A conceptual model is developed to better understand vadose zone vapor-phase diffusion within the mesas of the Pajarito Plateau at Los Alamos National Laboratory. We focus on 1,1,1-trichloroethane (TCA) vapor transport from a liquid-waste disposal site. The conceptual model incorporates several physical processes, including partitioning of TCA into the liquid phase, saturation-dependent diffusion, diffusion through asphalt, and interaction with the atmosphere. Three-dimensional numerical simulations that use the conceptual model of TCA transport are then calibrated to pore-gas monitoring data. Adjustable parameters in the numerical simulations are limited to (i) the vapor-phase diffusion coefficients for the different geologic units, asphalt cover, and land–atmosphere boundary layer and (ii) the fixed concentrations in the two shaft clusters. By including all of the components of the conceptual model in our numerical simulations we were able to achieve a reasonable match between the simulated plume and site data for two alternate conceptual models of asphalt, one with asphalt as a diffusive barrier and one without asphalt as a diffusive barrier. A goodness-of-fit analysis shows that the best-fit simulations are highly correlated to 132 data points from 21 boreholes. The simulations demonstrate that diffusive behavior describes the general characteristics of the current subsurface vapor plume. Effective vapor-phase diffusion coefficients used in the simulations that best fit the data suggest that barometric pumping is not contributing to diffusion in the deep vadose zone; however, it is likely that barometric pumping is occurring in fractures near the mesa edge. We conclude that asphalt is most likely acting as a barrier to diffusion at this site. The numerical simulations validate that the conceptual model developed for this study is a useful tool for analyzing TCA transport within the mesas of the Pajarito Plateau.