Hydrogeologic Characteristics and Groundwater Sustainability of a Deep Bedrock Aquifer System in Illinois

Population growth and climate change has made groundwater an increasingly important freshwater resource. This study uses MODFLOW to estimate sustainable yield of a deep St. Peter Aquifer in Bloomington, IL. The city of Bloomington has installed two high capacity wells into the St. Peter Sandstone to meet its growing water demands. The St. Peter Aquifer is confined, receiving almost no modern recharge and is experiencing overexploitation in parts of Northern Illinois. I hypothesize that existing fast depletion of the deep St. Peter occurs due to lower-than-expected aquifer parameters of the aquifer. Further, current pumping of groundwater from the St. Peter, plus the new wells could compromise long-term sustainability. The objective of this study is to characterize the aquifer and assess long-term sustainability of this aquifer with projected increases in demand. This study modeled changes in the water level of the St. Peter Aquifer to estimate sustainable yield using MODFLOW. The regional model found that the Sandwich fault Zone exacerbated the decrease in water level in wells near the fault zone. Lack of complexity in the regional model caused it to underperform compared to the Illinois Groundwater Flow Model (IGWFM) developed by the Illinois State Water Survey. Interpolation of the regional model onto the local model showed pumping had no affect on distant well, so pumping from areas far from the Bloomington has no affect on the wells. Since distant pumping has no effect on distant wells, the local model was set up and calibrated independently of the regional model. Grid refinement was applied to the local model to isolate the effects of pumping wells, making for easier calibration. Once the model was calibrated, an uncertainty analysis showed that hydraulic conductivity was the most sensitive parameter. A prediction model based off of project water demand for the city of Bloomington showed that the well’s sustainable yield is approximately 19 years. This means that Bloomington could pump from the St. Peter Aquifer using these wells before going dry