National Water-Quality Assessment Program

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

The Clean Water Act, the Safe Drinking Water Act, and other legislation have been implemented over the last 20 years to ensure that the people of the United States are provided water that is safe for drinking, swimming, and fishing. Some of the protective measures considered by water-resources managers are quite expensive. Use of these measures could result in higher water utility rates or might involve restrictions on the types and amounts of chemicals applied to nearby land. Scientific information about where, when, and how chemicals enter water supplies can help managers tailor protection strategies to fit the need, and so minimize costs and restrictions.

Although the Clean Water Act has provided funding to curtail nutrient contamination from point sources, primarily sewage-treatment plants, a large percentage of nutrient contamination is caused by nonpoint sources, such as atmospheric deposition, agricultural runoff, and seepage from septic systems. Contamination from nonpoint sources is more difficult to control and has only recently been addressed in national legislation.

(25K GIF)

Nitrate, ammonia, and total phosphorus concentrations are elevated downstream from urban areas. (Lights of Portland, Oregon, reflected in the Willamette River; photograph by Dennis Wentz).

Determining where water-quality problems are most likely to occur is the key to devising cost-effective watershed-management strategies. Our findings imply that management strategies need to incorporate some flexibility in different regions of the Nation to provide the greatest benefit for the lowest cost. For example, soil drainage characteristics are a useful guide to where ground water or surface water is most at risk to contamination from nutrients applied at the land surface. Ground water in areas of well-drained soils is vulnerable to contamination as a consequence of surface application of chemicals and warrants more complete protection strategies than in areas of poorly drained soils. We found that nitrate concentrations generally were low in ground water under poorly drained soils, even in NAWQA study areas where fertilizer was heavily applied at the surface. Watershed management of surface water, rather than ground water, might be a priority in these areas.

Ground-water protection strategies also could vary with the depth of wells and geologic characteristics of the area. Only 1 percent of public-supply wells in NAWQA study areas were found to exceed the EPA drinking-water standard for nitrate. In contrast, 12 percent of domestic-supply wells in agricultural areas exceeded the standard. Domestic wells are generally more shallow than public-supply wells, and more frequent exceedances of the nitrate standard are to be expected. Areas where domestic-supply wells are prevalent, and whose geologic characteristics allow easy transmittal of chemicals to ground water, may warrant protective measures not necessary for other parts of the Nation.

Science Can Identify When Nutrient Problems Require Special Management

Concentrations of nutrients sometimes have a distinct seasonal pattern in streams. Concentrations often are highest during storm events soon after application of fertilizers upstream. Other agricultural chemicals in streams follow similar patterns. Protection strategies in areas where these chemicals are of concern might need to be in force only during certain seasons, such as during the spring runoff period.

Science Can Evaluate Whether and By How Much Pollution Control Programs Decrease Nutrient Concentrations

Reducing the amount of nutrients applied to the land could improve the local quality of water. Agricultural scientists are currently considering such methods as varying the timing of fertilizer applications to minimize runoff into streams or pumping high-nutrient, shallow ground water for use as a fertilizer source. Fertilizer management plans are becoming more common as farmers better account for other sources of nutrients present in the soils, such as from nitrogen fixation by soybeans or manure application to cropland. Accounting for these additional sources of nitrogen when determining fertilizer application rates could decrease the excess nutrients in the soil and the amount of nutrients available to surface streams and ground water.

Recent improvements in wastewater treatment have decreased ammonia concentrations downstream from many urban areas. During this treatment, ammonia is converted to nitrate. Consequently, concerns about fish toxicity have decreased, but the potential for eutrophication is not diminished because total nitrogen concentrations remain constant.

Some municipalities have implemented voluntary or mandatory phosphate-reduction programs. Monitoring of phosphate loads has been helpful in determining whether or not these innovative programs make a difference. In Atlanta, for example, the amount of phosphorus in treated sewage effluent decreased by 83 percent in 4 years (1989-93) due to a combination of voluntary and mandatory reductions in phosphate detergents. At the same time, phosphorus loads in the Chattahoochee River downstream from Atlanta decreased by 53 percent.

In this and other cases, water-quality studies can provide scientific data on the economic and ecological benefits of water-management strategies. Without such data, an accurate analysis of benefits compared to costs would not be possible.

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/h5.html