Predictive approaches for the transport of colloids (microscopic, waterborne particles) through unsaturated soils are unavailable. Consequently, it is impossible to evaluate the risks associated with the transmission of colloid-sized contaminants through the vadose zone and into drinking-water aquifers or to inform management decisions intended to minimize these risks. Our work is aimed at addressing these deficiencies. In particular, we (the PI, a postdoctoral associate, and a Ph.D. student) will:
(1) measure the effects of infiltration rate and soil texture on colloid transport through vadose-zone soils during transient flow; (2) elucidate the responses of colloid mobility within vadose-zone soils to changes in colloid size and composition and pore-water chemistry; and (3) develop a mathematical model that is suitable for quantifying colloid mobilization and transport during transient, unsaturated flow through geologic media.
Our work will begin with laboratory experiments on the movement of mineral colloids through real vadose-zone soils. This laboratory component will involve column experiments designed to illuminate the sensitivity of colloid mobilization, transport, and deposition during soil imbibition and drainage to changes in soil texture, pore-water ionic strength, and the physicochemical properties of the colloids themselves. The column experiments will be supplemented by pore-scale visualization experiments that will reveal the principal mechanisms of colloid mobilization and retention under the physicochemical conditions examined in the column experiments. These experiments will also elucidate the changes in air-water configuration that occur during transient flow and that presumably drive colloid release. The visualization experiments, together with the column experiments, will be used to guide the development of a model for coupled transient porewater flow, colloid transport, and colloid mass transfer. This model will be based on modern conceptualizations of air-water configuration within unsaturated porous media and will approximate the effects of multiple mechanisms on colloid mobilization, rather than treating mobilization as a lumped process without a physical basis. By the end of the project period, a comprehensively evaluated model will be created that is suitable for making inferences on colloid mobility within unsaturated soils. The overall findings from this research should gain the attention of hydrologists, soil scientists, aquatic chemists, and engineers interested in two-phase flow phenomena, the fate and transport of engineered nanoparticles in geologic environments, soil formation processes, and the transmission of pathogens and colloid bound pollutants through unsaturated soils and towards the water table.
