Inhibition of Acetoclastic Methanogenesis by Crude Oil from Bemidji, Minnesota

By Ean Warren, Barbara A. Bekins, and E. Michael Godsy

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Abstract

The shallow ground water at a site near Bemidji, Minnesota is contaminated with crude oil spilled from a broken pipeline in 1979. With a continued source of dissolved crude oil components, the geochemical conditions in the aquifer have evolved into aerobic, iron-reducing, and methanogenic redox zones. The methanogenic zone starts within the crude oil-contaminated region and extends more than 60 meters downgradient. The methanogenic numbers in the aquifer are low, but variable, depending on the subpopulation of methanogen. In areas close to the crude-oil source, hydrogen- and formate-utilizing methanogens are found in numbers more than one hundred times higher than acetate-utilizers. The acetate-utilizers are found only well below the non-aqueous phase oil and further downgradient. This pattern of methanogen distribution suggests that growth of acetate-utilizers is limited near the source. Laboratory results suggest that toxicity of the dissolved crude-oil is an explanation.

Serum bottle assays were conducted using crude oil in a mineral salts solution inoculated with an enriched methanogenic consortia from a cresote-contaminated site in Pensacola, Florida. Acetate, hydrogen, and formate were added and gas volume change was monitored. Hydrogen- and formate-utilization were unaffected by the crude oil whereas acetate utilization was significantly inhibited. The distribution of aquifer methanogens together with the toxicity assays form a consistent picture with the hypothesis that acetoclastic methanogenesis is inhibited in the vicinity of the oil at the Bemidji site. Because acetate degradation has been widely documented as the rate-limiting step in anaerobic waste treatment processes, it is likely that the inhibition of acetoclastic methanogenesis by the crude oil affects the overall methanogenic degradation rates of the petroleum hydrocarbon contaminants.