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WASHINGTON -- No more oil is likely to leak into the Gulf of Mexico now that efforts to plug the blown-out well are succeeding, the government's point man on the spill declared Wednesday. A relieved President Barack Obama said the fight to stop the leak is 'finally close to coming to an end.'

At the White House, National Incident Commander Adm. Thad Allen said oil company BP's effort to plug the leak was progressing, giving officials 'high confidence' that there will soon be no more oil leaking into the environment. The upbeat assessment came as a government report released Wednesday said only about a quarter of the spilled oil remains in the Gulf. The rest has been contained, cleaned up or has otherwise disappeared.

Obama's team, however, was careful to emphasize that much work remains, from cleanup to damage assessment to help for hurting families. Obama said people's lives 'have been turned upside down' by the spill.

And White House spokesman Robert Gibbs told reporters, 'There's a lot of reasons why there's no 'Mission Accomplished' banner.'

'There's a lot of work to do,' he said. 'We're not leaving the area, and more importantly, we're not leaving behind any commitment to clean up the damage that's been done and repair and restore the Gulf.'

Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, said the spill's effect on wildlife will continue for 'years and possibly decades to come.'

BP PLC earlier Wednesday announced it had reached a significant milestone when mud that was forced down the well held back the flow of crude in a procedure known as a 'static kill.'

Government officials defended the credibility of their report saying about 75 percent of the oil is gone. They said that description is based on direct measurements of the spill as well as estimates, and that the instruments they've used to capture the scope of the disaster have improved since it began April 20.

In Congress, lawmakers pressed scientists to explain what effects a chemical used to get rid of some of the oil will have on the Gulf's ecosystem.

BP applied nearly 2 million gallons of a chemical dispersant to the oil as it spewed from the broken underwater well. The aim was to break apart the oil into tiny droplets so huge slicks wouldn't tarnish shorelines and coat marine animals, and to make the oil degrade more rapidly.

The government report released Wednesday shows that 9.6 percent of the estimated 172 million gallons of oil released into the Gulf of Mexico was dispersed by the chemicals.

Sen. Sheldon Whitehouse, D-R.I., called use of the chemicals a 'grand experiment.' He said it was unclear whether it would limit damage from the spill, or cause greater harm.

Paul Anastas, the assistant administrator for the Office of Research and Development at the U.S. Environmental Protection Agency, said that while the effects of such a large quantity of dispersants are unknown, tests so far have not found dispersants near coasts or wetlands. Laboratory tests conducted by the EPA comparing the chemicals to oil alone and to mixtures of oil and dispersants also show that they are not more toxic.

'When you look at all of the tools to combat this tragedy ... dispersants have shown to be one important tool in that toolbox,' Anastas told lawmakers.

But several independent scientists testifying before the panel Wednesday faulted the EPA testing.

'A laboratory experiment ... doesn't help us understand much of the environmental chemistry or its effects on other parts of the ecosystem,' said Ronald Kendall, director of the Institute of Environmental and Human Health at Texas Tech University.

The chemical -- Corexit 9500 -- was on a federal list of preapproved dispersants, but in May the EPA directed BP to use less of the toxic chemical because its long-term effects were unknown.

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