COLLOID MOBILIZATION AND DEPOSITION IN UNSATURATED POROUS MEDIA

An improved understanding of the processes that govern the movement of colloid-sized particles through unsaturated porous media is requisite to addressing issues related to the migration of radionuclides and pesticides through the vadose zone, pollution of groundwater supplies by microbial pathogens, and soil-profile development. Four processes affect the movement of colloids through unsaturated media: mobilization, advection, dispersion, and deposition. Our work is aimed at advancing current knowledge of the mechanisms that govern colloid mobilization and deposition in homogeneous and heterogeneous porous media. This research relies on laboratory-based observations made at the pore and column scales and on mathematical modeling. Results of our experiments with homogeneously packed sand columns indicate that colloid deposition rates are sensitive to colloid mineralogy, vary directly with porewater ionic strength, and decrease with increasing moisture content. Observations from experiments with porous-medium micromodels reveal that colloid immobilization in the column experiments is controlled by attachment to air-water interfaces, straining within thin-water films that surround mineral grains of partially saturated pores, and storage within stagnant-water zones that branch from advecting porewater regions. Although release of retained colloids is slow during steady flow, mobilization is rapid during transient flow and reflects the release of strained colloids from expanding thin-water films and from immobile-water zones that reconnect with advecting porewater regions during porous-medium imbibition. The deposition and mobilization mechanisms that control colloid mobility within homogeneous media also influence colloid transport through partially saturated columns containing structured heterogeneity, but colloid transfer between zones of differing permeability complicates descriptions of this transport. Our ongoing research is devoted to elucidating properties of the soil-water-colloid system that influence this inter-domain exchange and to developing a mathematical model appropriate for quantifying variably saturated flow and colloid transport through structured, heterogeneous media.