Rathfelder et al. [1991] modeled soil vapor extraction in two dimensions with a mobile gas phase, immobile oil and water phases, sorption, n components, and equilibrium or nonequilibrium versions. Citing Baehr et al. [1989], equilibrium modeling of field-scale venting seemed appropriate. Nonequilibrium was represented by source terms with a linear driving force. Implicit point-centered finite differences led to nonlinear equations solved sequentially by Newton-Raphson iteration, using a direct sparse linear-equation solver. Benson et al. [1993] also developed a two-dimensional n-component equilibrium model with a mobile gas phase. The flow equation was solved by block-centered finite differences, and the transport equations and phase equilibria were solved simultaneously. Brusseau [1991] modeled one-dimensional advective and dispersive gas transport with rate-limited sorption in the presence of an immobile liquid, emphasizing heterogeneity by considering advective and nonadvective regions in the soil and permitting equilibrium or nonequilibrium partitioning in each region. Culver et al. [1991] included mobile gas and water, surface volatilization, and vapor sorption in a two-dimensional finite-element LEA model. Ong et al. [1992] incorporated experimentally-derived vapor-sorption relationships into this model. Armstrong et al. [1994] extended the model of Mendoza and Frind [1990] to include nonequilibrium partitioning. The two-dimensional finite-element model also accounted for liquid sorption and gas advection and diffusion, with an immobile water phase.