In Situ Measurement of Effective Vapor-Phase Porous Media Diffusion Coefficients

Knowledge of the effective vapor-phase porous medium diffusion coefficient is important for many applications, including risk-based volatilization and vapor migration calculations and remediation performance estimates. A procedure for measuring the effective vapor-phase porous medium diffusion coefficient in situ is presented here. The approach utilizes transient changes in volume-averaged concentrations of an inert tracer gas, rather than changes in point concentration measurements, to determine the effective vapor-phase porous medium diffusion coefficient D v eff. Measurements are made over intermediate time frames (minutes − hours), and the procedure is easily altered to probe scales ranging from approximately 0.1−1 m. The data reduction leads to the determination of D v eff/(θv)1/3, where θv denotes the air-filled porosity. Given this reduced dependence on θv (relative to approaches based on transient point concentration changes), in many settings it would not be necessary to make independent measurements of moisture content and total porosity to determine D v eff. For example, if θv falls in the range 0.13 ≤ θv ≤ 0.43 cm3-vapor/cm3-soil, then using an assumed value θv = 0.28 would contribute to less than a 23% error in determining D v eff from experimental data. The theory, a general protocol, and an example field protocol are presented along with sample field data.