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Minnesota Stormwater Manual

When infiltration is not authorized under a stormwater permit
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File:Permit constraints plan view.png
Schematic illustrating construction stormwater permit prohibitions for infiltration. In this schematic, infiltration is not allowed at the site because it receives stormwater discharges from a vehicle fueling area and from an industrial facility where infiltration is prohibited. Infiltration would also not be allowed in the blue shaded area because it is located within a DWSMA.
File:Permit constraints profile view.png
Schematic illustrating construction stormwater permit prohibitions for infiltration. This schematic describes the limitations for infiltration. Groundwater flow beneath the site is from right to left. Native soil is shown in brown and the infiltration practice is in blue. Note that the separation distances from the infiltration practice are the shortest distances from the practice to the feature requiring separation (e.g. bedrock, active karst, etc.).

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.

Each of these is discussed in greater detail below.

Discharges from vehicle fueling areas

Vehicle fueling areas represent a relatively high risk for spills involving highly mobile and toxic chemicals. Gasoline compounds in particular are mobile and contain chemicals that represent a significant risk to human health and the environment (e.g. benzene). Standard designs for infiltration practices will not be adequate to treat runoff with high concentrations of these chemicals and the chemicals will readily leach through the underlying soil, thus representing a risk to groundwater. For more information, see the section potential stormwater hotspots.

Shallow bedrock and shallow water table

Stormwater infiltrating below an infiltration practice may require additional treatment even after passing through the media or soil in the infiltration practice. The distance needed to provide adequate treatment varies with soil and pollutants in the infiltrating water. For example, soils with high clay or organic matter contents typically retain pollutants better than sandy soils having little organic matter. A separation distance of 3 feet is commonly used for other types of infiltrating water, such as water infiltrating from a septic drain field. A separation distance of 5 feet is recommended, particularly in sandy soils, but a minimum of 3 feet separation from either bedrock or the seasonally high water table is required. For more information see the pages on shallow soils and depth to bedrock and shallow groundwater.

Discharges from industrial facilities where infiltration is prohibited

Infiltration is prohibited at certain industrial facilities where infiltration represents a potential risk for groundwater contamination. This prohibition exists for 5 Standard Industry Code (SIC) sectors.

Although the prohibition at these sites does not apply to infiltration practices operating prior to April 5, 2010, the prohibition in the construction stormwater permit applies to all facilities operating under these 5 sectors. Infiltration is not allowed at any site receiving stormwater discharges from a facility operating within one of these 5 sectors. For more information, see the page on infiltration at permitted industrial sites

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