2003 Volume 94 Pages 3-9
Under natural conditions, forces such as gravity, temperature, and pressure gradients in the soil coupled with diffusive fluxes will control the fate of gaseous contaminants. The assumptions underlying the basic equations of diffusive and advective gas transport processes in soil are discussed. To test the traditional equations for gas transport, laboratory experiments were conducted to explore the transport of a dense gas (freon-113) through columns of air-dry Oso-Flaco sand with a large concentration of freon-113 maintained at the inlet to the columns. Gas densities (concentrations) were monitored at the inlet and the outlet and within the columns during transport. Significant differences in fluxes and density profiles were observed for the three primary flow directions (horizontal, vertically upward, vertically downward) at high source densities. Numerical models based on the standard Darcy-Fickian transport equation did not fit the measured fluxes. Slip flow was found to be significant relative to Darcy advective flow, but did not account for the discrepancy between model simulations and data. Further theory development was necessary in order to ascertain why the standard equations did not adequately describe the diffusive and advective transport processes for dense gases. New equations governing the transport of gaseous chemicals in porous media were derived by applying the method of volume averaging to the point equations for mass and momentum flux. The form of the new transport parameters provide possible explanations for discrepancies between experimental and numerical modeling results for systems where neither diffusive nor advective driving forces dominate.
