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Comprehensive Stormwater Management Plan Northeast Lino Lakes Drainage Improvement Project WSB Project No. 2929-79 Page 20 1. Located outside of a vulnerable Drinking Water Source Management Area (Minnesota Department of Health 2014); 2. Must have a type A or B hydrologic soil group; 3. Depth to water table greater than 24-inches; and 4. Located outside of the National Wetland Inventory wetlands. Only 19.3 percent of the CSMP area meet the majority of these four criteria for infiltration. RCWD Rule C.6 requires that, where feasible, infiltration must be used. However, most the CSMP area will need to use alternative BMPs to treat stormwater runoff. For the purposes of this conceptual analysis, we have assumed NURP stormwater ponds, with a TP removal factor of 0.50 will be used. The summary of the required water quality volumes is presented in Table 6-2. Table 6-2. Assumed Water Quality Treatment Summary (Lino Lakes) Impervious Surface Constructed to Date 38 acres Proposed Impervious Surface 706 acres Area Suitable for Infiltration 218.5 ac Required Volume Reduction [1.1” runoff from impervious] 11.8 ac-ft Required NURP Volume [2.5” runoff from drainage area] 89.4 ac-ft Required Water Quality Volume for Lino Lakes Full Build Out 101.2 ac-ft Estimated TP Reduction through Infiltration and Ponding 60% Proposed TP Loading with Stormwater Treatment 6.62 lbs 6.2.1 Volume Reduction Practices As discussed above, Rule C.6 requires that where feasible, infiltration must be provided for stormwater runoff from new and/or reconstructed impervious surfaces. The proposed impervious surface area is based on the City of Lino Lakes 2030 Comprehensive Plan. Water quality treatment volume required for phosphorus removal is calculated as follows: Required WQ Treatment Volume [ac-ft] = Aimpervious * 1.1 inches / TP Removal Factor / 12 in/ft Infiltration practices are allocated a TP Removal Factor of 1.0. On-site infiltration is not feasible for most of the CSMP are due to poor soils, high groundwater and the presence of wetlands. Using NRCS soils data and the National Wetland Inventory, only 19 percent of the area may be suitable for infiltration practices within Lino Lakes. As development proceeds within the CSMP area, developers will be required to demonstrate that infiltration is not feasible on-site before utilizing the regional NURP basins or other BMP alternatives (Appendix F for infiltration requirements by parcel). 6.2.2 NURP Basins Stormwater ponds used for water quality treatment must use a TP Removal Factor of 0.5, per Rule C.6, and sized per the NURP criteria, which includes a permanent dead pool sized to store the runoff volume from a 2.5-inch rainfall event of the tributary area.