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United States Department of Agriculture

Agricultural Research Service

Title: Straining and Attachment of Colloids in Physically Heterogeneous Porous Media

Authors
item Bradford, Scott
item Bettahar, Mehdi - P-DOC,USSL,RIVERSIDE,CA
item Simunek, Jirka - UC RIVERSIDE, CA
item Van Genuchten, Martinus

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 20, 2004
Publication Date: May 20, 2004
Citation: Bradford, S.A., Bettahar, M., Simunek, J., Van Genuchten, M.T. 2004. Straining and attachment of colloids in physically heterogeneous porous media. Vadose Zone Journal. 3:384-394.

Interpretive Summary: Colloids particles such as disease causing bacteria and viruses pose a serious threat to food and water supplies in the United States. Most research to date on colloid transport and fate has been conducted in uniform soil systems. Natural soil systems are much more complex due to variations in soil texture (size). This manuscript reports on research designed to explore the role of variations in soil texture on colloid transport and fate. Our data and computer simulations indicates that colloid mobility is significantly affected by the colloid and soil size, and soil texture variations. This information will provide scientists and engineers with an improved understanding of colloid transport and fate in natural soils systems, and should therefore facilitate the accurate assessment of contaminant potential and the development of more efficient remediation strategies.

Technical Abstract: Colloid transport studies were conducted in water saturated physically heterogeneous systems to gain insight into the processes controlling transport in natural aquifer and vadose zone (variably saturated) systems. Stable monodispersed colloids (carboxyl latex microspheres) and porous media (Ottawa quartz sands) that are negatively charged were employed in these studies. The physically heterogeneous systems consisted of various combinations of a cylindrical sand lens embedded in the center of a larger cylinder of matrix sand. Colloid migration was found to strongly depend upon colloid size and physical heterogeneity. A decrease in the peak effluent concentration and an increase in the colloid mass removal in the sand near the column inlet occurred when the median grain size of the matrix sand decreased or the size of the colloid increased. These observations and numerical modeling of the transport data indicated that straining was sometimes an important mechanism of colloid retention. Experimental and simulation results suggest that attachment was more important when the colloid size was small relative to the sand pore size. Transport differences between conservative tracers and colloids were attributed to flow bypassing of finer-textured sands, colloid retention at interfaces of soil textural contrasts, and exclusion of colloids from smaller pore spaces. Colloid retention in the heterogeneous systems was also influenced by spatial variations in the pore water velocity. Parameters in straining and attachment models were successfully optimized to the colloid transport data. The straining model typically provided a better description of the effluent and retention data than the attachment model, especially for larger colloids and finer-textured sands. Consistent with previously reported findings, straining occurred when the ratio of the colloid and median grain diameters was greater than 0.5%.

Last Modified: 9/22/2014
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