Long‐term time‐lapse surface and borehole electrical resistivity monitoring of natural recharge‐ induced contaminant plume behavior

Abstract

Summary Geophysical measurements, and electrical resistivity tomography (ERT) data in particular, are sensitive to properties that are related (directly or indirectly) to hydrological processes. The challenge is in extracting information from geophysical data at a relevant scale that can be used quantitatively to refine hydrological models and interpret plume behavior. We consider the use of ERT data for examining the impact of recharge on subsurface contamination at the southern-most corner of the S-3 ponds at the Oak Ridge Integrated Field Research Challenge (IFRC) site, TN. The approach we have developed involves the coupled inverse modeling of time-lapse borehole ERT data and hydrogeochemical data, including water level and solute concentration data. The time variations in resistivity are due to changes in total dissolved solids (primarily nitrate), saturation, pH, and temperature. The current approach is to use our understanding of the borehole ERT response to recharge-related processes at the local scale to aid in interpretation of more spatially extensive surface ERT data. We consider a large dataset of time-lapse borehole and surface ERT data collected at the site in 2008 and 2009. We find that the system response to short-term heavy rainstorms is different from the response to seasonal and annual fluctuations and that there is a very good agreement between resistivity structures recovered from borehole and surface ERT data. These results suggest that we can transfer derived insights about petrophysical models and plume behavior across scales, thus permitting remote quantification of recharge-induced processes (such as nitrate dilution) over field-relevant scales.