Enhanced Characterization of the Mississippi River Valley Alluvial Aquifer Using Surface-Geophysical Methods - a Pilot Study near Money, Mississippi

Author(s): Adams, R.; Kress, W.; Minsley, B.; Kass, M.

The Mississippi River Valley alluvial (MRVA) aquifer is a complex and poorly understood near-surface aquifer system used to supply irrigation for agriculture across the alluvial plain of the Lower Mississippi River basin. The thickness and extent of the aquifer units are typically determined by evaluating geophysical and driller logs from test holes at spatially discrete points. Surface-geophysical data, along with borehole-geophysical and lithologic data from test holes, can be used to provide high-resolution three-dimensional characterization of the aquifer system. In 2016, the U.S. Geological Survey (USGS) conducted a pilot study to demonstrate the use of surface-geophysical methods for delineation of near-surface geologic features, characterization of alluvial aquifer properties, and evaluation of surface water/groundwater exchange in the MRVA. The area chosen for this pilot was a 100-acre plot in Money, Mississippi. The study approach integrated waterborne and terrestrial resistivity and nuclear magnetic resonance (NMR) surveys to develop a three-dimensional geoelectrical model of the site. This integrated approach helped define the 100-150 feet of sand aquifer and the contact of the clay-confining unit beneath it. Shallow terrestrial-resistivity surveys confirmed that the clay-rich loam at the land surface continues as a clay-rich alluvial deposit approximately 25-50 ft thick beneath the study area. The presence of this relatively impermeable layer above the alluvial aquifer has the potential to limit vertical recharge from precipitation or irrigation. The NMR survey was used to determine that the aquifer volume consists of 30% water with two-thirds of that available for use. Comparisons of the waterborne- and terrestrial-resistivity surveys were used to identify that a hydraulic connection or potential for water exchange, between the Tallahatchie River and the MRVA is possible. These geophysical observations provide a more accurate understanding of the local hydraulic properties and hydrology of the MRVA aquifer at this site, and will contribute new data to constrain a regional, numerical groundwater model.

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