Nearly 3 million liters (some 771,000 gallons) of a chemical dispersant ejected into oil and gas from BP’s Deepwater Horizon oil spill last spring and summer lingered until at least September, a new study shows. The chemicals moved in concert with plumes of oil deep beneath the Gulf of Mexico’s surface.
David Valentine of the University of California, Santa Barbara and his colleagues periodically sampled plume water that flowed at depths of 1,000 meters or more between May and September 2010. They shipped these samples to chemist Elizabeth Kujawinski at the Woods Hole Oceanographic Institution in Massachusetts and her colleagues for analysis.
With rare exception, they report online January 26 in Environmental Science & Technology, the dispersant did not degrade but instead moved with the plumes until they were lost to dilution in the Gulf’s depths.
“If the dispersant worked, it should have been associated with the liquid oil — that is, moving off laterally into the deepwater plume. Which is where we found it — and the only place,” Kujawinski says. “We did not see it below the plume or even sloughing off the top of it.”
To scout for the dispersant, known as Corexit 9500A, Kujawinski focused on an active ingredient known as DOSS, or dioctyl sodium sulfosuccinate. It accounted for 10 percent by weight of the dispersant mixture, which was released at rates ranging from around 13,000 to 80,000 liters per day.
Prior to capping the well, plume concentrations of DOSS hovered in the low parts per million range, after which it diminished to parts per billion concentrations. DOSS levels in the plume matched what would have been expected if the dispersants remained with the oil. That, Kujawinski says, suggests no biodegradation of DOSS — and shows why remnants of dispersant applications could be detected 300 kilometers from the wellhead and even two months after their last application.
“When you read about Corexit, it’s supposed to biodegrade,” observes Carys Mitchelmore of the University of Maryland’s Center for Environmental Science in Solomons. But specific rates have not generally been reported, she adds. So the dispersant’s apparent persistence in the new paper is somewhat unexpected.
Then again, Mitchelmore notes, “Corexit is made up of multiple chemicals, so each might have different biodegradation rates.” The aquatic toxicologist says she would like to see are data showing whether Corexit enhanced the ultimate breakdown of BP’s oil.
“The jury’s still out on the role of dispersants in oil degradation,” she says. “Some say they enhance it, others say they inhibit it.”
Like Mitchelmore, Beth McGee of the Chesapeake Bay Foundation in Annapolis, Md., served on a 2005 National Academy of Sciences assessment of oil-spill dispersants. Clearly, McGee says, undersea use in the Deepwater Horizon spill constitutes “uncharted territory.”
“Dispersants typically degrade fairly rapidly,” McGee says. “So the new data leave me fairly surprised.” And, she adds, the results suggest that novel uses — such as injecting them a mile below the surface where it’s cold and there’s no light — deserve study, if only to answer questions prompted by the BP spill.