The Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES), Phase II Stormwater Program for Municipal Separate Storm Sewer Systems (MS4s) is the second part of a program designed to improve the quality of the nation's streams, rivers, lakes, and estuaries by managing stormwater runoff from urban and suburban areas, construction projects, and industrial sites. 

OVERVIEW:  Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it 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 and develop a stormwater management program.

PHASE I AND PHASE II:  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. There are approximately 750 Phase I MS4s.

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. There are approximately 6,700 Phase II MS4s.

Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.

WHAT IS AN MS4?  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 (including storm drains, pipes, ditches, etc.);
• Not a combined sewer; and
• Not part of a Publicly Owned Treatment Works (sewage treatment plant).

(This information was taken in part from the epa.gov website)