Joel Massmann

Effects of atmospheric pressures on gas transport in the vadose zone

Temporal variations in barometric pressure due to weather patterns may induce air intrusion into the subsurface. This air intrusion can affect monitoring activities aimed at characterizing the composition and movement of gases in the vadose zone. Expressions are presented to estimate gas fluxes due to the combined effects of Knudsen diffusion, multicomponent molecular diffusion, and viscous flow. These expressions are used to evaluate the validity of the single-component advection-dispersion equation for simulating gas transport in the presence of atmospheric pressure variations. The single-component equation provides reasonable results when used to simulate transport in media with relatively high gas permeability. Computer simulations of vertical transport at sites with homogeneous soils indicate that “fresh” air can migrate several meters into the subsurface during a typical barometric pressure cycle. Horizontal pressure gradients can develop at sites with near-surface heterogeneities. These gradients may cause fresh air to intrude meters or tens of meters into the vadose zone during a storm event.

Uncertainties in cleanup times for soil vapor extraction.

Uncertainties in cleanup times for soil vapor extraction systems are estimated using mass removal curves generated through Monte Carlo analyses. The primary source of uncertainty is assumed to be soil permeability. Four general model components are coupled to quantify uncertainties: (1) a geological uncertainty model, (2) a spill distribution model, (3) a vapor flow model, and (4) a mass removal model. Spatial variability in permeability has a more significant effect on cleanup time than does overall average permeability because of the effects of channeling. For the examples considered in this study, field measurements of air conductivity have a relatively small effect on uncertainties in cleanup time. The examples also suggest that surface covers can significantly reduce cleanup time in cases where the extraction wells are not immediately within the zone of contamination and for relatively large spills. If wells are completed within the contaminated zone, however, these covers can actually increase average cleanup times.