Stormwater / MS4


Municipalities that meet certain criteria must obtain NPDES permit coverage for discharges of stormwater from their municipal separate storm sewer systems (MS4s). In Pennsylvania, there is one Large MS4, one Medium MS4, and 1,059 Small MS4s. Additional Small MS4s may be designated by DEP when determined to be appropriate using DEP’s designation criteria.

MS4s must apply for NPDES permit coverage or a waiver if they are located in an urbanized area as determined by the latest Decennial Census by the U.S. Census Bureau, or if they are designated as needing a permit by DEP.

For NPDES permits issued after 2017, MS4s that discharge to surface waters impaired for certain pollutants or that discharge to waters in the Chesapeake Bay watershed were required to develop Pollutant Reduction Plans (PRPs) or TMDL Plans.