In the early days of the response to the Gulf of Mexico oil spill, BP and U.S. government officials turned to a relatively unproven strategy: spraying chemical dispersants on the ocean to break apart the slick.
The game plan was quickly rolled out over the protests of environmental groups, which questioned the chemicals' effect. The government pressed on and said the dispersants would send the oil underwater, where it would naturally degrade. Officials later began injecting dispersants at the source of the leak itself.
The surface, it seems, is now largely oil-free. Fishing bans have been lifted, and early this month the Obama administration proclaimed that the oil was largely "gone" and that only 26 per cent of the 760 million litres of leaked oil remained.
But now, in a peer-reviewed study published Thursday in the academic journal Science, a group of American scientists have pinpointed what may be the effect of the strategy. They found an invisible 35-kilometre plume of oil, oxygen-starved in cold, dark water 1,100 metres deep and degrading at one-tenth the rate of the oil at the surface.
Based on 57,000 samples gathered by an unmanned submarine, the study's findings are the first firm evidence of an underwater plume in the area, and come after a pair of university reports this week that also contradicted the government's claim. All suggest a grim reality - the oil that leaked into the Gulf of Mexico won't soon be gone.
Now, high-profile institutions are questioning the White House's suggestion that the worst is over.
"If you asked me - and I have been studying oil spills for 15 years - whether or not you would see oil subsurface, I would say no, doesn't oil float?" said Christopher Reddy of the Woods Hole Oceanographic Institution in Massachusetts, one of the authors of the study. "So this is an important aspect in the basic science world."
The plume itself, measured over 10 days in June, is odourless and invisible (the authors debated whether even to use the world "plume" to describe it) but testing revealed high levels of hydrocarbons and allowed researchers to map its size.
"Visually these water samples look like clean spring water and had no smell. That doesn't mean that they're not harmful. It's just that you know the perception that this plume is a river of chocolate syrup flying along at 1,100 metres is just not true," Dr. Reddy said.
Researchers cautioned that they have no way of knowing whether it's the only plume in the area. It is well below the depth at which most fish live, but other sea life will be impacted.
Co-author Ben Van Mooy said "the plumes could stick around for quite a while," as microbes that would typically break it apart can't get oxygen at that depth. The researchers expect it to linger for months or more.
"We absolutely should be concerned that this material is drifting around for who knows how long. They say months in the [research]paper, but more likely we'll be able to track this stuff for years," said Monty Graham, a scientist at the Dauphin Island Sea Lab in Alabama who was not involved in the study.
On Monday, the University of Georgia came forward with evidence that 79 per cent of the oil is still in the water column. The University of South Florida said a day later that oil had settled in plankton on the ocean floor far from the spill site. Combined with Thursday's report, some locals are now doubting the government's claim.
"I'm sure there's still oil out there somewhere, underneath the surface or whatever," said David Legnon, a charter fisherman in Venice, La., the town that served as the staging ground for much of the oil spill response.
"They don't see anything on top of the water. So, the oil's 'gone.' That's what they're saying. But they don't really know. We have these other people - marine biologists and whatever - saying there's still 79 per cent of the oil out there. So who do you believe?"
On Thursday evening, after the publication of the report, U.S. officials sought to qualify their two-week-old claims about how much oil is left. Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, said that while the deep-water oil isn't breaking apart as rapidly as first hoped it is still disappearing "relatively rapidly." Thad Allen, a retired Coast Guard admiral leading the U.S. government's response, said it was a difficult decision to use dispersants, about 6.8 million litres of which have now been sprayed into the Gulf.
But officials decided to "accept the implication of the hydrocarbons in the water column rather than [nearer shore in]Barataria Bay or the Chandeleur Islands," Mr. Allen said.
Academics have repeatedly questioned government claims since the spill began. One example included the government's initial estimates of the size of the leak - academics said it was much larger and the government later agreed.
"One major misconception is that oil that has dissolved into water is gone and therefore harmless," University of Georgia marine science professor Charles Hopkinson said earlier this week. "The oil is still out there, and it will likely take years to completely degrade. We are still far from a complete understanding of what its impacts are."
With reports from Reuters and Associated Press