2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 3
Presentation Time: 8:30 AM


MOLINS, Sergi1, MAYER, K. Ulrich2 and AMOS, Rich T.1, (1)Earth and Ocean Sciences, Univ of British Columbia, 6339 Stores Rd, Vancouver, BC V6T 1Z4, Canada, (2)Earth and Ocean Sciences, Univ of British Columbia, 6339 Stores Rd, Vancouver, V6T 1Z4, Canada, smolins@eos.ubc.ca

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.