As Oil Becomes 'Mousse' Then 'Tarballs,' Chemistry Could Determine Coast's Fate

The Deepwater Horizon disaster is no Exxon Valdez, says marine chemist Edward Overton. Instead of a "black tide" of crude oil flushing into marshlands, Overton is looking for mostly "tarballs" to invade the Gulf Coast's beaches and marshes.

Between the leaking wellhead—which is 1500 meters down on the sea floor, 65 kilometers out in the Gulf—and the coast, the oil is transforming. It first forms a foamy "mousse" that quickly loses its more volatile, more toxic components. That looks like roof tar, says Overton, professor emeritus at Louisiana State University, Baton Rouge, and a chemical hazard assessment contractor for the National Oceanic and Atmospheric Administration. "It's getting less toxic, but it's staying sticky."

By the time the spill reaches the coast, it's likely to be mostly tarballs and tar mats that are more likely to kill plants and animals by sticking to them than by outright toxicity, Overton says. "Mother Nature is helping us because we're dealing with a sticky material that won't cover large areas." Still, "we've never had a spill of this magnitude on a marshland coast. It's hard to tell what the biggest impact will be."

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