COCODRIE, La.—Five million barrels of oil seems like a lot. That is approximately what spewed from the blowout at BP's Macondo well last year, about enough to fill an area the size of an American football field more than 90 meters deep—and much of it has gone missing.

"There's a lot of water out there for the oil to be in," notes toxicologist Scott Miles of Louisiana State University (L.S.U.), who has researched the fate of the oil from last year's spill. Where is it? "Your guess is as good as mine."

Last August 4, the federal government released an accounting of all that oil, updating it in November. According to that report, ships skimmed 160,000 barrels of oil and BP's various efforts captured another 820,000 barrels before it escaped into the sea. More than 400 burnings of concentrated slicks moved 260,000 barrels from water to air. The application of 1.8 million gallons of COREXIT dispersed 770,000 barrels of the oil spill while the simple mechanics of the oil and gas entering the water 1,500 meters down through a relatively narrow pipe dispersed another 630,000 barrels. Nearly a quarter of the oil evaporated or dissolved in the water or air. That leaves just 1.1 million barrels lost at sea—or hidden in the shoreline. After all, by August 20, no oil slicks from the Macondo well blowout could be detected on the surface of the Gulf of Mexico.

"The numbers are not unreasonable and it may very well be correct," says biologist Christopher D'Elia, dean of the School of the Coast and Environment at L.S.U. But "putting a budget together [for an oil spill] is really hard to do. Any one of these things could be off." And some scientists put the number of remaining oil much higher—as much as four million of those barrels are still out there, according to the most pessimistic estimates.

Oil eaters

There is no doubt that the hydrocarbon-eating microbes of the Gulf of Mexico had a feast last year. Their efforts may have disposed of the more than 1.4 million barrels of oil broken up into small droplets. "Where we have oxygen and light and heat, the oil will degrade rapidly," says marine and atmospheric scientist Alexander Kolker of the Louisiana Universities Marine Consortium. "Where we don't have those things it will be slower."

Those microbes did much of their work in the deep as vast plumes of oil formed under the water, contrary to the protestations of then BP chief executive Tony Hayward and the expectations of National Oceanic and Atmospheric Administration (NOAA) head Jane Lubchenco. Scientists documented several such plumes, including one that was 200-meters high, two-kilometers wide and 35-kilometers long. That plume persisted at least through June, suggesting that microbes were not working as quickly as might have been hoped—although other research found much higher rates of microbe oil-munching, starting with the natural gas components of the blowout. 

The oil that did make the trip all the way to the surface, took roughly two hours to do so and created a slick at least a kilometer away from the wellhead itself, depending on currents. And whether in the deep or on the surface, all of that oil found itself among some 643 quadrillion gallons of water—a lot of ocean to get lost in.

Yet, NOAA testing found enough of the more than 1,000 different hydrocarbons in oil to keep areas closed to fishing until April 19 this year—almost exactly a year after the spill started. "Oil is still present," explains biologist Martin Bourgeois of the Louisiana Department of Wildlife and Fisheries.

That is not entirely the fault of the consortium of companies involved in the Macondo well blowout. The Gulf of Mexico endures a slew of spills every year, in addition to the natural oil seeps that release as much as 1.4 million barrels of oil annually. Much of that oil—and likely the oil from BP's Macondo well blowout—ends up sinking to the seafloor, either directly or inside dead oil-eating microbes or oil-containing plankton. The biggest Gulf oil spill prior to BP's blowout—Ixtoc 1—saw roughly 25 percent of 3.5 million barrels of oil sink to the bottom. "Has this oil dropped to the floor?" Miles asks, noting that some deep-sea sampling suggests it has. "Oil may be getting heavier and heavier and dropping to the floor."

Persistent oil

It's not just the oil that persists, of course. At least one of the compounds in the 300 oil tankers worth of dispersants used to break up the spilled oil is still out there—dioctyl sulfosuccinate sodium salt, or DOSS, which is the active ingredient in COREXIT. "It didn't seem to be significantly degraded even through September," says chemical oceanographer Elizabeth Kujawinski of the Woods Hole Oceanographic Institution, who studied its fate in water samples. "This one component lasts. I don't know what happened to the rest."

And oil can still be found in the Louisiana marshland, in places like Bay Jimmy and Elmer's Island. That oil will be there for a very long time, based on the experience of previous oil spills such as the Florida oil barge mishap off the coast of Cape Cod or the Exxon Valdez tanker spill in Prince William Sound, Alaska. "The sediments provided a long-term refugium for toxic levels of oil," says biologist Andrew Whitehead of L.S.U.

And there's no question that oil and gas generally will remain a big part of Louisiana for the foreseeable future. Billboards promoting legal help with denied BP claims alternate with advertisements for work in the offshore oil and gas industry on the highways leading to the coast. "Our oil field is so important to our parish, we had to ask, 'Please let us drill again,' which is kind of hard to explain," says Terrebonne Parish President Michel Claudet. Despite the unknowns about the long-term effects of BP's Macondo well blowout, "we don't have any problems with them drilling again."

Ultimately, five million barrels is just not that much oil. Gulf of Mexico offshore oil wells produce 1.6 million barrels of oil every day and the U.S. consumes more than 20 million barrels daily. In fact, the Louisiana gulf coast endures spills year after year and month after month. "There are spills happening every day out in the Gulf of Mexico," notes Janelle Robbins, associate director for Waterkeeper. "There's a lot of oil unaccounted for, it's not gone. It's out there someplace but finding it is going to be tricky. And some people would rather not find it at all."

As the Congressional Research Service's Jonathan Ramseur wrote in his report on the fate of the oil (pdf) from December 16: "Months later it is unknown what happened to the oil that remained….It is debatable whether the fate of the remaining oil will ever be established conclusively."

Editor's Note: Reporting for this article took place as a result of a fellowship from the Metcalf Institute for Marine and Environmental Reporting at the University of Rhode Island.