Colloid-Filtration Theory for Vadose-Zone Systems

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0409174SaiersKnowledge of processes that govern the transport of colloid-sized particles through thevadose zone is required to predict the movement of sorbing contaminants within soils, to assessthe risks associated with the entry of pathogenic microbes into drinking-water aquifers, and toestimate the time scales for soil illuviation and soil-profile development. Colloid deposition atair-water and solid-water interfaces plays an important role in controlling the transport rates andporewater concentrations of colloids in the vadose zone. Despite the importance of colloids invadose-zone processes, a theory suitable for predicting colloid deposition in unsaturated porousmedia does not exist.The focus of this proposed research is to take steps towards filling this gap in knowledgeby defining, in a quantitative way, the fundamental relationships between colloid-depositionkinetics at air-water and solid-water interfaces and the physical properties of the soil-water-colloidsystem. These relationships will be derived through analysis of a suite of pore-scalesimulations of water flow and advective-diffusive colloid transport through partially saturatedvoid spaces. Modern models for air-water configuration in variably saturated porous media willbe used to specify the geometry of the porewater domains for which the pore-scale simulationsof colloid transport will be conducted. Regression analysis of the results of the flow-and-transportsimulations is expected to yield power-law equations that express the rate of colloidcollisions with air-water and solid-water interfaces as functions of fluid-flow velocity, colloidsize and density, porous-medium texture, and capillary pressure (which controls watersaturation). Except for the need to account for the presence of the air phase, the proposedapproach for deriving the power-law equations for colloid-interface collision rates isconceptually similar to published approaches used to formulate the colloid-filtration equationsfor water-saturated porous media.Predictions of colloid-deposition kinetics made with the power-law equations will becompared with measurements of colloid deposition made in laboratory column experiments. Inthese 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 newtheory and help direct its refinement.Findings from this study will advance current knowledge of mass-transport processes inpartially saturated geologic materials and substantially improve our understanding of key mass-transferreactions that affect porewater concentrations of colloids within the vadose zone. Thisproposed research represents a first attempt at deriving and testing a colloid-filtration theory forunsaturated porous media that is needed to make quantitative inferences regarding colloidmobility in the vadose zone. Such a theory should make an important contribution towardsaddressing critical water-quality issues of national concern that are related to the movement ofmicrobial pathogens and colloid-associated contaminants through near-surface environments.