Determining (Un)certainty in Modeling Colloid Transport by Visualization

Modeling transport and retention of colloids in the vadose zone has received considerable attention in recent years. In spite of the many colloid modeling studies, validation of the models has been limited to visulization with static micromodels and to matching simulated with experimental break through in soil columns. These modeling studies have invoked colloid sorption to the air-water interface as one of the mechanism for colloid retention. There is no independent validation that this is occurring in sand columns. In this presentation, we describe a pore-scale visualization technique to observe the distribution and retention of colloids in water between silica sand grains at the air-water interface and on the surface of the grains. This new method is adapted to the three-dimensional, sandy soil matrix with flowing water. Using this method we describe significant processes of colloid retention in unsaturated soils and compare them to what has been assumed in computational models. We found that colloid sorption to the air-water interface might occur but it was not a mechanism in the retention of colloids. Retention at the air-water-solid interface was significant and could explain the reduction in concentration of observed breakthrough of colloids. Our visualization method provides a critical link from the micromodel visualization studies with stagnant water to the transient sand column experiments. Ultimately the visualization method will decrease the uncertainty in modeling colloid transport in the vadose zone.