Geoelectrical monitoring of natural attenuation process in an organic contaminated site including artificial insulating films

Abstract

Hydrogeophysical methods such as electrical resistivity tomography (ERT) have been increasingly employed in the investigation of organic contaminated sites. However, their applicability during monitored natural attenuation (MNA) of organic pollutants (OPs) remains highly site dependent, owing to the variable and often contradictory electrical responses associated with OP migration and degradation. In addition, artificial structures introduced by common remediation efforts (e.g., insulating films) can significantly complicate the interpretation of electrical data, yet their potential influence on geoelectrical measurements has not been systematically evaluated. To assess the feasibility of applying geoelectrical methods in the context of MNA, we conducted a one-year time-lapse surface and cross-borehole ERT monitoring at an OP-contaminated site. After obtaining the geoelectrical data, an effective correction methodology was first developed to mitigate the signal distortions caused by subsurface insulating films. Then, based on long-term hydrochemical sampling at this study site, we confirmed that the subsurface electrical conductivity showed no significant correlation with the concentration of dissolved OPs in groundwater (as opposed to the free phase) due to their lack of ionic charge carriers. Meanwhile, through a variety of hydrochemical indicators related to biodegradation, we demonstrated the correlations between high-conductivity anomalies delineated by ERT and the biodegradation byproduct plumes of OPs, where metabolic compounds such as chloride and carbon dioxide lead to increases in groundwater electrical conductivity. These findings highlight the potential of geoelectrical methods to provide spatiotemporal insights into OP degradation, which is particularly essential for MNA-based remediation strategies.