Using Electrical Resistivity Imaging to Evaluate Permanganate Performance During an In Situ Treatment of a RDX-Contaminated Aquifer

摘要

The difficulty and cost of groundwater remediation are compounded by our inability to directly observe the subsurface. Traditionally, boreholes and monitoring wells have been used to characterize sites and assess the performance of remedial efforts. These techniques are expensive and, by themselves, are effectively random samples guided by the training and experience of the investigator. Electrical resistivity imaging (ERI) is a geophysical technique that infers subsurface water and soil electrical properties. Where the soil, the contaminant, and remediation compound are suitable, ERI quickly and economically provides the investigator with a spatially extensive, high-density, high-quality model of subsurface conditions. ERI has earned success in research and applications by reliably locating contaminant targets where traditional drilling and direct push methods have failed to locate contamination. In this demonstration project, ERI was used to monitor injection of in situ chemical oxidation (ISCO): an injection of sodium permanganate to mineralize RDX at the former Nebraska Ordnance Plant near Mead, Nebraska. Monitoring wells showed that the ISCO demonstration was, indeed, transforming RDX, but samples from seventeen available wells and eight direct push profiles at different locations did not provide enough data to construct the distribution and flow of the injected permanganate. ERI showed that the permanganate injection flowed against the regional groundwater gradient, and that the solution was able to sink below the monitoring well screens. Without geophysical observations, no information would have been available to explain the permanganate and RDX concentrations observed in the wells. The same data were used to guide the boring of additional holes; unguided drilling would be a costly and inefficient option, based on essentially random location choices. ERI demonstrated its usefulness and value in monitoring a permanganate injection.