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MN CSW Permit Section 16 Infiltration Systems

==SECTION 16 INFILTRATION SYSTEMS==

===16.1===
<big>Infiltration Systems.</big>

===16.2===
<big>[https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices Infiltration options] include, but are not limited to: [https://stormwater.pca.state.mn.us/index.php?title=Infiltration infiltration basins, infiltration trenches], rainwater gardens, [https://stormwater.pca.state.mn.us/index.php?title=Bioretention bioretention] areas without underdrains, [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) swales] with impermeable check dams, and natural depressions. If permittees utilize an infiltration system to meet the requirements of this permit, they must incorporate the design parameters in item [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_16_Infiltration_Systems#16.3 16.3] through item [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_16_Infiltration_Systems#16.21 16.21]. Permittees must follow the infiltration prohibition in item [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_16_Infiltration_Systems#16.14 16.14] anytime an infiltration system is designed, including those not required by this permit.</big>

===16.3===
<big>Permittees must design infiltration systems such that pre-existing hydrologic conditions of wetlands in the vicinity are not impacted (e.g., inundation or breaching a perched water table supporting a wetland).</big>

===16.4===
<big>Permittees must not excavate infiltration systems to final grade, or within three (3) feet of final grade, until the contributing drainage area has been constructed and fully stabilized unless they provide rigorous [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices erosion prevention] and [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices sediment controls] (e.g., diversion berms) to keep sediment and runoff completely away from the infiltration area.</big>

===16.5===
<big>When excavating an infiltration system to within three (3) feet of final grade, permittees must stake off and mark the area so heavy construction vehicles or equipment do not compact the soil in the infiltration area.</big>

===16.6===
<big>Permittees must use a [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment pretreatment device] such as a vegetated filter strip, forebay, or water quality inlet (e.g., grit chamber) to remove solids, floating materials, and oil and grease from the runoff, to the maximum extent practicable, before the system routes stormwater to the infiltration system.</big>

===16.7===
<big>Permittees must design infiltration systems to provide a [https://stormwater.pca.state.mn.us/index.php?title=Water_quality_criteria water quality volume] (calculated as an instantaneous volume) of one (1) inch of runoff, or one (1) inch minus the volume of stormwater treated by another system on the site, from the net increase of impervious surfaces created by the project.</big>

===16.8===
<big>Permittees must design the infiltration system to discharge all stormwater (including stormwater in excess of the water quality volume) routed to the system through the uppermost soil surface or engineered media surface within 48 hours. Permittees must route additional flows that cannot infiltrate within 48 hours to bypass the system through a stabilized discharge point.</big>

===16.9===
<big>Permittees must provide a means to visually verify the infiltration system is discharging through the soil surface or filter media surface within 48 hours or less.</big>

===16.10===
<big>Permittees must provide at least one [https://stormwater.pca.state.mn.us/index.php?title=Understanding_and_interpreting_soils_and_soil_boring_reports_for_infiltration_BMPs soil boring], test pit or [https://stormwater.pca.state.mn.us/index.php?title=Determining_soil_infiltration_rates infiltrometer test] in the location of the infiltration practice for determining infiltration rates.</big>

===16.11===
<big>For design purposes, permittees must divide field measured infiltration rates by 2 as a safety factor or permittees can use soil-boring results with the [https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates infiltration rate chart in the Minnesota Stormwater Manual] to determine design infiltration rates. When soil borings indicate type A soils, permittees should perform field measurements to verify the rate is not above 8.3 inches per hour.  This permit prohibits infiltration if the field measured infiltration rate is above 8.3 inches per hour.</big>

===16.12===
<big>Permittees must employ appropriate on-site testing ensure a [https://stormwater.pca.state.mn.us/index.php?title=Shallow_soils_and_shallow_depth_to_bedrock minimum of three (3) feet of separation] from the seasonally saturated soils (or from bedrock) and the bottom of the proposed infiltration system.</big>

===16.13===
<big>Permittees must design a maintenance access, typically eight (8) feet wide, for the infiltration system.</big>

===16.14===
<big>This permit prohibits permittees from constructing infiltration systems that receive runoff from vehicle fueling and maintenance areas including construction of infiltration systems not required by this permit.</big>

===16.15===
<big>This permit prohibits permittees from constructing infiltration systems where infiltrating stormwater may mobilize high levels of contaminants in soil or groundwater. Permittees must either complete the [https://stormwater.pca.state.mn.us/index.php?title=File:Contamination_screening_checklist_for_stormwater_infiltration_July_2018.xlsx MPCA's contamination screening checklist] or conduct their own assessment to determine the suitability for infiltration. Permittees must retain the checklist or assessment with the SWPPP.

For more information and to access the MPCA's "contamination screening checklist" [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_Infiltration_and_soil/groundwater_contamination:_A_guide_to_the_Construction_Stormwater_Permit_requirements see the Minnesota Stormwater Manual].</big>

===16.16===
<big>This permit prohibits permittees from constructing infiltration systems in areas where [https://stormwater.pca.state.mn.us/index.php?title=Guidance_for_amending_soils_with_rapid_or_high_infiltration_rates 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.</big>

===16.17===
<big>This permit prohibits permittees from constructing infiltration systems in areas with less than [https://stormwater.pca.state.mn.us/index.php?title=Shallow_soils_and_shallow_depth_to_bedrock 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.</big>

===16.18===
<big>This permit prohibits permittees from constructing infiltration systems in areas of [https://stormwater.pca.state.mn.us/index.php?title=Soils_with_low_infiltration_capacity predominately Hydrologic Soil Group type D soils] (clay).</big>

===16.19===
<big>This permit prohibits permittees from constructing infiltration systems within a [https://stormwater.pca.state.mn.us/index.php?title=Guidance_for_using_the_Stormwater_Special_and_impaired_waters_search_tool Drinking Water Supply Management Area] (DWSMA) as defined in Minn. R. 4720.5100, subp. 13, if the system will be located:

:a. in an [https://stormwater.pca.state.mn.us/index.php?title=Guidance_for_using_the_Stormwater_Special_and_impaired_waters_search_tool Emergency Response Area (ERA) within a DWSMA] classified as having high or very high vulnerability as defined by the Minnesota Department of Health; or
:b. in an ERA within a DWSMA classified as moderate vulnerability unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater; or
:c. outside of an ERA within a DWSMA classified as having high or very high vulnerability, unless a regulated MS4 Permittee performed or approved a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater.

See "[https://stormwater.pca.state.mn.us/index.php?title=Guidance_and_recommendations_for_conducting_a_higher_level_of_engineering_review_for_stormwater_infiltration_in_DWSMAs_and_Wellhead_Protection_Areas higher level of engineering review]" in the Minnesota Stormwater Manual for more information.</big>

===16.20===
<big>This permit prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of [[Karst|active karst features]].</big>

===16.21===
<big>This permit prohibits permittees from constructing infiltration systems in areas that receive runoff from the following [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_at_permitted_industrial_stormwater_facilities industrial facilities] not authorized to infiltrate stormwater under the NPDES stormwater permit for industrial activities: automobile salvage yards; scrap recycling and waste recycling facilities; hazardous waste treatment, storage, or disposal facilities; or air transportation facilities that conduct deicing activities.</big>