Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Rigby J.R., Kress W., Minsley B.

The USDA is conducting a pilot study to investigate the potential for a managed aquifer recharge project to augment recharge to the Mississippi River Valley Alluvial Aquifer (MRVA). The conceptual design of the project proposes to use riverbank filtration to capture water from the Tallahatchie River for direct injection in the aquifer. The USGS Mississippi Alluvial Plain (MAP) project, as part of a larger regional effort, in collaboration with USDA has contributed extensive high-resolution geophysical investigations to the characterization of the hydrogeology of the project area including Airborne Electromagnetic (AEM) surveys, surface Nuclear Magnetic Resonance (NMR), and towed time-domain electromagnetic surveys (tTEM). The Resolve AEM survey provides the best composite image of the aquifer available to date and at a higher resolution than any previous method. The high-resolution airborne survey mapped an area 15 km by 30 km north and west of Greenwood, MS. Flight lines were oriented east-west and separated by 250m. The AEM provides a spatially extensive data set to help interpret the thickness of the MRVA, the elevation of the aquifer top and bottom, and the nature of material lying above and below the aquifer. The AEM survey results for the study area show the bulk of the MRVA between 10m and 45m below land surface. An extensive electrically conductive unit begins to appear broadly at 40-50 m below land surface in the eastern half of the project area though it is cut by a large-scale channel-like feature of electrically resistive material extending to depths of approximately 80 m below land surface. In the western half of the project area moderately resistive material extends deeper than in the eastern half suggesting the appearance of subcropping Tertiary units in contact or nearly in contact with the MRVA. The AEM survey has also provided extensive data on the electrical properties of the top 10 m of sediment below land surface commonly referred to as the confining layer of the MRVA. This layer is shown to be highly heterogeneous, suggesting that areal recharge from precipitation may also be spatially heterogenous and vary substantially. Results from near-surface methods gives a still higher resolution picture of the upper sediment layers but broadly agrees with results of the high-resolution AEM survey. These methods will be further compared with other geophysical methods and core logs to outline the evolving conceptual framework of the study area for the managed aquifer recharge project.