July 19, 2011

Study Identifies Chemicals In Gulf Of Mexico Oil Plume

The mystery over what happened to the oil and gas released during last year's Gulf of Mexico oil spill has been partially solved, according to new research published this week in the Proceedings of the National Academy of Sciences.

Researchers led by the Woods Hole Oceanographic Institution (WHOI) gathered oil and gas last year from the deep, 22-mile-long underwater oil plume as it escaped from the damaged wellhead of the Deepwater Horizon oil rig.   Using this data, they applied a new molecular model that better explains the diffusion of the spill under water.

The results were also helpful in explaining why some chemicals, but not others, made their way into the plume.

The data "provide compelling evidence" that the oil component of the plume sampled in June 2010 was essentially comprised of benzene, toluene, ethybenzene, and total xylenes"”together, called BTEX"”at concentrations of about 70 micrograms per liter (mg/L), said WHOI chemist Christopher Reddy, lead author of the study.

The 70 mg/L in the plume were "significantly higher than background," he said.


The findings "help explain and shed light on the plume formation and verify much of what we thought about the plume's composition."

WHOI Senior Scientist Judith McDowell said that acute toxicity levels of BTEX are in the range of 5 to 50 milligrams per liter (mg/L) for aquatic organisms.  But sublethal effects, including neurological impairment, are observed at lower levels, she added.

"In most instances the BTEX compounds are volatilized very quickly such that exposure duration is very short."


A key component of the study was a one-of-a-kind fluid sample the team collected directly from the broken riser at the Macondo well. To accomplish this, the team used an isobaric gas-tight sampler designed to gather fluids from deep-sea hydrothermal vents.

With the gas-tight sampler and other necessary equipment, the scientists were shuttled from their active research vessel to a smaller boat and brought to the Ocean Intervention III, operating above the Macondo well. They were then given 12 hours to do something never done before.

Using an oil industry remotely operated vehicle, they maneuvered the gas-tight sampler to the source of the spill to capture an "end-member" sample of fluid as it exited the riser pipe.

No other such sample exists.

By analyzing this sample, the scientists were able to determine what was in fluid spewing from the Macondo well before nature had a chance to change it, and the exact ratio of gas and oil in the fluid.


Using petroleum industry terms, they found a gas-to-oil ratio (GOR) of 1,600 cubic feet of gas per barrel of oil. This value is smaller than other proposed values, Reddy explained, suggesting "more oil may have been coming out of the well than other people calculated."



This raises a number of questions, he said, including, "Why are those chemical there in those concentrations? Why are they so abundant in the water?"





"So this story is really about, 'from pipe to plume: what chemicals got off the elevator to the surface and migrated to the plume.'"



"This paves the way to look at any environmental effects."

The study, which was funded by the National Science Foundation and the U.S. Coast Guard, was published online July 18 in the journal Proceedings of the National Academy of Sciences. 


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