Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: O'Reilly A., Wren D., Locke M., Rossell W., Mirecki J.

To assess the potential for managed aquifer recharge to mitigate declining groundwater levels in the Mississippi River Valley alluvial aquifer (MRVAA) and support irrigated agriculture in the Delta, the U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, in partnership with local stakeholders and U.S. Army Corps of Engineers, is conducting the Groundwater Transfer and Injection Pilot (GTIP) project. The system consists of one extraction well, a 1.8-mile pipeline, and two injection wells, with a design capacity of 1,500 gpm. Groundwater is filtered by passing through sands adjacent to the Tallahatchie River and subsequently is extracted, transferred, and then injected into a depleted section the MRVAA. Data collection at 17 observation wells and the river includes continuous groundwater level and monthly water quality samples. Prior to operation, observations indicate ambient groundwater was less mineralized at the extraction site than the injection site but both exhibited suboxic iron-reducing redox conditions, and groundwater levels responded to seasonal weather cycles, irrigation withdrawals, and river stage. Operation began in 2021, and two injection tests have been conducted for durations of 89 and 204 days, yielding total injected volumes of 550 and 575 ac-ft, respectively. During the first test the injection rate averaged 730 gpm/well with both wells running simultaneously, causing 6.7-ft increase in groundwater level at the injection site and 4.7-ft decrease at the extraction site. During the second test, both wells were run simultaneously for the first 13 weeks at a rate averaging 570 gpm/well, but due to declining capacity of the extraction well, injection wells were thereafter operated individually on a weekly schedule at 600 gpm. By the end of the second test, impacts were observed 1 mile from the injection wells where the groundwater level increased 0.3 ft. During both tests, differences in water chemistry between the river and extraction well indicate increased mineralization due to rock-water interactions and biogeochemical processes during riverbank filtration, whereas relatively small changes in water chemistry occurred in the MRVAA at the injection site. Clogging of the injection wells by iron bacteria terminated the first test; treatment with a dilute oxalic acid solution returned both wells to their prior capacities. No effects of clogging were observed during the second test. However, the extraction well experienced increasing drawdown over time, which was likely related to low river stage and possibly further impaired by formation of an adjacent sinkhole. Findings from the GTIP project demonstrate its technical feasibility in the Delta and the importance of considering the unique hydrogeology of the region, maintaining similar water chemistry between the injected water and ambient groundwater, and coordinating operating schedule with river stage and riverbank filtration capacity.