Comparison of kinetic and hybrid-equilibrium models simulating colloid-facilitated contaminant transport in porous media

The presence of colloidal particles in groundwater can enhance contaminant transport by reducing retardation effects and carrying them to distances fu rther than predicted by a conventional advective/dispersive equation with n ormal retardation values. When colloids exist in porous media and affect co ntaminant migration, the system can best be simulated as a three-phase medi um. Mechanisms of mass transfer from one phase to another by colloids and c ontaminants can be kinetic or equilibrium-based, depending on the sorption- desorption reaction rate between the aqueous and solid phases. When the rat e of sorption between the water phase and the solid phase(s) is not much gr eater than the rate of change in contaminant concentration in the water pha se, kinetic sorption models may better describe the phenomenon. In some cas es of modeling one or more mass transfer processes, a useful simplification may be to introduce the local equilibrium assumption. In this study, the l ocal equilibrium assumption for sorption processes on colloidal surfaces (h ybrid equilibrium model) was compared with kinetic-based models. Sensitivit y analyses were conducted to deduce the effect of major parameters on conta minant transport. The results obtained from the hybrid equilibrium model in predicting the transport of colloid-facilitated groundwater contaminant ar e very similar to those of the kinetic model, when the point of interest is not at contaminant and colloid source vicinities and the time of interest is sufficiently long for imposed sources.