Deep sea BP spill dispersants din't degrade for months

A fresh oil slick from the Deepwater Horizon spill, during June 2010.
Note that one drop of detergent was added to the oil slick, forming the cleared circle.
(Photo by David L. Valentine, University of California Santa Barbara)

From ScienceNews

Nearly 3 million liters [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 Jan. 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.

Breaking up is hard to do.
When oil and gas mixtures are ejected from a deep wellhead, liquid oil droplets of many different sizes form and rise toward the ocean surface.
Because the smaller droplets become as dense as the surrounding water deep below the surface--in this case at about 1,100 meters--they are swept away laterally by prevailing ocean currents (left panel).
When a dispersant is added at the depth of the wellhead, a component called a surfactant breaks up the oil into small droplets (middle panel).
If the dispersant works perfectly, virtually all the liquid oil is in these "neutrally buoyant" droplets and is carried away before ever reaching the surface and the droplets become small enough to be consumed, or "biodegraded," by bacteria.
In the Deepwater Horizon spill (right panel), scientists found evidence that the dispersant mixed with the small droplets in the deep-water hydrocarbon plume but also discovered the oil/dispersant mix had not yet biodegraded several months after the spill.
The study could not distinguish between oil droplets coated with surfactant (which would suggest the dispersant worked as planned) and surfactant floating freely on its own (suggesting the substance did not attach to the oil, as intended).
(Credit: Jack Cook, Woods Hole Oceanographic Institution)

“If the dispersant worked, it should have been associated with the liquid oil — that is, moving off laterally into the deep-water 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 [3,400 to 21,000 gallons] 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 [186 miles] 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, Maryland., 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.

Links :

• WHOI : First study of dispersants in Gulf spill suggests a prolonged deepwater fate
• NationalGeographic : Gulf spill dispersants surprising long-lasting
• NYTimes : Study: undersea dispersant in Gulf of Mexico lingered in Deepwater plume
• Wired : No progress on better chemicals for oil disaster cleanup / EPA orders to use less-toxic oil dispersant /