/AnMtgsAbsts2009.51983 Colloid and Cesium-137 Mobilization and Transport in Vadose-Zone Sediments Under Transient-Flow Conditions.

Wednesday, November 4, 2009: 10:55 AM
Convention Center, Room 401, Fourth Floor

Tao Cheng and James Saiers, School of Forestry & Environmental Studies, Yale Univ., New Haven, CT
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.