Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill
- aDepartment of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
- bEnvironmental Chemistry Modeling Laboratory, Swiss Federal Institute of Technology at Lausanne (EPFL), 1015 Lausanne, Switzerland;
- cApplied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; and
- dDepartment of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom
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Edited by John M. Hayes, Woods Hole Oceanographic Institution, Berkeley, CA, and approved June 10, 2011 (received for review January 25, 2011)

Abstract
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. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. 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. 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 C1-C5 hydrocarbons released to the water column was 1.7 × 1011 g. 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. The most abundant petroleum hydrocarbons larger than C1-C5 were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L-1. 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.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: creddy{at}whoi.edu.
Author contributions: C.M.R., J.S.S., S.P.S., and R.C. designed research; C.M.R., J.S.A., J.S.S., S.P.S., K.L.L., R.K.N., C.A.C., C.P.M., G.T.V., and B.A.S.V.M. performed research; J.S.S., J.F., and R.C. contributed sampling and logistics; C.M.R., J.S.A., J.S.S., S.P.S., K.L.L., R.K.N., J.F., G.T.V., B.A.S.V.M., and R.C. analyzed data; and C.M.R., J.S.A., J.S.S., and R.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1101242108/-/DCSupplemental.
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