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

Abstract

Executive SummaryThis 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 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) and unplanned releases from the B, BX, and BY tank farms and/or associated facilities. Electrically conductive contaminants are those that increase the ionic strength of pore fluids relative to native conditions. Most types of solutes released into the subsurface B-Complex were electrically conductive.The ERT data were collected and originally inverted as described in the 2007 report Surface Geophysical Exploration of the B, BX, and BY Tank Farms at the Hanford Site, 1 which provides 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 because the available inversion codes could not 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 effort was initiated in 2007 by the U.S. Department of Energy (DOE) Office of Science, with later support from the DOE Office of Environmental Management and the U.S. Department of Defense, to develop a highperformance 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. 2,3In 2012, under the Deep Vadose Zone Applied Field Research Initiative, the DOE Richland Operations Office and CH2M HILL Plateau Remediation Company commissioned an effort for Pacific Northwest National Laboratory 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 contaminan...

Evaluating Contaminant Flux from the Vadose Zone to the Groundwater in the Hanford Central Plateau. SX Tank Farms Case Study

“…However, ERT is sensitive to the presence of metallic infrastructure. Thus, use of ERT is challenging in tank farms, although recent advances in data inversion may help reduce the interference from known metallic infrastructure (Johnson and Wellman 2013 -It is important to determine whether disposal of water or contaminants outside the SX Tank Farm influence the contaminant flux beneath the tank farm. Thus, data for contaminant and water releases/discharges, contaminant and moisture distribution, and the distribution of subsurface properties for the surrounding area are important.…”

Groundwater characterization and monitoring at a complex industrial waste site using electrical resistivity imaging

“…Buried metallic infrastructure such as boreholes, underground piping, and tanks redistribute subsurface current flow during ERT measurements and can have a significant impact on ERT images, thereby reducing the utility of ERT. Johnson et al (2012) and Johnson and Wellman (2013) demonstrated a method of removing the effects of buried infrastructure by explicitly modeling the infrastructure in the ERT imaging algorithm in the E4D computational mesh (Johnson 2014). This method was used by explicitly incorporating borehole, underground piping, and underground tanks in the ERT modeling (Fig.…”

“…The cementation exponent m is dependent on the rate of change in pore complexity with porosity (Yue 2019), and on particle shape and orientation (Niu and Zhang 2018), and typically varies between 1.2 and 4.4 (Lesmes and Friedman 2005). A value of 1.8, which has been previously applied to represent Hanford sediments (Johnson and Wellman 2013), was used. The saturation exponent n is associated with the additional tortuosity due to the replacement of pore fluid with air (an insulator).…”

“…Johnson et al (2012) developed methods for incorporating constraints into an ERT inversion algorithm, in the form of discontinuous boundaries, known values, and independently determined spatial covariance information. Johnson and Wellman (2013) reinverted the resistivity data collected by Rucker and Fink (2007) using an algorithm that explicitly accounts for metallic infrastructure, including wells, pipes, and storage tanks, and reported significantly improved imaging results using the new inversion methods. Marinenko et al (2019) reported on numerical experiments to evaluate the potential effects of metal pipes located above and below the ground surface on the ability of ERT to identify low-resistance zones of permafrost thawing around gas production wells.…”

Feasibility Assessment of Long‐Term Electrical Resistivity Monitoring of a Nitrate Plume

“…We use a similar coupling of E4D with a flow and transport simulator to determine the feasibility of contaminant and remedial monitoring at a legacy nuclear waste facility, the Hanford 200 East Area B‐Complex in southeastern Washington State. Previous field hydrogeological and ERI studies have focused on the spatial distribution and transport of contaminants in the vadose zone, specifically nitrates due to the strong conductivity contrast with the native groundwater (Rucker and Fink ; Johnson and Wellman ; Oostrom et al ). No geophysical imaging field studies to date have focused on monitoring contaminant groundwater migration, owing in part to the water table being relatively deep (∼70 m below‐ground surface), which increases the infrastructure and implementation costs associated with ERI monitoring.…”