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**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. | ||
*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. | + | **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. |
*Media passes all tests. How to correct if perc failures occur after placement | *Media passes all tests. How to correct if perc failures occur after placement | ||
**It is difficult to provide a specific answer. It is essential to determine the cause of the failure. The manual unfortunately does not currently provide good information on remediating practices that are performing poorly. The [http://stormwaterbook.safl.umn.edu/infiltration-practices/maintenance-infiltration-practices University of Minnesota] has developed a page that provides useful information on this topic. | **It is difficult to provide a specific answer. It is essential to determine the cause of the failure. The manual unfortunately does not currently provide good information on remediating practices that are performing poorly. The [http://stormwaterbook.safl.umn.edu/infiltration-practices/maintenance-infiltration-practices University of Minnesota] has developed a page that provides useful information on this topic. | ||
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**We can't specifically answer this question, but the issue of P leaching was examined in setting P credits for green roofs. A technical document produced by the contractor provides some insight into this issue. [https://stormwater.pca.state.mn.us/index.php?title=File:Green_roof_pollutant_removal.docx See link]. For green roofs, it appears that they leach P for the first 4 to 10 years after construction and then may begin retaining P. | **We can't specifically answer this question, but the issue of P leaching was examined in setting P credits for green roofs. A technical document produced by the contractor provides some insight into this issue. [https://stormwater.pca.state.mn.us/index.php?title=File:Green_roof_pollutant_removal.docx See link]. For green roofs, it appears that they leach P for the first 4 to 10 years after construction and then may begin retaining P. | ||
*Can you talk a little about the use of stormwater-specific geotechnical borings versus interpretation of structural-specific borings (e.g. those for footings and foundations nearby a proposed practice) | *Can you talk a little about the use of stormwater-specific geotechnical borings versus interpretation of structural-specific borings (e.g. those for footings and foundations nearby a proposed practice) | ||
− | **Fundamentally the methods are similar, but for designing infiltration practices, it becomes more important to characterize the entire profile. | + | **Fundamentally the methods are similar, but for designing infiltration practices, it becomes more important to characterize the entire profile. Thus, nearly complete recovery in samples is essential. The focus on borings will be determining the vertical thickness of confining layers and the horizontal extent of these confining layers. This is one reason why in situ infiltration tests are recommended rather than borings. |
*Does the MPCA have a recommendation for when infiltration should not be used due to high blow counts/dense soils? What would be considered a high blow count? | *Does the MPCA have a recommendation for when infiltration should not be used due to high blow counts/dense soils? What would be considered a high blow count? | ||
**We looked for information on this and found a few papers that might be worth exploring. See [http://web.mst.edu/~rogersda/umrcourses/ge441/Subsurface%20Exploration%20Using%20the%20Standard%20Penetration%20Test%20and%20the%20Cone%20Penetrometer%20Test.pdf 1], [https://www.researchgate.net/publication/238180153_Use_of_SPT_Blow_Counts_to_Estimate_Shear_Strength_Properties_of_Soils_Energy_Balance_Approach 2], [http://web.mst.edu/~rogersda/umrcourses/ge441/NOTES%20for%20STANDARD%20PENETRATION%20TEST.pdf 3], [https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwiCqbTDmoPRAhVs6YMKHTjdAq0QFggzMAM&url=http%3A%2F%2Ffiles.engineering.com%2Fdownload.aspx%3Ffolder%3Deacd731d-8a93-4c08-b54c-57463d90e75a%26file%3DCohesive_and_Non_Cohesive_Soil_Parameters_ASCE_1996.pdf&usg=AFQjCNHoxcdfwbwsqOEt-EJix0mNkTzMkg&cad=rja 4]. In general, changes in the number of blow counts with depth provides a qualitative indication of the presence of a confining layer. | **We looked for information on this and found a few papers that might be worth exploring. See [http://web.mst.edu/~rogersda/umrcourses/ge441/Subsurface%20Exploration%20Using%20the%20Standard%20Penetration%20Test%20and%20the%20Cone%20Penetrometer%20Test.pdf 1], [https://www.researchgate.net/publication/238180153_Use_of_SPT_Blow_Counts_to_Estimate_Shear_Strength_Properties_of_Soils_Energy_Balance_Approach 2], [http://web.mst.edu/~rogersda/umrcourses/ge441/NOTES%20for%20STANDARD%20PENETRATION%20TEST.pdf 3], [https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwiCqbTDmoPRAhVs6YMKHTjdAq0QFggzMAM&url=http%3A%2F%2Ffiles.engineering.com%2Fdownload.aspx%3Ffolder%3Deacd731d-8a93-4c08-b54c-57463d90e75a%26file%3DCohesive_and_Non_Cohesive_Soil_Parameters_ASCE_1996.pdf&usg=AFQjCNHoxcdfwbwsqOEt-EJix0mNkTzMkg&cad=rja 4]. In general, changes in the number of blow counts with depth provides a qualitative indication of the presence of a confining layer. |
Below are questions submitted during the December 15, 2016 webinar on Infiltration. Answers include links and if appropriate, links to additional information.