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Minnesota Stormwater Manual

Stormwater infiltration
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Infiltration basin Detailed Cross Section
Schematic showing an infiltration basin, which is one of several stormwater control practices designed to infiltrate stormwater runoff. Infiltration practices capture stormwater runoff and allow it to infiltrate into the underlying soil. Pollutant removal occurs through a variety of mechanisms, including adsorption, absorption, plant uptake, and degradation. Note that inflow into the practice has undergone pretreatment and that once the practice is filled, runoff bypasses the practice rather than entering it.
The Construction Stormwater Permit prohibits infiltration of stormwater runoff “within 1,000 feet up-gradient or 100 feet down-gradient of active karst features”. Active karst is a terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface.The Construction Stormwater Permit prohibits infiltration of stormwater runoff in 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.The Construction Stormwater General permit prohibits permittees from constructing infiltration systems where infiltrating stormwater may mobilize high levels of contaminants in soil or groundwater. Infiltrated runoff has the potential to transport contaminants in the soil to the groundwater or mobilize contaminants in the groundwater. Stormwater at contaminated sites can be managed by surface capture and treatment, or as infiltration by avoiding contact between runoff and contaminated soils and groundwater.The Construction Stormwater Permit prohibits permittees from constructing infiltration systems in areas where soil infiltration rates (including amended soils) are field measured at more than 8.3 inches per hour unless they amend soils to slow the infiltration rate below 8.3 inches per hour. The primary concerns with infiltration rates above 8.3 inches per hour are (1) a diminished ability to attenuate pollutants due to the relatively short contact time between the soil and infiltrating stormwater and (2) a higher potential for rapid contaminant transport to groundwater systems (e.g., in the event of chemical spills).A minimum 100 foot horizontal separation is REQUIRED between any infiltration BMP and a sensitive public water supply well, and a 50 foot minimum horizontal separation is REQUIRED between any infiltration BMP and all other public water supply wells.The Construction Stormwater Permit requires a higher level of engineering review for proposed infiltration projects in areas overlying an Emergency Response Area (ERA) where the vulnerability of the DWSMA (Drinking Water Supply Management Area) is classified as moderate, or in areas outside the ERA where the vulnerability of the DWSMA (Drinking Water Supply Management Area) is classified as high or very high.Knowing the soils in the area of a proposed infiltration best management practice (BMP) helps determine the suitability and design of the BMP. Soils determine how quickly stormwater will infiltrate, affect plant growth, and affect the fate and transport of pollutants. Section 16.10 of the Construction Stormwater general permit states “Permittees must provide at least one soil boring, test pit or infiltrometer test in the location of the infiltration practice for determining infiltration rates”. The Minnesota Stormwater Manual recommends that soil borings or pits be dug to verify soil types and infiltration capacity characteristics and to determine the depth to groundwater and bedrock.Potential Stormwater Hotspots (PSHs) are activities or practices that have the potential to produce relatively high levels of stormwater pollutants. Designation as a PSH does not imply that a site is a hotspot, but rather that the potential to generate high pollutant runoff loads or concentrations exists. PSHs include locations where there is a potential risk for spills, leaks, or illicit discharges. Stormwater hotspots may also be areas which produce higher concentrations of pollutants than normally found in urban runoff. The Construction General Permit (CGP) prohibits infiltration in areas that receive discharges from vehicle fueling and maintenance and in 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.The Construction General Permit (CGP) prohibits infiltration when an infiltration system will be constructed in areas of predominately Hydrologic Soil Group D (clay) soils. Soils with low infiltration capacity (tight soils) are defined as soils with steady-state infiltration rates equal to or less than 0.06 inches per hour. These soils will typically be categorized under Hydrologic Soil Group (HSG) D.This infiltration practice appears to have a likelihood of mobilizing contaminants. A site investigation would include assessing the mobility of the contaminants, estimating the likelihood that contaminants would be transported to groundwater, and determining if there are receptors (e.g. wells, surface water) that would be impacted.This infiltration practice should be assessed to determine if there is adequate separation from the municipal supply well and if underlying materials need to be modified to slow the infiltration rate to 8.3 inches per hour or lessThis infiltration practice should be assessed to determine how deep the karst is and/or if underlying materials need to be modified to slow the infiltration rate to 8.3 inches per hour or lessThis infiltration practice should be assessed to determine if there is 3 or more feet of separation between the bottom of the practice and the underlying bedrockThis infiltration practice should be assessed to determine if there is 3 or more feet of separation between the bottom of the practice and the underlying perched water table or if the soils are not classified as Hydrologic Group D soilsimagemap stormwater constraints
Stormwater infiltration constraints - hover above a circle for text

This page contains links to several pages that address infiltration of stormwater runoff. Link to portal for Infiltration Practices.

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