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Geochemistry of the Mississippi River Valley Alluvial Aquifer and preliminary analysis of impacts from the groundwater transfer and injection pilot project in Shellmound, Mississippi
Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: O'Reilly A., Yarbrough L.

The Mississippi embayment aquifer system comprises six regional aquifers. The shallowest aquifer is the Mississippi River Valley alluvial aquifer (MRVAA), which is intensively pumped for agricultural irrigation in the Delta region of Mississippi. With long-term decreases in the water level and continuing groundwater withdrawals, long-term groundwater availability from the MRVAA is a concern. Thus, the U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory is implementing the Groundwater Transfer and Injection Pilot (GTIP) project to determine the effectiveness of using managed aquifer recharge (MAR) to support groundwater availability from the MRVAA in the Delta. The MAR technology being tested by the GTIP project uses source water obtained by riverbank filtration from the Tallahatchie River and transfers and injects the water into the MRVAA where water levels have declined. However, by inducing the infiltration of river water into the aquifer, the quality of the source water and groundwater must be examined. The water quality of the MRVAA in other locations has been previously reported, but these results do not reflect the water quality after transfer and injection. Thus, a new collection of chemical analysis is needed to determine the potential impact of groundwater transfer and injection on water quality. The water quality samples were collected at 17 observation wells, one extraction well, two injection wells, discharge pipe for injection well backflush water, three sites in Lake Henry and the river. For each collection site, values for pH, specific conductivity, temperature, and dissolved oxygen were also measured. One sampling event was conducted prior to operation in March 2021, and monthly sampling events were continued during two operational periods—April to July 2021 and February to August 2022—for a total of 13 sampling events. In 1978, the water quality of the MRVAA was analyzed in the Delta. These results showed that the groundwater had the following characteristics: a hard calcium bicarbonate type, dissolved iron concentration greater than 3 mg/L, elevated concentrations of manganese, and a range of dissolved solids concentrations (153-751 mg/L). Thus far, our sampling results have demonstrated the groundwater to be of calcium bicarbonate type in the MRVAA, with total iron and manganese concentrations exceeding 1.5 and 0.2 mg/L, respectively. Generally, the higher levels of iron in the MRVAA have occurred at observation wells west of the Tallahatchie River and northeast of Lake Henry. The total dissolved solids range from 86-622 mg/L. Variation of this range is presented in the observation wells. However, the lowest total dissolved solids are more prevalently identified at the Tallahatchie River and in Lake Henry at the sampling site where backflush water is discharged. Further work contributes to the understanding of natural and MAR induced evolution of water quality.

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