PNNL

Abstract

Long-term monitoring solutions at contaminated sites are necessary to track plume migration and evaluate the performance of remediation efforts. Electrical resistivity imaging (ERI) can potentially provide information about plume dynamics; however, the feasibility and likelihood of success is seldom evaluated before conducting a field study. Coupling flow and transport models with geoelectrical models provides a powerful way to assess the potential effectiveness of an actual ERI field campaign. We present a coupled approach for evaluating the feasibility of monitoring nitrate migration and remediation using 4D time-lapse ERI at a legacy nuclear waste facility. This kilometer-scale study focuses on depths below the water table (~70 m). A flow and transport model is developed to perform simulations of nitrate migration and removal via a hypothetical pump-and-treat system. A tracer injection is also simulated at the leading edge of the nitrate plume to enhance the conductivity contrast between the native subsurface and the groundwater fluids. Images of bulk conductivity provide limited information concerning plume migration while time-lapse difference images, which remove the static effects of geology, provide more useful information concerning plume dynamics over time. A spatial moment analysis performed on flow and transport and ERI models matches well during the tracer injection; however, inversion regularization smoothing otherwise limits the value in terms of locating the center of mass. We find that the addition of a tracer enables ERI to characterize plume dynamics during pump-and-treat operations, and late-time ERI monitoring provides a conservative estimate of nitrate plume boundaries in this synthetic study.