Self-potential for monitoring soil remediation by smouldering: a proof of concept

Near-surface soils contaminated with non-aqueous phase liquids, such as coal tar, crude oil, and chlorinated solvents, remain a serious problem. Smouldering remediation is a technique now being applied in the field for in situ destruction of non-aqueous phase liquids. Based on a selfsustaining exothermic reaction, smouldering remediation generates a hot region (>400 °C) that propagates through the subsurface. Self-potential is here considered for the first time as a nondestructive means for monitoring the smouldering remediation process. First, a series of sandbox experiments were conducted to investigate the magnitude of the thermoelectric coupling coefficient (CTE) for different sand sizes, water contents, and heat sources. Measured CTE values ranged from -0.47 mV/°C for coarse, water-saturated sand to -0.05 mV/°C for fine sand with a saturation of 30%. Next, self-potential measurements were conducted during several laboratory smouldering remediation experiments, examining the response as a function of both space and time. A significant self-potential anomaly was observed on the surface during the smouldering period. Moreover, the magnitude of the self-potential anomaly was demonstrated to be highly correlated to the separation distance between the (moving) reaction front and the (stationary) self-potential electrode positions. Overall, this research suggests that the self-potential method has a significant promise as a non-invasive monitoring tool for in situ smouldering remediation of contaminated sites.