Understanding the evolution of nearshore Deepwater Horizon oil spill residues and characterizing them using GC/MS/MS methods
John, Gerald Francis
Type of DegreeDissertation
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On April 20, 2010, Deepwater Horizon, a semi-submersible oil rig exploring for oil in the northern Gulf of Mexico (GOM), exploded. The resulting accident released about 4.8 million barrels of oil into the waters of GOM over a period of 87 days. Oil released from this accident impacted various beaches located along Florida, Alabama, Mississippi, Louisiana and Texas coastline. The weathered oil started arriving on the beaches of Alabama from early June 2010. The oil that washed ashore was in the form of highly viscous, neutrally buoyant, water-in-oil emulsion known as mousse. An unknown quantity of mousse interacted with near-shore sediments, settled down to seafloor transforming into submerged oil mats (SOMs) and surface residual balls (SRBs). SOMs and SRBs continue to impact the beaches of Alabama to this day. The overall goal of this study is to monitor SRBs deposition activities along Alabama’s beaches and evaluate the transformation of various PAHs trapped in these oil spill residues when they are exposed to sunlight. The first part of this study (Chapter 2) summarizes the field sampling activities completed along Alabama’s beaches over the past four years (from May 2010 to August 2014). Based on our field observations we conclude the following: (1) virtually all the tarballs found on Alabama’s beaches have the characteristics of MC252 oil, (2) the beaches wouldn’t return to the expected pre-spill background levels in the next five to ten year period, (3) chemical analysis shows the concentration levels of certain polycyclic aromatic hydrocarbons (PAHs), especially chrysene, almost remaining constant over the past four years; and (4) the presence of SRBs/SOMs continue to pose long-term environmental risk to the nearshore beach environment. The second part of this study (Chapter 3) discusses the effects of sunlight on weathering of five groups of PAHs present in MC252 oil and SOMs samples: – (1) C0- to C4-naphthalenes, (2) C0- to C4-phenanthrenes, (3) C0- to C3-dibenzothiophenes, (4) C0- to C3-fluorenes, and (5) C0- to C4-chrysenes. The PAHs present in both MC252 oil and SOM can be weathered by evaporation and photo-oxidation processes. Among the five groups of PAHs, C0 to C4-chrysenes trapped in SOMs have the least weathering rate. Chrysene and its alkylated homologues (C0- to C4-chrysenes) are resistant to various natural weathering processes and could potentially persist in the environmental for a very long time. Therefore, the third section of this effort (Chapter 4) focusses on developing a more efficient GC/MS/MS MRM analytical method for characterizing C0- to C4-chrysenes in oil spill residues. The developed method was used to quantify C0- to C4-chrysenes in MC252 oil and the estimated concentrations were close to that of values reported in the literature. The oil from Deepwater Horizon accident that washed ashore was predominantly in the form of water-in-oil (W/O) emulsion known as mousse. The environmental risks posed by these emulsions can be considerably reduced if this W/O emulsion can be dispersed rapidly in the deep ocean environment. COREXIT9500A, which was widely used to disperse the released oil, is ineffective in dispersing W/O emulsions. The fourth part of this dissertation (Chapter 5) focusses on developing a method to modify COREXIT9500A by adding polar additives (1-octanol and hexylamine) to enhance its ability to disperse W/O emulsions. Preliminary results show that the addition of these additives have the potential to enhance the effectiveness of COREXIT9500A to disperse W/O emulsions. The final section (Chapter 6) summarizes the key findings of this research and also points out some future research recommendations.