Application of Time-Lapse Electrical Resistivity Tomography for Monitoring Spatial and Temporal Amendment Behavior

Enhanced bioremediation has become one of the preferred remedies for hydrocarbon and/or solvent-contaminated sites. For example, one-third of all contaminated Department of Defense (DoD) sites have identified bioremediation or enhanced bioremediation, where amendments are used to stimulate microbial activity, as the primary remedial strategy. One of the main challenges associated with enhanced bioremediation is the validation of amendment delivery and remedial performance. Sampling methods are expensive and only provide point information; hence new tools are needed to enable more cost effective and reliable interpretation of amendment distribution and delivery.

Time-lapse electrical resistivity tomography (ERT) is a proven method of providing non-invasive, volumetric information about the distribution of bulk subsurface electrical conductivity (1/resistivity) in space and time. In general, amendments have a different fluid conductivity than the ambient ground-water conductivity. In amendment-based remedial applications, temporal variations in bulk conductivity can be caused by amendment migration, amendment degradation, and/or amendment-driven biogeochemical activity. By imaging changes in bulk conductivity, time-lapse ERT has the potential to provide relevant information concerning spatial and temporal amendment behavior.

We have developed and implemented an ERT-based approach for monitoring amendment behavior [Hydrogeophysical Performance Monitoring System (HPMS)], which is currently deployed at the former Brandywine Defense Reutilization Marking Office (DRMO), a DoD Superfund site now managed by Andrews Air Force Base, Brandywine, Maryland. At the core of this approach is a method to estimate changes in fluid conductivity from changes in bulk conductivity while accounting for spatial variability in both tomographic resolution and petrophysical calibration. The goals of our work are to (1) demonstrate the HPMS, and (2) provide site-specific insight into amendment behavior. Here, we describe the HPMS system and its implementation, and we discuss the associated resistivity inverse methodology and the results obtained to date from this system.