Gulf "oil cloud" triggers new dispersant concerns

MIAMI (Reuters) - A large undersea cloud of dissolved hydrocarbons discovered this week near the Gulf of Mexico oil spill raises fresh questions about toxic chemicals used to fight the spill and their environmental impact.

David Hollander, a University of South Florida oceanographer, headed a research team that discovered the six-mile (10-km) wide "oil cloud" while on a government-funded expedition aboard the Weatherbird II, a vessel operated by the university's College of Marine Science.

"We were collecting samples down to two miles below the surface," Hollander told Reuters in an interview on Friday.

"The plume or the cloud of dissolved hydrocarbons in the water was discovered northeast of the wellhead, about 35 kilometers (22 miles) to the northeast," he said.

It was the second major deepwater plume discovered since the April 20 blowout at BP Plc's Macondo well. Hollander said it was believed to stretch all the way from the wellhead to the site where it was first detected on Tuesday, in an area off the continental shelf south of Mobile, Alabama.

Hollander said scientists had yet to determine whether the dissolved hydrocarbons, found in oxygen-depleted waters, were the result of chemical dispersants used deep below the Gulf surface to break down oil from the leaking well.

But he said the contaminants -- which could eventually be pushed onto the continental shelf before shifting slowly down toward the Florida Keys and possibly out to the open Atlantic Ocean -- raised troubling questions about whether they would "cascade up the food web."

The threat is that they will poison plankton and fish larvae before making their way into animals higher up the food chain, Hollander said.

INVISIBLE THREAT

The underwater contaminants are particularly "insidious" because they are invisible, Hollander said, adding that they were suspended in what looked like normal seawater.

"It may be due to the application of the dispersants that a portion of the petroleum has extracted itself away from the crude and is now incorporated into the waters with solvents and detergents," he added.

"We think there could be both short-term and long-term implications ... There's a lot of unchartered territory that we're moving into with this oil spill," said Hollander.

He said dispersants, a cocktail of organic solvents and detergents, had never been used at the depth of BP's well before, and no one really knows how they interact physically and chemically under pressure with oil, water and gases.

"On the surface they're very readily or actively used and their behavior is well understood. That's not the case at all with their use in the subsurface and especially at a mile deep," Hollander said.

"A very-large-scale experiment is being conducted and we don't know the implications of it," he added.

Hollander said the amount of suspected dispersants in the cloud of hydrocarbons was likely to be known after about two weeks of further testing.

The U.S. National Oceanic and Atmospheric Administration funded the mission aboard the Weatherbird II along with the Florida Fish and Wildlife Service, Hollander said.

He said NOAA had dispatched a vessel on Thursday to probe the same subsurface plume discovered by his team.

In a statement issued on Monday, the U.S. Environmental Protection Agency said the underwater use of dispersants appeared to have been effective so far in breaking up oil from the BP spill and did not seem to have had any significant impact on aquatic life.

"EPA and the U.S. Coast Guard are taking steps that could reduce the volume of dispersants applied in the Gulf. While we do know dispersants are less toxic and shorter-lived than the oil, much remains unknown about their impact on the environment when used in these unprecedented volumes," the statement said.

Roughly 850,000 gallons (3.2 million liters) of dispersant had been used to combat the Gulf spill as of Thursday, including 150,000 gallons (570,000 liters) released below sea level.

(Editing by Jane Sutton)