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