The government and university researchers confirmed Tuesday that plumes of dispersed oil were spreading far below the ocean surface from the leaking well in the Gulf of Mexico, raising fresh concern about the potential impact of the spill on sea life.
The tests, the first detailed chemical analyses of water from the deep sea, show that some of the most toxic components of the oil are not necessarily rising to the surface where they can evaporate, as would be expected in a shallow oil leak. Instead, they are drifting through deep water in plumes or layers that stretch as far as 50 miles from the leaking well.
As a rule, the toxic compounds are present at exceedingly low concentrations, the tests found, as would be expected given that they are being diluted in an immense volume of seawater.
“It’s pretty clear that the oil that has been released is becoming more and more dilute,” Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, said in an interview. “That does not mean it’s unimportant — far from it. The total amount of oil out there is likely very large, and we have yet to understand the full impact of all that hydrocarbon on the gulf ecosystem.”
Scientists outside the government noted that the plumes appeared to be so large that organisms might be bathed in them for extended periods, possibly long enough to kill eggs or embryos. They said this possibility added greater urgency to the effort to figure out exactly how sea life was being affected, work that remains in its infancy six weeks after the Deepwater Horizon oil rig exploded.
“I’m glad to see the levels are low,” said Carys L. Mitchelmore, an aquatic toxicologist at the University of Maryland who was not involved in the research. “But we’re talking about a huge Gulf of Mexico here. I want to see evidence that this is one of the main plumes and there’s not something way more concentrated somewhere else.”
The announcement of test results came as the White House said President Obama would make his fourth trip to the region next week, visiting Mississippi, Alabama and Florida on Monday and Tuesday.
The results on the plumes came from samples taken by researchers at the University of South Florida, in St. Petersburg. NOAA helped finance the research and joined in Tuesday’s announcement.
The test results, from samples taken in late May aboard the research vessel Weatherbird II, appeared to confirm information first presented three weeks ago by another group of researchers, who found evidence of large plumes of dispersed oil droplets in the deep ocean.
Those scientists have not yet completed their analysis of water samples, but one of them, Samantha Joye of the University of Georgia, supplied additional information at a news conference on Tuesday, including instrument readings taken on her most recent research cruise.
Those readings suggest that a large plume, probably consisting of hydrocarbons from the leak, stretches through the deep ocean for at least 15 miles west of the gushing oil well, Dr. Joye said. The top of the plume is about 3,600 feet below the sea surface; the plume is three miles wide and as thick as 1,500 feet in spots, she said.
The University of South Florida researchers found an even larger plume stretching northeast of the oil well, with the hydrocarbons separated into two distinct layers in the ocean. One layer is about 1,200 feet below the surface, and the other is 3,000 feet deep, the scientists said.
The government’s confirmation of subsea oil plumes is significant in part because BP, the oil company responsible for the leak, had denied that such plumes existed, and NOAA itself had previously been cautious in interpreting the preliminary results from Dr. Joye’s group.
“The oil is on the surface,” Tony Hayward, BP’s chief executive, said last week. “There aren’t any plumes.”
Descriptions of the plumes from the two groups of scientists are filling in details of one of the most remarkable findings to come from the disaster: the realization that much of the oil in a deepwater blowout may remain below the surface.
The scientists say the plumes are not bubbles of oil, as many people have imagined them, but consist of highly dispersed or dissolved hydrocarbons. In some spots, enough oil is present to discolor the water, but in most places, water samples come up clear. Yet the dissolved hydrocarbons show up vividly on instruments, and they can be smelled in some samples.
“This so-called invisible oil, which people tend to have a hard time grasping, is detectable clearly using analytical methods,” said Ernst Peebles, a University of South Florida oceanographer who helped carry out the research.
Jeffrey Short, a marine scientist with Oceana, an advocacy group, said that even though the concentration of chemicals was low at any one locale, the magnitude of the plumes suggested a need for more research.
“We should, at a bare minimum, keep a much closer eye on how many plumes there are, how big they are, how long they last and what organisms they’re affecting,” Dr. Short said.
As the government wrestled with the many safety issues raised by the disaster, it imposed a drilling freeze late last month that halted virtually all new oil exploration in the gulf. On Tuesday, the Obama administration announced new standards that will allow resumption of drilling in water less than 500 feet deep.
All wells in water deeper than 500 feet remain under a moratorium for at least the next six months while a presidential panel studies the April 20 Deepwater Horizon explosion and makes recommendations on whether and how to resume such drilling.
The new rules address some of the problems that investigators believe contributed to the blowout of the BP well, including failure of the blowout preventer and improper design or application of the cement around the well bore.
“Oil and gas from the outer continental shelf remains an important component of our energy security as we transition to the clean-energy economy,” Interior Secretary Ken Salazar said Tuesday in a statement, “but we must ensure that offshore drilling is conducted safely and in compliance with the law.”
By JUSTIN GILLIS
John M. Broder and Henry Fountain contributed reporting.