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Mr. Michael Grochala May 23, 2025 Page 4 The NPDES MS4 permit requires permittees to provide post-construction water quality standards adopted at the local level. The MS4 permit requires permittees to meet the requirements of future TMDLs. Currently there is a TMDL Implementation Plan proposed for discharges to Peltier Lake and Hardwood Creek, they are identified as the Peltier Lake and Centerville Lake Excess Nutrients TMDL and the Hardwood Creek Impaired Biota (Fish) and Dissolved Oxygen TMDL. Depending on the location and proximity to impaired waters, development within the AUAR area may need to complete an anti- degradation analysis for the impaired water. Water Quantity and Quality Analysis A water quantity and quality analysis was completed for the existing and proposed conditions within the study area. This quantitative analysis uses the NRCS runoff curve number method to calculate runoff, and the results are summarized in Table 1 and Table 2. Table 1 summarizes the total runoff volumes for both proposed conditions scenarios, with and without volume reductions, compared to the existing condition. Table 1. Existin and Proposed Annual Runoff Volumes' Existing 2040 Scenario 1 2040 Scenario 1 2040 Scenario 2 2040 Scenario 2 conditions without Volume with Volume without Volume with Volume co Reduction Reduction2 Reduction Reduction2 (AC-FT) (AC-FT) (AC-FT) (AC-FT) (AC-FT) 11,193 11,357 4,026 11,365 3,787 'Annual runoff volumes are based on an average of 32 inches of rainfall for the state of Minnesota. 21.1 inches represents approximately 90%of all rain events in Minnesota(Minnesota Minimal Impact Design Standards, MPCA), therefore volume reduction of 90%was assumed in all land use areas for proposed scenarios, except Permanent Rural, Urban Reserve and Right-of Way, which are assumed to not require any future stormwater management. Table 2 summarizes the total pollutant loads for proposed conditions, with and without volume reductions, compared to the existing condition. Table 2. Total Suspended Solids and Total Phosphorus Annual Loads 2040 Scenario 2040 2040 Scenario 2040 Pollutant' Existing 1 without Scenario 1 2 without Scenario 2 conditions Volume with Volume Volume with Volume Reduction Reduction Reduction Reduction TSS 888 950 298 955 282 tons/ r TP 7,958 7,603 2,377 8,010 2,248 Ibs/ r 'Pollutant loading was determined using Event Mean Concentration values from the MPCA Stormwater Manual, based on Land Use classification for the AUAR area. M:\027919-000\Admin\Docs\Stormwater\MEMO-021725-Stormwater AP.docx Page 78 of 240