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+ | [[file:Permit constraints plan view.png|300px|thumb|alt=schematic of infiltration constraints|<font size=3>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.</font size>]] | ||
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+ | [[file:Permit constraints profile view.png|300px|thumb|alt=schematic of infiltration constraints|<font size=3>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.).</font size>]] | ||
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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 [https://stormwater.pca.state.mn.us/index.php?title=III._STORMWATER_DISCHARGE_DESIGN_REQUIREMENTS#III.D._PERMANENT_STORMWATER_MANAGEMENT_SYSTEM Construction Stormwater permit] includes several prohibitions to infiltration of stormwater. | 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 [https://stormwater.pca.state.mn.us/index.php?title=III._STORMWATER_DISCHARGE_DESIGN_REQUIREMENTS#III.D._PERMANENT_STORMWATER_MANAGEMENT_SYSTEM Construction Stormwater permit] includes several prohibitions to infiltration of stormwater. | ||
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*areas within a Drinking Water Supply Management Area (DWSMA) as defined in [https://www.revisor.mn.gov/rules/?id=4720.5100 Minn. R. 4720.5100], subp. 13., unless allowed by a local unit of government with a current MS4 permit; and | *areas within a Drinking Water Supply Management Area (DWSMA) as defined in [https://www.revisor.mn.gov/rules/?id=4720.5100 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. | *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. | ||
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+ | Each of these is discussed in greater detail below. | ||
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+ | ==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 [https://stormwater.pca.state.mn.us/index.php?title=Potential_stormwater_hotspots&action=submit potential stormwater hotspots]. | ||
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+ | ==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 [https://stormwater.pca.state.mn.us/index.php?title=Shallow_soils_and_shallow_depth_to_bedrock shallow soils and depth to bedrock] and [https://stormwater.pca.state.mn.us/index.php?title=Shallow_groundwater shallow groundwater]. | ||
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+ | ==Discharges from industrial facilities where infiltration is prohibited== |
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
Each of these is discussed in greater detail below.
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.
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.