Inverse three-dimensional groundwater modeling using the finite-difference method for recharge estimation in a glacial till aquitard

Knowledge of groundwater recharge rates is essential for developing sustainable groundwater resources management schemes and for assessing the susceptibility of the groundwater system to contamination by leachable nutrients and toxic compounds such as nitrates and pesticides. This study was carried out to develop a method for estimating groundwater recharge in a glacial till aquitard using inverse groundwater modeling based on the USGS modular finite–difference groundwater model. The three–dimensional model incorporated the effects of the various hydrogeologic properties, such as hydraulic conductivity, specific yield, storage coefficient, and porosity, and hydrologic processes influencing recharge such as evapotranspiration and subsurface drainage. The model also accounted for the spatial variability of hydraulic conductivity in the oxidized and unoxidized layers based on geostatistical analysis. The groundwater model was calibrated and validated using years with adequate groundwater data. Inverse modeling was consequently performed using the calibrated model and simulation results yielded generally fair agreement between observed and calculated head distribution. Simulation results indicated that the annual net groundwater recharge for the five–year simulation period considered ranged from 18.7 mm/yr to 33.2 mm/yr, constituting approximately 2.3% to 4.3% of the annual precipitation in the area. The recharge estimates are within the typical range of recharge for the humid Midwest. The finite–difference model could serve as an alternative method for estimating groundwater recharge in a glacial till aquitard.