Surface Water Discharges Section
Individual NPDES Permit for Municipal Separate Storm Sewer Systems (MS4s)

Urban stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), and is often discharged, untreated, into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a NPDES permit (Phase I or Phase II) and develop a stormwater management program. This permit type is based on population. For additional information, please refer to the EPA’s MS4 Website. 

• Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.
• Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges.