By Amanda Mascarelli

Almost three million litres of chemical dispersants were injected into the broken Deepwater Horizon oil well during last year's spill, and lingered for months after the well was successfully capped, researchers say.

The dispersants travelled with the deep-water oil plumes in the Gulf of Mexico at depths of 1,000-1,200 metres, and degraded very little between May and September, according to a study published this week in Environmental Science and Technology.

The finding has mixed implications. On the one hand, it is positive that the dispersants remained in deep waters and didn't float up through the water column, where they would have mingled with surface layers, says Elizabeth Kujawinski, a chemical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts, who led the study. "But the bad news is that it stayed there. It didn't really go away as quickly as maybe they had thought it would."

The fate of the chemical dispersants has been one of the biggest wild cards of the spill's legacy. Prior to the Deepwater Horizon disaster, dispersants had been used only to treat oil slicks at the surface and had never been tested in deep subsurface environments.

Many researchers expressed concern (see 'Debate grows over impact of dispersed oil') early on, when the decision was made to apply dispersants on such a large scale in the deep ocean, because so little was known about how the chemicals might interact with the oil and gas spewing from the wellhead, or how long the chemical brew would stick around.

Indeed, the non-profit Project on Government Oversight in Washington DC revealed this week that e-mail exchanges between scientists from government agencies, including the US Environmental Protection Agency (EPA) and National Oceanic and Atmospheric Administration (NOAA), that were tasked with leading the response to the Deepwater Horizon crisis seem to show that the Obama administration "may have ignored" EPA suggestions to use more conservative language in relation to the success of the chemical dispersants.

Deep impact

Corexit 9500A, the main dispersant that BP applied, both at the surface and in deep water, is a mixture of surfactants and hydrocarbon-based solvents. Dispersant works by breaking oil down into tiny droplets, which aids in its degradation and weathering.

The researchers measured one of Corexit 9500A's main ingredients, a surfactant called dioctyl sodium sulphosuccinate (DOSS), on cruises in the Gulf in May and June, and again in September, two months after the well had been capped. Their technique involved extraction of the target molecule onto a specialized cartridge, followed by mass spectrometry and liquid chromatography to precisely quantify DOSS levels.

The extraction step renders their method about 1,000 times more sensitive than the technique used by the EPA, says Kujawinski. During the disaster, the EPA reported that it could not detect the dispersant except in a very small subset of samples, which led some to speculate that it was breaking down rapidly.

But Kujawinski says that the EPA's detection method was adequate for its goal, which was to detect concentrations of dispersant that could be considered toxic. The concentrations that have been shown to be toxic for shallow-water organisms are in the range of milligrams per litre; the EPA's method allows detection of micrograms per litre concentrations, which are well within the bounds of toxicity.

"The EPA does not have the same mandate as a researcher who is trying to trace this material," says Kujawinski. Her team's goal was to track the fate of the dispersants even once they had become very dilute.

The research stops short of answering some key questions, such as how well the dispersants worked in aiding the breakdown of oil at depth.

"It's really the first step in what is going to have to be a larger collection of works to truly understand everything that was going on with the dispersant," says David Valentine, a geomicrobiologist at the University of California, Santa Barbara, and a co-author on the study.

Scott Socolofsky, a civil engineer at Texas A&M University in College Station, who was not involved with the study, says that the research lays an important foundation for understanding what chemical and biological processes were at play during and after the spill. "I think it's going to be a fundamental result that Deepwater Horizon researchers are all going to have to work off of," says Socolofsky.

Overexposure?

So far, the researchers say there is no indication that the dispersant was present at high enough concentrations to be toxic to the organisms that were exposed to it. But both Kujawinski and Valentine caution that it is too early to draw conclusions. For one, it is unclear whether currently available toxicology assessments can be extrapolated to deep-water ecosystems, says Valentine. Toxicological studies are traditionally based on short periods of exposure and on organisms that dwell in shallow waters. As yet, there are no studies that can mirror the chronic exposure that deep-water organisms were exposed to for months during and after the spill.

Valentine says that "there's still fuzziness" when it comes to applying classic toxicological studies to "this alien world of the deep". He explains that "they don't account for the fact that life in the deep ocean happens quite a bit differently: the temperature is quite cold, the organisms are under a number of perpetual stressors, and we don't know the sensitivity of these organisms to different compounds".

The researchers agree that only time will tell whether applying the dispersant at depth was the right decision. "It was a classic decision between bad and worse," says Valentine, "and I'm still not sure which one we chose."