In Gulf oil spill, how helpful – or damaging – are dispersants?

Two dispersants BP has been using to break up the oil spewing from an undersea wellhead in the Gulf of Mexico carry the federal stamp of approval. But they are not rated as effective or as safe for marine life as at least 12 other government-approved dispersants on the market.

Environmental groups are asking why this is the case, and they suggest BP may reduce damage to coastal habitats by breaking up the oil before it hits shore – but at the expense of the marine ecosystem further out in the Gulf.

In the four weeks since the Deepwater Horizon rig capsized after an explosion, BP has released 436,000 gallons of the two dispersants, Corexit EC9500A and Corexit EC9527A, the company says. Dispersants break up the escaped oil into molecular bits before it reaches shore.

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The Environmental Protection Agency (EPA) has pre-approved both for such emergencies. The effectiveness of Corexit EC9500A is rated as 55 percent, and the effectiveness of Corexit EC9527A is rated as 63 percent, according to the EPA. That ranks them behind 12 other dispersants (out of 18) that the agency has determined do a better job dispersing oil while protecting marine life.

As for toxicity, the EPA rates both products as either comparable in toxicity or 10 to 20 times more toxic than the 12 others on the list. This week, BP chief executive Tony Hayward told The Guardian newspaper that the amount was “tiny in relation to the total water volume” in the Gulf.

Reliance on dispersants, especially in response to a disaster on par with the 1989 Exxon Valdez oil spill in Alaska, is understandable, say environmental groups. What they don’t understand is why marine ecosystems are being sacrificed to save coastal habitats, a trade-off that wouldn’t be an issue if less toxic solutions were stockpiled.

Not enough is known about how the Corexit products will affect marine life, says Richard Charter, senior policy adviser for marine programs with Defenders of Wildlife, an advocacy organization in Washington. Not only is the size of the spill unique, but the Gulf environment presents conditions that EPA testing would not necessarily replicate in a lab.

“You now have a giant chemistry experiment being done in the Gulf of Mexico,” Mr. Charter says.

Dispersants in general are also unpredictable in this situation because it is uncertain where the molecules will travel and eventually settle, due to heavy tidal conditions and tropical storms, and what byproducts will form as a result.

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