After 96 days of incubation, endogenous rates of methanogenesis and sulfate reduction in the initial toxicity screening experiment appear to be slowing. Presently the headspaces of unamended methanogenic incubations contain 25-35% methane while unamended sulfate-reducing incubations have consumed nearly 25 mM sulfate. Positive control incubations were amended separately with benzoate (200 µmol) and toluene (85 µmol), To date, stimulation of activity in these incubations has only been seen definitively in benzoate incubations, in which both sulfate consumption and methane production above the unamended controls have approached the stoichiometrically expected amounts.
The primary biodegradation experiment examining whole gasoline under methanogenic, sulfate-reducing, and nitrate-reducing conditions is being monitored for methanogenesis and electron acceptor consumption. The high endogenous rates of terminal electron accepting activities have obscured any stimulation of activity which may have occurred due to gasoline amendments during this early phase of the incubation. In addition to gasoline, the potential for the degradation of Alaska North Slope crude oil is also being investigated in Ft. Lupton microcosms under all three electron-accepting conditions. At this early point in the incubations, no stimulation of activity due to any hydrocarbon amendments have been observed. However, in separate experiments examining the biodegradation of a series of alkanes as individual substrates, we have evidence for pentane, heptane, octane, and undecane biodegradation under both methanogenic and sulfate-reducing conditions. These results suggest that short-chain alkanes, major components of gasoline, may be being degraded in the gasoline-amended microcosms despite the absence of measurable increase in methanogenesis or sulfate reduction.