Published online 10 July 2010 | Nature | doi:10.1038/news.2010.347

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Debate grows over impact of dispersed oil

Researchers fear chemical is finding its way to shore and up the food chain.

It may look unhealthy, but the cure could be worse.USCG/Petty Officer 1st Class Tasha Tully

For years, Robert Twilley has worked to bridge the traditional academic divides between oceanography and coastal science.

"They really are not two separate systems," says Twilley, a coastal scientist from Louisiana State University in Baton Rouge. "Whatever you do offshore certainly has implications to the shoreline and bay estuary environments."

Now, Twilley is watching this lesson unfold before his eyes. As London-based BP continues to pour vast quantities of dispersants into the Gulf of Mexico, Twilley and many other scientists are growing increasingly concerned about the chemical soup that may be creeping onshore, as well as the poorly understood effects of dispersant in the water column at sea.

Initially, BP and the federal agencies involved in the spill response made the decision to use dispersant offshore to limit the amount of viscous oil washing up on beaches and into the wetlands, says Ed Overton, an environmental chemist at Louisiana State University. The idea behind the use of dispersants offshore, says Overton, "is you hold your nose and accept damage offshore to try to prevent damage onshore".

So far, more than 6.6 million litres of dispersant have been applied: more than 4 million litres offshore and more than 2.5 million litres at the site of the leak. On the surface, dispersants are sprayed from planes over the surface of the oil. To reach oil at depth, dispersant is pumped from a vessel at the surface down to a wand pointed into the oil flowing from the broken wellhead, some 1.5 kilometres deep. Before the Deepwater Horizon spill, dispersants had only been used to treat surface oil.

Therein lies the worry, says David Valentine, a geomicrobiologist at the University of California in Santa Barbara. "It's an experiment that's never been performed before—to dump that much of an industrial chemical into the ocean."

"My hunch is that when this thing started they wanted to keep the oil off the beaches, so they used the dispersants," says Samantha Joye, a biogeochemist at the University of Georgia in Athens. "But no one thought it would go on this long."

Now there are fears that that dispersed oil is making its way into shallow waters even as questions abound about the impacts on the water column at depth. We are trying "to unravel what's truth and what's purely speculation," says Twilley.

Increasing toxins

BP has been authorized to use dispersants only in waters deeper than 10 metres and no nearer to shore than five kilometres. So far, its closest application has been 14.6 kilometres from shore, and the vast majority of the dispersant has been applied at 28 kilometres or further, according to Arden Ahnell, BP's Director of Global Product Stewardship.

The US Environmental Protection Agency announced on 30 June that its initial round of toxicity testing on eight dispersants, including Corexit 9500, produced by Nalco of Naperville, Illinois — the product that BP is using to help break up the oil — found no "biologically significant" endocrine disrupting effects on the small estuarine fish and mysid shrimp that they tested. However, the EPA noted that it still needs to do further testing on the effects of the dispersants mixed with oil.

"We already know that dispersants are less toxic than oil if you compare the two," says Susan Shaw, a marine toxicologist and director of the Marine Environmental Research Institute in Blue Hill, Maine. "But because Corexit contains a petroleum solvent, we're actually putting petroleum solvent on top of a petroleum spill. So it's increasing the hydrocarbon in the water column."

Corexit 9500 is composed primarily of detergents that break globs of oil into tiny microdroplets, as well as a propanol-based solvent that is used in household cleaning products. Dispersion is thought to speed up oil degradation because tiny droplets can be more readily metabolized by oil-eating microbes.

Joye doesn't buy that claim: "It assumes that the dispersant doesn't impact the microbial community, and we have no idea if that's true or not," she says. "There's just as good a chance that this dispersant is killing off a critical portion of the microbial community as it is that it's stimulating the breakdown of oil."

Delivery system

Furthermore, says Shaw, the dispersant can increase the toxicity of the oil for those marine organisms that encounter it. "It's like a delivery system," says Shaw. "The [dispersed] oil enters the body more readily and it goes into the organs faster."

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"The real issue we've got in this spill is the massive amount of oil that just keeps coming," says Overton. "We've gone past any normal use of dispersants."

Still, Overton is not convinced that dispersed oil is making its way to shallow waters. "The whole idea of dispersing only offshore is to do it only where there's so much water that it gets dispersed below dangerous levels very quickly," says Overton.

Twilley is part of a science and engineering review team that is providing guidance to the Coastal Protection and Restoration Authority of Louisiana, and he says the lines of communication are open with the EPA to develop a unified position for a bioremediation plan.

Meanwhile, Twilley says, more field testing is needed to monitor the actions of the dispersants and oil, alone and in combination.

"The complication here is the trade-offs," says Twilley. "There are certain things with dispersants that are of benefit, and there are negatives, and we're having problems evaluating those trade-offs." 

See http://www.nature.com/oilspill for more.

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