PNNL

Abstract

Despite remediation efforts at the former nuclear weapons facility at the Hanford site (Washington State), leaching of uranium (U) from contaminated sediments to the ground water persists at the Hanford 300 Area. Flooding of contaminated capillary fringe sediments due to seasonal changes in the Columbia River stage has been identified as a reason of continued U supply to ground water. We investigated the release of U from Hanford capillary fringe sediments to pore water. Contaminated Hanford sediments were packed into reservoirs of centrifugal filter devices and saturated with Columbia River water for 3 to 84 days at varying solution-to-solid ratios (1:3, 1:1, 5:1, 10:1, 25:1 mL:g). After specified times, samples were centrifuged to a gravimetric water content of 0.11 ± 0.06 g g-1. Within the first three days, there was an initial rapid release of 6-9% of total U from the sediments into the pore water, independent of the solution-to-solid ratio. After 14 days of reaction, however, the experiments with the narrowest solution-to-solid ratios (1:3 and 1:1 mL:g) showed a decline in dissolved U concentrations. The removal of U from the solution phase was accompanied by removal of Ca and HCO3-. Geochemist workbench simulations, conducted using measured solution concentrations from experiments, indicated that calcite could precipitate in the 1:3 solution-to-solid ratio experiment. After the rapid initial release in the first three days for the 5:1, 10:1, and 25:1 solution-to-solid ratio experiments, there was sustained release of U into the pore water. Up to 22% of total U was released on day 84 for the 25:1 solution-to-solid ratio reaction. This sustained release of U from the sediments had diffusion-limited kinetics.