Water becomes polluted in a number of ways. Industries and municipal wastewater treatment plants discharge wastes, via a pipe, into surface waters. Polluted runoff is one example and exists in rural, suburban, and urban environments. Other examples of nonpoint source pollution include airborne toxics, contaminated groundwater from the leaching of pollutants, and contaminated sediments. Other types of impairment include alteration of physical habitat like channel modification and streamflow alteration.
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.