Hydrogeology and Ground-Water Availability in Southwest McLean and Southeast Tazewell Counties Part 2: Aquifer Modeling and Final Report

Abstract

An assessment of the aquifer characteristics and ground-water availability has been completed for one of the largest sand-and-gravel aquifers in Illinois, the Sankoty-Mahomet Sand aquifer, in the confluence area of the Mahomet and Mackinaw Bedrock Valleys in McLean and Tazewell Counties. Based on data gathered during the field portion of the study, a ground-water flow model was developed that simulates the effects of a hypothetical well field for various locations within the study area. It was used to estimate the quantity of ground water that could be withdrawn from the aquifer within the study area. The model was calibrated to match water levels measured in area wells in 1994 and to match the baseflow gains and losses in the Mackinaw River and Sugar Creek. A hypothetical well field, pumping 15 million gallons a day (mgd), was simulated at four locations, The results varied from a maximum drawdown of 8 feet in the Hopedale scenario to 55 feet in the Armington scenario. If a well field similar to the well fields modeled were installed in the study area, as many as 400 private wells may be impacted. In certain areas near the Mackinaw River, a well field would greatly reduce the ground-water portion of baseflow entering the Mackinaw River. Pumping three of the well fields together, at a total rate of 37.5 mgd, indicated that the aquifer should be able to sustain withdrawals in excess of 37.5 mgd if the pumpage is distributed in the study area