A COMPARISON OF GEOLOGIC INFLUENCE ON PHOSPHORUS LOADING THROUGH LACUSTRINE GROUNDWATER DISCHARGE IN THREE LAKES IN WESTERN WISCONSIN

Lake eutrophication in Wisconsin and throughout the upper Midwest has resulted in severe environmental and economic impacts across the region. Most lake eutrophication events are attributed to excess phosphorus loading. Historically is has been assumed that phosphorus is primarily contributed from agricultural sources and migrates through sediment runoff. Recent investigations have demonstrated that phosphorus is elevated in both surface water and groundwater in western Wisconsin, and that bedrock geology may be a significant contributor of phosphorus to the hydrologic system. Furthermore, a case study in Barron County demonstrated that groundwater with P contamination resurfaced into surface water through lacustrine groundwater discharge. In response to these findings, case studies were expanded to include lakes positioned in three different geologic settings common to western Wisconsin- thick sand and gravel overlying Cambrian Mt. Simon sandstone (Barron County), shallow Cambrian Mt. Simon sandstone (Eau Claire County), and thick glacial deposits overlying the Ordovician Prairie du Chien Group (St. Croix County). This study provides a preliminary understanding of how geologic setting may influence groundwater P loading in the investigated lake systems.