Geochemical Processes Controlling Migration of Tank Wastes in Hanford's Vadose Zone

Abstract

Nuclear wastes from Hanford's processing for separation of plutonium are stored in massive, buried, single‐shell tanks in 18 tank farms. These so‐called tank wastes were initially thermally hot because of radioactive decay, and many exhibited extreme chemical character in terms of pH, salinity, and radionuclide concentration. At present, 67 of the 149 single shell tanks are suspected to have released over 1.9 million L of tank waste to the vadose zone, with most leak events occurring between 1950 and 1975. Boreholes have been placed through the largest vadose zone plumes to define the extent of contaminant migration and to develop conceptual models of processes governing the transformation, retardation, and overall transport of tank waste residuals. Laboratory studies with sediments so collected have shown that ion exchange, precipitation and dissolution, and surface complexation reactions have occurred between the tank wastes and subsurface sediments, moderating their chemical character and retarding the migration of select contaminants. Processes suspected to facilitate the far‐field migration of immobile radionuclides including stable aqueous complex formation and mobile colloids were found to be potentially operative but unlikely to occur in the field, with the exception of cyanide‐facilitated migration of 60Co. Certain fission product oxyanions (Mo, Ru, Se, Tc) and nitrates are the most mobile of tank waste constituents because their adsorption is suppressed by large concentrations of waste anions, the vadose zone clay fraction is negative in surface charge, and, unlike Cr, their reduced forms are unstable in oxidizing environments. Reaction/process‐based transport modeling is beginning to be used for predictions of future contaminant mobility and plume evolution.

Rates and mechanisms of uranyl oxyhydroxide mineral dissolution

“…Uranyl (UO 2 2+ ) oxyhydroxide minerals are commonly formed in natural environments from the weathering of U (IV) oxide solids under oxidizing conditions (Hazen et al, 2009), corrosion of spent nuclear fuel in a geological repository , or potential precipitation from liquid uranium (U) waste released directly into the subsurface (Zachara et al, 2007). In the absence of dissolved phosphate or silicate, U(VI) has been observed to form a variety of crystalline and amorphous oxyhydroxide mineral phases in oxic subsurface environments (Morris et al, 1996;Hunter and Bertsch, 1998) and in synthetic acidic-tocircumneutral wastewaters (Kanematsu et al, 2014).…”

Evaluation of Soil Flushing for Application to the Deep Vadose Zone in the Hanford Central Plateau

“…The Hanford and Ringold Formation deposits have similar mineralogy (Zachara et al 2007a). The gravels are mainly basalt, the sands are a split between basalt and granite, and the silt is dominated by quartz, feldspars, and micas.…”

“…Unconsolidated sediments, between 30 and 122 m thick, overlie the Columbia River Basalt Group lava flows at the 200 Area (Zachara et al 2007a). A laterally extensive, low permeability formation called the CCU, or Cold Creek Unit, is a mixture of caliche (or calcrete), a fine-to coarse-grained, calcium-carbonate cemented paleosol, and a cohesive, compact, massive to laminated, and stratified finegrained sand and silt (e.g., sandy mud).…”

“…The Hanford formation contains a sequence of interbedded sand and mud, coarse to medium sand, and sandy gravel (Oostrom et al 2008). The Ringold Formation is a sedimentary sequence of fluvial-lacustrine clay, silt, sand, and granule to cobble gravel deposited by the ancestral Columbia River, and the water table is located within the Ringold Formation at the 200 Area (Zachara et al 2007a). …”

Surface Geophysical Exploration - Compendium Document

“…The low kd of this anion allows it to migrate virtually unimpeded through the vadose zone and nitrate plumes typically show large low resistivity signatures. On the other hand, cesium (a cation and gamma ray emitter) was released in small quantities ionically, and has a much higher partition coefficient that causes it to partially sorb onto the Hanford sediments (Zachara et al, 2007 andSteefel et al, 2003, "Cesium Migration In Hanford Sediment: A Multisite Cation Exchange Model Based on Laboratory Transport Experiments"). The cesium-based salts, therefore, do not have a signature much different than the expected background resistivity conditions.…”