GROUNDWATER AVAILABILITY IN THE LAKE MICHIGAN BASIN

Groundwater availability in the Lake Michigan Basin was studied as part of the Great Lakes Basin Pilot for a national assessment of water availability and use. A groundwater-flow model was used to quantify groundwater availability by simulating the response of the system to changes in anthropogenic and environmental stresses. Groundwater availability was quantified by determining the status and trends of groundwater fluxes and groundwater in storage in the system. The study area for the groundwater-flow model extends over 83,000 square miles and includes land below and around Lake Michigan. The model extends vertically from land surface to Precambrian bedrock in twenty model layers of variable thickness and includes heterogeneous glacial deposits overlying bedrock units dipping from the Wisconsin Arch into the Michigan Basin. Because of salinity in the Michigan Basin, the USGS density-dependent flow model SEAWAT was used for the simulations. This transient model has over two million finite difference cells and simulates the response of the system to multiple pumping centers that stress several different aquifers in the basin. Regional recharge was estimated through time from 1864-2005 using a soil-water balance approach and applied to the model in 12 stress periods. Calibration of the model was challenging, and the model provided opportunities to test techniques to calibrate large regional models with hundreds of parameters and thousands of observations. The model was used to show the source of water to wells and changes in the groundwater flow system from 1864-2005. Direct discharge of groundwater to Lake Michigan was studied and found to be approximately two percent of the total groundwater discharge to surface water; the remaining 98 percent is groundwater discharged to streams and other inland surface-water bodies. Because the regional model is too coarse to examine local groundwater/surface-water interaction, a local-scale inset model was developed within the regional model to demonstrate how it may be used in the context of stream-aquifer interaction.