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

摘要

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. CH{sub}4 concentrations in soil gassamples from areas with high residual refinery waste concentrations were found to fluctuate greatly in response to seasonal changes in groundwater levels. The 14{sup left}C content of the CH{sub}4 was low (0.03-0.10 times modern), indicating that it waspredominantly formed from the refinery wastes. Theδ13{sup left}C andδD values of the CH{sub}4 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 aerobicdegradation of the hydrocarbons caused by influx of oxygen-enriched rainwater or anaerobic hydrocarbon degradation. In the vadose zone, theδ13{sup left}C and hi values of the CH4 increased as its concentration decreased, indicating that the CH4 wasbeing oxidized by methanotrophic bacteria. This is confirmed by large decreases in theδ13{sup left}C values and 14{sup left}C content of coexisting CO{sub}2. The results of this study show that soil microorganisms can utilize highly weatheredhydrocarbons to produce significant concentrations of CH{sub}4. They also demonstrate how easily misleading conclusions about levels of intrinsic bioremediation can be drawn from spatially or temporally limited sample sets.