m |
m |
||
Line 37: | Line 37: | ||
**Yes. Vegetation has multiple benefits, including protecting the practice, providing additional retention of water and pollutants, and providing habitat and aesthetic value. | **Yes. Vegetation has multiple benefits, including protecting the practice, providing additional retention of water and pollutants, and providing habitat and aesthetic value. | ||
*Can you talk more about sub grade infiltration practices? | *Can you talk more about sub grade infiltration practices? | ||
− | + | **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 is 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. | |
− | 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]. | |
− | Can you compare annual maintenance costs of infiltration basins vs. subgrade infiltration? | + | *Can you compare annual maintenance costs of infiltration basins vs. subgrade infiltration? |
− | + | **The manual does not provide specific information on cost for different infiltration practices. [https://www.pca.state.mn.us/sites/default/files/p-gen3-13x.pdf this paper] may provide some useful information. Also see [https://www.lrrb.org/PDF/200523.pdf 1], [https://www3.epa.gov/npdes/pubs/usw_d.pdf 2 (Table 6-10)], and [https://www.novaregion.org/DocumentCenter/Home/View/1675 3]. | |
− | Design: Please clarify the bottom of the infiltration practice. Is it the floor of the basin or the bottom of the planting medium, ripped area, and abutment to the in situ soil. | + | *Design: Please clarify the bottom of the infiltration practice. Is it the floor of the basin or the bottom of the planting medium, ripped area, and abutment to the in situ soil. |
− | + | **If engineered media are utilized, the bottom of the practice is the base of the engineered media. If native soils are used, the base would be the depth to which excavation occurred. [https://stormwater.pca.state.mn.us/index.php?title=File:Illustration_of_depth_to_bedrock_or_wt.jpg This diagram] may be useful and includes an example where soils were ripped to alleviate compaction. | |
*Are you posting this presentation on line for us to review? | *Are you posting this presentation on line for us to review? | ||
**Yes. See [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_Manual_webinars link] | **Yes. See [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_Manual_webinars link] | ||
− | 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. |
Below are questions submitted during the December 15, 2016 webinar on Infiltration. Answers include links and if appropriate, links to additional information.