Although nitrogen and phosphorus occur naturally and are essential for the growth of plants and animals, excessive nutrients in water can adversely affect human health and the environment. Elevated concentrations of nitrogen and phosphorus in streams and ground water of the Potomac River Basin often result from human activities such as manure and fertilizer applications.
Nutrient inputs to the Potomac River Basin are related to land use. Agricultural areas receive the largest amounts of nutrients because manure and fertilizer applications comprise 45 percent of nitrogen and 93 percent of phosphorus inputs.
In most waters of the Potomac River Basin, concentrations of nutrients do not pose a threat to human health or wildlife.
( p. 7 )
Elevated nitrogen concentrations in streams and ground water are common in areas of intensive row cropping, such as the northeastern part of the Potomac River Basin, and areas underlain by carbonate bedrock, such as the Great Valley.
( p. 8 )
Organic nitrogen and phosphorus concentrations are typically low, except in streams during high flows .
( p. 9 )
Tributary streams draining agricultural areas yield the greatest quantities of nitrogen to the Potomac River; streams draining agricultural and urban areas yield the greatest quantities of phosphorus.
( p. 9 )
Pesticides can make water unfit to drink and cause adverse ecological effects in streams. Commonly used pesticides are present in ground water in the Potomac River Basin, but in most cases at concentrations that are not threatening to human health.
( p. 10 )
More pesticides were detected in streams than in ground water, but only rarely at concentrations threatening to aquatic life.
( p. 11 )
Pesticides were commonly detected in agricultural areas of the Potomac River Basin, particularly in areas of intense crop production such as the corn-producing northeastern counties and in the Great Valley. Samples from forested areas rarely contained detectable pesticides.
( p. 12 )
Pesticides were frequently present in streams in urban areas; insecticide concentrations were greatest in urban streams.
( p. 13 )
Maximum concentrations of most pesticides occur in streams during the spring and early summer months, coincident with their application to fields, although atrazine and metolachlor are present year round in streams in agricultural areas.
( p. 14 )
Spring floods carry large amounts of nutrients and pesticides. A June 1996 flood on the Potomac River at Washington, D.C., carried an estimated 3,300 pounds of atrazine and 3,300,000 pounds of nitrogen.
( p. 14 )
Higher concentrations of agricultural chemicals are found in streams in the Great Valley than in other agricultural areas, presumably because carbonate bedrock permits relatively rapid movement of these chemicals through ground water to streams.
( p. 15 )
Chlorinated organic compounds, mercury, and lead are present in streambed sediment at concentrations that have some potential to adversely affect aquatic life.
Although its use was banned in 1988, chlordane was detected in streambed sediment from 13 of 26 sites, including 4 sites at which concentrations pose a high potential for adverse effects on aquatic life.
( p. 17 )
The use of DDT was banned in 1972 but it was detected in streambed sediment at most sites, although concentrations typically pose little risk to aquatic life.
( p. 17 )
Mercury from an industrial plant in Waynesboro, Va., possibly over a period of decades, has caused elevated concentrations of mercury in sediments downstream from the plant in the Shenandoah River, a major Potomac tributary.
( p. 18 )
The organic compounds and metals present in streambed sediment have been incorporated into the food chain.
( p. 18 )
Radon is present in ground water throughout the Potomac River Basin.
The Federal drinking-water standard for radon is currently under review by the U.S. Environmental Protection Agency; however, radon levels in 69 percent of ground-water samples were greater than a previously proposed standard of 300 picocuries per liter.
Radon in ground water is related to rock type, and levels are highest in eastern parts of the basin underlain by crystalline and siliciclastic rocks.
Despite an estimated 44-percent increase in population in the Potomac River Basin from 1970 to 1990, total phosphorus concentrations in the Potomac River at Washington, D.C., have decreased since 1979, and nitrogen concentrations have apparently stabilized.
Different forms of nitrogen show differing patterns in long-term trends in the Potomac River at Washington, D.C.; ammonia plus organic nitrogen concentrations have decreased, whereas nitrate concentrations have increased.