WATER AVAILABILITY IN THE LAKE MICHIGAN BASIN

The USGS has constructed a regional ground-water flow model of the Lake Michigan Basin and surrounding areas as part of the National Water Availability and Use— Great Lakes Basin Pilot. The transient two-million-node model incorporates multiple aquifers and pumping centers with cones of depression that extend into deep saline waters. The model simulates the exchange between a dense surface-water network and heterogeneous glacial deposits overlying stratified bedrock of the Wisconsin Arch and Michigan Basin. These elements pose challenges for the model setup, the handling of variable density conditions, and calibration strategy. The final model provides a multi-state platform for quantifying the regional sources and sinks of ground water (including recharge, pumping, and groundwater flow to water bodies — all elements of the ground-water budget that change with time) and for mapping the direction and magnitude of flows in a series of aquifers (including the locations of ground-water divides at different depths on both sides of Lake Michigan and their movement in response to pumping). The necessarily coarse resolution of the regional model imposes limitations on its applicability, but new techniques (LGR, GFLOW/MODFLOW coupling) enhance our ability to embed fine-scale models inside it and, in this way, to address local water-availability issues while leveraging the insight gained from the regional model.