HR: 1400h
AN: H33B-06    [Abstracts]
TI: Reactive Transport Modeling of Vadose Zone Contamination: Feedback between Reactions and Gas-Phase Transport
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: The unsaturated zone acts as a buffer zone for contaminants on their way to the water table but can also attenuate the emission of contaminants leaving the subsurface environment through the gas phase. A reactive transport model that includes multicomponent gas transport has been developed to investigate the processes that contribute to the generation and attenuation of contaminants in the unsaturated zone. In particular, the model is suitable to study the feedback processes between advective-diffusive gas transport and geochemical reactions. The model is also able to estimate diffusive and advective contributions to gas transport in multicomponent systems. Two model applications are presented that investigate gas transport and reactions in mine tailings and at a site with organic contamination. In mine tailings, atmospheric oxygen transported into the sediment column is consumed in the oxidation of sulfide minerals. Gas volume loss caused by the consumption of atmospheric oxygen drives advective fluxes. In the absence of carbonate minerals, the advective component accounts for 16 % of the net oxygen flux into the column, while, in a carbonate-rich system, advection accounts for 10 % of the net oxygen flux. Dissolution of carbonate minerals has a moderating effect on advective gas transport since carbon dioxide can partially compensate for the depletion of oxygen. At an oil spill site, volatilization and degradation of organic contaminants cause advective and diffusive fluxes of organic vapors away from the source zone. At early stages, volatilization dominates and oxidation of these organic vapors attenuates the emission of contaminants to the atmosphere. The contribution of advection to organic vapor fluxes is significant initially but decreases with time. At later stages, the oil source becomes depleted of its most volatile fraction, and anaerobic degradation of aromatic compounds and heavier n-alkanes results in the production of methane. Up to 15 % of methane produced is transported away from the source zone by advection. Subsequently, methane is oxidized in the presence of atmospheric oxygen. A reduction of total gas pressure during oxidation results in the development of advective gas fluxes toward the oxidation zone. As a consequence, the distribution of non-reactive gases such as argon or nitrogen in the unsaturated zone can be used as indicators of these processes.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1847 Modeling
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Joint Assembly