Scientists are raising concerns about a new federal report that says most of the oil from the Gulf of Mexico spill has been dispersed or contained.

The report (pdf) released yesterday attempts to clear up a mystery surrounding the fate of the millions of gallons of oil that gushed from the failed BP PLC well before it was capped in mid-July. The report says all but 26 percent of the spilled oil is accounted for.

Much of the oil that remains is being degraded or cleaned up on Gulf Coast beaches, the report states.

"The bottom line here is we can account for all but about 26 percent of the oil, and of that, much of that is in the process of being degraded and cleaned up on the shore," Jane Lubchenco, administrator of the National Oceanic and Atmospheric Administration, said yesterday at the White House.

But some scientists say such a statement is misleading.

"For months, there has been a great huge shadow over the Gulf; that shadow has lifted, and that is good, and we're all glad and relieved," said Ian MacDonald, a professor of biological oceanography at Florida State University who has been tracking the flow of the spill. "But we shouldn't lift our guard, and we shouldn't pretend it is over."

The report is based on a calculation made earlier this week by a team of federal scientists that about 206 million gallons of oil had leaked from the well. The scientific team can calculate the precise amount of oil that has been contained, but in other recovery efforts like burning and skimming, calculations are less precise.

The report acknowledges it is based on the "best available scientific estimates," but scientists are concerned that the report was presented with more certainty than it likely deserves.

"I find it troubling to give these very precise numbers, very precise estimates, for things that are just extrapolations," said MacDonald.

Pedro Alvarez, chairman of Rice University's civil and environmental engineering department, agreed. "I'm not trying to say it's inaccurate; I'm trying to highlight that there's uncertainty."

"We don't know exactly how much was released," he added. "It's not like when you have a tank with a known inventory."

Alvarez thinks the federal science team could have better reflected that uncertainty.

But Lubchenco yesterday defended the report and deflected questions about the scientists' concerns.

"We believe that these are the best direct measurements or estimates that we have at the moment," she said. "We have a high degree of confidence in them."

The scientists are also concerned about the amount of oil that has been described as having disappeared -- a claim that scientists say is not backed up by either the report or the conditions in the ocean. Carol Browner, President Obama's top energy and climate adviser, yesterday morning said the initial assessment showed that "more than three-quarters of the oil is gone."

"The vast majority of the oil is gone," she said on NBC's "Today" show.

But the scientists say that although much of the oil is out of sight, it is still present in the water. The report estimated that 26 percent of the oil has dispersed -- either naturally or through the use of chemical dispersants. Like bacon grease that disperses when dishwater detergent is put into a pan, the dispersed oil is less visible but is not gone from the ocean.

The dispersed oil, Lubchenco said yesterday, "is in the process of being very rapidly degraded naturally, and so Mother Nature is assisting here considerably." She noted that "diluted and out of sight doesn't necessarily mean benign."

MacDonald said the oil left in the water column is still significant.

"There are 10 Exxon Valdez spills that are still in the water -- that has not disappeared but is still in the water being biodegraded," said MacDonald. "They're hoping Mother Nature, the old girl, is going to take care of that stuff for us. Gee whiz, I hope she can do it without croaking."

Alvarez agreed. "What they're not saying is that probably most of the oil is still in the water. That's my main beef."

That dispersed oil could still have significant effects on marine life, such as bluefin tuna. The imperiled tuna swim, mate and spawn in the Gulf in the spring and summer. Larvae or juvenile bluefin tuna could be exposed to the dispersed oil.

Some environmentalists and marine biologists have predicted it could affect a whole year class of the fish, and the effects may not be fully known for decades.

There is also still significant residual oil that is buried in marine sediments or coastal soils. Once buried and not exposed to the same wave and solar action, it may break down more slowly, scientists say. The oil could eventually come to the shore as tar balls or form sticky mats.

Still, Alvarez says there are trade-offs.

"When you disperse oil in the water ... it migrates easier, and therefore it reaches more eco-receptors, more marine life," he said. But "the dose would be lower. The oil would be reaching organisms at lower concentrations."

Still, he added, "Dilution should not be the solution to pollution."

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