Feasibility Study of Electrical Monitoring of In-situ Thermal Desorption in Remediation of Contaminated Soil

Lands once occupied by industrial facilities often suffer from contamination of organic matters that leak into the soil within tens of meters below the surface. In-situ thermal desorption (ISTD) is a treatment procedure that removes the contaminant by heating the soil in subsurface and extracting the evaporated organic matters. ISTD relies on many steel pipes vertically buried in the soil to deliver heat to the entire volume of treatment. The effectiveness of remediation was conventionally assessed by taking temperature measurements - the organic matters are considered evaporated at a certain temperature. We propose to use geophysical electrical method to monitor ISTD because the water in the soil evaporates and the soil's electrical resistivity can be significantly increased as the result of heating. This phenomenon has been confirmed by a pilot study that involved two lines of conventional ERT survey in an ISTD project at a site polluted by coal tar. In our present experiment, 169 heating steel pipes distribute evenly over a 13 x 13 grid at a site with a 3 m spacing and serve as long electrodes for both current injection and potential measurement. While more sophisticated configurations can be used, we adopt the "pole-pole" array, in which the pipes take turns to act as the A electrode and the non-energized pipes are used as M electrode for potential measurement; the B and N electrodes are both at the infinity. The procedure is numerically modeled by a 3D dc resistivity code (RESnet) that uses equivalent resistor network for an efficient simulation of hundreds of steel pipes. Our results have shown that even simple data stacking can reveal a volume 15 m below the surface that is under-heated. Our method can be an effective real-time monitoring tool in assessing the progress of ISTD compared with the temperature probing. Significant fuel and time can be saved if the heating can be directed from the overheated region to the under-heated. Adding point electrodes on the surface can provide vertical resolution that aids 3D imaging of the entire ISTD volume.