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 /(θ v ) 1/3, where θ v denotes the air-filled porosity. Given this reduced dependence on θ v (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 θ v falls in the range 0.13 ≤ θ v ≤ 0.43 cm 3 -vapor/cm 3 -soil, then using an assumed value θ v = 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.