The design of vapor-extraction remedial systems and the analysis of their performance can be improved by using models that can simulate the chemical and physical processes affecting the occurrence and movement of multiple-compound vapor-phase chemical mixtures. Previous models fall into two categories: (1) multiple-compound phase distribution models which are either nondimensional (no transport) or one-dimensional (column experiments); and (2) multidimensional, single-compound transport models. In this paper, a model is presented that couples the steady-state vapor flow equation, the advection-diffusion transport equation, and a multiple-compound, multiphase chemical partitioning model. The numerical implementation allows spatially variable fields of permeability, confining layer permeability, and initial contaminant concentrations. Based on the concentrations of each chemical compound, the model calculates whether a nonaqueous phase liquid (NAPL) is present, and calculates the chemical phase distribution by the appropriate equilibrium partitioning formulation (Henry's Law or Raoult's Law). The user can specify the location and discharge rates of any number of extraction or injection wells, including zero wells, in which case the simulation will solve transport by diffusion only. The remediation, by vapor extraction, of hypothetical fuel hydrocarbon spills was simulated to investigate the error introduced by failing to account for natural (nonideal) conditions. The nonideal conditions include inhomogeneous soil permeability, leakage of atmospheric air into the subsurface (as from a bare ground surface), and irregular contaminant distribution. The model was also run in the pure diffusion mode to simulate the transport of benzene to the ground surface, and to show the limitations of single-compound vapor flux models when a multicompound NAPL (such as gasoline) represents the source of benzene.
