AN EVALUATION OF PHOSPHORUS LOADING THROUGH LACUSTRINE GROUNDWATER DISCHARGE ON A STRATIFIED DRAINAGE LAKE IN WESTERN WISCONSIN

Phosphorus loading throughout the Midwest United States has led to widespread lake eutrophication, with estimated $4.2 billion in economic impact in the United States. Despite historic assumptions that P is dominantly contributed by anthropogenic sources and generally immobile in groundwater systems, this study demonstrates that P is highly mobile in the subsurface and that the regional geology may be a notable source. This case study focuses on Mud Lake in Barron County, WI, a stratified drainage lake positioned in glacial sand and gravel deposits overlying Cambrian sandstone, with surrounding agriculture land use practices.

Analyses included groundwater quality, surface water quality, measures of groundwater discharge, and sediment geochemistry, with samples collected at eight to ten different sites around the perimeter of Mud Lake. On site measurements of water quality using an Aqua TROLL 600 Multiparameter Sonde include temperature, pH, specific conductivity, oxidation-reduction potential, dissolved oxygen, and nitrate. Laboratory measurements include dissolved phosphorus, iron, and manganese. Initial data from 2018 document groundwater P concentrations up to 790 ppb, and estimate groundwater contributions of 7,000 lbs (3,175 kg) per year, which may contribute to internal loading from sediments as well as flow directly into the lake remaining in dissolved form.