Agricultural runoff

DEFINITION: Water that flows into rivers, lakes, and other bodies of water from agricultural land and operations

Runoff from land used for agriculture, which often contains the residues of chemical and organic fertilizers and pesticides, is one of the major sources of water pollution around the world.

The water that flows into streams from farmland after rain or snowmelt carries the of the herbicides, fungicides, insecticides, and fertilizers that farmers have used on the land. Many of the wastes produced by cattle, hogs, sheep, and poultry that are raised on feedlots flow into nearby streams. The water that returns to nearby rivers and lakes after it is used to irrigate farmland may be polluted by salt, agricultural pesticides, and toxic chemicals. These materials and chemicals that are carried with soil eroding from agricultural land and transported by water degrade the quality of streams, rivers, lakes, and oceans.

Agricultural runoff enters rivers and lakes from farmland spread over large areas. Since such runoff is a nonpoint source of water pollution, it is more difficult to control than discharges from factories and treatment plants. The chemicals in agricultural runoff have contaminated in many areas of the United States. Rivers pick up and dissolved salts from agricultural runoff as they flow to the oceans. Salt in the Colorado River, for example, increases from about 50 milligrams per liter to 800 mg/l as the river flows down from its headwaters to Mexico.

Nutrients such as potassium, phosphates, and nitrogen compounds from organic wastes and fertilizers are carried by agricultural runoff into rivers and lakes. In the process known as eutrophication, excessive nutrients in bodies of water stimulate the growth of plants such as pond weeds and duck weeds, plant-like organisms called algae, fish and other animals, and bacteria. As more of these organisms grow, more also die and decay. The decay process uses up the oxygen in the water, depriving the fish and other aquatic organisms of their natural supply of oxygen. Some types of game fish, such as salmon, trout, and whitefish, cannot live in water with reduced oxygen. Fish that need less oxygen, such as carp and catfish, will replace them. If all the oxygen in a body of water were to be used up, most forms of life in the water would die. Eutrophication that results from human activities, such as agriculture, is known as cultural eutrophication. In the late 1950s Lake Erie, 26,000 square kilometers (10,000 square miles) in area, was reported to be eutrophic. Thanks to stringent pollution-control measures, the lake improved into the 1990s. However, despite a reduction in the amount of phosphorus particles running off into Lake Erie, dissolved phosphorus residue left behind from past runoffs lingered in the lake’s bottom sediment and began to cause a re-eutrophication of the lake in the 2010s.

Mitigating Environmental Impacts

The 1987 amendments to the Clean Water Act represent the first comprehensive attempt by the US government to control caused by agricultural activities. In 1991 the US Geological Survey began to implement the full-scale National Water-Quality Assessment (NAWQA) program. The long-term purposes of NAWQA are to describe the status and trends in the quality of the nation’s water resources and to provide a scientific understanding of the factors affecting the quality of these resources. In October, 1997, an initiative intended to build on the environmental successes of the Clean Water Act was announced. It focused on runoff from farms and ranches, city streets, and other diffuse sources. The plan called for state and federal environmental agencies to conduct assessments every two years.

Irrigation is necessary for survival in many developing countries. Governments struggle to build advanced agricultural systems, and developments in agriculture have improved food production around the world. However, the increasing reliance on fertilizers and other agricultural chemicals has contributed to the pollution of rivers and lakes. Therefore, interest has shifted to farming with reduced use of chemicals. Scientists have turned their attention to developing organic ways to grow food that require less fertilizer and fewer pesticides, and many farmers rotate their crops from year to year to reduce the need for chemical fertilizers. Instead of spraying their crops with harmful pesticides, some farmers combat damaging insects by releasing other insects or that prey upon the pests. Scientists have also developed genetically engineered plants that are resistant to certain pests. Other strategies for minimizing pollution caused by agricultural runoff include maintaining buffer zones between irrigated cropland and sites where wastes are disposed, restricting application of manure to areas away from waterways, avoiding application of manure on land subject to erosion, reusing water used to flush manure from paved surfaces for irrigation, constructing ditches and waterways above and around open feedlots to divert runoff, constructing lined water-retention facilities to contain rainfall and runoff, applying solid manure at a rate that optimizes the use of the nitrogen it contains for a given crop, and allowing excess to evaporate by applying it evenly to land.

Watersheds are areas of land that drain to streams or other bodies of water. Most nonpoint pollution-control projects focus their activities around watersheds because watersheds integrate the effects that land use, climate, hydrology, drainage, and vegetation have on water quality. In the United States, the National Monitoring Program was initiated in 1991 to evaluate the effects of improved land management in reducing in selected watersheds. Federal agencies involved in this program include the Environmental Protection Agency (EPA), the Department of Agriculture, the US Geological Survey, and the US Army Corps of Engineers. A composite index was constructed and published to show which watersheds had the greatest potential for possible degradation of water quality from combinations of pesticides, nitrogen, and sediment runoff. The EPA sets criteria on water quality to help states set their own site-specific standards to control pollution and thus reduce nutrient loading to rivers and lakes.

Bibliography

Copeland, Claudia. The Clean Water Initiative. Washington, D.C.: Congressional Research Service, Library of Congress, 1998.

Hill, Marquita K. “Water Pollution.” In Understanding Environmental Pollution. 4th ed. New York: Cambridge University Press, 2020.

Hunt, Constance Elizabeth. Thirsty Planet: Strategies for Sustainable Water Management. New York: Zed Books, 2004.

"Lake Erie Eutrophication Exacerbated by Release of Sediment Phosphorus during Anoxia." National Centers for Coastal Ocean Science, 27 Oct. 2021, coastalscience.noaa.gov/news/lake-erie-eutrophication-exacerbated-by-release-of-sediment-phosphorus-during-anoxia/. Accessed 22 July 2024.

Sullivan, Patrick J., Franklin J. Agardy, and James J. J. Clark. “Water Pollution.” In The Environmental Science of Drinking Water. Burlington, Mass.: Elsevier Butterworth-Heinemann, 2005.