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

Abstract

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 (minuteshours), 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 e θ v e 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.

Proof-of-Concept Study of an Aerobic Vapor Migration Barrier Beneath a Building at a Petroleum Hydrocarbon-Impacted Site

“…Real-time pressure differential transducers (Pace Scientific, 3,7,8,9,11,12,13,16,17,18,19,20,22, B, C, D, E, and J). The differential pressures were referenced to Location F at 3.2 m BGS.…”

Identification of application‐specific critical inputs for the 1991 Johnson and Ettinger vapor intrusion algorithm

“…Primary Inputs Reasonably Estimated from Correlations and Secondary Inputs: {D T eff and D crack eff } The primary inputs D T eff and D crack eff can be measured (Johnson et al 1998a), but they are most often calculated using Equations 3 and 4 and the secondary inputs D air , D H 2 O , H i , h v , h m , and h T . Therefore, the discussion here focuses both on reasonable ranges of values for D air , D H 2 O , H i , h v , h m , and h T and on expectations for reasonable ranges of D T eff and D crack eff .…”

A variance decomposition approach to uncertainty quantification and sensitivity analysis of the Johnson and Ettinger model

“…In comparison to soil permeability, this parameter was shown to play a more important role for all soil types and source depths. Field measurements of total effective diffusivity are not commonly collected during vapor intrusion investigations; however, consideration of this parameter warrants additional attention and has been highlighted by others as important for vapor intrusion (Johnson, et al, 1998; Hers et al, 2009; Johnston and Gibson, 2011). Our results, not only identify the range of variability to be expected depending on the scenario at hand, but also mark the important cases where special consideration should be given when estimating the input parameters to which the J&E model is most sensitive (i.e.…”