Effect of colloids on volatile contaminant transport and air-water partitioning in unsaturated porous media

Abstract. The presence of mobile colloids in subsurface environments can enhance subsurface contaminant migration by reducing retardation effects. We developed a mathematical model based on mass balance equations to describe the transport and fate of colloidal particles and a volatile contaminant in an unsaturated porous medium. When colloids are present in an unsaturated medium, the system representation include four phases: an aqueous phase, a carrier phase, a stationary solid matrix phase, and the air phase. Colloidal mass transfer between the aqueous and solid matrix phases and between the aqueous phase and the air-water interface, and the contaminant mass transfer between aqueous and colloidal phases and between the aqueous phase and the air-water interface are represented by kinetic expressions. Nondimensionalized governing equations are solved to analyze colloid and contaminant transport in an unsaturated column. A sensitivity analysis of the transport model was utilized to assess the effects of several parameters on model behavior. Results show that the effect of colloids on a volatile contaminant transport is highly dependent on the properties of the contaminant and the colloidal surfaces. The presence of an air-water interface retards the volatile contaminan