A municipal separate storm sewer system (MS4) is a means of transportation, individually or in a system, (e.g. roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, storm drains, etc.) that are:
- owned or operated by a public entity (e.g. cities, townships, counties, military bases, hospitals, prison complexes, highway departments, universities, etc.) with jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes. This includes special districts under State law (sewer, flood control, or drainage districts, etc.), an authorized Indian tribal organization, or a designated and approved management agency under section 208 of the Clean Water Act;
- designed or used for collecting or transporting stormwater;
- not a combined sewer; and
- not part of a publicly owned treatment works.
Only certain MS4s in Minnesota are subject to stormwater regulation under the Clean Water Act and Minnesota Rule 7090. The MPCA regulates the following:
- An MS4 located fully or partially within an urbanized area as determined by the latest Decennial Census and owned or operated by a publicly owned entity that has the potential resident capacity, bed count occupancy, or average daily user population of 1,000 or more.
- The entire jurisdiction of a city or township that is located fully or partially within an urbanized area as determined by the latest Decennial Census and owns or operates an MS4.
- An MS4 owned or operated by a municipality with a population of 10,000 or more.
- An MS4 owned or operated by a municipality with a population of at least 5,000 and discharges or has the potential to discharge stormwater to one of the following:
- A water identified as an outstanding resource value water as identified in Minn R. 7050.0180, subparts 3 and 6.
- A water identified as a trout lake or trout stream as identified in Minn. R. 6264.0050, subparts 2 and 4.
- A water listed as impaired under section 303(d) of the Clean Water Act, 33 U.S.C. § 1313.
Owners or operators of MS4s that meet the above conditions must submit an MS4 General Permit to the MPCA. Under the permit, the owner or operator is required to reduce the amount of sediment and pollution carried from stormwater to surface and ground water to the maximum extent practicable. Additionally, the permit requires the system owner or operator to develop a stormwater pollution prevention program (SWPPP) that incorporates best management practices (BMPs) to reduce stormwater impacts most appropriate to their MS4.