Diffusive release of uranium from contaminated sediments into capillary fringe pore water.

Despite remediation efforts at the former nuclear weapons facility, leaching of uranium (U) from contaminated sediments to the ground water persists at the Hanford site 300 Area. Flooding of contaminated capillary fringe sediments due to seasonal changes in the Columbia River stage has been identified as a source for U supply to ground water. We investigated U release from Hanford capillary fringe sediments by packing sediments into reservoirs of centrifugal filter devices and saturating them with Columbia River water for 3 to 84days at varying solution-to-solid ratios. After specified times, samples were centrifuged. Within the first three days, there was an initial rapid release of 6-9% of total U, independent of the solution-to-solid ratio. After 14days of reaction, however, the experiments with the narrowest solution-to-solid ratios showed a decline in dissolved U concentrations. The removal of U from the solution phase was accompanied by removal of Ca and HCO(3)(-). Geochemical modeling indicated that calcite could precipitate in the narrowest solution-to-solid ratio experiment. After the rapid initial release in the first three days for the wide solution-to-solid ratio experiments, there was sustained release of U into the pore water. This sustained release of U from the sediments had diffusion-limited kinetics.