Later models have attempted to describe species conservation within the context of physically nonideal transport. Some models have attempted to account for non-ideal sorption effects within the solid phase by rate-limited sorption models. Brusseau 2j presented an unsteady-state model which incorporated rate-limited sorption, allowed for two pore domains ('advective' and 'non-advective') and considered convective-dispersive transport to govern gas-phase transport. Gierke et al. 12 developed an unsteady-state model which included a macropore gas domain and a micropore water-filled domain. Their model accounted for air Convection and dispersion, diffusion through an immobile water region, air-water interfacial VOC mass transfer, and sorption at the solid-water interface. The Freundlich isotherm was assumed for VOC sorption from the micropore water to the solid. They solved their model by orthogonal collocation. The model presented by Cho 23 included the velocity profile as predicted by the 'air' flow equation uncoupled to interfacial mass-transfer of VOC and solution of the full VOC species equation for the gas-phase in the soil interstices, and had a simplified treatment of the species contained in the non-flowing phases contiguous to the soil gas. His treatment of mass transfer in this regard was to pose a 'lumped' soil region where an initial constant rate of VOC transfer eventually yields, at some critical point, to a falling-rate mechanism. In the initial period, conditions external to the solid, such as air flow rate, determine the mass transfer of VOC. In the latter period, parameters internal to the solid, such as diffusion, control. The mass transfer expression between mobile and immobile phases is described by a first-order concentration-driving-force expression. His adoption of this mass transfer scheme was based upon the similarity of VOC removal with drying of pulps and sandy mixtures.