Methane production and consumption monitored by stable H and C isotope ratios at a crude oil spill site,Bemidji, Minnesota

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.