Towards efficient in situ gas thermal desorption remediation assisted with cross-borehole resistivity tomography

Abstract

Tracking the spatial and temporal characteristics of contaminant distribution and energy consumption is crucial for optimizing in situ gas thermal desorption (GTD) remediation efficiency at NAPL-contaminated sites. This study utilized time-lapse cross-borehole resistivity to monitor the in situ GTD remediation process at a field-scale NAPL-contaminated site. By integrating stratigraphic and sampling data, areas with resistivity values exceeding 50 Ω·m before remediation were identified as two contaminated layers. After remediation, the high-resistivity anomalies disappeared, and the resistivity distribution offered a more distinct delineation of the stratigraphy. It was observed that resistivity decreased with heating under stable groundwater levels, which was attributed to temperature increases and contaminant removal. The resistivity error coefficient k served as a correction factor to assess the impact of contaminant concentration on resistivity. During heating, k for the contaminated layer increased significantly to 31 %, whereas k for the non-contaminated layer remained within 3 %. Accordingly, the 3D distribution of contaminants over 10 mg/kg during remediation was captured with the resistivity changes. Concentrations in contaminated layers 1 and 2 decreased to below 10 mg/kg during days 25–30 and 20–25, respectively. Furthermore, based on variations in contaminant concentration, the relationship between effective energy utilization per cubic meter for contaminant removal and resistivity changes was established. As a result, areas where the ratio of effective energy utilization (∆E′) to statistical energy (∆E) exceeded 50 % were delineated, which can be employed to estimate the temporal and spatial distributions of energy consumed for contaminant removal. The volume of effective energy utilization peaked at 8.35 m³ between days 10 and 15. This can guide the optimization of heating strategies through fixed-depth heating, thereby enabling more targeted energy expenditure. In summary, this study provides new insights into tracking the temporal and spatial distributions of contaminants and energy use, which is crucial for improving remediation strategies and preventing energy waste.