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Title: STRAINING OF COLLOIDS AT TEXTURAL INTERFACES

Author
item Bradford, Scott
item SIMUNEK, JIRKA - U.C. RIVERSIDE, CA
item BETTAHAR, MEHDI - PARSONS ENG, CALIF.
item Tadassa, Yadata
item Van Genuchten, Martinus
item Yates, Scott

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2005
Publication Date: 10/11/2005
Citation: Bradford, S.A., Simunek, J., Bettahar, M., Tadassa, Y.F., Van Genuchten, M.T., Yates, S.R. 2005. Straining of colloids at textural interfaces. Water Resources Research. 41:W10404, doi:10.1029/2004WR003675.

Interpretive Summary: Natural soil and aquifer systems frequently contain layers and lenses of contrasting soil texture (size). In comparison to uniform soils, relatively few studies have examined the transport behavior of colloids, such as disease causing microorganisms, at textural interfaces. Laboratory and computer studies were conducted to investigate mechanisms of colloid deposition in layered sands. Retention of colloids in small soil pores (straining) was most pronounced when water flowed from a coarser to a finer textured soil, especially for larger colloids and greater contrasts in soil size. A computer model to predict colloid transport was modified to describe the filling and accessibility of small pores (straining sites) to colloids. Findings from this study have important implications for the design of efficient water treatment techniques based upon soil passage, as well as the transport and fate of microorganisms and colloid-associated contaminants in heterogeneous soils.

Technical Abstract: Although natural soil and aquifer systems often contain layers and lenses of contrasting soil texture, relatively little research has focused on the mechanisms of colloid retention at textural interfaces. Saturated column studies were undertaken to characterize the straining behavior of negatively charged latex colloids (1.1 and 3.0 micron) at soil textural interfaces. Mechanisms of colloid transport and retention were deduced from measured effluent concentration curves, final spatial distributions in the columns, mass balance information and numerical modeling. Transport and deposition of colloids were found to be highly dependent upon the textural interface. Deposition of colloids in a given sand was always most pronounced at the soil surface. Here colloids enter a new pore network and are more likely to encounter smaller pores or dead-end regions of the pore space that contribute to straining. Less deposition occurred at textural interfaces within the column than at the soil surface. We believe that this is due to the fact that advection, dispersion and size exclusion tends to confine colloid transport to the larger pore networks, thus limiting accessibility to straining sites. Increasing the textural contrast at an interface produced greater colloid deposition when water flowed from coarser to finer textured sands. Conversely, when water flowed from finer to coarser textured sands, little deposition occurred. Numerical modeling indicates the need to account for blocking (filling) and accessibility of straining sites in layered systems. A previously developed straining model was modified to account for this behavior.