High-resolution airborne electromagnetic, magnetic, and radiometric survey near Shellmound, Mississippi

Abstract

As part of the Mississippi Alluvial Plain Regional Water Availability Study, the U.S. Geological Survey acquired 2,364 line-kilometers (km) of airborne electromagnetic (AEM), magnetic, and radiometric data near Shellmound, Mississippi in March 2018. The Shellmound region hosts a managed aquifer recharge (MAR) pilot project, developed by the Agricultural Research Service of the U.S. Department of Agriculture, investigating the use of bank filtration along the Tallahatchie River as a source for recharge in areas of significant groundwater decline by direct injection into the Mississippi River Valley Alluvial aquifer (MRVA) about 3 km from the extraction gallery. Understanding the structure of the aquifer, including both shallow and deep confining units, is important for the success of this pilot MAR study and will be even more important for potential future large-scale MAR projects and groundwater model development efforts. The purpose of this survey is to contribute high-resolution information a bout subsurface geologic structure to inform groundwater models, water resource infrastructure studies, and local decision making. The existing understanding of the base of the MRVA is based on lithologic and borehole geophysical logs in the survey. Inverted resistivity models derived from the AEM data indicate that resistive (>50 ohm-m) areas are well correlated to log picks associated with the top/bottom of the MRVA. A 12–84 m thick aquifer layer has been interpreted from the AEM data based on resistivity thresholds, which will help further refine the hydrostratigraphy throughout the survey area. The AEM data also indicate a discontinuous, electrically conductive surficial confining layer that is up to 22 m thick and more persistent in the western half of the survey, where backswamp deposits are indicated in the surficial geology map. This conductive layer is not indicated in lithologic logs, possibly because the log interpretive focus was on identifying the depth to the contact between the base of MR VA and the underlying confining unit, but it may be an important barrier to recharge and will be included as input into future groundwater models. The eastern limit of the conductor generally corresponds with the transition to point bar deposits. The trends in the radiometric data also correlate well with the mapped surficial geology and soil survey maps with a generally elevated abundance of potassium near the existing river channels.