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**The Construction Stormwater Permit prohibits infiltration of stormwater runoff “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”. The question implies that active karst has been identified at a site. Investigation methods in karst areas are discussed [https://stormwater.pca.state.mn.us/index.php?title=Karst#How_to_investigate_for_karst_on_a_site here]. Higher engineering review is not defined in the permit, nor is it clear what the intent of the higher review is. Presumably a higher engineering review would indicate that there is sufficient confining material between the bottom of the practice and the nearest underlying aquifer. This would be based on the presence of some confining layer of sufficient thickness to attenuate pollutants should the practice fail. This type of analysis should be conducted by a licensed geoscientist or engineer. Additional information may be found at [https://stormwater.pca.state.mn.us/index.php?title=Karst#General_stormwater_management_guidelines_for_karst_areas General stormwater management guidelines for karst areas] and at the [http://www.dnr.state.mn.us/waters/groundwater_section/mapping/sensitivity.html MNDNR's webpage on groundwater sensitivity]. | **The Construction Stormwater Permit prohibits infiltration of stormwater runoff “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”. The question implies that active karst has been identified at a site. Investigation methods in karst areas are discussed [https://stormwater.pca.state.mn.us/index.php?title=Karst#How_to_investigate_for_karst_on_a_site here]. Higher engineering review is not defined in the permit, nor is it clear what the intent of the higher review is. Presumably a higher engineering review would indicate that there is sufficient confining material between the bottom of the practice and the nearest underlying aquifer. This would be based on the presence of some confining layer of sufficient thickness to attenuate pollutants should the practice fail. This type of analysis should be conducted by a licensed geoscientist or engineer. Additional information may be found at [https://stormwater.pca.state.mn.us/index.php?title=Karst#General_stormwater_management_guidelines_for_karst_areas General stormwater management guidelines for karst areas] and at the [http://www.dnr.state.mn.us/waters/groundwater_section/mapping/sensitivity.html MNDNR's webpage on groundwater sensitivity]. | ||
*Does the manual address infiltration in D soils - as required by several watershed districts? | *Does the manual address infiltration in D soils - as required by several watershed districts? | ||
− | **The Construction Stormwater permit prohibits infiltration in areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit. Meeting the 1 inch volume requirement and the 48 hour drawdown requirement on D soils would require a very large infiltration practice. The manual | + | **The Construction Stormwater permit prohibits infiltration in areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit. Meeting the 1 inch volume requirement and the 48 hour drawdown requirement on D soils would require a very large infiltration practice. The manual provides [https://stormwater.pca.state.mn.us/index.php?title=Soils_with_low_infiltration_capacity#What_are_general_stormwater_management_guidelines_for_sites_with_low_infiltration_capacity_soils.3F management guidelines for sites with D soils]. |
*How much avg square ft cost for install and maintenance of this example system? | *How much avg square ft cost for install and maintenance of this example system? | ||
**[https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_BMPs_%E2%80%93_selection_considerations This table] provides general cost information for different infiltration systems. [https://stormwater.pca.state.mn.us/index.php?title=Cost-benefit_considerations_for_infiltration This page] was created in the original manual and provides some basic information, although the information is likely dated. For an in-depth discussion of costs for various stormwater management practices, see [http://watertech.rutgers.edu/Publications/Member-Submitted%28Peer-Reviewed%29/John_Gulliver/Weiss-Gulliver-Erickson-2007.pdf Weiss et al (2007)]. | **[https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_BMPs_%E2%80%93_selection_considerations This table] provides general cost information for different infiltration systems. [https://stormwater.pca.state.mn.us/index.php?title=Cost-benefit_considerations_for_infiltration This page] was created in the original manual and provides some basic information, although the information is likely dated. For an in-depth discussion of costs for various stormwater management practices, see [http://watertech.rutgers.edu/Publications/Member-Submitted%28Peer-Reviewed%29/John_Gulliver/Weiss-Gulliver-Erickson-2007.pdf Weiss et al (2007)]. | ||
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**The manual does not discuss infiltration in areas of high road salt application. areas of salt storage are discussed and may be considered [https://stormwater.pca.state.mn.us/index.php?title=Potential_stormwater_hotspots stormwater hotspots]. This is a topic that deserves some attention and it may be appropriate to designate transportation areas with high road salt application as stormwater hotspots. Specific guidance needs to be developed, however. | **The manual does not discuss infiltration in areas of high road salt application. areas of salt storage are discussed and may be considered [https://stormwater.pca.state.mn.us/index.php?title=Potential_stormwater_hotspots stormwater hotspots]. This is a topic that deserves some attention and it may be appropriate to designate transportation areas with high road salt application as stormwater hotspots. Specific guidance needs to be developed, however. | ||
*What storm event needs to infiltrate within 48 hours when including the bounce in the drawdown depth? | *What storm event needs to infiltrate within 48 hours when including the bounce in the drawdown depth? | ||
− | **The Construction Stormwater permit requires that Permittee(s) must design the project so that the water quality volume of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site (i.e. infiltration or other volume reduction practices) and not discharged to a surface water. | + | **The Construction Stormwater permit requires that Permittee(s) must design the project so that the water quality volume of one (1) inch of runoff from the new impervious surfaces created by the project is retained on site (i.e. infiltration or other volume reduction practices) and not discharged to a surface water. This volume is considered to be captured instantaneously by the BMP and must be infiltrated within 48 hours. |
*Was mentioned that mndot grade 2 compost was not recommended for filtration practices due to nutrient leaching. Are there alternative recommendations other than peat moss? | *Was mentioned that mndot grade 2 compost was not recommended for filtration practices due to nutrient leaching. Are there alternative recommendations other than peat moss? | ||
**The manual does not recommend that specific composts be used or not used. To reduce the likelihood of exporting phosphorus from BMPs that have an underdrain, the P content in the media must be less than 30 mg/kg per Mehlich III (or equivalent) test. this would equate to a media with about 5 percent organic matter with a P concentration of about 0.06 percent in the organic matter. We agree that the P content of different sources of organic matter will likely vary and that over time, less P is likely to leach from the media. We are hoping to eventually have more specific information about media and different sources of organic matter. | **The manual does not recommend that specific composts be used or not used. To reduce the likelihood of exporting phosphorus from BMPs that have an underdrain, the P content in the media must be less than 30 mg/kg per Mehlich III (or equivalent) test. this would equate to a media with about 5 percent organic matter with a P concentration of about 0.06 percent in the organic matter. We agree that the P content of different sources of organic matter will likely vary and that over time, less P is likely to leach from the media. We are hoping to eventually have more specific information about media and different sources of organic matter. |
Below are questions submitted during the December 15, 2016 webinar on Infiltration. Answers include links and if appropriate, links to additional information.