Urban areas are blanketed with roads, sidewalks, parking lots and buildings that seal the ground off from water that would otherwise be absorbed and filtered by soils and plants. Whenever it rains, water runs through the streets, down storm drains and into local waterways without being cleansed of all the impurities it picks up along the way. This stormwater can be managed, however, by replacing typical pavements with porous asphalt, cement and pavers that allow water to pass through them. This reduces flooding and pollution by allowing the water to be absorbed and filtered naturally by the underlying soil. These "pervious" pavements perform best in pedestrian walkways, sidewalks, driveways, parking lots and low-volume roadways.
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(Sources: U.S. Environmental Protection Agency. “National Pollutant Discharge Elimination System: Pervious Concrete Pavement.” Accessed Online May 8, 2012.; U.S. Environmental Protection Agency. “National Pollutant Discharge Elimination System: Porous Asphalt Pavement.” Accessed Online May 8, 2012.; U.S. Environmental Protection Agency. “Greening EPA: Permeable Pavers.” Accessed Online May 8, 2012.; Images courtesy EPA.)