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Stormwater runoff occurs when water from rain or snowmelt flows over the ground. Impervious surfaces such as driveways, sidewalks and streets prevent stormwater from naturally soaking into the ground.Development typically replaces the natural landscape with impervious surfaces like roads, roofs and parking lots. When stormwater cannot sink into the ground, it has nowhere to go but to flow across the surface of the ground.
Stormwater washes pollutants off roads, lawns and other surfaces and carries them into the nearest body of water. These pollutants can harm fish and wildlife populations, kill native vegetation, contaminate drinking water supplies, and make recreational areas unsafe.
Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it is often discharged untreated into local water bodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a National Pollutant Discharge Elimination System (NPDES) permit and develop a stormwater management program.