CSIA Isotope Forensics of Lightly Refined Petroleum Products in Groundwater

Present day industrial activities introduce significant amounts of petroleum-based products into the natural environment from events such as leaking storage tanks and pipelines and oil spills associated with routine shipping operations. Consequently petroleum hydrocarbons are the most commonly detected organic contaminants in groundwater. Molecular fingerprinting is a conventional approach to identify the source of a particular contaminant plume or event, while during migration the molecular profiles of contaminant plumes often change due to mixing with other sources and physical/biological weathering, making a conclusive match between the source and the pollutant difficult. Because of their resistance to microbial degradation, triterpanes and steranes are commonly used as biomarkers for source correlation. However, such biomarker compounds are generally absent from lightly refined petroleum products.

Compound Specific Isotope Analysis (CSIA) has become a powerful tool for studying the sources and environmental fate of organic contaminants. The isotopic ratios of the contaminants are useful tracers for their sources, while the isotopic fractionation can be used to quantitatively assess the progress of an environmental process such as biodegradation. The primary objective of this study is to examine the effectiveness of CSIA tool for isotope forensics on petroleum products leakage. Totally seven gasoline samples were from a few subsurface plumes, with certain underground storage tanks as known sources. These gasoline samples were investigated using our established CSIA methodology in addition to statistic methodology developed using both isotopic and molecular indices. Carbon CSIA has been successful in many cases, but for petroleum contaminants, the relatively small amount of fractionation associated with carbon isotopes has limited its applicability: difference in d13C values of individual compounds among oils from different sources is too small for definitive differentiation. Wide compositional variations among these samples demonstrate that the hydrogen isotopic compositions of petroleum hydrocarbons are potentially better tracers to their origins and could be more useful for bioremediation studies than carbon isotopic compositions. Statistical terms introduced in this study, “Sample Pairs STDEV” and “Dissimilarity Coefficient”, are proved to be powerful tools in distinguishing among petroleum hydrocarbon samples.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California