Advective and dispersive contributions to gas phase transport in porous media

Gas-phase and aqueous-phase transport experiments were conducted to explore the advective and dispersive contributions to gas phase transport in unsaturated and saturated porous media with varying water content. Soil was placed a in 90 cm column, and sulfur hexafluoride was used as the tracer gas for vapor-phase transport experiments, while Rhodamine-WT fluorescent dye was used as the tracer in saturated transport experiments. In saturated water-phase transport, molecular diffusion is essentially negligible to the dispersion due to mechanical mixing as the liquid moves around the particles and through the soil matrix. For vapor-phase transport in an unsaturated soil, molecular diffusion is the main cause of dispersion except for cases in which advection rates are high relative to the pore space in the soil matrix. Mechanical mixing can become important in gas-phase transport for transitional water content in the partially saturated zone. With increasing moisture content, water takes up more and more pore-space in the soil matrix, restricting the cross-sectional area available for gas-phase flow. This results in a gradual acceleration of gas-phase advective flows to the point where both mechanical mixing and molecular diffusion are significant. However, more data are needed to fully explain vapor phase transport. There is great need for more research to clarify how gaseous material reach the surface from the soil structure. This is needed to improve detection and collection of gaseous material below the surface. In this study, dispersive experiments will provide a basis for describing the relationship between soil moisture and dispersion. The results indicate that at low soil water content, dispersivity is not influenced by soil water content. At higher soil water contents, dispersivity is greatly influenced by soil water content. These results differ at various water contents.