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National Pollutant Discharge Elimination System (NPDES)

Stormwater Discharges from Municipal Sources

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Quick links to relevant information on stormwater discharges from municipal sources: 


Polluted stormwater runoff is commonly transported through municipal separate storm sewer systems (MS4s), and then often discharged, untreated, into local water bodies.

An MS4 is a conveyance or system of conveyances that is:
  • owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.,
  • designed or used to collect or convey stormwater (e.g., storm drains, pipes, ditches),
  • not a combined sewer, and
  • not part of a sewage treatment plant, or publicly owned treatment works (POTW).

To prevent harmful pollutants from being washed or dumped into MS4s, certain operators are required to obtain NPDES permits and develop stormwater management programs (SWMPs). The SWMP describes the stormwater control practices that will be implemented consistent with permit requirements to minimize the discharge of pollutants from the sewer system.

Phase I MS4s
Phase II MS4s

The 1990 Phase I regulation requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.

There are approximately 855 Phase I MS4s covered by 250 Individual Permits

The 1999 Phase II regulation requires small MS4s in U.S. Census Bureau defined urbanized areas, as well as MS4s designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. Phase II also includes non-traditional MS4s such as public universities, departments of transportation, hospitals and prisons.

Most of the 6,695 Phase II MS4s are covered by statewide General Permits, however some states use individual permits. There are three Watershed Permits that cover 3 Phase I and 40 Phase II MS4s.

Urbanized Area Maps for NPDES MS4 Phase II Stormwater Permits