AN ASSESSMENT OF NUTRIENT LOADING THROUGH LACUSTRINE GROUNDWATER DISCHARGE IN MUD LAKE, WISCONSIN

Phosphorus-laden runoff from farm fields, barnyards, suburban lawns, urban areas and wastewater treatment plant discharge has been associated with contamination of surface water throughout the upper Midwest. This nutrient loading into the lacustrine system results in eutrophication events have been implicated in significant degradation of surface water quality across the region. Although phosphorus has been thought to be immobile in groundwater systems, a recent investigation throughout western Wisconsin has shown elevated levels in regional aquifers, and suggests that phosphorus may be accumulating and actively transporting throughout the groundwater system.

This study seeks to quantify phosphorus loading through lacustrine groundwater discharge in Mud Lake in Chetek and Cameron, Wisconsin. Mud Lake is one of five interconnected lakes, all of which experience intense seasonal eutrophication events. The geology is characterized as Cambrian sandstone (some exposed) overlaid by sand and gravel glacial deposits. Mud Lake covers 358.5 acres and has an average depth of 5.2 feet. Lake bottom sediments are composed mainly sand with some gravel (K= 6.7x10^-2 cm/s) with a few sites exhibiting a heightened presence of silt (K= 5.0x10^-5 cm/s). Ten (10) mini-piezometers (5 ft length, 1/2” ID, 4” screen) were installed around the perimeter of the lake. Surface water, groundwater, and soil were sampled every 2 to 3 weeks to measure standard water quality, nitrate, and dissolved and sorbed metal concentrations. Measurements of vertical hydraulic gradient, hydraulic conductivity, and water quality are used to estimate nutrient flux into Mud Lake through lacustrine groundwater discharge.