WASHINGTON -- As President Obama and BP officials cheered Wednesday's breakthrough of a 106-day effort to plug the oil giant's blown-out well along with a new government report that claimed much of the spilled oil is gone, lawmakers pressed federal scientists for answers on the effects a chemical used in the cleanup operation will have on the Gulf of Mexico's ecosystem.
BP reported Wednesday that mud forced down the well overnight was pushing the crude back down to its source for the first time since the Deepwater Horizon rig exploded off Louisiana on April 20, killing 11 workers.
President Obama, while noting that people's lives "have been turned upside down," declared to union officials in Washington that the operation was "finally close to coming to an end."
"Our recovery efforts, though, will continue," he said. "We have to reverse the damage that's been done. We will continue to work to hold polluters accountable for the destruction they've caused. We've got to make sure that folks who were harmed are reimbursed and we're gonna stand by the people of the region however long it takes until they're back on their feet."
The containment effort isn't over. Crews performing the so-called "static kill" effort overnight now must decide whether to follow up by pumping cement down the broken wellhead. Federal officials said they won't declare complete victory until they also pump in mud and then cement from the bottom of the well, and that won't happen for several weeks.
BP applied nearly 2 million gallons (7.6 million liters) 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.
A report released Wednesday by the National Oceanic and Atmospheric Administration shows that 9.6 percent of the estimated 172 million gallons (652 million liters) of oil released into the Gulf was dispersed by the chemicals.
The rest of the nearly three-quarters of the oil -- more than 152 million gallons -- has either been collected at the well by BP's cap, naturally deteriorated, evaporated or dissolved.
That leaves about 53.5 million gallons in the gulf. The amount remaining -- or washed up on the shore -- still is more than four times the size of the 1989 Exxon Valdez spill.
Federal officials say about a quarter of the oil evaporated or dissolved in the warm gulf waters, the same way sugar dissolves in water. Another one-sixth naturally dispersed, because of the way it leaked from the well. Another one-sixth was burned, skimmed or dispersed using chemicals.
Sen. Sheldon Whitehouse, a Democrat, 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.
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.
While Corexit was used in previous oil spills, BP for the first time applied the chemical beneath the water surface, where the oil was coming out of the well. Typically, dispersant is applied to oil pooled on the surface. And never before had such a large amount of the dispersant been used.
The Associated Press contributed to this report.