Thompson Lake Water Quality Improvement and WRAPS Implementation

Thompson Lake is a shallow lake located in the City of West St. Paul, within the Lower Mississippi River Watershed Management Organization (LMRWMO). The 7-acre lake receives runoff from a 175-acre urban watershed that was developed in the 1950's and currently provides very little stormwater treatment. The eastern shoreline is within Thompson County Park, which features a fishing pier, event center, and miles of regionally connected trails. The west shoreline of the lake abuts the St. Croix Lutheran School. Thompson Lake is impaired for recreational use due to excessive nutrients, and is on the MPCA's 2014 Impaired Waters list. A watershed restoration and protection strategies (WRAPS) study and total maximum daily load (TMDL) developed from 2012 to 2014 identified watershed runoff as the primary source of phosphorus to the lake; the TMDL identified a phosphorus waste load reduction of 30% necessary to achieve MPCA water quality standards. The proposed project will treat stormwater runoff prior to discharge into Thompson Lake, thereby reducing phosphorus concentrations in the lake, improving lake clarity, and leading to removal from the Impaired Waters list. The project includes construction of a series of upland stormwater basins at the north end of the lake, inside Thompson County Park, to treat runoff from 83% of the watershed (145 acres). The project will include sediment forebays at storm sewer outfalls, a stormwater wetland/pond, and a stormwater reuse irrigation system that draws water from the wetland and irrigates nearby parkland. Water quality modeling indicates that the proposed project will reduce phosphorus loading to the lake by 39%, achieving the waste load reduction identified in the TMDL. The project will provide additional public benefits including native habitat enhancement, education opportunities, trail improvements, and improved aesthetics. The project will be completed in partnership with the City of West St. Paul and Dakota County.