Modeling of colloid facilitated contaminant transport in unsaturated porous media

Vita.The presence of mobile colloids in subsurface environments can enhance the contaminant migration in groundwater by reducing retardation effects. Because mobilized colloids have similar surface properties to that of stationary soil matrix in groundwater aquifer, colloids can easily sorb contaminants and n-migrate to distances farther than predicted by the advective-dispersive transport equation. When such carriers exist in a porous medium, the system can be idealized as three phases: an aqueous phase, a carrier phase, and a stationary solid matrix phase. However, when these materials are present especially in a vadose zone, the system should include one more phase, i.e., the air phase, in the analysis of the system. In the work reported, a mathematical model was developed to describe the transport and fate of the colloidal particles and a contaminant in unsaturated porous media. The model is based on mass balance equations in a four-phase medium. Colloidal mass transfer between aqueous and solid matrix phases, and aqueous phase and the air-water interface, and the contaminant mass transfer between aqueous and colloidal phases, and aqueous and air phases are represented by kinetic expressions. Governing equations are non-dimensionalized and solved to analyze the phenomena of colloid and contaminant transport. The model results matched successfully with experimental data of Wan and Wilson (1994a). The presence of the air-water interface retards the colloid transport significantly. Results also show that the retardation of contaminant transport by colloid is highly dependent on the properties of the contaminant and the colloidal surfaces. The four-phase system can be assumed to be the local equilibrium when the Damkohler numbers for mass transfer are larger than about 30. The presence of colloids reduces the retardation of the 30. The presence of colloids reduces the retardation of the volatile contaminant migration via the mass transfer across volatile contaminant migration via the mass transfer across volatile contaminant migration via the mass transfer across volatile contaminant migration via the mass transfer across the air-water interface, probably due to the interaction between contaminant and colloid. The equilibrium assumption for the contaminant mass transfer across the air-water interface may be valid for the volatile contaminant with the Henry's constant less than I