Re-Inversion of Surface Electrical Resistivity Tomography Data from the Hanford Site B-Complex

Abstract

This report documents the three-dimensional (3D) inversion results of surface electrical resistivity tomography (ERT) data collected over the Hanford Site B-Complex. The data were collected in order to image the subsurface distribution of electrically conductive vadose zone contamination resulting from both planned releases of contamination into subsurface infiltration galleries (cribs, trenches, and tile fields), as well as unplanned releases from the B, BX, and BY tank farms and/or associated facilities. Electrically conductive contaminants are those which increase the ionic strength of pore fluids compared to native conditions, which comprise most types of solutes released into the subsurface B-Complex. The ERT data were collected and originally inverted as described in detail in report RPP-34690 Rev 0., 2007, which readers should refer to for a detailed description of data collection and waste disposal history. Although the ERT imaging results presented in that report successfully delineated the footprint of vadose zone contamination in areas outside of the tank farms, imaging resolution was not optimized due to the inability of available inversion codes to optimally process the massive ERT data set collected at the site. Recognizing these limitations and the potential for enhanced ERT characterization and time-lapse imaging at contaminated sites, a joint effortmore » was initiated in 2007 by the U.S. Department of Energy – Office of Science (DOE-SC), with later support by the Office of Environmental Management (DOE-EM), and the U.S. Department of Defense (DOD), to develop a high-performance distributed memory parallel 3D ERT inversion code capable of optimally processing large ERT data sets. The culmination of this effort was the development of E4D (Johnson et al., 2010,2012) In 2012, under the Deep Vadose Zone Applied Field Research Initiative (DVZ-AFRI), the U.S. Department of Energy – Richland Operations Office (DOE-RL) and CH2M Hill Plateau Remediation Company (CHPRC) commissioned an effort for the Pacific Northwest National Laboratory (PNNL) to re-invert the ERT data collected over the B-Complex using E4D, with the objective to improve imaging resolution and better understand the distribution of vadose zone contamination at the B-Complex. The details and results of that effort as documented in this report display a significant improvement in ERT image resolution, revealing the nature and orientation of contaminant plumes originating in former infiltration galleries and extending toward the water table. In particular, large plumes originating in the BY-Cribs area appear to have intercepted, or are close to intercepting the water table after being diverted eastward, possibly by the same low permeability unit causing perched water north of the B-Tank Farm boundary. Contaminant plumes are also evident beneath the BX-Trenches, but do not appear to have intercepted the water table. Imaging results within the tank farms themselves are highly biased by the dense network of electrically conductive tanks and dry wells, and are therefore inconclusive concerning contaminant distributions beneath tanks. However, beneath the diversion boxes, the results do reveal highly conductive anomalies that are not associated with metallic infrastructure, and may be diagnostic of extensive contamination. Overall, the parallel ERT inversion provides additional detail concerning contaminated zones in terms of conductive anomalies. These anomalies are consistent with waste disposal histories, and in several cases reveal lateral contaminant transport caused by heterogeneity within the vadose zone.« less