What is stormwater?
Stormwater is rainfall or melted snow that runs off 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 include vehicle exhaust products, brake and tire dust, oil and grease, sediment, salt, pet waste, and trash. Because stormwater is untreated, these pollutants enter our waterways.
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, 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.) having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes.
The MS4 General Permit is designed to reduce the amount of sediment and pollution that enters surface and ground water from storm sewer systems to the maximum extent practicable. Stormwater discharges associated with MS4s are subject to regulation under the National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS). 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.
MnDOT MS4 permits
- MnDOT Metro District MS4 program and resources
- MS4 out-state district application (PDF)
What is a total maximum daily load?
A total maximum daily load (TMDL) is the maximum amount of a pollutant a water body can receive without violating water quality standards, and an allocation of that amount to the pollutant’s sources. Once a water body is added to the Impaired Waters List (303(d)) for not meeting a water quality standard, the next step is to conduct a TMDL assessment, where MPCA identifies all sources of the pollutant and determines how much each source must reduce its contribution in order to meet the standard.