What is stormwater?
Stormwater is rainfall or melted snow that runs off and is often routed to impervious surfaces, such as roadways, rooftops, sidewalks, and parking lots. Impervious, or hard surfaces, prevent the stormwater from naturally soaking into the ground where the soil would filter out pollutants. As it flows, stormwater picks up pollutants from these surfaces. Common pollutants that are picked up include: oil and grease, sediment, nutrients, salt and pet waste.
What is a Municipal Separate Storm Sewer System?
A Municipal Separate Storm Sewer System (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).
When did MnDOT become a MS4?
In 1972, the Clean Water Act (CWA) 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 (NPDES) 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.
How does MnDOT’s MS4 program reduce pollutants in stormwater?
The Storm Water Pollution Prevention Program (SWPPP) 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 (BMPs) for each of these six minimum control measures. To see more detailed information about these BMPs, follow the SWPPP links below. Contact us with any questions.
MnDOT MS4 NPDES permits
- MS4 out-state district application (PDF)