ADVECTIVE-DISPERSIVE TRANSPORT OF DENSE ORGANIC VAPORS IN THE UNSATURATED ZONE .1. MODEL DEVELOPMENT

Vapors from organic liquids that are suspended as residual in the unsaturated zone can migrate considerable distances through the aerated pore space and thus cause extensive contamination of the groundwater. In order to investigate the various transport mechanisms that play a role in the migration of these vapors, a numerical model is developed. The model includes and distinguishes between the processes of diffusion, advection due to density gradients, and advection due to the vapor mass released by vaporization at the source. Phase partitioning to the soil moisture is represented as an equilibrium retardation process that both slows plume development and makes contaminants available for transport by infiltrating water. The formulation is analogous to that for density‐dependent transport in the saturated zone. Axisymmetric coordinates are used in order to represent localized residual‐saturation solvent sources. Application to a three‐dimensional laboratory experiment shows that inclusion of density‐driven advection provides a better match with the observations. A simulation of the migration of 1,1,1‐trichloroethane vapors in a highly permeable coarse sand demonstrates that advection may be a very effective transport mechanism. Copyright 1990 by the American Geophysical Union.