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 t ransport 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 equilibri um phase partitioning. The model includes both diffusion and density-i nduced advection and allows for the incorporation of spatial heterogen eities and nonisothermal conditions. Numerical sensitivity analyses we re conducted to further evaluate the relative influence of various tra nsport parameters on vapor migration. Use of measured field values as input parameters resulted in a very good match between the experimenta l results and numerical simulations. In both experiments, vapor plumes spread several meters from the source and downward to the capillary f ringe within only a few days. Seasonal. temperature variations were fo und to have a significant impact on the rate and total mass of vapor t ransport, and variations in organic carbon content, and to a lesser ex tent moisture content, exerted the greatest control on retardation of vapor migration. Transport was diffusion dominated, but density-induce d advection was an appreciable component of net transport under summer conditions, when vapor concentrations were higher. Geologic condition s at the site made overall transport relatively insensitive to the gro und surface boundary condition.