What do you get when you mix oil and dispersants? A mixture that doesn’t seem to be more toxic than oil alone, the EPA said yesterday. Their statement came after a second round of testing eight oil dispersants.

The EPA tested the response of two sensitive Gulf species, the mysid shrimp and a small fish called the inland silverside, which they exposed to mixtures of dispersants plus oil and to oil alone.

The results indicate that the eight dispersants tested are similar to one another based on standard toxicity tests on sensitive aquatic organisms found in the Gulf. These results confirm that the dispersant used in response to the oil spill in the Gulf, Corexit 9500A, is generally no more or less toxic than the other available alternatives. [EPA statement]

Chemical dispersants help break down oil, in theory putting it in a form easier for microbes to consume. Still, dispersants are toxic, and BPs unprecedented use of huge amounts of Corexit worried EPA officials, who were uncertain of the chemical’s long-term effects. Reportedly, BP and the United States Coast Guard have not used dispersants since July 19, when the leaking well was successfully fitted with a temporary cap.

The tests prove that the oil itself, not the dispersants, is “enemy No. 1,” Paul Anastas, EPA assistant administrator for research and development, told reporters on a conference call. [CNN]

Still, Anastas says EPA scientists have more research to do on the lingering effects of dispersants:

“The type of acute toxicity we’re discussing today is only one part of the hazard,’’ he said. Another is the health effects of the breakdown products of the dispersant, which the agency has yet to investigate. [New York Times]

As we reported yesterday, since the temporary cap was put in place oil slicks have been quickly disappearing from the surface of Gulf waters. Some say that the oil evaporated, that sunlight broke it down, or that the dispersants helped microbes eliminate it:

Adm Thad Allen, the national incident commander, said: “It’s becoming a very elusive bunch of oil for us to find.” … By some estimates, up to 40 per cent of the oil may have evaporated as soon as it reached the surface. Experts said that warm surface water and weeks of sunlight had broken up the crude, along with strong winds and waves during storms last week. The Gulf’s waters also contain bacteria that have always degraded oil that seeps naturally from the ocean floor. [The Telegraph]

But the fact that the oil is quickly disappearing from the water’s surface doesn’t have everyone celebrating. As Discovery News reports, researchers have seen such vanishing acts before only for the oil to reappear on shore, as happened with the 1979 Ixtoc I oil gusher which also used dispersants. Larry McKinney, who worked on that spill, said he fears that the oil will mix with sediment and sink to the continental shelf.

“It’s a race,” McKinney said. “Can the microbial activity eat up the oil before it mixes with sediments and sinks? … BP used a lot of dispersant and the oil went someplace,” McKinney said. [Discovery News]

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Image: NASA