Minnesota Department of Transportation

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Project development

Environmental process | Environmental review

Municipal Separate Storm Sewer (MS4)


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 program.

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 permits. 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 Districts (Duluth, East Grand Forks, Elk River Area, La Crescent, Mankato, Moorhead, Rochester, and St. Cloud).

MnDOT’s MS4 permits apply to the entire Metro District and 2010 U.S. Census Bureau Boundary for the other cities.  

When to use this subject

The MS4 permit has requirements from design through maintenance. As of December 2021, there are new stormwater treatment requirements for existing impervious area in reconstruction projects that occur within the MS4 boundary. These separate thresholds and requirements than the NPDES Construction Stormwater permit.

How this subject fits into the overall project development process

The MS4 permit is ongoing in the areas stated above. MS4 requirements which may require space for stormwater treatment and budget, should be considered during the planning and scoping phases, and implemented during the design phases.

Organizations involved

  • MnDOT:
    • Environmental Assessment Unit
    • Project Managers
    • District Maintenance