A REACTIVE TRANSPORT MODEL TO SIMULATE COUPLED GAS FLOW AND REACTIONS

Gas-phase advection in the unsaturated zone may be due to a variety of processes including barometric pressure fluctuations, density gradients, and geochemical reactions. Here, we investigate the potential importance of reaction-induced gas advection, which is commonly neglected in reactive transport models. Gas advection has been implemented into the existing reactive transport model MIN3P by directly substituting the advective flux terms into the transport equations as a function of gas species concentrations.

The resulting model has been validated by comparison with an analytical solution as well as with literature examples. The model has been used to investigate (1) the effects of methane oxidation on gas transport in landfill top soils and (2) the effects of methane production and consumption on gas transport at a crude oil spill site. Model results were successfully fit to experimental and field data and show that reaction processes can have a strong feedback on gas transport. In example (1), the model calculations indicate that a 90-cm landfill cover soil is capable of attenuating 70% of CH₄ released. While the advective component contributes up to 81% of the total CH₄ flux, and up to 89% of the total CO₂ flux, diffusion remains as the dominant mechanism to supply atmospheric O₂ for methane oxidation into the cover. In example (2), the model was used to estimate the CH₄ flux in the vadose zone. Simulations suggest that the advective component contributes up to 10.5% of the total methane flux, and 72.8% of the total CO₂ flux. In this case, advection (caused by pressure changes due to CH₄ oxidation) effectively supplies atmospheric O₂ into the soil column for further CH₄ oxidation (up to 7% of total flux). Consequently, reaction-induced gas advection appears to be a process of importance in such environments.