Pilot Study for the Characterization of the Mississippi River Valley Alluvial Aquifer near Money, Mississippi
Monday, December 4, 2017: 5:10 p.m.
The Mississippi River Valley alluvial (MRVA) aquifer is a near-surface aquifer system used to supply irrigation for agriculture across the Mississippi Alluvial Plain. 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 measurements with nuclear magnetic resonance (NMR) soundings to develop a three-dimensional geoelectrical model of the site. Combining the results of all three methods into a three-dimensionally model improves the resolution of all of the techniques and enhances the characterization of aquifer properties. This integrated approach helped define the 100-150 feet of the sand and gravel aquifer and the contact of the underlying clay-confining unit. Shallow terrestrial-resistivity surveys confirmed that the clay-rich loam at the land surface continues as a clay-rich alluvial deposit approximately 25-50 feet thick beneath the southern portion of 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 materials have an effective porosity of between 15 and 25% with two-thirds of the porosity consisting of mobile water that is unbound to fine-grained materials. 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.