Geophysical Surveys to Characterize Geologic Controls on Aquifer Recharge and Surface Water–Groundwater Exchange
Tuesday, December 5, 2017: 1:40 p.m.
The U.S. Geological Survey (USGS) developed a groundwater-flow model of the Mississippi Embayment Regional Aquifer System (MERAS) that incorporated multiple aquifers including the Mississippi River Valley alluvial (MRVA) aquifer. In addition to groundwater withdrawal, two major fluxes in the model are recharge from precipitation and surface water-groundwater exchange. In order to determine appropriate values for recharge to the MERAS model, the USGS has utilized two published datasets- the geomorphology of Quaternary deposits and local soil surveys. At a regional scale, recharge in the MERAS model correlate well with large-scale geomorphological features. However, there is little spatial variability, so local-scale variations in recharge are not adequately represented. Higher resolution data such as soil coverages provide a more spatially-variable estimates of recharge, but, soil-survey data often characterize the shallow soil horizon and do not reflect the generalized geomorphological features in which the horizon lies. In addition, streambed sediments may differ greatly from the mapped geomorphologic areas and shallow soils due to alteration from stream mechanics. Thus, geomorphologic maps and soil information are both types of surficial information that may not accurately reflect the underlying hydrogeology that controls infiltration of recharge water or the composition of streambed sediments.
In 2016-17, the USGS conducted several waterborne geophysical surveys to characterize the near-surface (<12 m) lithology that controls recharge to the MRVA aquifer and surface water-groundwater exchange at selected locations within the Mississippi Alluvial Plain (MAP). Two-dimensional vertical profiles of resistivity identified differences in geoelectrical properties of the streambed. High resistivity values are associated with coarse grained sediments and low values are indicative of fine grained materials. These resistivity-derived lithologies were then transformed using several techniques to inform the estimated hydraulic conductivity of the simulated streambed and refine the characterization of streamflow interactions in the MERAS groundwater-flow model.