Stormwater runoff occurs when rain, snowmelt or irrigation flows over land or impervious surfaces (e.g. parking lots, building rooftops.) without filtering into the ground. As a result, stormwater runoff has the potential to accumulate debris, chemicals, sediment and other pollutants and flow untreated into surface waters. Stormwater runoff is also referred to as non-point source pollution. The National Pollutant Discharge Elimination System (NPDES) Storm Water Program, administered by the EPA, regulates and permits stormwater discharges from three major potential sources: municipal separate storm sewer systems (MS4s), construction activities, and industrial activities.

CSO Photo

By Robert Lawton [CC BY-SA 2.5], via Wikimedia Commons

Municipal Separated Storm Sewer Systems (MS4) Entities


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