Isotopic evidence for biological controls on migration of petroleum hydrocarbons

The isotopic compositions of potential metabolic byproducts of petroleum hydrocarbon biodegradation in soil gas and groundwater samples from a shallow plume of aviation gas (AVGAS) were analyzed to assess levels and pathways of intrinsic bioremediation occurring at the site. Gasoline range organic compounds (GROs) in soil gas samples from the original source area were low (<1000 ppm), but GRO levels under an adjacent, asphalt-covered parking lot exceeded 100,000 ppm. Soil gas CH4 was >20% in the central part of the plume and correlated well with GRO concentrations. The 14C contents of the CH4, associated soil gas CO2 and dissolved inorganic carbon compounds (DIC) in the groundwater were all less than 0.1 times modern, indicating they were primarily formed from degradation of AVGAS and not other potential carbon sources such as degradation of natural organic matter or dissolution of carbonate shells. The δD and δ13C values of the CH4 indicate that it was produced via acetate fermentation. The δ13C values of CO2 and DIC in the central part of the plume were high, suggesting that a large fraction of the CO2 in that area was also produced by acetate fermentation. At the down-gradient edges of the plume, CH4 levels dropped to zero and the δ13C values of CO2 were much lower (−26‰), indicating that aerobic degradation of the AVGAS was dominant in this area. Beyond the down-gradient edges of the plume, the 14C contents of both groundwater DIC and soil gas CO2 were significantly below modern levels, implying that most of the carbon there was derived from hydrocarbons and had migrated beyond the edge of the plume. These data show that methanogenic activity in the central part of the plume was slowly degrading the AVGAS, but aerobic activity at the edges of the plume was effectively limiting migration of the hydrocarbons.