Public Education and Outreach: Inform residents and businesses about pollution prevention.
MS4 Essentials
Understanding municipal separate storm sewer systems and minimum control measures
Municipal Separate Storm Sewer Systems (MS4s) are a network of storm drains, pipes, ditches, and outfalls that collect and convey stormwater runoff separately from sanitary sewage.
These are regulated under the National Pollutant Discharge Elimination System, created in 1972 under the Clean Water Act. In 1987, the Clean Water Act was amended to include stormwater.
About MS4 Permits
The MS4 permits are issued to the largest cities and municipalities and requirements vary state by state. Phase I MS4 Permits were first implemented in 1990 and are given to systems which serve populations over 100,000 people. These permits are more stringent and with more requirements than Phase II. Phase II MS4 permits were first implemented in 1999 and are given to systems with a population of over 50,000 people.
The total number of MS4 can be difficult to track, and the most accurate numbers can be found on specific state websites. There are approximately 25 Phase I MS4s in the Chesapeake Bay Watershed, and over 1,300 Phase II MS4s. In addition, MS4s are tied to the Chesapeake Bay Total Maximum Daily Load (TMDL) milestones, and permittees must meet specific load reductions consistent with their state Watershed Implementation Plans (WIPs).
There are also non-traditional MS4 permits, given to large public institutions or large federal facilities, such as universities, airports, military bases, and correctional facilities. Nearly 40% of impervious land cover in the Chesapeake Bay watershed is unregulated, including most rural communities.
Minimum Control Measures (MCMs)
Each MS4 permit requires a Stormwater Management Program (SWMP) built around six MCMs.