Minnesota Department of Transportation

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Municipal Separate Storm Sewer System (MS4)

Twin Cities Metro

stabalizing

What is MS4?

Man stabilizing ground around creek
Worker stabalizing embankment.

MS4 stands for Municipal Separate Storm Sewer System. MS4 programs are required by the U.S. Environmental Protection Agency (EPA) and the Minnesota Pollution Control Agency (MPCA) to reduce pollution from stormwater to surface waters and groundwater. Municipalities with populations of 50,000 or greater, smaller designated cities, and other public entities with significant stormwater drainage systems have been selected to have MS4 programs. Other public entities that have been selected include universities, counties and state transportation departments. MnDOT Metro District is one of these designated MS4 programs.

How are pollutants from stormwater discharges reduced?

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 practicable extent. This is required as part of a permit MnDOT must hold due to the Clean Water Act.

Measures MnDOT takes 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.

For each of these six minimum control measures, the MnDOT Metro District adheres to Best Management Practices (BMPs). Reference the MS4 Background Page for more information regarding these BMPs.

When did MnDOT have to 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 Pollution 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 was required to apply for a NPDES permit to discharge stormwater. The Water Resource Engineering Section is responsible for MnDOT Metro District’s MS4 Program.

Additional background info