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**We're not sure what this question refers to, but the page [[Understanding and interpreting soils and soil boring reports for infiltration BMPs]] provides information on what information to collect with soil borings. The critical factor to consider in conducting borings when designing an infiltration BMP is to identify any restrictive layer in the soil profile. A SWPPP could utilize information from the manual in describing how BMP design and investigations will be conducted, or simply reference the manual. | **We're not sure what this question refers to, but the page [[Understanding and interpreting soils and soil boring reports for infiltration BMPs]] provides information on what information to collect with soil borings. The critical factor to consider in conducting borings when designing an infiltration BMP is to identify any restrictive layer in the soil profile. A SWPPP could utilize information from the manual in describing how BMP design and investigations will be conducted, or simply reference the manual. | ||
*What is the general thought of infiltration in areas that are in active karst features (less than 50 feet of soil cover) but it is not located in the DWSMA or Wellhead protection area? I have heard the term higher engineering review to determine if infiltration can be done in Karst areas but what does the higher enginnering review actually consist of? | *What is the general thought of infiltration in areas that are in active karst features (less than 50 feet of soil cover) but it is not located in the DWSMA or Wellhead protection area? I have heard the term higher engineering review to determine if infiltration can be done in Karst areas but what does the higher enginnering review actually consist of? | ||
− | **The Construction Stormwater Permit | + | **The 2013 Construction Stormwater Permit prohibited 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 2018 CSW permit no longer allows the option of infiltration under these conditions. 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 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]. | **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]. | ||
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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. This volume is considered to be captured instantaneously by the BMP and must be infiltrated within 48 hours. | + | **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. Note that all water delivered to the BMP must be removed from the BMP within the drawdown time, either through infiltration or through infiltration and outflow through an overflow pipe. |
*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. | ||
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**Not sure what this question is asking. Infiltration basins, trenches, dry wells, and underground practices are [https://stormwater.pca.state.mn.us/index.php?title=Infiltration discussed together in the manual]. When there are differences between the practices they are pointed out. Examples include [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_infiltration#Information_tables information tables]. | **Not sure what this question is asking. Infiltration basins, trenches, dry wells, and underground practices are [https://stormwater.pca.state.mn.us/index.php?title=Infiltration discussed together in the manual]. When there are differences between the practices they are pointed out. Examples include [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_infiltration#Information_tables information tables]. | ||
*Many sites we work in have saturated soils within 3' of the existing ground and require mound septic systems. However, the soil borings show a C soil. How are we suppose to provide filtration when this in the case? | *Many sites we work in have saturated soils within 3' of the existing ground and require mound septic systems. However, the soil borings show a C soil. How are we suppose to provide filtration when this in the case? | ||
− | **Infiltration is prohibited in the Construction Stormwater permit | + | **Infiltration is prohibited in the Construction Stormwater permit if the seasonal high water table is within 3 feet of the bottom of the practice. See [https://stormwater.pca.state.mn.us/index.php?title=Glossary#S the definition of saturated soil]. An underdrain system is recommended in these situations. The permit requires filtration systems with less than three feet of separation to include in impermeable liner ([https://stormwater.pca.state.mn.us/index.php?title=III._STORMWATER_DISCHARGE_DESIGN_REQUIREMENTS#III.D._PERMANENT_STORMWATER_MANAGEMENT_SYSTEM Part III.D.1.h]). |
*Can you point out and discuss the underground systems/ | *Can you point out and discuss the underground systems/ | ||
**Infiltration basins, trenches, dry wells, and underground practices are [https://stormwater.pca.state.mn.us/index.php?title=Infiltration discussed together in the manual]. When there are differences between the practices they are pointed out. Examples include [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_infiltration#Information_tables information tables]. | **Infiltration basins, trenches, dry wells, and underground practices are [https://stormwater.pca.state.mn.us/index.php?title=Infiltration discussed together in the manual]. When there are differences between the practices they are pointed out. Examples include [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_infiltration#Information_tables information tables]. | ||
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*We are in sandy soils with some basins infiltrating at 10+ inches/hr. Is there a reccomended maximum infiltration rate for pollutant removal? | *We are in sandy soils with some basins infiltrating at 10+ inches/hr. Is there a reccomended maximum infiltration rate for pollutant removal? | ||
**The Construction Stormwater permit prohibits infiltration in 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. The reason for this prohibition is the likelihood that pollutants will not be attenuated by the practice. We have looked for information in the literature to support this and for guidance on how to modify soils that have excessive infiltration rates, but we have been unable to find good information on the topic. Amending with compost and/or clay is a likely mechanism for reducing infiltration rates, but we cannot yet quantify these effects. | **The Construction Stormwater permit prohibits infiltration in 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. The reason for this prohibition is the likelihood that pollutants will not be attenuated by the practice. We have looked for information in the literature to support this and for guidance on how to modify soils that have excessive infiltration rates, but we have been unable to find good information on the topic. Amending with compost and/or clay is a likely mechanism for reducing infiltration rates, but we cannot yet quantify these effects. | ||
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+ | [[Category:Level 3 - General information, reference, tables, images, and archives/Reference/Training, webinars, and workshops]] |
Below are questions submitted during the December 15, 2016 webinar on Infiltration. Answers include links and if appropriate, links to additional information.
This page was last edited on 6 December 2022, at 14:28.