The National Pollutant Discharge Elimination System (NPDES) Stormwater Program is a federal program of the Environmental Protection Agency (EPA) that regulates storm water discharges from three potential sources:
Municipal separate storm sewer systems (MS4s)
Most stormwater discharges are considered point sources, concentrated runoff from site, and require NPDES permits. This permitting mechanism is designed to prevent stormwater runoff from washing harmful pollutants into local surface waters such as streams, rivers, lakes or coastal waters.
A municipal separate storm sewer system (MS4) is a drainage system owned by a municipality, in this case the City of Greenville, that carries stormwater runoff away from buildings, roads, and other development to alleviate flooding. More about MS4