Vapor transport of trichloroethylene in the unsaturated zone: Field and numerical modeling investigations

Vapor transport of chlorinated solvents in the unsaturated zone may be an important mechanism for the spread of contamination at spill sites and may be a significant factor controlling the extent of groundwater contamination. Two field experiments were carried out at the Canadian Forces Base Borden field site to provide detailed monitoring of the transport behavior of trichloroethylene vapors in the unsaturated zone. Experiments were conducted for both winter and summer conditions and under different surface boundary conditions. The observed results were simulated using a Fickian-based numerical model with linear equilibrium phase partitioning. The model includes both diffusion and density-induced advection and allows for the incorporation of spatial heterogeneities and nonisothermal conditions. Numerical sensitivity analyses were conducted to further evaluate the relative influence of various transport parameters on vapor migration. Use of measured field values as input parameters resulted in a very good match between the experimental results and numerical simulations. In both experiments, vapor plumes spread several meters from the source and downward to the capillary fringe within only a few days. Seasonal. temperature variations were found to have a significant impact on the rate and total mass of vapor transport, and variations in organic carbon content, and to a lesser extent moisture content, exerted the greatest control on retardation of vapor migration. Transport was diffusion dominated, but density-induced advection was an appreciable component of net transport under summer conditions, when vapor concentrations were higher. Geologic conditions at the site made overall transport relatively insensitive to the ground surface boundary condition.