Time-Lapse Geoelectrical Imaging of a Controlled Ethanol Release in Ottawa Sand

Ethanol is now widely used in petrochemical-based fuels as a fuel oxygenate, or as the primary ingredient in E85 gasoline. Increased production and use of ethanol resulted from environmental concerns over methyl-tertiary-butyl-ether (MTBE), a toxic water-soluble gasoline additive, and the need for renewable sources of non-petrochemical equivalents. Although replacement of MTBE by ethanol is driven by environmental concerns, ethanol itself poses problems if released into the subsurface. Subsurface releases of significant quantities of ethanol may initially reduce microbial populations, mobilize pre-existing subsurface contamination in soil and groundwater, and potentially form explosive conditions through methanogenesis; hence, there is a growing need for rapid assessment of subsurface releases to allow for expedited remedial action. Surface and borehole geophysical methods are capable of providing rapid results to assess the extent of a release; little work, however, has been done to understand the geoelectrical signature of ethanol in the subsurface. Here, we use a cross-borehole electrode array to monitor the three-dimensional (3D) geoelectrical response of a controlled ethanol injection into a closed hydrologic tank model initially containing Ottawa sand and tap water. During the course of the ethanol injection, full geoelectrical data sets were collected at approximately 40-min intervals. Inversion was carried out with a 3D parallel inverse modeling algorithm. In all, 14 3D images show the evolution of the ethanol distribution in the tank during and immediately following the injection of ethanol. Results suggest that geoelectrical field measurements would be useful to delineate and monitor an ethanol release.