A key ingredient of the chemical dispersants released last summer at BP's spewing wellhead persisted in the Gulf of Mexico's deep waters for two months and was carried by currents nearly 200 miles, according to a study released Wednesday.
The research, conducted by scientists from California universities and the Woods Hole Oceanographic Institution in Massachusetts, sheds light on the fate of the 771,000 gallons of dispersants — which had never before been released at such depths — but does not address their effect on deep-sea life, or whether they worked as intended.
BP applied about 2 million gallons of dispersants to break down the oil during the four months it gushed into the gulf and to keep it from reaching sensitive shore environments. More than half that amount was sprayed on the water's surface, but 771,000 gallons were pumped into the clouds of oil and gas that billowed from the seabed, nearly a mile below.
Dispersants have a history of use on shallow spills, but their deep-sea application was untested, leading to some criticism over the federal government's decision to allow it.
Analyzing water samples collected on three gulf research cruises last summer, the scientists measured a surfactant contained in the formulation of the Corexit dispersant applied at the seafloor. They found that the compound did not readily degrade and remained in fine clouds of oil and gas that drifted at depths of 3,000 to 3,600 feet.
The ingredient, dioctyl sodium sulfosuccinate, was detected in water samples as far as 190 miles from the wellhead and in samples taken two months after BP stopped applying dispersants.
Researchers said the quantities measured were far below levels shown to be toxic in lab tests. "We would be hard-pressed to state that, given the present state of knowledge, what we found is toxic," said lead author Elizabeth Kujawinski, an associate scientist at Woods Hole.
Still, she added, "we don't really know what effect it had on the environment."
Like detergent in a washing machine, surfactants in dispersants are designed to bind with both the oil and the water in a spill, reducing the surface tension that causes petroleum to form a sheen on the water's surface and allowing it to form fine particles that are more easily broken down by natural processes.
But the scientists said they could not discern from their samples whether the Corexit succeeded in reducing the size of oil droplets in deep waters or had just dissolved and drifted along with the hydrocarbons.
"All we have is vague idea of what happened," said David Valentine, a UC Santa Barbara geochemistry professor and one of Kujawinski's collaborators.
He cautioned that although the compound levels in the gulf samples did not approach levels found harmful in lab tests, deep-sea organisms could be more vulnerable. "It's a very different and in many ways more stressed life."
Concentrations of the sodium sulfosuccinate dropped progressively as the amount of time after release increased and the distance from the wellhead grew.
But the paper, published in Environmental Science & Technology, concluded that that phenomenon resulted from dilution rather than decomposition by the environment.
Noting that dispersants are a complex mixture of solvents and surfactants, Kujawinski said the fate of the other ingredients was not necessarily the same.
Some environmentalists and scientists questioned the widespread use of dispersants in the BP spill, saying it could have unknown environmental consequences and was more than anything a political move, intended to keep oil from washing up on the populated Gulf Coast, heavily dependent on tourism.
The use of Corexit was also criticized by those who said less toxic dispersants were available.
The U.S. Environmental Protection Agency launched various studies in the wake of the spill, examining how effective dispersants were and how they broke down in the environment. "We will share these results when the studies are complete," the agency said in a statement.