An Assessment of Nutrient Loading through Lacustrine Groundwater Discharge in Mud Lake, Wisconsin

Abstract

Mud Lake experiences severe lake eutrophication throughout the summer months. Although anthropogenic nutrient sources have been identified as a contributing factor, an ongoing investigation at UW-Eau Claire has demonstrated that the regional bedrock geology may also be a source of phosphorus (P) to the hydrologic system. Furthermore, despite historic assumptions that P is immobile in groundwater systems, the study provides evidence that P is highly mobile in the subsurface. The objective of this study is to understand the mobility of P in groundwater and its impact on Mud Lake. This entails the collection of groundwater, surface water, and sediment samples at nine different sites around the perimeter of Mud Lake. At each location, hydraulic head measurements were collected inside and outside of each monitoring well to determine vertical gradients. Measured water quality parameters include temperature, pH, nitrate, specific conductivity, oxidation-reduction potential, and dissolved oxygen. After field collection, water samples underwent filtration with .45-micron filters and were preserved with nitric acid. A .20-micron filtering process was also conducted on groundwater samples to test for colloidal (0.20-0.45 micron) P transport. Preliminary results report an average P flux of 43.6 kg/day with groundwater P concentrations up to 790 ppb.