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Polluted stormwater runoff is commonly transported through a Municipal Separate Storm Sewer System (MS4), and then often discharged, untreated, into local water bodies.

An MS4 is a conveyance or system of conveyances that is:
• Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.
• Designed or used to collect or convey stormwater (e.g., storm drains, pipes, ditches).
• Not a combined sewer, and
• Not part of a sewage treatment plant, or publicly owned treatment works (POTW).

To prevent harmful pollutants from being washed or dumped into MS4s, certain operators are required to obtain NPDES permits and develop stormwater management programs (SWMPs). The SWMP describes the stormwater control practices that will be implemented consistent with permit requirements to minimize the discharge of pollutants from the sewer system.

Public Notice of Intent to Issue Coverage Under a General Permit
MS4 Permit TMDL Application
MS4 Part 2 Permit Application
   - North Twin MPCA
   - Middle Twin MPCA
   - South Twin MPCA
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City of Crystal