Time-lapse electrical resistivity tomography mapping of DNAPL remediation at a STAR field site

Abstract

Time-lapse Electrical Resistivity Tomography (ERT), a surface geophysical technique, was applied for the first time to directly map in-situ DNAPL destruction at a field site. Time-lapse ERT was used to map remediation of coal tar at a site undergoing full-scale application of Self-sustaining Treatment for Active Remediation (STAR). STAR is an in situ thermal treatment based on smoldering combustion that destroys dense non-aqueous phase liquids (DNAPLs) while generating heat, water, and combustion gases. ERT was used to provide spatially and temporally resolved data to complement the conventional discrete and overview data normally collected during STAR operations. A STAR treatment cell targeting coal tar at 2.4 mbgs and below the water table was monitored with 2D surface resistivity surveys conducted before, during and after treatment. Two 36-electrode lines, each 21 m long, were employed. Relative resistivity ratio maps and discrete (nodal) resistivity signal analysis both proved useful in numerous ways. For example, ERT was demonstrated to map likely treated coal-tar zones. This was accomplished by identifying a characteristic, local, temporal resistivity signal pattern corresponding to STAR-induced subsurface temperature and liquid saturation changes. In addition, ERT identified the subsurface areas (i) effectively captured by the vapor extraction system, (ii) possibly missed by the vapor capture system where gases built-up, and (iii) where and when re-infiltration of groundwater occurred post-treatment. In addition, the overall average subsurface resistivity provided a useful metric to correlate with traditional site measures of treatment such as the cumulative amount of DNAPL mass treated. This study provides a successful proof of concept however the maximum potential of the approach is expected with a full 3D survey. Overall, the resistivity surveys provided valuable continuous mapping of the subsurface, including the locations of DNAPL mass destruction, and showed that ERT is promising for supporting evaluations of in situ remediation programs.