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What is an MS4?

MS4 stands for Municipal Separate Storm Sewer System. The MS4 is a nationwide stormwater management program overseen by the Environmental Protection Agency (EPA). In Pennsylvania, the program is administered by the Department of Environmental Protection (DEP).

A storm system uses infrastructure such as underground pipes, ditches, inlets and retention basins to collect and convey untreated stormwater from built up areas into nearby streams, rivers and oceans. The “separate” part of the name indicates that the stormwater system is separated from the sewer system. There are combined sewer & storm systems (“combined systems”), but they are not part of the MS4 program.

Who is part of the MS4 program?

Generally, municipalities (the “M” part of MS4) such as cities, towns, and boroughs, oversee separate stormwater systems. Large organizations such as universities and prisons may also be part of the MS4 program. There are a few different types of MS4s, but all become part of the program based on US Census population data. When population density is high enough to be considered an Urbanized Area by the Census, a municipality falls under the MS4 program. If you live in a rural area, even if your municipality has a stormwater management system, you probably do not fall under the MS4 program.

What are the elements of an MS4?

When a municipality is part of the MS4 program, they must obtain a National Pollution Discharge Elimination System (NPDES) permit to discharge stormwater legally into local waterways. NPDES permits are also commonly referred to as MS4 permits or stormwater permits. To meet the terms of the NPDES permit, municipalities must develop a Stormwater Management Program (SWMP). Communities discharging into a water body the DEP defines as “impaired,” such as the Chesapeake Bay, must also develop a Pollution Reduction Plan (PRP).

Every MS4 is unique, which makes every SWMP unique. However, every SWMP includes the same six focus areas that the Environmental Protection Agency considers essential for success, called Minimum Control Measures or “MCMs":

  • Public Education and Outreach
  • Construction Site Erosion Control
  • Public Participation and Involvement
  • Post Construction Stormwater Management
  • Illicit Discharge Detection and Elimination
  • Pollution Prevention and Good Housekeeping.


Penn State Extension: (includes video)

PA DEP, NPDES Frequently Asked Questions: (PDF)

Video Education Series hosted by PA DEP:

Map of PA MS4 municipalities: (PDF)

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  1. Toni Pogue

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