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1. When using infiltration for volume reduction, the following requirements <br />must be met: <br /> <br />(1) Infiltration volumes and facility sizes shall be calculated using the <br />appropriate hydrological soil group classification and design <br />infiltration rate from Table 1. Select the design infiltration rate <br />from Table 1 based on the least permeable soil horizon within the <br />first five feet below the bottom elevation of the proposed <br />infiltration BMP. <br /> <br />(2) The applicant may complete double-ring infiltrometer test <br />measurements at the proposed bottom elevation of the infiltration <br />BMP to the requirements of ASTM D3385. The measured <br />infiltration rate shall be divided by the appropriate correction <br />factor selected from the Minnesota Stormwater Manual. This test <br />must be completed by a licensed soil scientist or engineer. <br /> <br /> <br />(3) The infiltration area shall be capable of infiltrating the required <br />volume within 48 hours for surface and subsurface BMPs. <br /> <br />(4) Infiltration areas shall be limited to the horizontal areas subject to <br />prolonged wetting. <br /> <br />(5) Areas of permanent pools tend to lose infiltration capacity over time <br />and shall not be accepted as an infiltration practice. <br /> <br />(6) Stormwater runoff must be pretreated to remove solids before <br />discharging to infiltration areas to maintain the long term viability <br />of the infiltration areas. <br /> <br />(7) Design and placement of infiltration BMPs shall be done in <br />accordance with the Minnesota Department of Health guidance called <br />“Evaluating Proposed Stormwater Infiltration Projects in Vulnerable <br />Wellhead Protection Areas.” (Final version to govern.) <br /> <br />2. Volume reduction practices (e.g., infiltration or other) to retain the water <br />quality volume on-site must be considered first when designing the <br />permanent stormwater treatment system. This permit does not consider wet <br />Table 1. Design Infiltration Rates <br />Soil Group Soil Textures ASTM Unified Soil <br />Class Symbols Rate <br />A Gravel, sand, sandy gravel, or silty <br />gravel, loamy sand, sandy loam <br />GW, GP 1.63 in/hr <br />GM, SW, SP 0.80 in/hr <br />B Loam, Silt loam SM 0.60 in/hr <br />ML, OL 0.30 in/hr <br />C Sandy clay loam GC, SC 0.20 in/hr <br />D Clay loam, silty clay loam, sandy <br />clay, silty clay, or clay CL, CH, OH, MH 0.00 in/hr <br />Source: Minnesota Stormwater Manual, November 2005.