Oil dispersant lingers in Gulf

UC SANTA BARBARA (US) — A major component of the nearly 800,000 gallons of chemical dispersant used in the Gulf to breakup oil from last year’s Deepwater Horizon spill is not degrading. Now researchers question its potential long-term effects.

The dispersant was injected directly into the oil and gas flow coming out of the wellhead nearly one mile deep in the Gulf of Mexico.

The first study to access how well the strategy worked at breaking up oil droplets indicates that a major component of the dispersant itself was contained within an oil-gas-laden plume in the deep ocean and had still not degraded some three months after it was applied. Findings are reported in the journal Environmental Science & Technology.

“This study provides a first critical look at what happened to all that dispersant injected directly into the gush of oil at the sea floor,” says David Valentine, a geochemist at the University of California, Santa Barbara. “Key components of the dispersant were trapped in subsurface plumes of oil and gas, and were not rapidly biodegraded.”

Toxic toll
The results also raise questions about what impact the deep-water residue of oil and dispersant—which some say has its own toxic effects—might have had on the environment and marine life in the Gulf.

Woods Hole Oceanographic Institution (WHOI) chemist Elizabeth Kujawinski says the study offers scientists the first environmental data on the fate of dispersants in the spill. Their report represents the first peer-reviewed research to be published on the dispersant applied to the Gulf spill and the first data in general on deep application of a dispersant.

“These data will form the basis of toxicity studies and modeling studies that can assess the efficacy and impact of the dispersants. We don’t know if the dispersant broke up the oil. We found that it didn’t go away, and that was somewhat surprising,” says Kujawinski.

Some previous studies had indicated that dispersant applied to surface oil spills can help prevent surface slicks from endangering marshes and coastlines.

“By knowing how the dispersant was distributed in the deep ocean, we can begin to assess the subsurface biological exposure, and ultimately what effects the dispersant might have had,” says Valentine, whose team collected the samples for Kujawinski’s laboratory analysis.

Sampling science
Kujawinski and the other researchers found one of the dispersant’s key components, called DOSS (dioctyl sodium sulfosuccinate), was present in May and June—in parts-per-million concentrations—in the plume from the spill more than 3,000 feet deep. The plume carried its mixture of oil, natural gas and dispersant in a southwest direction, and DOSS was detected there at lower (parts-per-billion) concentrations in September.

Chemical tankers used for dispersant transport, stored in a Gulf of Mexico port facility. (Credit: David Valentine)

Using a new, highly sensitive chromatographic technique that she and WHOI colleague Melissa Kido Soule developed, Kujawinski reports those concentrations of DOSS indicate that little or no biodegradation of the dispersant substance had occurred. The deep-water levels suggested any decrease in the compound could be attributed to normal, predictable dilution.

They found further evidence that the substance did not mix with the 1.4 million gallons of dispersant applied at the ocean surface and appeared to have become trapped in deepwater plumes of oil and natural gas reported previously by other WHOI scientists and members of this research team.

The team also found a striking relationship between DOSS levels and levels of methane, which further supports their assertion that DOSS became trapped in the subsurface.

Though the study was not aimed at assessing the possible toxicity of the lingering mixture—Kujawinski says she would “be hard pressed to say it was toxic”—it nevertheless warrants toxicity studies into possible effects on corals and deep-water fish such as tuna, she said. The Environmental Protection Agency and others have already begun or are planning such research, she added.

Bad vs. worse
Kujawinski cautions that “we can’t be alarmist” about the possible implications of the lingering dispersant. Concentrations considered “toxic” are at least 1,000 times greater than those observed by Kujawinski and her colleagues, she says. But because relatively little is known about the potential effects of this type of dispersant/hydrocarbon combination in the deep ocean, she adds, “We need toxicity studies.”

“The decision to use chemical dispersants at the sea floor was a classic choice between bad and worse,” Valentine says. “And while we have provided needed insight into the fate and transport of the dispersant, we still don’t know just how serious the threat is. The deep ocean is a sensitive ecosystem unaccustomed to chemical irruptions like this, and there is a lot we don’t understand about this cold, dark world.”

Kujawinski says the good news is that the dispersant stayed in the deep ocean after it was first applied. “The bad news is that it stayed in the deep ocean and did not degrade.”

The work was funded by WHOI and the National Science Foundation. The instrumentation was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.

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