By Daniel Cressey

Since the Deepwater Horizon oil rig exploded and sank in April, vast quantities of dispersant chemicals have been sprayed onto the resulting slick. The chemicals--alongside corralling floating oil with booms and setting it aflame in "controlled burns"--are an attempt to reduce the environmental damage of oil still spewing out of a ruptured well in the Gulf of Mexico at the rate of 5,000 barrels (about 800,000 liters) per day.

What do dispersants do?

Dispersants do not remove oil from the area. Rather, they help large globs of oil "disperse" into smaller pieces--hence their name--which are easier for sea-living microbes to break down.

They work on the same principle as kitchen washing-up liquids. Both are made up mainly of surfactant molecules, which have heads that are attracted to water and tails that are repelled by water.

The molecules embed themselves at interfaces between oil and water. This lowers surface tension at the interface--that is, the difficulty of disrupting molecules of oil and molecules of water from clinging to their own kind rather than mixing.

"Lowering the interfacial tension reduces the energy required to mix the oil as discrete droplets into the water phase," says David Horsup, division vice-president for research and development at Nalco Energy Services, which manufactures the dispersant products currently in use in the Gulf of Mexico. "Dispersing the oil into small droplets allows naturally present microbes to then consume those oil droplets, and the dispersant rapidly to biodegrade."

Which dispersants are being used in the Gulf of Mexico?

So far, at least 325,000 gallons (over 1 million liters) of two types of dispersants have been used: Corexit 9500 and Corexit EC9527A.

"One is designed for light, fresh oil or oil which has been released very early on and hasn't been weathered," explains Horsup. "The other is designed for heavier oils and those that have been weathered for a few days."

What are the environmental impacts of dispersants?

Dispersants are themselves unpleasant chemical concoctions, although today's products are less damaging than the toxic solvents used in earlier spills such as the Torrey Canyon disaster. The main problem with dispersants relates to the fact that they help to spread oil more widely into the environment.

This helps to prevent large amounts of oil fouling beaches and coating animals, but although surface-dwelling animals may benefit from a reduction in oil exposure, animals in other parts of the oceans and those at the sea floor will experience more pollution.

"It's a trade-off, and no one will tell you using dispersants won't have an effect. You're trading one species for another," says Carys Mitchelmore, an environmental chemist at the University of Maryland Center for Environmental Science in Solomons and a co-author on a 2005 U.S. National Academies report on dispersants.

In one study, not yet published, Mitchelmore and her colleagues exposed soft corals to crude oil and the Corexit 9500 dispersant currently in use. Growth rates fell significantly.

Overall, however, there is still a dearth of knowledge about the impact of dispersants, especially if used in the volumes now being applied in the Gulf.

"The long-term effects are really unknown," says Mitchelmore. "The dispersant has inherent toxicity. And these oil droplets tend to be the same sort of size as food particles for filter-feeding organisms."

So far, dispersants have mainly been applied to surface slicks by spraying from planes. Could they be targeted to the source of the oil leak?

Underwater robots are now being used in the Gulf to apply dispersants directly to oil as it leaks from the sea floor. Such underwater use has never been attempted before at the depth of the current spill.

Eric Adams, an environmental engineer at the Massachusetts Institute of Technology and another author on the 2005 report, notes that, at the surface, wind and waves help dispersants to mix with and break up oil. Underwater, it is harder to ensure mixing.

"In some sense it's easier to apply it [dispersant] at the surface," says Adams, who has done theoretical work on deep-sea application systems for dispersants. But the advantage of underwater application, which can reach oil at its source, makes the experiment worth trying, he thinks.

Some have criticized test runs of deep-sea dispersant use at the Deepwater Horizon site by the U.S. Environmental Protection Agency (EPA), on the grounds that research so far has focused on the impact of dispersants in the upper parts of the ocean.

But the attempt does appear to be working. "Aerial observations indicated that the slick on the surface was significantly reduced once you inject the chemical right at the well head," Horsup says. "You get very rapid dispersion of the oil into the water column."

The EPA admits that the impact of underwater use on the environment is "still widely unknown." However, after the initial test runs, the go-ahead was given this week for further underwater applications.

What does the future hold?

In the short term, chemical companies worldwide are scrambling to keep up with the massive demand for dispersants resulting from the accident. Horsup says that suppliers are currently keeping up with demand, but that if demand increases markedly stockpiles may become depleted.

In the longer term, the Deepwater Horizon accident response has begun what is, to all intents and purposes, a giant experiment on dispersant use.

The 2005 National Academies report raised numerous questions about dispersant toxicity and the final fate of dispersed oil that need answers. Both Adams and Mitchelmore say not much progress has yet been made on those questions. The events in the Gulf of Mexico may push this research forwards