Exploratory laboratory research was performed to evaluate the fate of two organic contaminants in the unsaturated zone of the Bandelier Tuff at Los Alamos National Laboratory chemical waste site. Studies concentrated on vapor phase diffusion for trichloroethylene and orthoxylene known to occur in significant amounts at the site. This research was undertaken in support of the geohydrological characterization of the unsaturated zone and ultimately to facilitate environmental transport predictions. In these vapor phase diffusivity studies a laboratory method was developed for direct measurement of gaseous diffusion through undisturbed cores of Bandelier Tuff. Core preparation required careful handling to maintain the in situ structure of the tuff samples, which were mostly cohesionless. Effective diffusion coefficients were calculated from a one-dimensional steady-state solution of Fick's first law. Effective diffusion coefficients for the target organics were determined for samples from two locations within each of two lithologic units of the Bandelier Tuff at two average extreme moisture contents, 1 to 3% and 12 to 15% (by mass). The higher moisture content decreased the flux of the organics by approximately one order of magnitude. Differences among effective diffusion coefficients between sampling locations and lithologic units were not significant for each organic at a 5% significant level. Results provide an insight on the effect of moisture content, but data are needed to further develop or validate a relationship between diffusion and moisture content.
