Dispersants, the chemicals used to fight last year's Deepwater Horizon oil spill, have not disintegrated, posing a threat to the marine eco system, says a new study.
Approximately 3 million gallons of oil or refined petroleum products are spilled into U.S. waters every year. Oil dispersants (chemical agents such as surfactants, solvents, and other compounds) are used to reduce the effect of oil spills by changing the chemical and physical properties of the oil. By enhancing the amount of oil that physically mixes into the water, dispersants can reduce the potential that a surface slick will contaminate shoreline habitats. Although called for in the Oil Pollution Act of 1990 as a tool for minimizing the impact of oil spills, the use of chemical dispersants has long been controversial.
Nearly 800,000 gallons of chemical dispersant were injected directly into the oil and gas flow coming out of the wellhead nearly one mile deep in the Gulf of Mexico. Woods Hole Oceanographic Institution (WHOI) chemist Elizabeth B. Kujawinski and her colleagues report 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.
"This study gives our colleagues the first environmental data on the fate of dispersants in the spill," said Kujawinski, who led a team that also included scientists from UC Santa Barbara. "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," she added. "We found that it didn't go away, and that was somewhat surprising."
The study, which appears online Jan. 26 in the American Chemical Society (ACS) journal Environmental Science &Technology, is 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, according to ACS and Kujawinski. Some previous studies had indicated that dispersants applied to surface oil spills can help prevent surface slicks from endangering marshes and coastlines.
Kujawinski and her colleagues found one of the dispersant's key components, called DOSS (dioctyl sodium sulfosuccinate), was present in May and June ó 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.
Using a new, highly sensitive chromatographic technique that she and WHOI colleague Melissa C. 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 ó Kujawinski said she would "be hard pressed to say it was toxic" ó it nevertheless warrants toxicity studies into possible effects on corals and deep-water fish such as tuna, she said. The EPA and others have already begun or are planning such research, she added.
David Valentine of UC Santa Barbara and a co-investigator in the study, said, "This work provides a first glimpse at the fate and reactivity of chemical dispersants applied in the deep ocean. 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."
"The results indicate that an important component of the chemical dispersant injected into the oil in the deep ocean remained there, and resisted rapid biodegradation," said Valentine, whose team collected the samples for Kujawinski's laboratory analysis. "This knowledge will ultimately help us to understand the efficacy of the dispersant application, as well as the biological effects."
"The decision to use chemical dispersants at the sea floor was a classic choice between bad and worse," Valentine said. "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."
"The good news is that the dispersant stayed in the deep ocean after it was first applied," Kujawinski says. "The bad news is that it stayed in the deep ocean and did not degrade."