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Stormwater is of concern for two main issues - one related to the volume and timing of runoff water ( Flood control and water supplies) and the other related to potential contaminants that the water is carrying ( Water Pollution ). Because Impervious Surfaces (parking lots, roads, buildings) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding when the stormwater collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream base flow in dry weather. REGULATION IN UNITED STATES In the United States , the EPA is charged with Regulating stormwater as part of the Clean Water Act (CWA). Under the CWA , discharges to "Waters of the United States" require National Pollution Discharge Elimination System (NPDES) permits. NPDES permits for point source discharges have been required since the 1970s. In 1987, the CWA was expanded to require NPDES permits for discharges of non-point source discharges such as stormwater runoff. This 1987 expansion was promulgated in two phases - Phase I and Phase II. Phase I required that all Municipalities of 100,000 persons or more, industrial dischargers, and Construction sites of 5 Acre s or more have NPDES permits for their stormwater discharges. Phase I permits were issued in much of the U.S. in 1991. Phase II required that all Municipalities , industrial dischargers, Construction sites of 1 Acre or more, and other large property owners (such as School Districts ) have NPDES permits for their stormwater discharges. Phase II rules came into effect in 2003. The goal of the CWA is to restore all "Waters of the United States" to their "fishable" and "swimmable" conditions. Point source discharges, which originately mostly from municipal Wastewater ( Sewage ) and industrial wastewater discharges have been Regulated for over two decades. Pollutant loadings from these sources are tightly controlled and limited. However, thousands of water bodies in the U.S. remain impaired meaning that they contain pollutants at levels higher than is considered safe by EPA for the intended beneficial use of the water. Thus, the last source of pollutants entering these waters is termed polluted runoff, and has been known as non-point source pollution. Daily human activities result in deposition of pollutants on Roads , Lawns , Roofs , Farm fields, etc. When it Rains or we Irrigate , Water runs off and ultimately makes its way to a River , Lake , or the Ocean . While there is some Attenuation of these pollutants before entering the receiving waters, the quantity of Human activity results in large enough quantities of pollutants to impair these receiving waters. The 1987 amendments to the CWA were designed to reduce the pollutants that enter the receiving waters from diffuse or non-point sources, aka stormwater. See also: Impervious surface Permeable paving Reference Ferguson, Bruce K., 1998, Introduction to Stormwater, New York: John Wiley and Sons |