Evaluation of petroleum hydrocarbon weathering on coastal Louisiana beaches and salt marshes following the Deepwater Horizon oil spill using ramped pyrolysis – gas chromatography – mass spectrometry
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In spring of 2010, the Deepwater Horizon oil spill polluted hundreds of miles of coastline in the Gulf of Mexico. A combination of human-mediated and natural weathering processes then altered the chemical composition (i.e. toxicity) of this spilled crude oil over time and space. One of the most important, yet challenging, aspects of oil spill science is to quantify these chemical changes in natural environments. In this study we evaluate the chemical transformation of petroleum hydrocarbons from the Deepwater Horizon spill on a coastal Louisiana beach and salt marsh from 2010-2012. Using gas chromatographic analysis, we quantify the depletion of n-alkanes, polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs and hopanes relative to source oil to evaluate weathering trends across spatial and temporal differences. We report overall depletion of low molecular weight (LMW) n-alkanes and PAHs in all locations with time. The magnitude of depletion at any given time, however, depends on the sampling location, whereby the sites with the highest wave energy have the highest percentage of compound depletion. Oiled sediment from an enclosed bay shows highest retention of high molecular weight (HMW) PAHs, which may have been contributed from sources other than the Deepwater Horizon spill. This provides information regarding where petroleum hydrocarbons are likely to persist in coastal environments, which can be used to inform policy makers and responders for future petroleum pollution. In addition, we confirm these results with the novel application of ramped pyrolysis – gas chromatography – mass spectrometry (Py-GC-MS). We show that bulk flow Py-GC-MS can quantify overall weathering degree of oil samples, and that thermal slicing Py-GC-MS can quantify specific petroleum hydrocarbons as well as qualify changes in non-GC amenable petroleum hydrocarbons with weathering. Our data suggests an increase in HMW (i.e. resin and asphaltene) petroleum fractions and oxygenated products with weathering. This analysis not only elucidates weathering trends with current samples, but also illustrates the analytical capacity of this method for future petroleum hydrocarbon investigations.