Colloid mobilization and transport within unsaturated porous media under transient-flow conditions

[1] Contaminant-scavenging colloids are mobilized in the vadose zone during infiltration events, characterized by transient flow regimes. In the research reported here we develop a mathematical model in order to investigate the influences of flow transients on colloid mobilization in unsaturated media. The model solves coupled equations for unsteady pore water flow, colloid release, advective-dispersive colloid transport, and redeposition of pore water colloids. The immobile-phase colloid population is discretized into a series of compartments, each of which are assigned a value of critical moisture content (Theta(cr)) according to a piecewise linear density function. A compartment is activated when the model-calculated moisture content exceeds the Theta(cr) for the compartment, whereupon colloids are released at a rate proportional to the product of the pore water velocity and the colloid concentration within the compartment. We fit solutions of the model equations to data on the mobilization and transport of silica colloids within columns of unsaturated quartz sand. The model accurately reproduces measured pulse-type colloid releases induced by successive step-change increases in flow rate and moisture content. This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.