Isotopic evidence for biological controls on migration of petroleum hydrocarbons

The isotopic compositions of potential metabolic byproducts of petroleum hy drocarbon 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 c ompounds (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 plu me and correlated well with GRO concentrations. The C-14 contents of the CH 4, associated soil gas CO2 and dissolved inorganic carbon compounds (DIC) i n 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 car bonate shells. The delta D and delta(13)C values of the CH4 indicate that i t was produced via acetate fermentation. The delta(13)C values of CO2 and D IC in the central part of the plume were high, suggesting that a large frac tion 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 de lta(13)C values of CO2 were much lower (-26 parts per thousand), indicating that aerobic degradation of the AVGAS was dominant in this area. Beyond th e down-gradient edges of the plume, the C-14 contents of both groundwater D IC and soil gas CO2 were significantly below modern levels, implying that m ost of the carbon there was derived from hydrocarbons and had migrated beyo nd 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 acti vity at the edges of the plume was effectively limiting migration of the hy drocarbons. (C) 1999 Elsevier Science Ltd. All rights reserved.