PNNL

Abstract

This report describes geochemical, microbial, and physical characterization of Hanford vadose zone boreholes C9552, C9487, and C9488 (located in the BY Cribs, B-7AB, and B-8 waste sites, respectively) for measurement of contaminants of concern (uranium, technetium, iodine species, chromate, cyanide, other co-contaminants) and evaluation of attenuation processes and other factors that affect transport of these contaminants. Uranium contamination was low to moderate in most cores (0.3 to 1.0 ug/g) with high uranium (54 ug/g) in C9487 58’ depth. The uranium surface phase distribution was 3 to 12% aqueous/adsorbed, 4 to 40% carbonate-associated phases, and 40 to 80% oxides/silicates associated. Leach rates decreased over time reflecting high initial release of the aqueous/adsorbe fraction followed by slow release of a portion of uranium in carbonates. The uranium leached mass correlated well with the release rate (i.e., sediments with high U contamination had a higher aqueous/adsorbed fraction and a higher release rate). The iodine contamination ranged from 0.014 to 0.054 ug/g, of which a small fraction was aqueous and adsorbed (5 to 15%), a high fraction carbonate associated (60% to 75%, partially mobile), and a smaller fraction Fe/Mn oxide associated (25% to 40%, low mobility). Long-term batch and column leaching showed initial rapid release of half of the iodine mass (predominantly as iodide), followed by slower release from carbonates (predominantly as iodate). This behavior is consistent with low reported adsorption of iodide, moderate sorption of iodate, and the slow dissolution of carbonates, which can incorporate iodate. The leached mass in most sediments was dominated by iodide, with two sediments dominated by iodate. Experiments with 100 ug/L iodate addition showed a reduction to iodide at differing rates in hundreds of hours. Some B-Complex sediments had low Tc-99 contamination (