Jeff Haller for The New York Times
WASHINGTON — The government is expected to announce on Wednesday that three-quarters of the oil from the Deepwater Horizon leak has already evaporated, dispersed, been captured or otherwise eliminated — and that much of the rest is so diluted that it does not seem to pose much additional risk of harm.
A government report finds that about 26 percent of the oil released from BP’s runaway well is still in the water or onshore in a form that could, in principle, cause new problems. But most is light sheen at the ocean surface or in a dispersed form below the surface, and federal scientists believe that it is breaking down rapidly in both places.
On Tuesday, BP began pumping drilling mud into the well in an attempt to seal it for good. Since the flow of oil was stopped with a cap on July 15, people on the Gulf Coast have been wondering if another shoe was going to drop — a huge underwater glob of oil emerging to damage more shorelines, for instance.
Assuming that the government’s calculations stand scrutiny, that looks increasingly unlikely. “There’s absolutely no evidence that there’s any significant concentration of oil that’s out there that we haven’t accounted for,” said Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, the lead agency in producing the new report.
She emphasized, however, that the government remained concerned about the ecological damage that has already occurred and the potential for more, and said it would continue monitoring the gulf.
“I think we don’t know yet the full impact of this spill on the ecosystem or the people of the gulf,” Dr. Lubchenco said.
Among the biggest unanswered questions, she said, is how much damage the oil has done to the eggs and larvae of organisms like fish, crabs and shrimp. That may not become clear for a year or longer, as new generations of those creatures come to maturity.
Thousands of birds and other animals are known to have been damaged or killed by the spill, a relatively modest toll given the scale of some other oil disasters that killed millions of animals. Efforts are still under way in Louisiana, Mississippi, Alabama and Florida to clean up more than 600 miles of oiled shoreline. The government and BP collected 35,818 tons of oily debris from shorelines through Sunday.
It remains to be seen whether subtle, long-lasting environmental damage from the spill will be found, as has been the case after other large oil spills.
The report, which is to be unveiled on Wednesday morning, is a result of an extensive effort by federal scientists, with outside help, to add up the total volume of oil released and to figure out where it went.
The lead agency behind the report, the oceanic and atmospheric administration, played down the size of the spill in the early days, and the Obama administration was ultimately forced to appoint a scientific panel that came up with far higher estimates of the flow rate from the well. Whether the new report will withstand critical scrutiny is uncertain; advocacy groups and most outside scientists had not learned of it on Tuesday.
The government announced early this week that the total oil release, from the time the Deepwater Horizon exploded on April 20 until the well was effectively capped, was 4.9 million barrels, plus or minus 10 percent. That estimate makes the Deepwater Horizon disaster the largest marine spill in history. It is surpassed on land by a 1910 spill in the California desert.
As the scientists did their calculations, they were able to rely on direct measurements of the fate of some of the oil that spewed from the broken well. For example, BP and its contractors succeeded in capturing about 17 percent of it with various containment mechanisms, the report says.
The outcome for much of the oil could not be directly measured, but had to be estimated using protocols that were scrutinized by scientists inside and outside the government, Dr. Lubchenco said.
The report calculates, for example, that about 25 percent of the chemicals in the oil evaporated at the surface or dissolved into seawater in the same way that sugar dissolves in tea. (The government appears to have settled on a conservative number for that estimate, with the scientific literature saying that as much as 40 percent of the oil from a spill can disappear in this way.)
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