Comparisons of diffusive and advective fluxes of gas phase volatile organic compounds (VOCs) in unsaturated zones under natural conditions

Diffusive flux is traditionally treated as the dominant mechanism of gas transport in unsaturated zones under natural conditions, and advective flux is usually neglected. However, some researchers have found that pressure-driven and density-driven advective flux may also be significant under certain conditions. This article compares the diffusive, pressure-driven and density-driven advective fluxes of gas phase volatile organic compound (VOCs) in unsaturated zones under various natural conditions. The presence of a less or more permeable layer at ground surface in a layered unsaturated zone is investigated for its impact on the net contribution of advective and diffusive fluxes. Results show although the transient advective flux can be greater than the diffusive flux, under most of the field conditions the net contribution of the advective flux is one to three orders of magnitude less than the diffusive flux, and the influence of the density-driven flux is undetectable. The advective flux contributes comparably with the diffusive flux only when the gas-filled porosity is less than 0.05. The presence of a less permeable layer at ground surface slightly increases the total flux in the underlying layer, while the presence of a more permeable layer at ground surface significantly increases the total flux in it. When the magnitude of water table fluctuation is less than 1 cm, and the period is greater than 0.5 day, the fluctuation of the water table can be simulated by fixing the water table position and setting a fluctuating moving velocity at the water table.

► Average advective flux is one to three orders of magnitude less than diffusive flux.
► The density-driven flux is negligible in natural attenuation.
► Average advective flux is comparable with diffusive flux only when θgθg is very small.
► A more permeable layer at ground surface greatly increases the total flux in it.