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.