Enlarge AFP/Getty Images A new study reports precise measurements of a large oil plume that escaped from the Deepwater Horizon blowout. IMPACT OF OIL SPILL ON GULF IMPACT OF OIL SPILL ON GULF CLICK HERE for an interactive map showing beach closings, reader's thoughts and photos, important numbers and the impact on wildlife.

Gulf oil plume extended 21 miles

An undersea oil plume from the Gulf of Mexico spill stretched for at least 21 miles from the leak site, research out Thursday shows.

Reported in the journal Science, the study answers some of the questions about the fate of the crude, about 4.9 million barrels worth, in the Gulf. The size was mapped using a robot sub that plied the waters from June 19-28, when the approach of Hurricane Alex forced an end to its work before it could find an end to the plume.

"The evidence we collected clearly shows a plume existed," says lead study author Richard Camilli of the Woods Hole (Mass.) Oceanographic Institution. "The data suggest the plume extended much further than we tracked it." He said the persistence of the oil cloud was "unexpected," and may affect future estimates of the Gulf spill's size.

An estimated 207 million gallons of oil spilled into the Gulf of Mexico after the Deepwater Horizon rig, operated by BP, exploded April 20 of Louisiana's coast. The explosion killed 11 workers. A temporary cap placed on the well July 15 has stopped any more oil from leaking.

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In early June, former BP chief Tony Hayward cast doubt on oil hiding underwater in the spill. But National Oceanic & Atmospheric Administration (NOAA) scientists soon confirmed the existence of the deep-sea oil. The new study defines the size and chemistry of the plume:

• It was more than 21 miles long, 650 feet thick and 1.2 miles wide.

• It was headed southwest from the spill at 0.17 miles-per-hour. The plume bore scant signs of oil-eating microbes.

• Clear to the naked eye, the cloud held perhaps 6% of the toxic chemicals such as benzene released from the spill.

"There is still a lot of oil out there," says Nancy Rabalais of the Louisiana Universities Marine Consortium in Chauvin. "The plume study shows we have to continue deep-sea monitoring, not just shoreline efforts."

Ocean chemistry likely condensed the cloud through a combination of pressure, current, temperature and saltiness about 3,600 feet down, Rabelais suggests, "but clearly a lot more study is needed." About 57,000 chemical measurements were made in the study.

"The data clearly illustrate a large, continuous deep hydrocarbon plume," sprang from the spill, says oceanographer John Ryan of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., who headed a May effort that reported signs of deep-sea oil.

The researchers did not find depleted oxygen levels within the plume, which would have created a deep water "dead zone," threatening Gulf fishing. Microbes that eat oil are taking much longer to do their work deep in the water, and many months may pass before such a zone blossoms, warns the study.

The National Science Foundation funded the research under three "rapid-grant" awards, part of a $13 million federal research effort in the Gulf. The oil measurements were collected following Coast Guard evidence guidelines that were needed to assess environmental damages from the spill.

Study co-author Chris Reddy said there may have been other oil plumes in the Gulf, but this is the one they tracked.

"I don't discount other findings of (other) plumes," Reddy said.