MS4 / Stormwater Management

Program Overview

Polluted stormwater runoff is often transported to municipal separate storm sewer systems (MS4s) and ultimately discharged into local rivers and streams without treatment. The Environmental Protection Agency (EPA) Stormwater Phase II Rule establishes an MS4 Stormwater management program that is intended to improve the nation’s waterways by reducing the quantity of pollutants that storm water picks up and carries into storm sewer systems during storm events. 


Common pollutants include:

  • Oil and grease from roadways
  • Pesticides from lawns
  • Sediment from construction sites
  • Carelessly discarded trash such as cigarette butts, paper wrappers, and plastic bottles

When deposited into nearby waterways through MS4 discharges, these pollutants can impair the waterways, thereby discouraging recreational use of the resource, contaminating drinking water supplies, and interfering with the habitat for fish, other aquatic organisms, and wildlife.

National Pollutant Discharge Elimination System

In 1990, EPA promulgated rules establishing Phase I of the National Pollutant Discharge Elimination System (NPDES) Stormwater 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, to implement a stormwater management program as a means to control polluted discharges from these MS4s. 
  • The Phase II program extends coverage of the NPDES stormwater program to certain “small” MS4s but takes a slightly different approach to how the stormwater management program is developed and implemented.
Storm Water Management