Gaseous Transport Mechanisms In Unsaturated Systems: Estimation Of Transport Parameters

Understanding gas transport in subsurface systems is necessary for the successful application of innovative technologies, such as soil vapor extraction (SVE), bioventing (BV), and in-situ air sparging (IAS), to remediate subsurface-contaminated hazardous waste sites. In general, gas transport in natural porous media occurs via several flux mechanisms including: viscous flow, free-molecule or Knudsen flow, continuum or ordinary diffusion (molecular and nonequimolar fluxes), surface flow or diffusion, and thermal flow. The relative importance of these mechanisms depends on factors related to the permeate gases (molecular weight, and viscosity) and the porous media (the type of porous media, porosity, moisture content, and bulk density). Three models are currently used to quantify the diffusive transport of gases in natural porous media, namely, Fick\u27s first law, the Stefan-Maxwell equations, and the dusty gas model (DGM). The application of these models requires the quantification of many transport parameters, such as gas permeability coefficients, the effective binary diffusion coefficients, and Knudsen diffusion coefficients. These parameters are estimated from either laboratory or field measurements. Parameters determined from in-situ experiments tend to be much higher than the corresponding estimates from laboratory tests, while Knudsen diffusion coefficients have never been estimated in the field. These differences are attributed to the inclusion of macropores, fractures and heterogeneities in the field, and to the failure to consider all gas flux mechanisms that may have occurred. The DGM combines these mechanisms in a rigorous manner and can be applied to multicomponent gaseous systems under isobaric and non-isobaric conditions. Thus, it has been suggested as a framework for assessment and quantification of gas transport in subsurface systems and for the analysis of in-situ and laboratory tests to estimate the needed gas transport parameters. © 2006 American Chemical Society