Colloids are ubiquitous agents of nutrient and contaminant transport in the subsurface. Beyond what is predicted by two-phase models, mobile colloids have been shown to facilitate the transport of contaminants having a high affinity for their surfaces. Additionally, the migration of microorganisms responsible for contaminant degradation is also a colloidal transport process. Understanding colloid transport processes is therefore essential for predicting the migration and breakdown of soil-borne contaminants, and for the design and implementation of bioremediation systems. Heterogeneity in the environment is ubiquitous, occurring at textural transitions, the capillary fringe, and at wetting and draining fronts, and has profound impact on colloidal transport. The objective of this study is to observe the effects of meso-scale heterogeneity on the movement of colloids in variably-saturated porous media. Laboratory experiments on colloid transport have been conducted largely in columns where data collection has been limited to effluent break through and post-experimental destructive sampling, which has not allowed observation of the effects of spatial and temporal variability upon colloidal transport processes. The light-transmission system which will be used and further advanced in this work holds great promise for elucidation of transport dynamics resulting from meso-scale heterogeneity. This visualization technique is well-suited to observing the effects of saturation transitions and physical heterogeneities on colloidal transport. Heterogeneities explored will include the water content variation found at the capillary fringe, textural variation as layering of different media grades perpendicular to flow paths, and variation of the flow regime to mimic precipitation-driven infiltration events. The impact of particle size and colloid concentration will be explored within each regime.
