Colloid-facilitated transport of cesium in variably saturated Hanford sediments.

Radioactive 137Cs has leaked from underground waste tanks into the vadose zone at the Hanford Reservation in south-central Washington State. There is concern that 137Cs, currently located in the vadose zone, can reach the groundwater. In this study, we investigated whether, and to what extent, colloidal particles can facilitate the transport of 137Cs at Hanford. We used colloidal materials isolated from Hanford sediments. Transport experiments were conducted under variably saturated, steady-state flow conditions in repacked, 20 cm long Hanford sediment columns, with effective water saturations ranging from 0.2 to 1.0. Cesium, pre-associated with colloids, was stripped off during transport through the sediments. The higher the flow rates, the less Cs was stripped off, indicating in part that Cs desorption from carrying colloids was a residence-time-dependent process. Depending on the flow rate, up to 70% of the initially sorbed Cs desorbed from colloidal carriers and was captured in the stationary sediments. Less Cs was stripped off colloids under unsaturated than under saturated flow conditions at similar flow rates. This phenomenon was likely due to the reduced availability of sorption sites for Cs on the sediments as the water content decreased and water flow was divided between mobile and immobile regions.