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As water moves over yards, streets, house, driveways, etc., it picks up pollutants and carries them with it. Most runoff is directed to a storm drain that drains to the river or stream. Thus, any pollutants on the land will be carried to the river potentially harming wildlife and possible drinking water sources.
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Stormwater is what occurs following a precipitation event. Rain or snow falls on the ground and as the water moves downhill over the land it becomes "stormwater."
In 1987 Congress amended the Clean Water Act to require the U.S. EPA to establish requirements for storm water discharges. Phase I Stormwater regulations began in 1992 followed by Phase II in 2003. Phase I regulates industrial activity and municipal separate storm sewer systems (MS4s) with populations of 100,000 or more. Phase II regulates MS4s serving populations less than 100,000 and construction activities.