In contrast, MW-2 contained approximately 23% seawater, presumably due to turbulent mixing of ascending oil, gas, and water at the collection site a few meters above the LMRP. Owing to its collection from within the LMRP, sample MW-1 contained predominantly petroleum hydrocarbons (gas and oil) and a minor amount (approximately 5% vol/vol) of an aqueous fluid of seawater composition. Compositional data for released oil is also necessary for forensic analyses when distinguishing Macondo well oil from hydrocarbons released from other sources in the Gulf of Mexico. Gases are of particular interest because the gas fraction represents a large component of the carbon released, and it was biodegraded rapidly in the water column ( 3, 4). Moreover, such information provides direct constraints on estimates of the total mass of individual hydrocarbons released to the environment and the flow rates at the site of the spill ( 5). Compositional information for petroleum (gas and oil) released by the well at the seafloor is essential for evaluating the fates of hydrocarbons in the sea. This spill demonstrates the importance of interwoven chemical, physical, and biological processes in regulating the transport and fate of hydrocarbons in the deep marine environment. Relative to oil spills occurring at the sea surface, petroleum hydrocarbons experienced a unique set of processes following their release at 1.5-km depth ( 1– 4). Comparison of the endmember gas and oil composition with the composition of water column samples showed that the plume was preferentially enriched with water-soluble components, indicating that aqueous dissolution played a major role in plume formation, whereas the fates of relatively insoluble petroleum components were initially controlled by other processes.ĭuring the 3 mo following April 20, 2010, the Macondo well emitted several million barrels of gas and oil at the seafloor of the Gulf of Mexico following the sinking of the Deepwater Horizon drilling platform. The most abundant petroleum hydrocarbons larger than C 1-C 5 were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L -1. The endmember gas and oil compositions then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1,100 m. Based on the measured endmember gas-to-oil ratio and the Federally estimated net liquid oil release of 4.1 million barrels, the total amount of C 1-C 5 hydrocarbons released to the water column was 1.7 × 10 11 g. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1,600 standard cubic feet per petroleum barrel. ![]() ![]() Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. Quantitative information regarding the endmember composition of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for determining the oil flow rate, total oil volume released, and trajectories and fates of hydrocarbon components in the marine environment.
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