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.
"We do not know with certainty the adverse effects it might cause on undersea life."
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."
"The persistence of BTEX at depth poses an interesting question as to the potential effects of these compounds on mid-water organisms."
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.
"Getting this sample was probably the most dramatic and thrilling thing I have done in my life," Reddy said.
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."
Analyzing samples from the Macondo well and those they collected from the plume in June 2010 aboard the research vessel Endeavor, the researchers found that BTEX represented about 2 percent of the oil that came out of the well, but "nearly 100 percent of what was in the plume," Reddy said.
"A small, selective group of compounds took a right-hand turn" after exiting the well and formed the 3,000-foot-deep plume, he added.
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?"
The answers have to do with the tendency of those chemicals that "like" to dissolve in water to migrate to the plume, Reddy said. Unlike other substances emanating from the well that degrade or evaporate in the water or at the surface, the compounds in the plume showed little evidence of biodegrading when the researchers examined the plume in June 2010.
"Oil and gas experienced a significant residence time in the water column with no opportunity for the release of volatile species into the atmosphere," the researchers reported.
"Hence water-soluble petroleum compounds dissolved into the water column to a much greater extent than is typically observed for surface spills."
"We needed to have an 'end-member' sample, so that we could compare how nature affected the hydrocarbons as they left the riser pipe," he said.
"So this story is really about, 'from pipe to plume: what chemicals got off the elevator to the surface and migrated to the plume.'"
The findings have "direct implications for the ecotoxicological impact of plumes," Reddy said.
"Now that we know the compounds were there for a certain time, we need to look at what that would mean to ocean life."
"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.
On the Net: