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Crude oil from offshore deposits in the Miocene Monterey Formation is commonly processed at facilities along the California coast. This formation is known for natural crude oil seepage (NCS), manifested at a California oil and gas processing facility (the site) as small pools on the ground surface, discharge from an adjacent bluff, and as free product in a hand-dug well. In addition, hydrocarbons are also present in site soils due to historical leaks at the facility, referred to here as anthropogenic petroleum hydrocarbons (APH). An investigation using gas chromatography/mass spectrometry (GC/MS) and isotopic techniques compared the chemical signatures of NCS and APH in soil to determine if it was possible to separate the populations. The NCS samples have a well-developed unresolved complex mixture (UCM) compared to the APH samples, yet lighter polycyclic aromatic hydrocarbons (PAHs) in the NCS are more numerous and in higher concentrations than in the APH samples, possibly indicating a different degradation trajectory. Other useful discriminatory parameters appear to be the greater abundance of: tricyclic terpanes (in NCS), C27-diasteranes (in NCS), C28-ergosteranes (in APH), C27- and C29-monoaromatic steranes (in NCS), and C20- and C21-triaromatic steranes (in NCS). Other parameters, e.g., δ13C and δ D, had no apparent discriminatory power. This study demonstrates that NCS appears chemically distinct from APH at this site. However, it would be necessary to obtain empirical evidence for other sites with similar concerns due to vagaries in facility-specific sources, degradation rates, etc. [ABSTRACT FROM AUTHOR]

This study investigates the presence and concentration of light hydrocarbon gases in soil vapor located immediately above the capillary fringe of a petroleum-contaminated aquifer. A correlation was observedfor the linear regression plot of redox potential versus detectable C2+C3 alkane concentrations for a limited number of sampling points. C2+C3 alkanes were not detected at points were redox potentials in groundwater exceeded -260 millivolts. The predominance of methanogenic processes in this redox range, as well as the observed C2+C3 concentration ratios, suggest that ethane and propane gases in soil vapor may bebiogenically produced as well as a result of volatilization from NAPL. [ABSTRACT FROM AUTHOR]