FY24 CWF Alimagnet Lake Alum Treatment Project

The Vermillion River Watershed, in partnership with the Cities of Apple Valley and Burnsville, will continue to implement successful projects that result in improved water quality in Alimagnet Lake and systematically reduce the phosphorus load contributing to the lake's nutrient impairment. The VRWJPO and partners will perform an alum treatment in Alimagnet Lake to reduce the amount of internal phosphorus load within the lake. Alimagnet Lake is a nutrient (phosphorus) impaired water in Apple Valley and Burnsville with significant public use that receives stormwater runoff from a 985-acre urban subwatershed. The TMDL for Alimagnet Lake was completed in 2015 and identified both external and internal phosphorus load reductions needed to meet water quality standards. Once the TMDL and WRAPS were approved, the VRWJPO and Cities completed a subwatershed assessment (SWA) that identified targeted, prioritized, and measurable practices to reduce external load and have systematically been implementing projects identified in the SWA. The partners have reduced external phosphorus load by 64.4 lbs, which is more than the needed external phosphorus load reduction identified in the TMDL. With the external load addressed, the focus is now shifting to the internal phosphorus load. Per BWSR CWF Grant Policy requirements for internal phosphorus load projects, the VRWJPO and Cities completed an alum treatment feasibility study for the lake. The result of the study provides significant information about the lake alum treatment including treatment planning elements, dosing and timing recommendations for the treatment, a cost estimate, and an estimated internal phosphorus load reduction of 115 lbs, which equates to 11.5 lbs/year assuming a 10-year lifespan. The alum treatment's estimated pollutant reduction would bring the in-lake concentrations of total phosphorus to levels below the water quality standards, so the lake could be de-listed.

Alimagnet Lake is one of five lakes identified as a recreational lake in the Vermillion River Watershed Management Plan. The lake is a high priority for restoration for the VRWJPO and partner cities. The lake has a very engaged lake association that also wants improved water quality and have been active in addressing lake water quality issues. The lake is surrounded primarily by homes, but the cities share a large park on the lake's southern shore, which gets significant use from pet owners, cross-country skiers, walkers, snowshoers, skijorers, canoeists, kayakers, and cyclists. Publicly-accessible greenspace directly adjacent to a recreational lake is a rarity in the Twin-Cities Metropolitan Area, so restoration of the lake is a high priority for public use.