TRANSPORT OF COLLOIDS IN UNSATURATED POROUS MEDIA

Colloid transport in the subsurface is of much concern to protection of water supply due to introduction of pathogens such as bacteria, protozoa and viruses, as well as the potential for transport of colloid-bound toxic chemicals. In the past three decades, the vast majority of the studies focused on water-saturated (groundwater) environments, even though most pathogens and toxicants enter groundwater via transport through the vadose zone which is only partially water-saturated. Further, almost all of studies of unsaturated systems are limited to steady-state flow, while in nature, steady-state flow seldom takes place in natural subsurface environments. Flow in partially saturated porous media is dominated by transient wetting and drying events (e.g., storms, flushing toilets, and drainage). The work presented here uses a novel experimental approach to evaluate colloid transport with transient wetting fronts. Evaluations are made on the relative importance of water saturation, colloid surface charge, solution surface tension, and the role of electrostatic and capillary forces in colloid retention. The results measured under unsaturated transient flow condition are compared to those observed under saturated steady-state flow condition, which mainly deals with solid-liquid interfacial reaction. The results obtained by sectioning the horizontal columns show that total resident colloid concentrations decreased exponentially with the travel distance, and the relationship was sensitive to changes of ionic strength and surface tension of solution. Increase of ionic strength increased colloid concentrations near the colloid inlet but decreased the concentrations in the sand far from the inlet. Decrease of solution surface tension was found to cause increase of colloid mobility, i.e., more colloids traveled to farther place, though the travel speed substantially slowed down relative to the water solution. The results of this study suggest that both electrostatic force and capillary force are involved in the colloid retention and transport. Mechanistic study on the transient transport of colloids can improve our understanding of pathogen transport and facilitate development of novel strategies for site-specific management of pathogen contamination under natural flow conditions.