ABSTRACT
Equilibrium defines the direction of transport and, by virtue of an interface having effectively zero volume or accumulation, defines the ratio of concentrations in the immediate vicinity of an interface. Determining the preferred phase of a contaminant is the first stage in analyzing for the contaminant's potential for environmental impact. Diffusion is an important mass transport process, especially in vertical transport in the subsurface where low permeability and pressure gradients limit advective transport. Diffusion coefficient values for a large number of chemical species in air and water are available in W. J. Lyman et al., R. K. Sinnott, and L. J. Thibodeaux. An effective diffusion coefficient in a porous medium is function of the free air or free water diffusion coefficients and the physical properties of the solid matrix. In the unsaturated region, this solid matrix is intimately combined with air and water phases in the pore spaces.
