When infiltration is not authorized under a stormwater permit
Designed infiltration of stormwater is highly encouraged and generally preferred by regulatory agencies and stormwater practitioners. However, under certain conditions designed infiltration of stormwater presents an unacceptable risk to groundwater. Part III.D of the Construction Stormwater permit includes several prohibitions to infiltration of stormwater.
Infiltration is prohibited when the infiltration system will be constructed in
- areas that receive discharges from vehicle fueling and maintenance;
- areas with less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock;
- areas that receive discharges from industrial facilities which are not authorized to infiltrate industrial stormwater under an NPDES/SDS Industrial Stormwater Permit issued by the MPCA;
- areas where high levels of contaminants in soil or groundwater will be mobilized by the infiltrating stormwater;
- areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit;
- areas within 1,000 feet up‐gradient, or 100 feet down‐gradient of active karst features unless allowed by a local unit of government with a current MS4 permit;
- areas within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, subp. 13., unless allowed by a local unit of government with a current MS4 permit; and
- areas where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour or as allowed by a local unit of government with a current MS4 permit.