3D Characterization and Time-Lapse Imaging of the Desiccation Treatability Test at the Hanford BC-Cribs and Trenches Site using High Performance Computing applied to Electrical Resistivity Imaging - 12271
Contaminated vadose zone materials are a potential source of long-term groundwater contamination at many sites across the Department of Energy (DOE) complex. Deep vadose zone contamination presents a particularly challenging remedial problem due to the difficultly of locating contaminants and the expense of access and ex situ treatment. In situ remediation techniques, whereby remedial amendments must be delivered to contaminated soils, have been identified as a potential alternative. However, amendment delivery is typically uncertain and post delivery remedial performance is often not well understood due to limited information available from sparsely spaced boreholes. Recent advancements in electrical resistivity tomography (ERT) are being used to address these challenges at the Hanford site by providing remote, three-dimensional images of contaminant distribution and four-dimensional (three spatial dimensions plus the time dimension) images of in-situ vadose zone remediation processes. These capabilities were recently demonstrated with a large scale surface characterization effort and during a smaller scale desiccation treatability test at the Hanford BC-Cribs area. The results of these efforts demonstrate the utility of leveraging high-performance computing resources to process ERT data for 3D reconnaissance mapping of subsurface contaminants and for detailed 3D monitoring of vadose zone remediation efforts. Two examples of ERT imaging at a former waste disposal facility have been demonstrated at different scales; a large scale reconnaissance survey spanning many hundreds of meters and a detailed monitoring application spanning a few tens of meters. Although ERT is applicable at many scales, resolution is generally governed by proximity to electrodes. For instance, the BCCT reconnaissance survey demonstrated herein lost resolution with depth (i.e. away from surface electrodes), and was unable to detect a high conductivity anomaly at approximately 75 m below ground surface in the cribs area; the deep anomalies were detected during highly focused borehole sampling. Conversely, the desiccation site characterization and monitoring demonstrated high resolution at a small scale, but only because of close proximity to survey electrodes. Although proximity to electrodes is not the only factor governing ERT resolution, it is often the primary factor for determining how an ERT system should be effectively deployed for a given application. Regardless of resolution limitations, the utility of ERT for remotely detecting wastes and monitoring subsurface processes is evident. ERT imaging is not a final detection or monitoring solution alone, but provides a necessary capability to remotely detect subsurface conditions and understand subsurface processes. Such a capability can significantly increase the efficacy and reduce the cost of remediation by guiding sampling efforts and informing remedial system design and operation. However, given the computational demands of 3D ERT characterization and monitoring inversions, the full imaging capability enabled by contemporary ERT survey systems can generally only be realized with high performance computing resources, both hardware and software. (authors)