Reporting from Atlanta — With a thick shot of cement plugging the last worrisome part of BP's troubled well, government officials were one final pressure test away from declaring the source of America's worst offshore oil spill dead as early as Sunday.
If successful, the so-called "bottom kill" will close a chapter on a disaster story that began with a deadly explosion in the Gulf of Mexico and quickly became a central crisis for the Obama White House, a confirmation of the oil industry's inability to handle a major spill, and a reminder of the often risky and dangerous work required to feed the nation's fossil-fuel addiction.
Despite the known tally of the tragedy — the 11 men killed in the April 20 rig blowout, the lost season for the tourism and seafood industries, the thousands of dead creatures — a full accounting remains studded with unknowns, largely because it will take years to understand the disaster's effects on the environment.
"This spill began with a bang, ends with a whimper, and leaves a number of issues still screaming for attention," Rep. Edward J. Markey (D-Mass.), chairman of the House Select Committee on Energy Independence and Global Warming, said in a statement Saturday.
Much of the oil now floats beneath the ocean's surface. But with the public focused on the nation's economic malaise — and Washington lawmakers locked in partisan stasis —it remains to be seen whether this outsized environmental disaster will bring about major changes in the oil industry, energy policy, public opinion and politics.
"Moderate to heavy oil impacts" continue to hit roughly 109 miles of coastline, much of it in Louisiana, according to the federal government. But elsewhere in the gulf states, there are signs of a return to normal: Ribbons of surface oil have largely vanished, sunbathers are back on Florida beaches, seafood delicacies appear on New Orleans menus, and more fishing grounds open daily.
Yet unanswered questions stain the future: What will new federal regulations on the oil industry look like? Will they strangle jobs? Will tourists return? Will Americans again feel secure eating gulf seafood? Will BP really make good on its promise to pay for billions in potential damages?
"It's very confusing and dismal down here right now," said lifelong oysterman Dave Cvitanovich, who, like many Louisiana watermen, was drafted into BP's ad hoc cleanup industry that included flotillas of locals. "A hurricane comes and goes, and a week later you're back on your feet. But who knows whether this problem will still be around three months or a year from now?"
Of the estimated 4.9 million barrels of oil that gushed from the well, 25% was burned, skimmed or piped to tanker ships. A second 25% has evaporated or dissolved, according to government estimates.
Another 25%, classified by the government as "residual oil," consisted of light sheens on the water, thick goo on the shore and tar balls. The tar balls, though not harmful to humans, are likely to wash up on shore for some time.
"There's going to be a lot of oil out there that's going to show up on beaches for 10 years, 20 years," said R. Eugene Turner of Louisiana State University's Department of Oceanography and Coastal Sciences. "Tourists aren't going to like that at all."
The final 25% of the oil — the equivalent of four Exxon Valdez spills —- is of greatest concern to scientists. It is drifting 3,000 to 4,300 feet below the gulf's surface, in vast clouds of atomized droplets that could alter links in the chain of life.
This "dispersed" oil was broken into droplets, about the width of a hair, either when it shot at high speeds from the well's broken pipe or when it came into contact with the 1.8 million gallons of the controversial chemical dispersant Corexit.
The 5,000-foot depth of the leak exacerbates the mystery: Close to the surface, the Gulf of Mexico's warmth invigorates bacteria that have evolved to feed off natural oil seeps. But scientists are not sure what happens with oil-eating microorganisms in the darker, colder depths.
While one recent study showed that bacteria were slowly breaking down the oil, another found they were causing its "rapid decline." Last week, a third study said bacteria were mainly digesting natural gas that spewed from the well, but it didn't rule out that they were also consuming oil.
Kendra Daly, an ocean researcher at the University of South Florida, said the droplets could kill many plankton, a crucial component of the ocean food chain, possibly creating a cascade effect that could reduce the populations of other, larger species. Already researchers have found some evidence of oil harming plankton in an important spawning area off the Florida panhandle.