Improved time-lapse electrical resistivity tomography monitoring of dense non-aqueous phase liquids with surface-to-horizontal borehole arrays

• Time-lapse ERT proposed as a non-destructive DNAPL site monitoring tool
• Novel surface-to-horizontal borehole (S2HB) ERT proposed for DNAPL investigations
• Initial, theoretical benefit of S2HB ERT provided by simulated DNAPL remediation scenario
• Improved S2HB ERT mapping of NAPL changes demonstrated in a laboratory experiment

Electrical resistivity tomography (ERT) has long-standing potential to improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs). However, ERT is rarely used at DNAPL sites due to the complexity of the DNAPL target coupled with the inherent limitations of traditional (surface and cross-hole) ERT configurations. Horizontal boreholes are being increasingly used in remedial strategies at contaminated field sites. This paper presents a novel surface-to-horizontal borehole (S2HB) ERT configuration for DNAPL site investigations. This array configuration is combined with four-dimensional (4D) inversion methods (applied on two-dimensional (2D) time-lapse monitoring datasets) to explore the potential benefit of S2HB ERT for mapping the spatial and temporal evolution of DNAPL mass during remediation. A field scale DNAPL remediation scenario was first simulated by a coupled DNAPL-ERT model. This demonstrated that S2HB ERT may provide significant improvements over surface ERT, particularly for delineating DNAPL mass removal at depth. A laboratory experiment was then performed to validate the S2HB ERT approach in a physical system. The experiment confirmed that 4D S2HB ERT provides improved time-lapse monitoring of NAPL changes. Confidence in the ERT responses obtained from the experiment was increased by direct comparison to the actual distribution of NAPL mapped by excavation. Independent simulation of the experiment with the DNAPL-ERT model demonstrated that the model is reliable for simulating real systems. This initial study demonstrates significantly improved resistivity imaging with surface-to-horizontal borehole ERT and its potential as a non-destructive site characterization tool for mapping DNAPL mass changes during remediation.