Transport and Fate of Colloids in Physically Heterogeneous Porous Media

Soil column experiments were conducted to explore the influence of physical heterogeneity on the transport and fate of colloid particles in saturated porous media. The physically heterogeneous systems consisted of various soil combinations of a cylindrical soil lens embedded in the center of a larger cylinder of matrix soil. Colloid migration was found to strongly depend upon colloid size and the physical heterogeneity. Pore straining of colloids lead to a decrease in the peak effluent concentration and an increase in the inlet mass removal when the mean grain size of the matrix sand decreased or the size of the colloid increased. In heterogeneous systems, some colloid retention also occurred as a result of the presence of relatively stagnant flow zones. Colloid mass removal by pore straining was minimized in the heterogeneous systems as a result of flow bypassing when a finer textured lens was embedded in a coarser matrix soil. The transport differences between conservative tracers and colloid particles tended to increase in heterogeneous systems as a result of flow bypassing, pore straining, and size exclusion. Depending upon the colloid and soil size, size exclusion sometimes resulted in earlier colloid breakthrough and less concentration tailing compared to bromide. Transport parameters obtained from the homogeneous experiments were used in conjunction with a first-order attachment/detachment advective dispersion transport model to simulate the heterogeneous experiments. Simulations provided a reasonable description of tracer transport, but tended to overestimate the colloid transport potential, thus suggesting that the model should be refined to more realistically account for pore straining and size exclusion.