Colloid and Cs-137 Mobilization and Transport in Vadose Zone Sediments under Transient Flow Conditions

The transport of colloid-associated contaminants through the vadose zone is a challenging problem that has received considerable attention over the last decade. The objective of this work is to elucidate the effects of perturbations in flow on the mobilization and transport of mineral colloids and a radionuclide (cesium-137) within variably saturated sediments. Measurements made during simulated rainfall onto laboratory columns packed with sediments collected from Washington's Hanford Site reveal that high concentrations of in-situ colloids were mobilized with the passage of a drying or wetting front, but colloid mobilization was slow during steady flow. Cs-137 was mobilized with the colloids and its transport occurred almost exclusively in the colloidal form. Significant amount of colloid-associated Cs-137 were desorbed as the cesium-bearing colloids traveled through a zone free of Cs-137 contamination. This desorbed Cs-137 was quickly immobilized by adsorption onto the sediments. Although the Cs-137 concentration on the eluted colloids (Cs-137 mass per colloid mass) was independent of the porewater flow velocity, the effluent colloid concentration, and therefore the total effluent Cs-137 concentration (Cs-137 mass per effluent volume), were strongly influenced by porewater flow velocity during flow transients. These results suggest that the movement of Cs-137 in vadose zone sediments is controlled by colloid mobilization and transport, as well as Cs-137 desorption from the mobile colloids. We used the data from the column experiments to test a new model for colloid mobilization, transport, and re-deposition. The model solves coupled equations for transient porewater flow, advective- dispersive colloid transport, and rate-limited colloid mobilization and deposition. Both the model simulations and experimental observations reveal that colloid mobilization rates during imbibition decreased sharply with decreases in porewater flow velocity, while mobilization rates during drainage were relatively insensitive to porewater flow velocity, and for a given flow velocity, mobilization rates were greater during drainage than imbibition. The above model for colloid mobilization and transport is being modified to account for the interactions between Cs-137 and the colloids, porewater, and sediments in the system so that colloid- facilitated transport of Cs-137 can be predicted. Findings from this work provide sorely needed data on colloid-associated contaminant mobilization and transport within real sediments and a means for quantifying the phenomena.