USING AN INSET GROUNDWATER-FLOW MODEL TO EVALUATE THE EFFECTS OF MANAGED AQUIFER RECHARGE ON GROUNDWATER LEVELS NEAR SHELLMOUND, MISSISSIPPI

The U.S. Geological Survey, in cooperation with the Mississippi Department of Environmental Quality, developed a high-resolution inset groundwater-flow model in the Mississippi Delta as part of a pilot study to evaluate the effect of managed aquifer recharge on groundwater levels in the Mississippi River Valley Aquifer (MRVA). Groundwater withdrawals from the MRVA to irrigate crops have been vital to support agricultural production in the region since the early 1900s, but decades of substantial groundwater level declines have heightened concerns about the long-term sustainability of the aquifer. To understand the effects of groundwater irrigation and managed aquifer recharge on the aquifer, a high-resolution airborne electromagnetic (AEM) survey was completed to better characterize the aquifer extent and heterogeneity of its hydrologic properties. The managed aquifer recharge pilot project included the extraction of groundwater using a high capacity well adjacent to the Tallahatchie River and re-injection into the aquifer one-half mile west of the extraction site. A numerical groundwater-flow model was developed to simulate the effect of the managed aquifer recharge on groundwater levels. The model incorporated the high resolution AEM data and was constructed using MODFLOW 6 as an inset model to a larger semi-regional groundwater-flow model which provided inputs including boundary conditions and aquifer properties. The model used 100-meter cells and uniform vertical thickness layers except at land surface and the aquifer base. The AEM data were processed at multiple vertical resolutions and allowed for the development of various vertical layering configurations of the model. The model was calibrated, using PESTPP-IES, to measured groundwater levels, stream discharge, and stream stage data. Preliminary results simulated by the optimal 17-vertical layer discretization version model showed that continuous transfer of water at a rate of 1500 gallons per minute led to an increase in groundwater levels of up to 4 meters around the injection site with irrigation and areal groundwater recharge similar to the average of 2010-2015 rates.