Three‐dimensional self‐potential inversion for subsurface DNAPL contaminant detection at the Savannah River Site, South Carolina

Abstract

Self‐potential (SP) data are collected over an area of approximately 250 m2 at the Department of Energy Savannah River Site where there is known subsurface dense nonaqueous phase liquid (DNAPL) contamination. Nonpolarizing electrodes are used to measure the SP signal on a two‐dimensional (2‐D) surface grid with 2‐m spacing in both horizontal directions, and four borehole arrays with 3.7‐m electrode spacing and 25.6‐m total depth. The primary contaminants, tetrachloroethylene (PCE) and trichloroethylene (TCE), are known to undergo redox reactions in the environment. Variations in the subsurface redox conditions are proposed as an electrochemical source for the SP signals measured in this investigation. A 3‐D self‐potential inversion algorithm is used to find an electrical current source model, taking into account the resistivity structure derived from a 3‐D spectral induced polarization survey at the same field location. The sources and sinks of electrical current can be related to the zones of relative high or low redox potential and are therefore interpreted in the context of contaminated areas. These results are reasonably correlated with DNAPL concentration data obtained from several ground‐truth well measurements, indicating that the SP sources can be an indicator of contaminated areas where electrochemical source mechanisms are active. In several cases, however, the SP sources and contaminant concentrations are not correlated, reflecting the spatial variability of biogeochemical parameters in the Earth that control the SP response in addition to concentration. More extensive geochemical ground‐truth information is therefore needed to validate the self‐potential source inversion methodology and develop it as a predictive tool in the context of contaminated sites.