HR: 17:00h
AN: H14D-04    [Abstracts]
TI: Processes and Parameters Controlling the Extent of Methanogenic Conditions in the Unsaturated Zone of a Crude Oil Spill Site
AU: * Molins, S
EM: smolins@eos.ubc.ca
AF: University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AU: Mayer, K
EM: umayer@eos.ubc.ca
AF: University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AB: Gas concentrations measured in the vadose zone at a crude oil spill site near Bemidji, MN, show that a large area near the oil body is currently dominated by methanogenic conditions. Away from the oil body methane concentrations decrease as it is degraded by methanotrophic bacteria under aerobic conditions. Numerical simulations have been conducted to quantify the contributions of the relevant transport and reaction processes to the production and attenuation of methane in the vadose zone. Methane is generated in the vadose zone by anaerobic degradation of oil and is also added by fluxes from the capillary fringe and the saturated zone. Gas diffusion and advection contribute to the transport of methane in the lateral direction and towards the ground surface. Attenuation of methane concentrations occurs through aerobic oxidation in the presence of methanotrophic bacteria. Critical parameters were varied within bounds provided by field data and previous studies. Simulation results confirm that the layered sediment structure present at the site plays a significant role in explaining the observed distribution of gases in the vadose zone. The presence of a low permeability lens in the area upgradient from the source results in higher moisture contents, limiting diffusion of oxygen into the zone of methane production, and contributes to the spread of methane. Diffusion was identified as the most significant transport mechanism for gases in the vadose zone. However, field-observed zones of depleted and enriched N2 and Ar concentrations could only be explained by the development of advective fluxes induced by reactive processes (methanogenesis and methanotrophy). The zones of gas production are characterized by slightly increased total gas pressures and low concentrations of N2 and Ar, while zones of gas consumption show slightly depressed total gas pressures and high concentrations of N2 and Ar. The simulations suggest that the advective flux that develops between these zones contributes up to 15% of the total methane flux.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1847 Modeling
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting