A laboratory study was conducted to evaluate the validity of equilibrium assumptions for steady-state vapor-phase transport in homogenous, well-packed porous media in the presence of water infiltration. A volatile, slightly water-soluble compound, trichlorethylene (TCE) was chosen for the experiments. The movement of the compound was from a reservoir of solute dissolved in water, through a tension saturated zone and overlying unsaturated zone, to the soil surface. The experimental data and modeling results suggest that numerical models based on the assumption of thermodynamic equilibrium between the liquid- and gas-phase concentrations are inappropriate to describe this system. A model that assumes thermodynamic equilibrium between phases and uniform infiltration velocities predicts negligible gas-phase flux at any of the infiltration rates applied to our sand columns. We have observed fluxes ranging from 0.0464 μg h-1 cm-2 for an infiltration rate of 0.210 cm-1 to 0.006 μg h-1 cm-2 for an infiltration rate of 1.34 cm h-1. An equilibrium model adapted for a nonuniform velocity,field, i.e. one that assumes all nonuniformities can be described by a dispersion coefficient, predicts that dissolved-phase dispersive phenomena will dominate solute transport of a slightly soluble compound in soils during infiltration. This dispersive transport acts counter to the average velocity field, and it is physically irrational that dispersion counter to the average flow is the predominant transport mechanism. Therefore, we propose a nonequilibrium model in which the infiltration flow regime is assumed to be nonuniform and the fluid phases are not assumed to in in equilibrium with each other. The method requires the knowledge of a gas-liquid mass-transfer coefficient for volatile compounds in a porous medium. Values of this mass-transfer coefficient have not been previously reported.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Water Science and Technology