CONTROL OF THE DISTRIBUTION OF GLACIAL LAKE SEDIMENTS BY BURIED TOPOGRAPHY IN EAST-CENTRAL WISCONSIN

Glacial Lake Oshkosh formed in east-central Wisconsin during the Wisconsin Glaciation whenever the southwestern margin of the Green Bay Lobe of the Laurentide Ice Sheet was advancing or retreating. Fine-grained lacustrine sediment deposited in this proglacial lake potentially serves as a regional aquitard because of its low hydraulic conductivity and thickness. In Waupaca County, where this study was located, the demand for groundwater is sufficient that it is important to understand the subsurface distribution of this aquitard. For example, in one location a rotosonic borehole drilled for this study revealed that the aquitard is almost 120 feet thick. To determine the distribution of the aquitard in Waupaca County, we examined more than 2,000 well construction reports and created a GIS database that included depth to bedrock as well as thickness and composition of various glacial strata overlying bedrock. Kriging was used to stratigraphically track the extent and thickness of the lacustrine deposits that form the aquitard in the county. A bedrock elevation map reveals a northeast-southwest-trending trough through the center of the county. Further analysis of the well data revealed that the thickest portion of the aquitard lies within this bedrock trough. This has important implications for recharge of bedrock aquifers beneath this area of thick aquitard.