Independent technical evaluation and recommendations for contaminated groundwater at the department of energy office of legacy management Riverton processing site

The U.S. Department of Energy Office of Legacy Management (DOE-LM) manages the legacy contamination at the Riverton, WY, Processing Site – a former uranium milling site that operated from 1958 to 1963. The tailings and associated materials were removed in 1988-1989 and contaminants are currently flushing from the groundwater. DOE-LM commissioned an independent technical team to assess the status of the contaminant flushing, identify any issues or opportunities for DOE-LM, and provide key recommendations. The team applied a range of technical frameworks – spatial, temporal, hydrological and geochemical – in performing the evaluation. In each topic area, an in depth evaluation was performed using DOE-LM site data (e.g., chemical measurements in groundwater, surface water and soil, water levels, and historical records) along with information collected during the December 2013 site visit (e.g., plant type survey, geomorphology, and minerals that were observed, collected and evaluated). A few of the key findings include:  Physical removal of the tailings and associated materials reduced contaminant discharges to groundwater and reduced contaminant concentrations in the near-field plume.  In the mid-field and far-field areas, residual contaminants are present in the vadose zone as a result of a variety of factors (e.g., evaporation/evapotranspiration from the capillary fringe and water table, higher water levels during tailings disposal, and geochemical processes).  Vadose zone contaminants more » are widely distributed above the plume and are expected to be present as solid phase minerals that can serve as “secondary sources” to the underlying groundwater. The mineral sample collected at the site is consistent with thermodynamic predictions.  Water table fluctuations, irrigation, infiltration and flooding will episodically solubilize some of the vadose zone secondary source materials and release contaminants to the groundwater for continued down gradient migration – extending the overall timeframe for flushing.  Vertical contaminant stratification in the vadose zone and surficial aquifer will vary from location to location. Soil and water sampling strategies and monitoring well construction details will influence characterization and monitoring data.  Water flows from the Wind River, beneath the Riverton Processing Site and through the plume toward the Little Wind River. This base flow pattern is influenced by seasonal irrigation and other anthropogenic activities, and by natural perturbations (e.g., flooding).  Erosion and reworking of the sediments adjacent to the Little Wind River results in high heterogeneity and complex flow and geochemistry. Water flowing into oxbow lakes (or through areas where oxbow lakes were present in the past) will be exposed to localized geochemical conditions that favor chemical reduction (i.e., “naturally reduced zones”) and other attenuation processes. This attenuation is not sufficient to fully stabilize the plume or to reduce contaminant concentrations in the groundwater to target levels. Consistent with these observations, the team recommended increased emphasis on collecting data in the zones where secondary source minerals are projected to accumulate (e.g., just above the water table) using low cost methods such as x-ray fluorescence. The team also suggested several low cost nontraditional sources of data that have the potential to provide supplemental data (e.g., multispectral satellite imagery) to inform and improve legacy management decisions. There are a range of strategies for management of the legacy contamination in the groundwater and vadose zone near the Riverton Processing Site. These range from the current strategy, natural flushing, to intrusive remedies such as plume scale excavation of the vadose zone and pump & treat. Each option relates to the site specific conditions, issues and opportunities in a unique way. Further, each option has advantages and disadvantages that need to be weighed. Scoping evaluation was performed for three major classes of technologies – contaminant removal technologies, contaminant stabilization technologies, and natural flushing. The intent of the scoping evaluation is to provide an initial set of options for consideration by LM as they finalize plans to address the Riverton groundwater plume. Three technologies were conditionally recommended: 1) continued natural flushing, 2) groundwater pump and treat with plume scale irrigation to help flush out vadose zone contamination, or 3) in situ stabilization using structured geochemical zones to supplement the naturally reduced zones already present at the site. « less