Modeling of Ground‐Water Contamination Caused by Organic Solvent Vapors

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Abstract
Mathematical models are used to evaluate the potential for ground‐water contamination resulting from vapor transport of volatile organic solvents in the unsaturated zone. A two‐dimensional numerical model for density‐driven flow and transport of vapors shows that trichloroethylene (TCE) vapors can be expected to spread rapidly from a residual source above an unconfined aquifer in an unsaturated, sandy deposit. Sensitivity analyses show that the shape of the vapor plume is particularly sensitive to the ground surface boundary condition and the material permeability. The shape of the vapor plume is insensitive to the mass flux by diffusion through the capillary fringe, although the flux may be significant in terms of ground‐water quality because the drinking‐water standard is very low. The vapor plume simulations and a plug‐flow infiltration model are used to calculate a source function for a ground‐water transport model in order to estimate the potential for ground‐water contamination. Ground‐water simulations show that an impermeable surface cover could reduce the flux of contaminants to the aquifer by preventing infiltration through the vapor plume. However, for the conditions modeled, significant ground‐water contamination can be expected to occur regardless of whether the ground surface is covered or not. Contaminants can reach the saturated zone from a residual source either by liquid‐phase diffusion through the capillary fringe or by migrating as a vapor beyond the lateral limits of a cover and subsequently dissolving and being flushed to the saturated zone by infiltration.