Fingerprinting simulated marine oil spills with gasoline-range compound specific isotope correlation




Kory, Michael David

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Environmental liability cases involving spilled or released petroleum products into ocean ecosystems require oil identification techniques that are unambiguous and conclusive, even in situations where oils have been subjected to secondary environmental processes, such as, evaporation and dissolution. The ability and functionality of the Compound Specific Isotope Correlation (CSIC) technique is tested to determine its reliability to characterize released petroleum using the carbon isotope ratios (13C/12C) of the individual gasoline-range compounds (C5-C9). In particular, this thesis studies the potential of CSIC as a robust diagnostic tool, to identify and correlate marine releases of oil with their sources, especially those having undergone evaporative weathering. Three crude oils (Alberta Sweet Mixed Blend, Lacula and Louisiana) added to synthetic seawater were exposed to mechanically simulated wave energy and controlled evaporative weathering at 10 oC. Time-series sampling of the gasoline-range vapour fractions from the headspace employed Solid Phase Micro Extraction (SPME). SPME-Continuous Flow-Isotope Ratio Mass Spectrometry (SPME-CF-IRMS) determined the molecular abundances and stable carbon isotope ratios (δ13C) of the gasoline-range compounds of the original and weathered oils. Evaporation rates over the maximum 20 hour period varied for the 3 study oils. Most (74%) of the individual compounds measured in the oils display a δ13C enrichment with progressive evaporation with approximately half of the compounds in all 3 oils showing fractionation of the carbon isotopes ≤ 0.5‰ within measurement precision. The magnitude of carbon isotope shift observed in compounds pre-vs. post-weathering ranges from 0 to 2.8 ±0.6‰. There is no clear relationship identified between the degree of 13C enrichment in the oils and groupings such as chemical class, structure or carbon number. The overall weighted average 13C enrichment for all compounds in the 3 oils is approximately 1‰. Toluene was the only compound consistently exhibiting comparatively high 13C enrichment (1.6‰, 1.8‰ and 2.8‰) in all 3 oils after evaporative weathering. Hierarchical Cluster Analysis (HCA) treatment of the CSIC data set can reliably discriminate between the 3 oils despite evaporative weathering and δ13C changes. HCA is also able to unambiguously relate the three weathered oils back to their respective original unweathered oil. Diagnostic shifts in δ13C of individual compounds in an oil may potentially be used to trace weathered oils back to the source, and possibly give a estimation of time since release. However the typically rapid rate of evaporation for the gasoline-range fractions limits the time that an oil can be successfully identified by CSIC.



carbon isotope, crude oil, fingerprinting, oil spill