A core ingredient of the dispersant injected deep underwater at BP's runaway oil well remained trapped in an undersea plume of oil, methane and other hydrocarbons, resisting decay even as it became vanishingly dilute, scientists reported yesterday in the first detailed study of undersea dispersants in the Gulf of Mexico.
During the Deepwater Horizon oil spill, the government and BP PLC injected some 771,000 gallons of dispersant directly into the stream of oil and gas erupting from the Macondo well, more than a mile underwater. The injections, which ended in mid-July, were an attempt to break apart the oil, keeping it far removed from sensitive shorelines while also increasing the amount of crude available for a bacterial feeding frenzy.
While a large portion of the oil and gas escaping from the well stayed in the dark deep -- subsequently consumed by microbes and spread thin over the Gulf's vastness -- it remains uncertain whether dispersants, or simple physics, caused this plume. And while that question remains unresolved, it is now clear that the dispersants stayed put, closely following the hydrocarbon plume's southwest tack, some 3,600 feet below the surface.
During this snaking undersea voyage, at least one dispersant component, a complex surfactant essential to divvying up oil, seems to have resisted microbial decay for several months, instead spreading in practically undetectable concentrations up to 180 miles from the well, according to Liz Kujawinski, a chemist at the Woods Hole Oceanographic Institution and lead author of the study, to be published in the journal Environmental Science & Technology.
The surfactant's stubborn refusal to degrade -- which could be as much a function of its low numbers as its biochemistry -- came as somewhat of a surprise to Kujawinski, who, like most of the public, she said, "had the impression that the dispersant was supposed to have degraded faster."
When Kujawinski began to review past research, however, she saw few studies looking at the degradation of her chosen surfactant, dioctyl sodium sulfosuccinate, or DOSS, and none of those related to the cold, alien landscapes of deep water. What studies did exist were conflicting. "Turns out," she said, "there's not a whole lot of data out there on what should have happened."
"This was a pretty classic example of not having relevant scientific studies to base our expectations," added Dave Valentine, a geochemist at the University of California, Santa Barbara, who co-authored the dispersant paper. "Yet everybody [had] expectations."
Valentine led scientific expeditions into the Gulf during early June and September, those trips providing grist for a series of previously published papers finding that nearly all of the hydrocarbon gas -- methane, ethane, propane -- escaping from BP's well remained underwater, digested by microbes over a few-month span. And while Valentine focused on the gas, he made sure to sample for dispersants, too.
Over several months, Kujawinski analyzed samples taken during Valentine's June trip, which stayed close to the spill's center, comparing the surfactant concentrations to methane. As Valentine's work has shown, methane did not begin to degrade until some days removed from the wellhead. The two chemicals moved together in concentration, and this ratio, among other comparisons, indicated that nearly all of the DOSS remained underwater in amounts that hewed closely to the average reported injection rate.
By Valentine's second trip to the Deepwater Horizon site, in September, U.S. EPA was reporting that it could no longer find any undersea dispersants. More than a month after their last application, they had effectively disappeared. It was a fair assessment to make at the time, Kujawinski said, since EPA's coarser, high-speed tools focused on detecting hazardous thresholds, rather than minute concentrations.
Using advanced lab-based imaging, however, Kujawinski found that DOSS remained in the September water samples, lingering in the parts per billion, 1,000 times below any known effects likely to prove toxic to undersea life. (By September, all the methane had vanished.)
While DOSS only constitutes some 10 percent of the dispersant, Corexit 9500A, used at the wellhead, it is one of the more toxic ingredients, and extrapolated dispersant concentrations also fell below safety thresholds, Kujawinski said.
This drastic reduction in DOSS -- the highest level Kujawinski measured in June was 12 parts per million -- was driven largely by simple dilution, rather than bacterial degradation.
Comparing her figures to the concentrations expected from known water dynamics and the steady undersea currents measured by Woods Hole researchers last summer, Kujawinski estimated that the surfactant would hit such low numbers without any of the bacterial assistance seen in degrading methane and oil, she said.
There are several reasons the bugs could have demurred, Valentine said. Unlike the gas and oil seeps that pepper the Gulf floor, surfactant is a largely foreign compound, so few microbes would have been primed to attack it. And the DOSS was in such low numbers that it may not have warranted the energy needed to adapt to it.
Microbes are smart consumers. "It has to be worth their time," he said.
Good news, bad news
For man and bug alike, the surfactant's complex structure also made it a difficult subject, for dining or study.
Half of the molecule likes to be in oily substances, while the other half likes to be in water, Kujawinski said. It cannot easily be separated from samples, like traditional liquids or gases, and it will stick to Teflon-lined containers.
You wouldn't want your dispersant to behave otherwise, Valentine added. "These are the same properties that make it effective in breaking up oil droplets," he said.
As for whether the deepwater dispersants did their job and caused the oil to divide? That's a physics question, Kujawinski said, outside the bounds of her study. (Using high-resolution video, several physical scientists are tackling the problem.)
The dispersant could have been taken up in methane and missed the oil completely, or it could be associated with the small oil droplets in the deepwater plume.
Both dispersant scenarios seem plausible, she said.
Kujawinski does not want to raise alarm with her study, which will likely provide the baseline calculations for future measures of what effect, if any, dispersants had on life in the Gulf's deep waters. While concentrations were low, the length of exposure hasn't been seen in past toxicity work, and needs to be considered, she said.
"The good news is that the dispersant stayed in the deep ocean after it was first applied," Kujawinski added in a statement. "The bad news is that it stayed in the deep ocean and did not degrade."