Mobilization and transport of in situ colloids during drainage and imbibition of partially saturated sediments

Colloid mobilization and transport in the vadose zone have important implications to the migration of many contaminants that adsorb to mineral surfaces. The objective of this study is to elucidate the influences of transient flow events on the mobilization of in situ colloids from water-unsaturated natural sediments. Results from our laboratory column experiments demonstrate that sand pack drainage and imbibition mobilized substantial quantities of mineral colloids and that effluent colloid concentrations peaked concomitantly with the passage of the wetting and drying fronts. In these experiments, drainage events occurred first and mobilized higher concentrations of in situ colloids than the following imbibition events, and the mass of colloids mobilized was sensitive to pore water ionic strength, as well as to the rates of imbibition and drainage. The data from the column experiments were used to test a new mathematical model that is based on the assumption that colloid mobilization is driven by pore-scale changes in air-water configuration that occur during porous medium drainage and imbibition. The simulated changes in air-water configuration are predicted on the basis of the measured water retention characteristics of the porous medium. Agreement between measured and modeled concentrations of colloids is generally good, and the parameters that govern the kinetics of colloid mobilization and redeposition vary in a discernible fashion with changes in ionic strength, imbibition rates, and drainage rates.