Colloid transport and remobilization in porous media during infiltration and drainage

Abstract

Colloids are potential vectors of many contaminants in porous media. Understanding colloid transport is critical for assessing the migration of contaminants (e.g., pathogens) in the vadose zone. In this study, a series of column experiments were conducted to investigate the coupled effects of flow velocity, water content, and solution ionic strength on transport and remobilization of a model colloid (montmorillonite clay) in a model porous medium (Accusand) during transient unsaturated flow and steady-state saturated flow. The unsaturated transport experiments included a series of infiltration and drainage pulses (e.g., infiltration with colloids, followed by drainage of colloid suspensions, followed by infiltration with a colloid-free solution and drainage of the solution). Saturated flow experiments included only the infiltration of the colloid and colloid-free solutions. Tests were repeated for a variety of solution ionic strengths. Results showed that colloid transport was more sensitive to changes in solution ionic strength at low infiltration rates, and the effect of infiltration rate was more significant at high ionic strength. As a result, increased flow velocities and water content, resulting from high infiltration rates, enhanced colloid transport and remobilization under ionic strength conditions (e.g., 100 mM) that would otherwise lead to strong colloid retention. This observation conceptually suggests that chemical threshold values for preventing colloid movement in porous media might be larger for transient flow conditions than for uniform flow conditions. In addition, drainage was found to induce remobilization of the retained colloids, suggesting transport of colloids even after termination of injection. Overall, the study experimentally highlights the complicated interdependence of the effects of water content, flow velocity, and solution chemistry on colloid transport and remobilization.