The oil dispersants being used in the Gulf of Mexico spill may help destroy the oil a little faster, but their primary purpose is to alter its destination so that the oil stays in the deep ocean rather than reaching the shore.
Scientists don't know much about the oil's ultimate effect in the deep water, but most agree that it will have a much larger biological effect if it reaches the coast, which is teeming with wildlife.
"You're transferring the pollution, if you will, but under the right circumstances it's probably favorable," said E. Eric Adams, who specializes in environmental fluid mechanics at the Massachusetts Institute of Technology. "You use it when you are more concerned about [life] on the surface and shorelines than in the water column. It's a tradeoff," said Adams, who was on a National Academy of Sciences panel that investigated dispersants in 2005.
"Well offshore is the best place to use it," added Beth McGee, a water quality scientist at the Chesapeake Bay Foundation who was also a member of the National Academy panel. "You may be creating a toxic environment for fish, but you are preventing it from washing up on shore, which would have a much greater environmental impact. In the case of the gulf…you do everything you can to prevent it from reaching the shore."
The panel concluded that the dispersants themselves are much safer than those used a decade ago, but that much more research needs to be done on the effects of the dispersed petroleum.
The key ingredient of dispersants is a surfactant or, in layman's terms, a detergent. It is often mixed with an organic solvent or an alcohol to help it mix more thoroughly with the oil. Just like the detergent in your washing machine, it binds to both the oil and the water, reducing the surface tension that causes petroleum to form a sheen on the water's surface and allowing it to form fine particles.
These particles dissolve in water, allowing the spill to disperse downward rather than shoreward. They also make it much easier for sunlight and microorganisms to attack and destroy the oil. While untreated oil can persist for years or decades, finely dispersed oil can be degraded in days to months, McGee said.
Wave activity (again, as in the case of a washing machine) is required to help the surfactant disperse the oil. Dispersants weren't of much value in the 1989 Exxon Valdez oil spill, for example, because there wasn't enough wave activity for them to work effectively.
At the other extreme, however, bad weather also interferes with their effectiveness, said environmental chemist Carys L. Mitchelmore of the University of Maryland Center for Environmental Science in Solomons. "If there is bad weather and big waves," which has been the case in the gulf, "you can't use it."
Dispersants need to be used within the first 24 to 48 hours after the oil reaches the surface. Once the petroleum begins weathering and the lighter components evaporate, it becomes much harder to disperse the residue. These factors — weathering and big waves — are two reasons why control efforts at the site of the spill are now focusing on injecting the dispersant directly into the oil at the wellhead.
That approach, however, poses additional problems. Not only does the depth make it difficult, but a large volume of dispersant is required. Typically, a dispersant is added at a concentration of about one part of surfactant to 20 to 40 parts of petroleum, and the well is releasing hundreds of thousands of gallons per day.
Not everyone thinks the dispersants are a great idea. Chemist Lawrence Duffy of the University of Alaska, who studied the Exxon Valdez spill, thinks the dispersants "are more a public relations measure than a preventive one. …It has an effect on our psychology because we don't see the oil slick," but the oil is still there.
"It's better to have it on the surface so you can go ahead and scoop it up," he added.