The Use of Waterborne Resistivity Profiling to Quantify Hydraulic Conductivity of 150 Kilometers of Streambed in the Mississippi River Alluvial Plain

In fresh water aquifers, the geoelectric resistivity of earth materials commonly has a positive correlation with hydraulic conducitivity. In June of 2016, the US Geological Survey used waterborne continuous-resistivity profiling to map the shallow (< 10m) subsurface distribution of geoelectrical properties as a proxy for streambed hydraulic conductivity for reaches of the Tallahatchie (60km), Quiver (48km), and Sunflower (39km) Rivers in central Mississippi. Two-dimensional vertical profiles of resistivity were used to identify differences in geoelectrical structure of the streambed specifically between the larger, more-incised Tallahatchie River and the smaller, less-incised Quiver and Sunflower Rivers. Preliminary results show that mean apparent resistivity (Rhoa) on the Tallahatchie is 65 ohmm higher than on the Quiver and Sunflower Rivers. This difference in mean Rhoa is affected by lower resistivity water in the two smaller streams. However, lithologic differences among the streams are discernable in the variability of Rhoa. Distribution of Rhoa along the river profile is highly variable in the Sunflower River, with a standard deviation of 38 ohmm. This is about 52% greater than that of the Quiver at 23 ohmm and Tallahatchie at 27ohmm. Although the Tallhatchie and Quiver have significantly different water column resisitivities, the variability between the two streams is more similar than the highly variable Sunflower. In regional groundwater flow models, the hydraulic conductivity of streambed materials is typically an estimated parameter because of difficulty in obtaining a data-supported value in real-world conditions. The resistivities from this work will be used to scale streambed hydraulic conductivity for incorporation into the hydrogeological framework of a regional groundwater flow model, which may be used to guide policy decisions. Future studies will continue the development of geophysical methods to improve this model.