By Richard A. Lovett

Chemicals used to reduce oil slicks during the Deepwater Horizon oil spill in the Gulf of Mexico may have rendered the oil more toxic than official reports suggest, according to a Canadian toxicologist.

Peter Hodson, an aquatic toxicologist from Queen's University in Kingston, Ontario, presented his case on 9 November at a meeting of the Society of Environmental Toxicology and Chemistry in Portland, Oregon, in a session that highlighted the uncertain effects of such treatments on marine wildlife.

The chemicals, known as dispersants, are used to reduce the surface tension of spilled oil, allowing wind and waves to break it into microscopic droplets. These droplets disperse through sea water rather than floating in massive oil slicks that can blow on to shorelines. They are also more easily attacked by oil-eating bacteria.

Danger dispersed?

But until it is degraded by such bacteria, the dispersed oil becomes mixed into the water rather than sitting on top of it. This means that its toxic constituents, most notably polycyclic aromatic hydrocarbons (PAHs), are likely to have a greater effect on marine wildlife. "It's definitely a trade-off," says Charlie Henry, the National Oceanic and Atmospheric Administration's lead scientific support coordinator for the emergency response to the oil spill.

Official statements by the US Environmental Protection Agency (EPA) have reported that the oil-dispersant mix resulting from the Deepwater Horizon spill is no more toxic than the oil itself. Although that is technically true, it is misleading, says Hodson, who has been studying the effect of dispersed oil on Atlantic herring embryos.

The problem, explains Hodson, is that the dispersed cloud of microscopic oil droplets allows the PAHs to contaminate a volume of water 100-1,000 times greater than if the oil were confined to a floating surface slick. This hugely increases the exposure of wildlife to the dispersed oil. "EPA was presenting only part of the risk equation," he told the meeting. "They're trying to sugar-coat the message. In trying to understand the risks of dispersed oil, we need to understand exposure."

Hodson's research suggests that fish embryos, still in their eggs, are extremely sensitive to dispersed oil. "Exposures as brief as an hour can have a negative effect on embryonic fish," he says. That, combined with the fact that for any some species, large numbers of fish can spawn at about the same time of year, means that an entire hatch could be decimated by a plume of contaminated water: "You could have a very large portion of the fish stock affected."

Hodson adds that although dispersal allows the oil to be broken down more quickly, it can still take up to a month for microbes to have a measurable impact on the amount of oil present.

Lingering problem

Worse, the toxic constituents of oil hang around longer than other components, another speaker told the meeting. "This idea that there's an oil biodegradation rate doesn't hold," says Ronald Atlas, a microbiologist at the University of Louisville, Kentucky, who has studied the aftermath of the 1989 Exxon Valdez spill in Alaska. Alkanes, the simple hydrocarbons that comprise the bulk of oil, are degraded more readily than the PAHs, he points out.

Not everyone is so worried, however. Paul Boehm, an oceanographer at scientific-consulting firm Exponent in Maynard, Massachusetts, has done numerous studies of the Ixtoc-1 spill that occurred off the coast of Mexico in 1979. He notes that dispersants were also used following that disaster and didn't kill huge numbers of fish. "I don't think we'll see tremendous mortalities in the open ocean," he says of the Deepwater Horizon spill.

Other speakers at the Portland meeting also noted that natural processes break up the oil too, and that the dispersants used aren't terribly effective anyway: "Only 8% of the oil was actually dispersed," says Mace Barron, a toxicologist at the EPA's Gulf Ecology Division in Gulf Breeze, Florida.