Updated Conceptual Model for the 300 Area Uranium Groundwater Plume

Abstract

SummaryThe 300 Area uranium groundwater plume in the 300-FF-5 Operable Unit is residual from past discharge of nuclear fuel fabrication wastes to a number of liquid (and solid) disposal sites. The source zones in the disposal sites were remediated by excavation and backfilled to grade, but sorbed uranium remains in deeper, unexcavated vadose zone sediments. Despite source term removal, the groundwater plume has shown remarkable persistence, with concentrations exceeding the drinking water standard over an area of approximately 1 km 2 . The plume resides within a coupled vadose zone, groundwater, river zone system of immense complexity and scale. Interactions between geologic structure, the hydrologic system driven by the Columbia River, groundwater-river exchange points, and the geochemistry of uranium contribute to persistence of the plume.The U.S. Department of Energy (DOE) recently completed a Remedial Investigation/Feasibility Study (RI/FS) to document characterization of the 300 Area uranium plume and plan for beginning to implement proposed remedial actions. As part of the RI/FS document, a conceptual model was developed that integrates knowledge of the hydrogeologic and geochemical properties of the 300 Area and controlling processes to yield an understanding of how the system behaves and the variables that control it. Recent results from the Hanford Integrated Field Research Challenge (IFRC) site and the Subsurface Biogeochemistry Scientific Focus Area Project funded by the DOE Office of Science were used to update the conceptual model and provide an assessment of key factors controlling plume persistence.Hanford IFRC research identified several refinements to the conceptual model that can be extended to the 300 Area. Studies at the IFRC site with geophysics, hydraulic testing, and tracers confirm the presence of a lower permeability intermediate zone within the Hanford formation that has a large impact on the hydrologic system. Resulting vertical borehole flows have implications for monitoring, persistence of the uranium plume, and remediation approach. Research at the IFRC site has shown that uranium is mobilized in a heterogeneous way from the lower vadose zone in concentrations exceeding the drinking water standard during the spring high water table event, showing large fluctuations in concentrations over the approximate 3-month period of spring snowmelt. The observed behaviors of uranium strongly impact monitoring results, and should be considered in their interpretation. Improved persistence calculations should address more accurate adsorption-desorption parameters (e.g., surface complexation) for the saturated zone derived from fitting of new IFRC field experiment data.

Drought Conditions Maximize the Impact of High‐Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

“…Both the river water and groundwater are rich in dissolved oxygen (~100% for river water and ~80% for groundwater). The solute concentrations of CH 2 O, HCO 3 − , and NO 3 − provided in Table , were taken from averaged field sampling results (Zachara et al, , ). DOC in the river water was assumed to be the main carbon source, that is, CH 2 O, for the biogeochemical processes in the HZ, while groundwater was the main source for NO 3 − .…”

Drought Conditions Maximize the Impact of High-Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

“…The solute concentrations of CH2O, HCO3 -, and NO3-, provided in Table 3, were taken from averaged field sampling results (Zachara et al, 2012;2016). DOC in the river water was assumed to be the main carbon source, i.e., CH2O, for the biogeochemical processes in the HZ, while groundwater was the main source for NO3 -.…”

Modulating factors of hydrologic exchanges in a large‐scale river reach: Insights from three‐dimensional computational fluid dynamics simulations

“…The other three subsurface layers, from the bottom of the alluvium layer to basalt, are the Hanford layer, the Ringold gravel layer, and the Ringold lower mud layer, extending to the lower boundary of the alluvium layer, 98 and 78 m above the sea level, respectively. This subsurface structure and its properties for the control case (see Table ) were obtained from available characterization data for the Hanford site (Newcomb & Brown, ; Newcomb, Strand, & Frank, ; Thorne et al, ; USDOE, ; Williams, Rockhold, Thorne, & Chen, ; Zachara, Freshley, Last, Peterson, & Bjornstad, ).…”