Upper Minnehaha Creek Watershed Nutrient and Bacteria TMDL Study

This Total Maximum Daily Load (TMDL) study addresses nutrient impairments in twenty lakes and an E. coli impairment in Painter Creek within the Minnehaha Creek Watershed District (MCWD), which is located within the Upper Mississippi River Basin. The MCWD covers approximately 178 square miles in Hennepin and Carver Counties, including parts of Minneapolis, Minnesota and its western suburbs. The watershed drains to Minnehaha Creek and ultimately the Mississippi River. The water bodies addressed in this study are located within a distinct hydrologic basin within the MCWD referred to as the "Upper Watershed," which drains through agricultural land and suburbs west of Minneapolis to Lake Minnetonka, which outlets into Minnehaha Creek. The goal of this TMDL is to quantify the pollutant reductions needed to meet State water quality standards for nutrients in the lakes and E. coli standards in Painter Creek. Fifteen of these lakes are defined as deep lakes for which the North Central Hardwood Forest ecoregion numeric water quality standards are a summer average total phosphorus concentration of 40 μg/L, 14 μg/L chlorophyll-a, and greater than 1.4 meter in Secchi depth. The other six lakes are shallow, for which the numeric water quality standards are a summer average total phosphorus concentration of 60 μg/L, 20 μg/L chlorophyll-a, and greater than one meter in Secchi depth. Nutrient budgets were developed for all twenty lakes along with lake response models to set the TMDL and Load and Wasteload Allocations. A robust lake and stream monitoring dataset was available and was the basis of the nutrient budget calculations. Wasteload reductions ranging from no reduction to a 93 percent reduction and load reductions ranging from no reduction to 79 percent reduction will be necessary to meet water quality standards. Flow and bacteria monitoring data recorded in Painter Creek were used to establish a load duration curve meeting the E. coli numeric standard of no more than 126 organisms per 100 mL as a geometric mean of not less than five samples representative of conditions within any calendar month, nor more than 10% of all samples taken during any calendar month individually exceed 1,260 organisms per 100 mL. A TMDL, Wasteload Allocations, and Load Allocations were established for five flow categories: high flow, wet, mid-range, dry, and low flow. No reductions are necessary for high flow, wet, and mid-range flows. A 31 percent reduction will be necessary during dry conditions and a 37 percent reduction under low flow conditions to meet E. coli concentration standards.