Storm Water is defined by the U.S. Environmental Protection Agency (EPA) as urban runoff and snowmelt runoff, consisting only of those discharges which originate from precipitation events. Storm water is precipitation that flows across a surface to a storm drain system or receiving waters.
Examples of this include: the water that flows off a building's roof when it rains (runoff from an impervious surface); the water that flows into streams when snow on the ground melts (runoff from a semi-pervious surface); and the water that flows from a vegetated surface when rainfall exceeds the rate at which it can infiltrate into the soil (runoff from a pervious surface).
When something is pervious, it has the ability to allow a liquid to pass through. When all factors are equal, runoff increases as the surface becomes more impervious. For example, grass fields are more pervious and absorb more runoff than an asphalt parking lot. During precipitation events in urban areas, rainwater picks up and transports pollutants through storm water systems, and ultimately to waters of the Commonwealth. Examples of pollutants are motor oil or fuels from vehicles, soils, and dust, winter de-icing materials, and spilled or dumped chemicals.