Preferential flow and solute/colloid transport in porous media

Preferential flow has important agricultural and environmental implications, as it facilitates transport of pollutants through vadose zone to groundwater. This research investigates the role of preferential flow on the solute and colloid transport in porous media through visualization technique, experiments and modeling. A light transmission method (LTM) was developed to visualize preferential flow and to measure fluid contents in transient three-phase flow (water-oil-air) in porous media. The LTM used the hue and intensity of light transmitted through a slab chamber to measure fluid contents. Preferential flow was simulated as water fingered flow entering partially air-saturated sand (Experimental set I). To investigate the preferential flow transport of solutes (chloride) and colloids (Cryptosporidium parvum oocysts) in porous media, laboratory experiments were performed. Experimental set II was carried out in sand columns, under conditions known to foster fingered flow. Experimental set III investigated the effect of gas-water interfaces using water-repellent sand barriers. Experimental set IV involved undisturbed soil columns subjected to macropores flow. The columns were subjected to artificial rainfall; and calves' feces containing oocysts and a chloride tracer were applied at the surface. The breakthrough of oocysts and Cl-, monitored in the effluent, demonstrated the importance of preferential flow - fingered flow and macropores flow - on transport of solutes and colloids through vadose zone. The transport of chloride and oocysts was modeled based on partitioning the soil profile in a distribution zone simulated as an exponential loss and a conveyance zone described with the convective-dispersive equation. The model simulated accurately the asymmetric breakthrough patterns.