ASSESSMENT OF MANAGED AQUIFER RECHARGE UTILIZING RIVERBANK FILTRATION AND GROUNDWATER TRANSFER AND INJECTION THROUGH PILOT TESTING AND REGIONAL HYDROGEOLOGIC MODELING

Groundwater depletion of the Mississippi River Valley alluvial aquifer (MRVAA) threatens sustainability of agroecosystems in the intensively cultivated Mississippi Alluvial Plain. The U.S. Department of Agriculture Agricultural Research Service partnered with local stakeholders, U.S. Army Corps of Engineers, and U.S. Geological Survey (USGS) to conduct and assess the performance of the Groundwater Transfer and Injection Pilot (GTIP) project in the Delta region of Mississippi (Delta). The GTIP project uses a riverbank filtration-based managed aquifer recharge (MAR) approach, consisting of extracting groundwater from one well near the Tallahatchie River, transferring water through a 3-km pipeline, and reinjecting the water into the MRVAA via two wells where water levels have substantially declined. The system has a design capacity of 95 L/s and began operation in 2021. Two injection tests were conducted for durations of 89 and 204 days, yielding total injected volumes of 678,000 and 709,000 m³, respectively. At the injection site, groundwater levels increased up to 2 m whereas major ion concentrations decreased 21% on average in the MRVAA injection zone. A 1,050-km² inset groundwater flow model was developed by USGS based on a regional parent model of the entire Delta. The inset model leverages a high-resolution airborne electromagnetic survey to test three alternative layering conceptualizations. The transient (1900–2018) inset model was developed using MODFLOW 6 and Soil Water Balance models. Using the PEST++ Iterative Ensemble Smoother, the three models were calibrated against groundwater levels, streamflows, and stream stage. Results indicate that the most detailed representation of MRVAA layers produced the best calibration. A forecast model indicates that under average irrigation and recharge conditions (2010–15), the GTIP project has the potential to increase groundwater levels as much as 3 m around the injection site by 2050, but sustained increase would require yearly repetition of water transfer rates. Findings of the pilot testing and hydrogeologic modeling demonstrate the GTIP project can increase the amount of water in the MRVAA under the unique hydrogeologic conditions in the Delta. To help assess the possibility of large-scale implementation of MAR throughout the Delta, a third injection test and further forecast modeling would be beneficial.