Straining of colloids at textural interfaces

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 deposition at textural interfaces. Saturated column studies were undertaken to characterize the straining behavior of negatively charged latex colloids (1.1 and 3.0 (mu)m) at textural interfaces. Mechanisms of colloid transport and retention were deduced from measured effluent concentration curves, final spatial distributions in the columns, mass balance information, microscopic examination of deposition behavior in micromodel experiments, 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 sand (inlet) 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 sand surface. We believe that this is due to the fact that advection, dispersion, and size exclusion tend 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.