MODELING VAPOR EXTRACTION AND GENERAL TRANSPORT IN THE PRESENCE OF NAPL MIXTURES AND NONIDEAL CONDITIONS

The design of vapor-extraction remedial systems and the analysis of th eir 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 fa ll into two categories: (1) multiple-compound phase distribution model s which are either nondimensional (no transport) or one-dimensional (c olumn experiments); and (2) multidimensional, single-compound transpor t 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 permeabil ity, confining layer permeability, and initial contaminant concentrati ons. Based on the concentrations of each chemical compound, the model calculates whether a nonaqueous phase liquid (NAPL) is present, and ca lculates the chemical phase distribution by the appropriate equilibriu m 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 ext raction, of hypothetical fuel hydrocarbon spills was simulated to inve stigate the error introduced by failing to account for natural (nonide al) conditions. The nonideal conditions include inhomogeneous soil per meability, leakage of atmospheric air into the subsurface (as from a b are 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-c ompound vapor flux models when a multicompound NAPL (such as gasoline) represents the source of benzene.