Colloid and Cesium-137 Mobilization and Transport in Vadose-Zone Sediments Under Transient-Flow Conditions.

Mobilization of mineral colloids in the vadose zone has the potential to enhance the transport of many contaminants. The objective of this study is to investigate the co- transport of radionuclides by mineral colloids under transient flow conditions. We performed column drainage and imbibition experiments and model simulation to study the release of in-situ colloids and cesium-137 from vadose zone sediments collected from Hanford, Washington. Our results show high concentrations of colloids and cesium-137 were mobilized with the passage of drying and wetting fronts. At porewater ionic strength of 0.16 mM, 77 mg colloids and 117 nCi cesium-137 were released during drainage and subsequent imbibition, and 93% of the eluted cesium-137 were in colloid- associated form. As the porewater ionic strength increased to 50 mM, 1.5 mg colloids and 41 nCi cesium-137 were released, and only 3% of the eluted cesium-137 were associated with colloids. Both our experimental results and model calculation show when the column length increased from 10.4 to 20.8 cm, the eluted colloid and cesium- 137 mass increased only 50%, indicating significant amount of the mobilized colloids re- deposited during migration. Our model simulation further demonstrates at higher porewater ionic strength and low flow velocity, which more resembles field conditions, co-transport of cesium-137 diminished as colloid re-deposition became more influential. Our results also reveal when contaminated colloids invaded an uncontaminated zone, cesium-137 transport was reduced due to both colloid re-deposition and cesium-137 desorption from the mobile colloids. Taken together, our findings demonstrate cesium- 137 transport in vadose-zone sediments is controlled by colloid mobilization, transport, and re-deposition, as well as cesium-137 adsorption/desorption. Geochemical conditions, e.g., porewater ionic strength, that influence the above processes will exert strong control on cesium-137 co-transport by colloids.