Integrated Airborne Geophysical Data for Mapping Saltwater in the Mississippi River Valley alluvial Aquifer, Northeast Louisiana

Salinity in groundwater reduces crop productivity, drinking water quality, and economics. Saltwater plume mapping can provide crucial information for groundwater management. However, delineating saltwater plumes in aquifers is challenging and highly uncertain due to a lack of water quality data from water wells. This study introduces an integrated framework to map saltwater plumes in the agriculturally intensive Mississippi River Valley alluvial aquifer (MRVA) in northeast Louisiana using airborne electromagnetic (AEM) resistivity data. However, AEM resistivity data cannot distinguish saline sands from fine-grained sediments as both exhibit low resistivity values. To overcome this ambiguity, a lithofacies model is first constructed using borehole data through indicator kriging. Then, a 3D resistivity model is constructed for sand facies by fusing AEM resistivity data from two different airborne EM systems through a cokriging method. Finally, chloride concentration distribution in the MRVA is inferred by a resistivity-to-chloride concentration regression formula that is established using nearly collocated wells and AEM data points. The resistivity model shows the stratified pattern with low resistivity at the MRVA base, which resembles saltwater stratification. The salinity model shows that the highest chloride concentration zone coincides with the Jurassic Mississippi Salt Basin, indicating saltwater originates from the deep saline formations. Additionally, saline river water may also contribute salinity to the alluvial aquifer through surface water-groundwater interactions. The delineated saltwater plumes reveal saltwater upconing and proximity to pumping wells, providing unprecedented information to the agriculture sector.