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Title: Colloid Transport in Unsaturated Porous Media: The Role of Water Content and Ionic Strength on Particle Straining

item Bradford, Scott
item Van Genuchten, Martinus

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2007
Publication Date: 11/17/2007
Citation: Torksaban, S., Bradford, S.A., Van Genuchten, M.T., Walker, S.L. 2007. Colloid Transport in Unsaturated Porous Media: The Role of Water Content and Ionic Strength on Particle Straining. Journal of Contaminant Hydrology. Vol 96:113-127

Interpretive Summary: An understanding of the transport and fate of colloids in unsaturated soil is needed to protect groundwater resources from colloidal contaminants such as pathogenic microorganisms. Experimental and modeling studies were conducted to examine the influence of water content and the chemical composition of the water on the migration and removal of colloids in unsaturated sand. Results indicate that the influence of water chemistry on the removal of colloids in small pores is enhanced at lower water contents and for smaller sand sizes. This observation can be explained by two factors. First, the number of colloids directed to small pores by flowing water is believed to depend on the chemical composition of the water. Second, a larger fraction of the water flows to this small pores as the water content of the sand decreases. This information should improved our ability to quantify microorganism transport in unsaturated soils.

Technical Abstract: Packed column and mathematical modeling studies were conducted to explore the influence of water saturation, pore-water ionic strength, and grain size on the transport of latex microsphere colloids (1.1 micron) in porous media. Experiments were carried out under unfavorable conditions in terms of colloid attachment to solid-water interfaces (SWI) and air-water interfaces (AWI) by using hydrophilic and negatively charged colloids and modifying the solution chemistry with a bicarbonate buffer to pH 10. DLVO calculations and complementary batch experiments were conducted and demonstrated that attachment of colloids to the SWI or AWI was insignificant across the range of the ionic strengths considered. The breakthrough curve and the final deposition profile were measured in each experiment showing that the retention was highly dependent on the suspension ionic strength, water content, and sand grain size. In contrast with conventional filtration theory, most of the colloids were found deposited close to the column inlet, and hyper-exponential deposition profiles were usually observed to decrease monotonically with depth. A mathematical model accounting for time and depth dependent straining produced a reasonable fit for both the breakthrough curves and final deposition profiles. Results suggest that straining is the primary mechanisms of colloid retention under both saturated and unsaturated conditions for the selected experimental conditions. The impact of straining increased with decreasing water content due to enhanced regions of flow stagnation. Increasing the solution ionic strength is also believed to enhance straining by leading the weakly associated colloids in secondary energy minimum to stagnation regions such as grain-grain junctions and dead-end pores and retaining the entrapped colloids in place.