INVERSE THREEDIMENSIONAL GROUNDWATER MODELING USING THE FINITEDIFFERENCE METHOD FOR RECHARGE ESTIMATION IN A GLACIAL TILL AQUITARD

Knowledge of groundwater recharge rates is essential for developing sustainable groundwater resourcesmanagement schemes and for assessing the susceptibility of the groundwater system to contamination by leachable nutrientsand toxic compounds such as nitrates and pesticides. This study was carried out to develop a method for estimatinggroundwater recharge in a glacial till aquitard using inverse groundwater modeling based on the USGS modularfinitedifference groundwater model. The threedimensional model incorporated the effects of the various hydrogeologicproperties, such as hydraulic conductivity, specific yield, storage coefficient, and porosity, and hydrologic processesinfluencing recharge such as evapotranspiration and subsurface drainage. The model also accounted for the spatialvariability of hydraulic conductivity in the oxidized and unoxidized layers based on geostatistical analysis. The groundwatermodel was calibrated and validated using years with adequate groundwater data. Inverse modeling was consequentlyperformed using the calibrated model and simulation results yielded generally fair agreement between observed andcalculated head distribution. Simulation results indicated that the annual net groundwater recharge for the fiveyearsimulation period considered ranged from 18.7 mm/yr to 33.2 mm/yr, constituting approximately 2.3% to 4.3% of the annualprecipitation in the area. The recharge estimates are within the typical range of recharge for the humid Midwest. Thefinitedifference model could serve as an alternative method for estimating groundwater recharge in a glacial till aquitard.