PNNL

Abstract

The Hanford Site 300 Area lies adjacent to the Columbia River, approximately 5 km north of Richland, Washington. Past waste disposal practices in the 300 Area resulted in vadose zone uranium contamination beneath former infiltration ponds and trenches. Stage-driven water table fluctuations and river water intrusion facilitate mobilization of uranium from contaminated sediments in regions periodically occupied by the water table (i.e., the periodically rewetted zone or PRZ), thereby raising groundwater uranium concentrations above the maximum allowable contaminant level for uranium. Enhanced attenuation of uranium using phosphate treatment was selected as part of the remedy in the 300 Area Record of Decision (ROD) (EPA and DOE, 2013). From September 4-20, 2018, in accordance with DOE/RL-2014-42-ADD1, CH2M Hill Plateau Remediation Company conducted an in situ uranium sequestration remedy by injecting a phosphate amendment through injection wells installed in a select region of the 300 Area PRZ and the lower vadose zone (LVZ), which is the region of the vadose zone directly above the PRZ. Real-time cross-borehole electrical resistivity tomography (ERT) was used to evaluate amendment delivery by imaging the spatial and temporal distribution of the change in subsurface electrical conductivity caused by amendment migration. ERT imaging surveys were conducted continuously using wellbore annulus electrodes installed in three clusters of injection and monitoring boreholes. On each cluster, surveys were conducted every 52 minutes and images were reported via website in near-real time. This report documents the ERT imaging operations and interpretation of imaging results in terms of the overall area impacted by the presence of amendment (including phosphate and/or carrier fluid) within the treatment area. Based on the interpretation, amendment delivery to the LVZ and PRZ was observed to be variable between each of the three imaging clusters, as shown in Figure S.1. Amendment transport in Cluster 1 exhibited significant lateral flow in comparison to Clusters 2 and 3, resulting in a widespread zone of amendment-impacted LVZ and PRZ sediment between injection wells. Cluster 2 exhibited enough lateral flow to impact the LVZ and PRZ near the injection wells, but only the PRZ appears to have been impacted by amendment in the region between injection wells. Cluster 3 exhibited the smallest amount of lateral flow and appears to have been impacted by amendment primarily near the injection wells. Imaging results below the water table suggest that more amendment entered the groundwater in Cluster 2 and Cluster 3 than in Cluster 1. Lateral transport within the three clusters is correlated with differences in bulk electrical conductivity between LVZ and PRZ sediments evident in baseline static ERT imaging. Each cluster exhibited relatively layered bulk conductivity structure within the LVZ and PRZ, which is consistent with layers of finer and coarser materials that tend to exhibit higher and lower levels of saturation respectively. Each cluster also exhibited lateral variations in bulk conductivity, which are diagnostic of changes in sediment properties or states (such as sediment texture, porosity, saturation, and pore fluid conductivity). Cluster 1 exhibited markedly higher baseline bulk electrical conductivity structure than Cluster 2 or Cluster 3, which may have been caused by horizontally continuous, relatively fine-grained zones of sediment that promote lateral transport. Overall, amendment coverage variability within the LVZ and PRZ appears to have resulted primarily from variability in hydrogeologic properties caused in part by legacy waste discharge operations and past remediation excavations.