ASSESSING POTENTIAL IMPACTS OF INCREASED GROUNDWATER WITHDRAWALS ON SURFACE WATER RESOURCES OF A WATERSHED IN SE WISCONSIN

Increased water demands from an expanding suburban population, coupled with concerns of water quality in the deep bedrock aquifer currently tapped by municipal wells, has led to proposals for new high capacity wells in the shallow sand and gravel aquifers of the Mukwonago River watershed, southeastern WI. Local residents and conservation groups however, have questioned the potential impact of these new wells on springs, wetlands, streams and lakes in a watershed identified by the Wisconsin Department of Natural Resources as an "outstanding water resource". Assessing the impact of pumping on these aquatic ecosystems requires a groundwater flow model that captures the essential interactions among subsurface hydrostratigraphic units and surface discharge features. An existing, telescoped model of the watershed derived from a regional scale flow model simulates relatively uniform groundwater discharge along all stream reaches. However, field measurements of stream flow indicate more focused discharge along the edge of a buried bedrock valley. Preferential flow zones in the shallow bedrock provide a mechanism by which discharge would be concentrated in this manner. Samples for Sr isotope analysis have been collected from three deep wells, twelve shallow wells, seven springs and numerous locations along the river. Results of these analyses are being used to test hypotheses related to preferential flow zones and to link surface water features to the aquifers through which water has flowed prior to discharge. The improved conceptual model of the system derived on the basis of flow measurements and Sr isotope sampling is being incorporated into a revised numerical model that can be used to address management questions.