Improved time-lapse ERT monitoring of dense non-aqueous phase liquids (DNPLs) with surface-to-horizontal borehole arrays

Electrical resistivity tomography (ERT) has long been recognized to exhibit the potential to significantly improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs). However, the technique has not become a common tool for mapping such contaminants due to the complexity of the DNAPL target coupled with the inherent limitations of the commonly used surface and cross-hole ERT configurations. The objective of this work is to evaluate the potential of surface-tohorizontal borehole (S2HB) ERT for mapping the spatio-temporal evolution of DNAPL mass during remediation. A coupled DNAPL-ERT model was employed to provide an initial, theoretical evaluation of S2HB ERT, relative to surface ERT, for monitoring the remediation of a realistic, field-scale DNAPL source zone. A laboratory experiment was then performed to demonstrate the S2HB ERT approach for a real system involving a changing NAPL distribution over time. Four-dimensional ERT inversion was employed for both numerical and experimental data to generate time-lapse resistivity images of the subsurface regions experiencing NAPL changes. Results demonstrate significantly improved resistivity imaging with S2HB ERT and its potential as a non-invasive DNAPL site tool for mapping DNAPL mass changes during remediation.