What is a Municipal Separate Storm Sewer System?
An MS4 is a conveyance or system of conveyances (roads with drainage systems, municipal streets, catch basins, curbs and gutters, ditches, man-made channels, storm drains, etc.) that is also owned or operated by a public entity (which can include cities, townships, counties, military bases, hospitals, prison complexes, highway departments, universities, etc.).
Stormwater discharges associated with MS4s are subject to regulation under the National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) MS4 Permit. The MS4 General Permit is designed to help reduce the amount of sediment and other pollution that enters surface and ground water from storm sewer systems to the maximum extent practicable. Through the MS4 General Permit, the system owner or operator is required to develop a Stormwater Pollution Prevention Program (SWPPP) that incorporates best management practices applicable to their MS4.
Municipalities with populations of 50,000 or greater, smaller designated cities, and other public entities with significant stormwater drainage systems are required to have MS4 programs. Other public entities that have been selected include universities, counties and state transportation departments. MnDOT is a regulated MS4 and has two separate MS4 permits—one for Metro District and one for Greater Minnesota, which includes seven urbanized areas across six other Districts (Duluth, East Grand Forks, La Crescent, Mankato, Moorhead, Rochester, and St. Cloud).
How MnDOT became an MS4
In 1972, the Clean Water Act came into effect, which prohibited the point discharge of any pollutant to waters of the United States unless the discharges were covered by a National Pollutant Discharge Elimination System permit. In 1987, Congress amended the CWA to require the EPA to establish phased requirements for NPDES permits. When Phase II became effective in 2003, the MnDOT Metro District and portions of the Greater Minnesota Districts were required to apply for a NPDES permit to discharge stormwater. The Water Resource Engineering Section is responsible for MnDOT Metro District’s MS4 Program and the Office of Environmental Stewardship assists the other Districts with their MS4 Program.
Storm Water Pollution Prevention Program
The Storm Water Pollution Prevention Program covers six minimum control measures to help reduce the discharge of pollutants from our storm sewer system, to the maximum extent practicable. The minimum control measures include:
- Public education and outreach
- Public participation/involvement
- Illicit discharge detection and elimination
- Construction site runoff control
- Post-construction site runoff control
- Pollution prevention/good housekeeping
MnDOT follows Best Management Practices for each of these six minimum control measures.