Experiments and Modeling of the Transport of Trichloroethene Vapor in Unsaturated Aquifer Material

A bench-scale reactor system was used to investigate mass-transfer dynamics and transport of trichloroethene (TCE) vapor in a column of unsaturated aquifer material under conditions of advective gas flow, at 25 °C and 90% relative humidity. Two gas flows (40 and 80 mL/min) and two relative vapor pressures of TCE (10% and 90% P/P o, where P is vapor pressure and P o is the saturation vapor pressure) were selected as experimental variables. Breakthrough curves were generated for week-long inputs of TCE-laden air and for short pulses of a nonsorbing tracer gas. Equilibrium sorption isotherms for TCE were also measured and used as tools for interpreting the column experiment results. Slow mass-transfer kinetics were observed in all of the transport experiments. Evidence from the breakthrough curves and the sorption isotherms suggested that, at 90% P/P o, a significant amount of TCE was condensed in pores or sorbed at the gas−water interface. Desorption and volatilization of interfacially sorbed TCE appeared to be rapid processes. The applicability of a recently developed mathematical transport model using a statistical γ distribution of desorption rate constants was tested using the experimental data. The γ distribution provides two adjustable parameters to account for sorption site heterogeneity and multiple mechanisms of sorption. When fit to the breakthrough curve obtained at high flow and high relative pressure, the model successfully predicted TCE frontal breakthrough and elution profiles at all other experimental conditions with no adjustable parameters. The predictive capability of the γ model was shown to be superior to that of two commonly used alternative model paradigms: the two-site first-order and two-site spherical diffusion models.