Seasonally-induced fluctuations in microbial production and consumption of methane during bioremediation of aged subsurface refinery contamination

Intrinsic bioremediation of 90-130 year old refinery wastes in shallow, saturated soils was studied over a 1-year period by measuring the compositions and isotopic ratios of soil gas and groundwater samples. CH4 concentrations in soil gas samples from areas with high residual refinery waste concentrations were found to fluctuate greatly in response to seasonal changes in groundwater levels. The C-14 content of the CH4 was low (0.03-0.10 times modern), indicating that it was predominantly formed from the refinery wastes. The delta(13)C and delta D values of the CH4 were consistent with formation via acetate fermentation. The source of the acetate to drive this reaction is not clear but could be due to either aerobic degradation of the hydrocarbons caused by influx of oxygen-enriched rainwater or anaerobic hydrocarbon degradation. In the vadose zone, the delta(13)C and delta D values of the CH4 increased as its concentration decreased, indicating that the CH4 was being oxidized by methanotrophic bacteria. This is confirmed by large decreases in the delta(13)C values and C-14 content of coexisting CO2. The results of this study show that soil microorganisms can utilize highly weathered hydrocarbons to produce significant concentrations of CH4. They also demonstrate how easily misleading conclusions about levels of intrinsic bioremediation can be drawn from spatially or temporally limited sample sets.