Advanced physical and chemical characterization of oil spill residues

Abstract

Oil spill contamination is a worldwide problem. On April 20th, 2010, the Deepwater Horizon (DWH) oil platform located off the coast of Louisiana exploded and sank after 2 days. The wellhead could not be sealed until July 15th, 2010, and this resulted releasing about 4.9 million barrels (650,000 metric tons) of crude oil into the Gulf of Mexico (GOM). It was reported that the DWH oil spill contaminated over 1,700 kilometers of GOM shoreline and impacted several amenity beaches, marshes and other ecologically sensitive coastal ecosystems along Texas, Louisiana, Mississippi, Alabama and Florida. More recently, on January 28th, 2017, another relatively smaller oil spill occurred when two cargo ships collided about two miles away from the Ennore Kamarajar shipping terminal near Chennai City, India, resulting in a major oil spill known as the Chennai oil spill. This accident released about 75 metric tons of the heavy bunker oil into the Bay of Bengal and contaminated 25 miles of the coastline extending from Chennai’s northern suburban town Ennore all the way to the southern suburban town Thiruvanmiyur. In this study, source identification and advanced characterization methods were completed to investigate the residues from these two oil spills. In the first part of the study, a simple 2-tier field testing protocol, which is based on unique physical characteristics of the oil spill residues, is developed for identifying DWH oil spill residues. A variety of oil samples originated from different oil spill events were tested using the protocol and the samples were classified into “DWH samples” and “non-DWH samples”. The results were verified by analyzing the samples with advanced chemical fingerprinting methods. The verification results matched the results derived from the field testing protocol. The proposed protocol is a reliable and cost-effective field testing approach for differentiating DWH oil spill residues from other types of petroleum residues. The second part of the study is to understand the impacts of burning and excessive heating on hopane biomarkers which are typically used for fingerprinting oil spill residues. In the study, laboratory-scale in situ burning (ISB) experiments were conducted using two types of oils: a model oil prepared using C30-αβ hopane standard, and a DWH reference crude collected from the MC252 well. Our experimental data show that C30-αβ hopane will decrease following an ISB event, although the diagnostic ratios of hopanes will remain stable. Therefore, while relative concentration of different types of hopanes can be used for fingerprinting, C30-αβ hopane cannot be used as a conservative biomarker. In the third part, thermal degradation patterns of hopane biomarkers at high temperatures were investigated by heating the oil samples in an oven. Our data show that C30-αβ hopane in crude oil starts to degrade at about 160 °C, which is lower than the temperature an oil slick will encounter during a typical ISB event. We also found the degradation level of C30-αβ hopane increases with the increase of heating temperature and heating time. The diagnostic ratios of hopanes also changed when the oil was heated at higher temperatures for longer times. Overall, heating is an important process that can degrade hopane biomarkers and it can also change their relative ratios and hence the fingerprint. In the fourth part, we completed a case study of 2017 Chennai oil spill based on both field-scale observational data and chemical characterization data. Our field survey shows that after the spill large amounts of oil was trapped within a relatively stagnant zones near the seawall-groin intersection regions, and this trapping pattern was unique to the Chennai oil spill. The initial cleanup efforts that used manual methods to skim the floating oil and scrub the oil contaminated rocks were relatively effective. The chemical characterization data studied the unique hopanes and steranes fingerprints for the source oil, which can be used for identification and tracking of the oil spill residues. Our experimental studies also show that evaporation is a significant weathering process. During initial hours, the volatile compounds depleted rather rapidly, resulting in the accumulation of heavy polycyclic aromatic hydrocarbons (PAHs) in the crude. Most of these highly toxic heavy PAHs are recalcitrant, and their long-term environmental impacts are largely unknown. In the final section, we summarize the key findings of this study and also point out some recommendations for future research.