Contaminant Monitoring with Electrical Resistivity Tomography: A New 3D Surface-to-Horizontal Borehole Configuration

ABSTRACT: Restoration of sites contaminated by dense non-aqueous phase liquids (DNAPLs) remains a major geoenvironmental challenge. The success of restoration programs relies on effective characterization and monitoring of the evolving DNAPL distribution over time. As conventional techniques such as soil coring and monitoring wells are costly, invasive, and suffer from low sampling density, non-invasive geophysical techniques are becoming increasingly popular. The electrical resistivity tomography (ERT) technique has shown significant promise for monitoring DNAPL mass changes over time but is hampered by decreasing resolution with increasing distance from the ERT electrodes. Surface ERT is limited at sites with deeper DNAPL distributions, while the common alternative with cross-hole ERT can require many vertical boreholes drilled into the distributed DNAPL. Recently, a novel approach with surface-to-horizontal borehole (S2HB) ERT was proposed, taking advantage of the increase of horizontal remediation wells deployed at DNAPL sites. While highly promising, S2HB ERT has only been demonstrated in 2D, with a single line of surface electrodes overlying a single line of horizontal borehole electrodes. Three-dimensional (3D) imaging is necessary to capture the full extent of complex, heterogeneous DNAPL distributions. The objective of this study is to evaluate the effectiveness of S2HB ERT in 3D, with a 2D surface grid of electrodes and one or more horizontal boreholes lined with electrodes. A suite of laboratory tank experiments was conducted with 3D S2HB ERT monitoring of changing DNAPL distributions. In all experiments, 3D surface ERT was also used to provide a reference for S2HB performance. Different configurations of 3D S2HB ERT were used, along with different DNAPLs and distributions, providing a robust assessment of 3D S2HB ERT. Initial results demonstrate that 3D S2HB ERT provides significantly improved monitoring of DNAPLs, with ongoing work investigating the relationship between the number of horizontal boreholes, resolving ability, and monitoring performance.