Influence of vapor-phase sorption and diffusion on the fate of trichloroethylene in an unsaturated aquifer system.

This research evaluates the influence of vapor-phase sorption and diffusion on the fate and transport of a common volatile pollutant, trichloroethylene (TCE). Vapor-phase sorption of TCE by a porous aluminum oxide surface coated with humic acids (to simulate an aquifer material) was observed to be highly dependent on moisture content. Linear partition coefficients for binding of TCE vapor under a range of unsaturated conditions were 1-4 orders of magnitude greater than the value measured for the saturated sorbent. In addition, laboratory measurement of the TCE diffusion coefficient through the simulated aquifer material indicated that an existing empirical formula used to estimate this parameter can be in error by as much as 400%. The significance of differences in sorptive partition coefficients and diffusion coefficients was examined with an existing one-dimensional vertical transport model for the unsaturated zone. Model calculations indicate that the common practice of assuming saturated partition coefficients apply to unsaturated conditions should be avoided to obtain accurate predictions of volatile contaminant transport.