Vermillion River Watershed TMDL Report

This Total Maximum Daily Load (TMDL) study was completed for the Vermillion River Watershed, which is a subwatershed in the Mississippi River – Lake Pepin 8 digit Hydrologic Unit (HUC) located in the Lower Mississippi River Basin. The study addresses one river turbidity impairment; 12 river/stream bacteria impairments; and nutrient impairments for two lakes, Alimagnet and East Lake. The Vermillion River Watershed covers approximately 364 square miles in Scott, Dakota and Goodhue Counties. The watershed drains to the Vermillion River and ultimately the Mississippi River near Lock and Dam 3 northwest of Red Wing, Minnesota. The water bodies addressed in this study are located in the upper portion of the Vermillion River Watershed, which drains a mixture of agricultural land and developed suburban land approximately 10-15 miles south of Minneapolis and St. Paul, Minnesota. The goal of this TMDL is to quantify the pollutant reductions needed to meet State water quality standards for turbidity, E. coli and nutrients in the impaired streams and lakes in the upper portion of the watershed. Flow, turbidity and total suspended solids (TSS) monitoring data recorded in the Vermillion River class 2A turbidity impaired reach (07040001-517) were used to establish a load duration curve. The curve displays the class 2A TSS numeric standard of 10 mg/L that may not be exceeded more than 10% of the time over a multiyear data window. A TMDL, waste load allocations (WLAs), and load allocations (LAs) were established for five flow categories along the flow duration curve: very high, high, mid, low, and very low flow conditions. A 45% reduction will be necessary during very high flow conditions, and an 8% reduction under high flow conditions to meet the proposed TSS concentration standard. Similar to turbidity, flow and bacteria monitoring data recorded throughout the 12 bacteria impaired reaches were used to establish load duration curves. The curves were set up to meet the E. coli numeric standard of no more than 126 organisms per 100 mL as a geometric mean within any calendar month. Additionally, no more than 10% of all samples taken during any calendar month may exceed 1,260 organisms per 100 mL. The TMDL WLAs and LAs for each bacteria impaired reach were established for the five flow categories described previously.