Polluted runoff is caused by rainwater or snowmelt flowing overland into nearby waterways. Water becomes polluted as it picks up silt from eroding soils, oils, toxic metals, and other wastes on roads from vehicles, and animal wastes and chemicals from farm fields and suburban lawns. Runoff can also be the result of agricultural irrigation. In urban areas, runoff from paved streets and parking lots carries grime, litter, and toxic metals that have accumulated on road surfaces into local waters via street gutters and storm drains. At times, used motor oil, anti-freeze, paints, and other harmful wastes are intentionally dumped into gutters and storm drains.
Polluted runoff needs to be controlled because the associated environmental problems can be severe.
The recent toxic outbreaks in the mid-Atlantic region of Pfiesteria , a microbe that causes illnesses in people and massive fish kills, is thought to be linked to nutrient pollution. Excess concentrations of nitrates in groundwater used for drinking can cause serious health effects, especially for infants. Agricultural runoff also can contain toxics like pesticides. Urban runoff contains numerous pollutants including metals that are toxic to fish and other aquatic life. Runoff carries fertilizers, pesticides and other chemicals, and sediments into adjacent water bodies or may contaminate groundwater.