A study was designed to determine if the intrinsic bioremediation of gas condensate hydrocarbons represented an important fate process in a shallow aquifer underlying a natural gas production site. For over 4 yr, changes in the groundwater, sediment, and vadose zone chemistry in the contaminated portion of the aquifer were interpreted relative to a background zone. Changes included decreased dissolved oxygen and sulfate levels and increased alkalinity, Fe(II), and methane concentrations in the contaminated groundwater, suggesting that aerobic heterotrophic respiration depleted oxygen reserves leaving anaerobic conditions in the hydrocarbon-impacted subsurface. Dissolved hydrogen levels in the contaminated groundwater indicated that sulfate reduction and methanogenesis were predominant biological processes, corroborating the geochemical findings. Furthermore, 10-1000-fold higher numbers of sulfate reducers and methanogens were enumerated in the contaminated sediment relative to background. Putative metabolites were also detected in the contaminated groundwater, including methylbenzylsuccinic acid, a signature intermediate of anaerobic xylene decay. Laboratory incubations showed that benzene, toluene, ethylbenzene, and each of the xylene isomers were biodegraded under sulfatereducing conditions as was toluene under methanogenic conditions. These results coupled with a decrease in hydrocarbon concentrations in the contaminated sediment confirm that intrinsic bioremediation contributes to the attenuation of hydrocarbons in this aquifer.
A combined geochemical and microbiological approach
was needed to delineate the biogeochemical processes
occurring in an aquifer contaminated by landfill leachate in
Norman, OK, where the important microbially mediated
reactions in an anoxic plume were iron reduction, sulfate
reduction, and methanogenesis. The highest rates of
sulfate reduction (13.2 μM/day) were detected near the
water table where sulfate levels were maximal (up to 4.6
mM). The enrichment of 34S in the sulfate pools (δ34S of SO4
2-
was 67−69‰), and dissolved hydrogen measurements
provided additional support for the importance of sulfate
reduction near the water table. Methane was detected in the
center of the plume where sulfate was depleted. Microbial
incubations demonstrated concomitant sulfate reduction
and methanogenesis in the anoxic portion of the plume.
Although high concentrations of soluble reduced iron
were detected throughout the aquifer and H2 levels were
indicative of iron reduction under steady-state conditions,
microbiological experiments showed that iron reduction was
active only at the edges of the sulfate-depleted portion
of the plume. This study demonstrates the benefits of using
a combined geochemical and microbiological approach
to elucidate the spatial distribution of biogeochemical
processes in contaminated aquifers.
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