An effective electrode configuration for the detection of DNAPLs with electrical resistivity tomography

Locating Dense Non-Aqueous Phase Liquids (DNAPLs) is often a bottleneck for the successful remediation and/or control of polluted sites. Electrical Resistivity Tomography (ERT) between boreholes can detect DNAPLs because of their high electrical resistivity. In applying ERT the definition of the 'best' measurement schedule is a poorly resolved problem. In two case studies, and with synthetic modeling, the cross-hole tripole-pole electrode configuration is compared with the more widely used circulating dipole-dipole and cross-hole dipole-dipole electrode configurations. The case studies involved ERT measurements between five boreholes at two polluted sites with unconsolidated sediments in the Netherlands. The outcome of the modeled data and the field data show that a cross-hole tripole-pole electrode configuration is more effective in locating DNAPLs than the other configurations. The tomographic image from the cross-hole tripole-pole configuration shows clear horizontal patterns that are in accordance with the sediment layers of the subsoil. The locations of the DNAPLs as indicated by the ERT measurements have been confirmed with groundwater samples. Still, the interpretation of the tomographic images should be done with care because ERT cannot distinguish DNAPLs from other high resistivity objects (e.g., massive building blocks) and in the top of some of the images high-resistivity computational artifacts are present that could be mistaken for DNAPLs. An important practical advantage of the cross-hole tripole-pole configuration is that the number of measurement points, and thus the measurement time, is less than half that of the other configurations.