses. Concentrations in many urban areas were high enough to exceed criteria or recommendations intended to protect aquatic life and prevent accelerated eutrophication.

In large rivers, such as the Potomac, Rio Grande, or Willamette, nutrient concentrations are usually low. The upstream basins of these rivers contain a mixture of land uses, and high-concentration runoff from agricultural and urban areas can be diluted with low-concentration runoff from undeveloped areas.

Nitrate concentrations in ground water generally decrease with depth

(5K GIF)
Nitrate concentrations are highest in "shallow" ground water (within about 100 feet of land surface). Ground water at greater depths might be protected from surface contamination by intervening rock layers or by the natural transformation of nitrate to nitrogen gas. Deep water also might be low in nitrate because it entered the ground before intensive fertilizer applications became prevalent.

(7K GIF)
Nitrate concentrations in shallow ground water in agricultural areas were generally higher than in other areas. Concentrations in 12 percent of the domestic-supply wells in agricultural areas exceeded the U.S. Environmental Protection Agency maximum contaminant level (MCL) of 10 mg/L for drinking water. Nitrate concentrations were higher in agricultural and urban areas than in undeveloped areas, such as forest and rangeland. However, concentrations generally were less than those for ground water in similar locations, and the drinking-water MCL was rarely exceeded.

(7K GIF)
Ammonia and total phosphorus concentrations were higher downstream from urban areas. At least 10 percent of the samples from urban sites contained sufficient ammonia to exceed the chronic exposure criteria for aquatic life. Total phosphorus concentrations exceeded the limit recommended by the U.S. Environmental Protection Agency for streams (0.1 mg/L) in samples from sites affected by urban or agricultural land uses.

(30K GIF)
The drainage basins of large rivers, such as the Connecticut River at East Haddam, Connecticut, include a mixture of land uses, and nitrate concentrations in these rivers generally were lower than in smaller streams draining basins that are predominantly agricultural or urban (photograph by Steve Garabedian).

(107K GIF)
In areas where woodland is intermixed with cropland, such as the Wild Rice River Basin of western Minnesota, concentrations of nitrate in ground water were lower than in areas of more intensive agriculture (photograph by Tim Cowdrey).

Soil Drainage

Movement of water from the land surface to aquifers and to streams is affected in part by soil drainage, the ability of soil to transmit water. Soil scientists classify soils by hydrologic group, based primarily on drainage characteristics. Soil hydrologic groups range from A (well-drained soils through which water moves rapidly) to D (very poorly drained soils through which water moves slowly).

Nitrate concentrations in ground water generally are highest beneath soils classified in hydrologic groups A and B, soils with rapid drainage. These soils provide easy pathways for the flow of water and nitrate to the water table. Poorly drained soils in hydrologic groups C and D impede the movement of nitrate to the subsurface in several ways. First, they are generally fine-grained silts and clays, which retard the downward movement of water and, therefore, of nitrate to the water table. Second, tile drains or ditches commonly are used in very poorly drained agricultural fields to remove excess water from the soil. This prevents some nitrate from ever reaching the ground water, instead directing it into nearby streams. In tile-drained areas of the Midwestern Corn Belt, such as in the White River Basin study unit in Indiana, nitrate concentrations in ground water were low, but concentrations in streams were high. Third, water in poorly drained soils is often low in oxygen, which restricts the chemical reaction that converts ammonia to nitrate and favors the chemical reaction that converts nitrate to nitrogen gas. In an extensive area of poorly drained soils on the coastal plain of the Albemarle-Pamlico study unit in North Carolina, nitrate concentrations in ground water were very low, but ammonia concentrations were high.

In areas where soils are very poorly drained, perforated pipelines are commonly used to remove excess water from croplands. Originally, these drainage pipes were made of ceramic tile. Modern pipes are made of flexible tubing, but are still referred to as "tile drains." Tile drains decrease the movement of water and nitrate to aquifers, instead directing seepage to nearby streams.

(40K GIF)
Ponded water on poorly drained soil in a cornfield near Random Lake, Wisconsin (photograph by Kevin Richards).

(39K GIF)
Installation of drainage pipe on a farm near Washington, Indiana (photograph by Jeffrey Martin).

(47K GIF)
Surface drain receiving tile drainage from cropland in the San Joaquin Valley of California (photograph by Marc Sylvester).

(111K GIF)
Surface drain receiving gravity drainage from cropland in the Rio Grande Valley near Derry, New Mexico (photograph by Lee Lewis).

(23K GIF)
Soil drainage is an important factor affecting the movement of water and nitrate to aquifers and streams. This map shows soils classified into four hydrologic groups, based on drainage characteristics. The data were obtained from digital maps produced by the Department of Agriculture's Natural Resources Conservation Service (formerly the Soil Conservation Service) in 1993.

(6K GIF)
Nitrate concentrations were highest in ground water beneath well-drained soils.

Geology

The type of geologic formations through which ground water passes can affect how easily water and nutrients move downward. Nitrate concentrations in shallow ground water beneath agricultural land differ among four broad types of formations in which the wells were sampled. Nitrate concentrations were highest in ground water from unconsolidated sands and gravels, the formation which, of the four, transmits water most easily. Concentrations were not quite as high in ground water from alluvium (river deposits) or carbonate rock (limestone). These formations do not allow water to move as rapidly down to ground water, though carbonate rock can be fractured or contain solution channels that provide quick connections to the subsurface. Concentrations were lowest in ground water from formations through which water moves very slowly, such as cemented sandstones and crystalline rock (such as granite).

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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
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