Environmental fate of petroleum biomarkers in Deepwater Horizon oil spill residues over the past 10 years.

The long-term fate of three groups of petroleum biomarker compounds (terpanes, steranes, and triaromatic steranes) was investigated in the Deepwater Horizon (DWH) oil spill residues collected from Alabama (USA) beaches over the past 10 years. This is the first study to investigate the long-term fate of these three groups of petroleum biomarkers in DWH oil spill samples over 10 years. We employed the highly recalcitrant C30 αß-hopane as an internal biomarker to quantify the degradation levels of different biomarker compounds, and also to estimate the overall weathering levels of DWH oil spill residues. The data show that four lower molecular weight tricyclic terpanes (TR21, TR22, TR23, and TR24), three lower molecular weight steranes (S21, S22, and C27), and all triaromatic steranes degraded over the 10-year study period. All other terpanes (including hopanes) and steranes remained recalcitrant. There have been contradicting literature data on the degradation levels of homohopanes, and this field study demonstrates that all the homohopanes remained recalcitrant after 10 years of natural weathering. Our data also show that despite some degradation, the relative diagnostic ratios of the biomarkers remained stable for all three groups of biomarkers over the 10-year period.

Understanding the thermal degradation patterns of hopane biomarker compounds present in crude oil.

In-situ burning (ISB) is a common oil spill response technique used for managing marine oil spills. The burnt residues generated from ISB can have several toxic compounds and therefore their impacts on aquatic ecosystem are of major environmental concern. When quantifying the fate of the toxic compounds in ISB residues, C30-αß hopane is routinely used as a conservative biomarker since it has shown to be resistant to most natural weathering processes. However, a recent laboratory study has shown that C30-αß and other hopane compounds have the potential to degrade when crude oil was physically burnt under controlled conditions. When crude oil is burnt, the temperature of the oil can raise up to 350-500 °C; however, so far, no one has studied the fate of hopanes when crude oil is simply heated to very high temperatures. In this study, we hypothesize that heating crude oil to very high temperatures would result in the degradation of hopane compounds. Results of our study show that C30-αß hopane in crude oil will start to degrade at around 160 °C and the degradation pattern follows first order kinetics. Other types of hopanes and their diagnostic ratios can also change when the oil is exposed to severe heating conditions. We conclude that removal of hopane biomarkers via thermal degradation is a possible depletion pathway during ISB. Therefore, caution should be exercised when using hopanes as conservative biomarker compounds for characterizing ISB residues.

Fate of hopane biomarkers during in-situ burning of crude oil - A laboratory-scale study.

In-situ burning (ISB) is a remediation strategy that is used for managing oil spills. ISB generates heavy residues that can submerge and negatively impact benthic environments. To track the fate of toxic contaminants in ISB residues, a conservative hopane biomarker, such as C30-αß hopane, is often used. Furthermore, diagnostic ratios of various hopanes are used for source oil identification. Use of these biomarkers assume that during ISB the quantity of C30-αß hopane will be conserved, and the diagnostic ratios of various hopanes will be stable. The objective of this study is to test the validity of these two assumptions. We conducted laboratory-scale ISB experiments using a model oil prepared from commercial C30-αß hopane standard, and a reference crude oil. Laboratory data collected under controlled burning conditions show that C30-αß hopane will not be conserved; however, the diagnostic ratios of hopanes will still remain fairly stable.

Long-term monitoring data to describe the fate of polycyclic aromatic hydrocarbons in Deepwater Horizon oil submerged off Alabama's beaches.

The 2010 Deepwater Horizon (DWH) catastrophe had considerable impact on the ∼ 50 km long sandy beach system located along the Alabama shoreline. We present a four-year dataset to characterize the temporal evolution of various polycyclic aromatic hydrocarbons (PAHs) and their alkylated homologs trapped in the residual oil buried along the shoreline. Field samples analyzed include the first arrival oil collected from Perdido Bay, Alabama in June 2010, and multiple oil spill samples collected until August 2014. Our field data show that, as of August 2014, DWH oil is still trapped along Alabama's beaches as submerged oil, predominately in the form of surface residual oil balls (SRBs). Chemical characterization data show that various PAHs present in the spilled oil (MC252 crude) weathered by about 45% to 100% when the oil was floating over the open ocean system in the Gulf of Mexico. Light PAHs, such as naphthalenes, were fully depleted, whereas heavy PAHs, such as chrysenes, were only partially depleted by about 45%. However, the rate of PAH weathering appears to have decreased significantly once the oil was buried within the partially-closed SRB environment. Concentration levels of several heavy PAHs have almost remained constant over the past 4 years. Our data also show that evaporation was most likely the primary weathering mechanism for PAH removal when the oil was floating over the ocean, although photo-degradation and other physico-chemical processes could have contributed to some additional weathering. Chemical data presented in this study indicate that submerged oil containing various heavy PAHs (for example, parent and alkylated chrysenes) is likely to remain in the beach system for several years. It is also likely that the organisms living in these beach environments would have an increased risk of exposure to heavy PAHs trapped in the non-recoverable form of buried DWH oil spill residues.