Polluted storm water runoff is often transported to storm drain systems, referred to by the permit as, municipal separate storm sewer systems (MS4s) and ultimately discharged into local rivers and streams without treatment.  EPA’s Storm Water Phase II Rule establishes an MS4 storm water management program that is intended to improve the Nation’s waterways by reducing the quantity of pollutants that are introduced into storm drain systems during storm events and non-stormwater activities.  Common pollutants include oil and grease, salts and deicing materials associated with transportation; pesticides, fertilizers and organic refuse from private and commercial landscape maintenance.  Sediment and various construction related pollutants from construction sites activities and also carelessly discarded trash, such as, paper, plastic bottles and yard waste.  When these pollutants are collected by the miles of roadway gutter and acres urban surfaces, the pollutant volumes are significant and impair the waterways, thereby impairing the recreational use of the water body, contaminating water supplies, and interfering with the habitat for aquatic organisms, and land wildlife. 

In 1990, EPA promulgated rules establishing Phase I of the National Pollutant Discharge Elimination System (NPDES) storm water program.   The Phase I program for MS4s requires operators of “medium” and “large” MS4s, that is, those that generally serve populations of 100,000 or greater, implement a storm water management program as a means to control polluted discharges from these MS4s.  The Storm Water Phase II Rule extends requirement of the NPDES storm water program to certain “small” MS4s but takes a slightly different approach to how the storm water management program is developed and implemented.