What is an MS4?
Municipal Separate Storm Sewer System refers to a collection of structures designed to gather stormwater and discharge it into local streams and rivers.
DEP MS4 Requirements
Municipalities and other entities such as universities and prisons that meet certain standards must obtain NPDES permit coverage for discharges of stormwater from their municipal separate storm sewer systems (MS4s).
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. EPA has posted Urbanized Area Maps from the 2010 Census data. Urbanized Area information from the 2000 Census can also be reviewed at the U.S. Census Bureau 2000 Urbanized Areas Information .
For the current permit term, MS4s that discharge to surface waters impaired for certain pollutants or that discharge to waters in the Chesapeake Bay watershed are required to develop Pollutant Reduction Plans (PRPS) or TM