Methane production and consumption monitored by stable H and C isotope ratios at a crude oil spill site,Bemidji, Minnesota |
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Institution: | 1. The Institute for the History of Material Culture, Russian Academy of Sciences, St. Petersburg, Russia;2. The State Historical Museum, Moscow, Russia;3. The Institute of Archaeology, Russian Academy of Sciences, Moscow, Russia;4. University of Groningen, Centre for Isotope Research (CIO), Groningen, Netherlands;1. The Archaeology Centre, University of Toronto, 19 Russell Street, Toronto, Ontario M5S 2S2, Canada;2. Department of Anthropology, University of Texas at Austin, SAC4.102, 2201 Speedway Stop C3200, Austin, TX 78712, USA;1. Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, CA 94583-2324, United States;2. Trihydro, 20 Myrtle St, Orono, ME 04473, United States;3. University of British Columbia, Dept. of Earth, Ocean and Atmospheric Sciences, 2207 Main Mall, Vancouver, B.C. V6T 1Z4, Canada. |
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Abstract: | Stable isotopic ratios of C and H in dissolved CH4 and C in dissolved inorganic C in the ground water of a crude-oil spill near Bemidji, Minnesota, support the concept of CH4 production by acetate fermentation with a contemporaneous increase in HCO3? concentration. Methane concentrations in the saturated zone decrease from 20.6 mg L?1 to less than 0.001 mg L?1 along the investigated flow path. Dissolved N2 and Ar concentrations in the ground water below the oil plume are 25 times lower than background; this suggests that gas exsolution is removing dissolved CH4 (along with other dissolved gases) from the ground water. Oxidation of dissolved CH4 along the flow path seems to be minimal because no measurable change in isotopic composition of CH4 occurs with distance from the oil body. However, CH4 is partly oxidized to CO2 as it diffuses upward from the ground water through a 5- to 7-m thick unsaturated zone; theδ13C of the remaining CH4 increases, theδ13C of the CO2 decreases, and the partial pressure of CO2 increases.Calculations of C fluxes in the saturated and unsaturated zones originating from the degradation of the oil plume lead to a minimum estimated life expectancy of 110 years. This is a minimum estimate because the degradation of the oil body should slow down with time as its more volatile and reactive components are leached out and preferentially oxidized. The calculated life expectancy is an order of magnitude estimate because of the uncertainty in the average linear ground-water velocities and because of the factor of 2 uncertainty in the calculation of the effective CO2 diffusion coefficient. |
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