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

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 contaminant migration because of mass transfer rate across the air-water interface, offsetting the facilitating effect of colloids. We tested the effects of varying Henry's constant and the contaminant mass transfer across the air-water interface. The equilibrium assumption for the contaminant mass transfer across the air-water interface may be valid for volatile contaminants with dimensionless Henry's constants less than one. As Henry's constant increases, the contaminant mass transferred across the air-water interface is approximately 20% larger at 46% air saturation than at 15% air saturation because of a larger volume of air. At 15% air saturation, air phase contaminant reaches the equilibrium concentration at about 18 pore volumes with a large dimensionless Henry's constant (H+ = 10). However, at 46% air saturation, it takes over 20 pore volumes to reach the equilibrium concentration, even with a smaller Henry's constant(H+ = 1).