Abstract
Petroleum compounds enter the marine environments from natural seeps, releases related to industrial petroleum extraction and processing, spills from transport and transportation accidents, and natural runoff from the use of fossil fuels. The Deepwater Horizon (DWH) oil spill was unique in that it resulted in the uncontrolled release of oil and natural gas into the Gulf of Mexico at a depth of 1500m below the Gulf's surface. This spill lasted 87days and resulted in an estimated release of 4.2 million barrels of oil and large quantities of natural gas. Crude oils are some of the most complex and diverse organic mixtures found in nature. They contain thousands of different compounds belonging to several compound classes, mainly hydrocarbons and their heteroatom-containing analogs (the so-called nonhydrocarbons, including species with nitrogen, sulfur, oxygen, and trace metal moieties). All crude oils generally contain the same types of chemical structures derived from their source organic material (marine or terrestrial). However, these compounds can be in highly variable proportions in crude oils drawn from different reservoir conditions and locations. Both the structures of hydrocarbons and their respective quantities change rapidly once the crude oil is released into the environment, making the circumstances associated with every spill unique. In general, smaller, lower-molecular-weight oil compounds are more susceptible to processes such as evaporation, dissolution, and biodegradation. In contrast, the heavier, more hydrophobic compounds tend to adhere to living organisms or particulates and persist. The presence of certain compounds in the source oil, such as polycyclic aromatic hydrocarbons (PAHs), as well as the formation of post-spill weathering products, will also determine the acute and chronic toxicity of the spilled oil. Natural processes can transform and ultimately degrade virtually all compounds in crude oils, with aerobic oxidation proceeding much faster than anaerobic degradation, although not all crude oil components are degraded at the same rate. Due to the submarine and offshore setting of the Macondo well (MW) blowout, components of the spilled oil were distributed throughout the northern Gulf of Mexico, the water column, offshore deep-water sediments, the water's surface, and the coastal beaches and marshes. The light and nonviscous nature of MW crude oil favored its removal through natural degradation (light-driven and microbial), evaporation, dissolution, and dispersal processes. While responders, with knowledge of the physical properties of the MW oil, executed their preplanned response efforts to contain spilled oil offshore, a significant proportion of the oil residues reached the more sensitive coastal areas in Louisiana. This chapter will elucidate the most important postspill fate processes involving compounds present in the MW crude oil.
