Chemical characterization and biomarker fingerprinting of Deepwater Horizon oil spill residues
Type of DegreeDissertation
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On April 20th, 2010, the Deepwater Horizon (DWH), a semi-submersible drilling rig, located about 120 miles off the coast of Alabama, exploring for oil in the Gulf of Mexico (GOM), experienced a catastrophic blowout and exploded. This event resulted in one of the largest marine oil spill disasters in U.S. history. Until the leaked oil gusher was capped on July 15th, 2010, approximately 4.9 million barrels of crude oil was released into the waters of GOM. About 40 days after the accident, the spilled oil started to wash along the shorelines of Florida, Alabama, Mississippi and Louisiana. The State of Alabama has approximately 50 km of sandy shoreline that are classified as amenity beaches. These beaches are priceless due to their economic and environmental values. To understand the environmental impacts of DWH oil on these amenity beaches, our group has been continuously monitoring this 50 km region over the past five years. The overall goal of this study is to characterize and fingerprint the field samples collected from these monitoring efforts. In the first part of this study we compare the chromatographic signatures of petroleum biomarkers present in DWH source crude, three other reference crude oils, emulsified mousse that arrived on Alabama’s shoreline in June 2010, and seven tar balls collected from Alabama beaches from 2011 to 2012. Characteristic of hopane and sterane fingerprints show that all the tar ball samples originated from DWH oil. The diagnostic ratios of various hopanes indicated an excellent match. Quantitation data for C30αβ-hopane concentration levels show that most of the weathering observed in DWH-related tar balls found on Alabama’s beaches is likely the result of natural evaporation and dissolution processes that occurred when the oil was transported across the Gulf of Mexico, prior to beach deposition. Based on the physical and biomarker characterization data presented in this study we conclude that virtually all fragile, sticky, brownish tar balls currently found on Alabama shoreline originated from the DWH oil spill. In the second part of this study 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 depletion rates of PAHs appear to have decreased significantly once the oil was buried within the partially-closed SRB environment. Depletion levels of several heavy PAHs have almost remained constant over the past 4 years. Our data also show that evaporation was the most likely 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 the submerged oil containing various heavy PAHs (for example, parent and alkylated chrysenes) is likely to remain in the beach system for several years. In the third part of this study we have analyzed the first-arrival oil spill residues collected from two Gulf of Mexico (GOM) beach systems following two recent oil spills: the 2014 Galveston Bay (GB) oil spill and the 2010 Deepwater Horizon (DWH) oil spill. This is the first study to provide field observations and chemical characterization data for the 2014 GB oil spill. The primary purpose of this chapter is to present the similarities and differences between these two oil spills. Our data show that both oil spills had similar shoreline deposition patterns; however, the physical and chemical characteristics of their residues differed considerably. In the final section we summarize the key outcomes of this research effort and also point out some future research directions.