Description: In this study, sediments collected from boreholes drilled in 2010 and 2011 as part of a remedial investigation/feasibility study were characterized. The wells, located within or around two process ponds and one process trench waste site, were characterized in terms of total uranium concentration, mobile fraction of uranium, particle size, and moisture content along the borehole depth. In general, the gravel-dominated sediments of the vadose zone Hanford formation in all investigated boreholes had low moisture contents. Based on total uranium content, a total of 48 vadose zone and periodically rewetted zone sediment samples were selected for more detailed characterization, including measuring the concentration of uranium extracted with 8 M nitric acid, and leached using bicarbonate mixed solutions to determine the liable uranium (U(VI)) contents. In addition, water extraction was conducted on 17 selected sediments. Results from the sediment acid and bicarbonate extractions indicated the total concentrations of anthropogenic labile uranium in the sediments varied among the investigated boreholes. The peak uranium concentration (114.84 µg/g, acid extract) in <2-mm size fractions was found in borehole 399 1-55, which was drilled directly in the southwest corner of the North Process Pond. Lower uranium concentrations (~0.3–2.5 µg/g, acid extract) in <2-mm size fractions were found in boreholes 399-1-57, 399-1-58, and 399-1-59, which were drilled either near the Columbia River or inland and upgradient of any waste process ponds or trenches. A general trend of “total” uranium concentrations was observed that increased as the particle size decreased when relating the sediment particle size and acid extractable uranium concentrations in two selected sediment samples. The labile uranium bicarbonate leaching kinetic experiments on three selected sediments indicated a two-step leaching rate: an initial rapid release, followed by a slow continual release of uranium from the sediment. Based on the uranium leaching kinetic results, quasi equilibrium can ...
Date: November 26, 2012
Creator: Wang, Guohui; Serne, R. Jeffrey; Lindberg, Michael J.; Um, Wooyong; Bjornstad, Bruce N.; Williams, Benjamin D. et al.
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