Cross-borehole electrical monitoring in groundwater remediation projects: understanding the flow path of remediation agents

In-situ remediation of contaminated groundwater often relies on the installation of a treatment zone degrading the pollution plume. In such projects, adequate delivery of remediation agents in the whole target volume is not trivial. Monitoring the spatial distribution of injected reagents is important for engineers and decision-makers, in order to determine the need of more careful injection in some areas. In this study, we use cross-borehole electrical resistivity tomography (XB-ERT) at two contaminated sites in Denmark to visualize the progressive spreading of two different chemical reagents: an oxidant and a reduction agent made of solid iron particles. Both reagents lead to significant electrical resistivity changes in the subsurface. We observe major differences in the spreading uniformity at the two sites, which we explain by different average permeability and reagent viscosity. We compare tomography results to induction logs in order to evaluate the reliability of the cross-borehole technique.