Knowledge of processes that govern the transport of colloid-sized particles through the vadose zone is required to predict the movement of sorbing contaminants within soils, to assess the risks associated with the entry of pathogenic microbes into drinking-water aquifers, and to estimate the time scales for soil illuviation and soil-profile development. Colloid deposition at air-water and solid-water interfaces plays an important role in controlling the transport rates and porewater concentrations of colloids in the vadose zone. Despite the importance of colloids in vadose-zone processes, a theory suitable for predicting colloid deposition in unsaturated porous media does not exist. The focus of this proposed research is to take steps towards filling this gap in knowledge by defining, in a quantitative way, the fundamental relationships between colloid-deposition kinetics at air-water and solid-water interfaces and the physical properties of the soil-water-colloid system. These relationships will be derived through analysis of a suite of pore-scale simulations of water flow and advective-diffusive colloid transport through partially saturated void spaces. Modern models for air-water configuration in variably saturated porous media will be used to specify the geometry of the porewater domains for which the pore-scale simulations of colloid transport will be conducted. Regression analysis of the results of the flow-and-transport simulations is expected to yield power-law equations that express the rate of colloid collisions with air-water and solid-water interfaces as functions of fluid-flow velocity, colloid size and density, porous-medium texture, and capillary pressure (which controls water saturation). Except for the need to account for the presence of the air phase, the proposed approach for deriving the power-law equations for colloid-interface collision rates is conceptually similar to published approaches used to formulate the colloid-filtration equations for water-saturated porous media. Predictions of colloid-deposition kinetics made with the power-law equations will be compared with measurements of colloid deposition made in laboratory column experiments. In these experiments, the sensitivity of colloid-deposition rates to variations in colloid diameter, grain-size, grain-size distribution, porewater velocity, and capillary pressure will be examined. Comparison of measured and predicted deposition rates will reveal deficiencies in the new theory and help direct its refinement. Findings from this study will advance current knowledge of mass-transport processes in partially saturated geologic materials and substantially improve our understanding of key mass-transfer reactions that affect porewater concentrations of colloids within the vadose zone. This proposed research represents a first attempt at deriving and testing a colloid-filtration theory for unsaturated porous media that is needed to make quantitative inferences regarding colloid mobility in the vadose zone. Such a theory should make an important contribution towards addressing critical water-quality issues of national concern that are related to the movement of microbial pathogens and colloid-associated contaminants through near-surface environments.
