The role of moving air-water interfaces in colloid mobilization within the vadose zone

The movement of colloid‐sized particles through the vadose zone has important water‐quality implications, yet the mechanisms that govern colloid mobilization are poorly understood. Results of our laboratory column experiments demonstrate that moving air‐water interfaces associated with a downward‐propagating drying front are capable of scouring kaolinite colloids from the surfaces of quartz sand. The efficiency of air‐water interfaces in capturing the immobile colloids increases with increasing drying‐front velocity and decreasing porewater ionic strength. A model that approximates the sand pack as a bundle of capillary tubes and that links pressure‐head changes during drainage to the rates of air‐water interface movement through the tubes reproduces the time‐series data on effluent‐kaolinite concentrations.