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P ollution is the release and spread of contaminants that degrade the environment and impair human health. Point source pollution has a discrete source, like an effluent pipe or smokestack, while nonpoint source pollution cannot be traced to a specific point of origin. Examples of nonpoint source pollution are pesticides from a farm field washing into a stream and soil erosion at a construction site. Controlling this type of pollution is challenging because there is no specific location to target and because inputs of nonpoint pollution are often intermittent.
Sources of nonpoint pollution are varied: streets and highways, farm fields and pastures, construction, logging, mining, lawns, and even pet wastes. Most pollutants from these sources are carried in runoff following a rainstorm or snowmelt. Nutrients, sediments, toxins, and pathogens are pollutants prevalent in runoff.
Phosphorus and nitrogen-essential nutrients for plant growth-become pollutants when excessive amounts of either enter an ecosystem. Sources of nitrogen and phosphorus pollution are agricultural and lawn fertilizers and animal wastes. In the United States and Europe, approximately 30 percent of phosphorus and 18 percent of nitrogen in agricultural fertilizers are taken up by crops. The remainder builds up in soils, washes into surface water, or leaches into groundwater.
Under naturally occurring conditions, nitrogen and phosphorus are relatively scarce in aquatic systems, thus limiting the growth of algae and plants. Populations of algae or of cyanobacteria (bluegreen algae) often proliferate following increases in nutrient levels. These algal blooms cloud water and limit light to submerged plants. Then, when these populations of algae die, the decomposition process uses much of the water’s dissolved oxygen. Lack of oxygen kills fish and clams, and alters the microorganism and invertebrate composition of the community. In marine ecosystems, nutrient enrichment leads to loss of biodiversity in seagrass beds and kelp communities. These ecosystems, when healthy, provide spawning ground and nurseries for many fish species as well as shelter and food for many marine mammals, so damage impacts many species.
Blooms of some microorganisms produce toxins. Red tides are overgrowths of aquatic microorganisms, so-named because they turn the water red or brown. These organisms synthesize toxins that damage the nervous system, can kill marine mammals and fish, and contaminate seafood. Some cyanobacteria responsible for freshwater algal blooms also produce toxins.
Excessive phosphorus is the primary cause of eutrophication, or premature aging, of lakes in the United States. Nitrogen is responsible for eutrophication in most estuaries and coastal waters. Marine areas that are highly impacted by eutrophication include the Gulf of Mexico, Chesapeake Bay, Long Island Sound, and the Florida Keys in North America, and the Adriatic, Baltic, Black, and North Seas in Europe. Extreme nitrogen enrichment in coastal waters results in what are called “dead zones,” or areas with very low oxygen that no longer support the life once found there. The Gulf of Mexico, which receives nutrient inputs from the Mississippi River, has a dead zone that expands to about 4,800 square miles (20,000 square kilometers) in the summer, an area the size of New Jersey.
Particles of silt, clay, and sand particles transported by water are called sediment. These soil particles are washed from fields, construction sites, and logging operations. As with algal blooms, sediment clouds water, blocks light, and limits plant growth. Some sediment particles block the gills of fish and aquatic insects, and bury clams, oysters, and other bottom dwelling organisms. Deposition of sediment in waterways and ports interferes with boat traffic and fishing. In addition, sediments often contain nutrients, which enrich the waters, as previously discussed.
Sediment deposition in Chesapeake Bay has increased fourto fivefold since the 1800s due to urban development, timber harvest, and agriculture. Research in the Chesapeake Bay watershed revealed that the most sediment is produced in the agricultural portions of the watershed, while the lowest input comes from the forested portions. In urban parts of the watershed, sedimentation is highest during building, but established developments continue to contribute sediments.
Pesticides usually enter surface water through runoff, but drifting spray from pesticide application also contributes to water contamination. Mining operations expose sulfur-containing rock to water and air, and this leads to the formation of sulfuric acid. The acid dissolves heavy metals, including mercury, lead, and copper, introducing these materials to runoff. These heavy metals are toxic to humans and wildlife.
Disease-causing bacteria and viruses enter water from manure, pet waste, and improperly treated human waste. These pathogens can contaminate seafood and render water unfit for human use. According to the National Resources Defense Council, 75 percent of U.S. beach closings in 2005 were due to unacceptable levels of bacteria.
Economic and social costs of nonpoint source pollution are high: fisheries are less productive, tourism is damaged, public health can be impaired, water treatment costs increase, and property values decrease.
Monitoring, at both the local and national levels, can provide vital information for water quality management. The National Water Quality Assessment (NAWQA) program is run by the U.S. Geological Survey (USGS). This long-term project monitors nutrients and pesticides in rivers and aquifers. The data gathered helps municipalities, agencies and scientists make management decisions for specific watersheds. Another monitoring program, run by the National Oceanic and Atmospheric Administration (NOAA), is the National Estuarine Eutrophication Survey.
Managing nonpoint pollution requires a variety of methods, depending on the source of pollutants. Vegetated buffer strips along water bodies trap sediment and remove nutrients from runoff. Efforts by farmers to determine levels of nutrient in their soils, then applying fertilizers only to meet current needs, reduces nutrients inputs. Construction site erosion is managed by silt fences to trap sediment and straw and hay mulches are used to hold soil in place until grass is established.
- Stephen Carpenter et al., “Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen,” Issues in Ecology (Summer, 1998);
- Robert Howarth et , “Nutrient Pollution of Coastal Rivers, Bays and Seas,” Issues in Ecology (Fall, 2000); National Oceanic and Atmospheric Administration, “Nonpoint Source Pollution,” oceanservice.noaa.gov;
- Natural Resources Defense Council, “Testing the Waters 2006: A Guide to Water Quality at Vacation Beaches,” www.nrdc.org;
- S. Department of Agriculture, “National Management Measures for the Control of Nonpoint Pollution from Agriculture” (Department of Agriculture, 2003);
- S. Geological Survey, “The Impact of Sediment on the Chesapeake Bay and Its Watershed,” chesapeake.usgs. gov;
- S. Geological Survey, “The National Water-Quality Assessment Program-Entering a New Decade of Investigations,” pubs.usgs.gov.