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Chemical plume in oil spill area

A 22-mile-long chemical plume from the Deepwater Horizon oil spill has been detected 3,000 feet below the ocean surface in the Gulf of Mexico.

The discovery, made by a robot submersible, shows that even deep sea ecosystems have not escaped the disaster.

Scientists still cannot say what the long-term effects will be.

The plume, containing a cocktail of petroleum hydrocarbons, is more than a mile wide and rises 650 feet above the ocean floor.

In June researchers tracked it moving slowly south-west of the source of the BP blowout at around 0.17 miles per hour.

Analysis of its composition confirmed that the plume was not the result of a natural oil seep. Oxygen readings suggest that bacteria are not breaking down the chemicals as quickly as expected.

Geochemist Dr Benjamin Van Mooy, a member of the US research team from Woods Hole Oceanographic Institution (WHOI) in Massachusetts, whose findings are published in the journal Science, said: "If the oxygen data from the plume layer is telling us it isn't being rapidly consumed by microbes near the well, the hydrocarbons could persist for some time. So it is possible that oil could be transported considerable distances from the well before being degraded."

Until now, attempts to detect and measure any deep sea plumes from the spill have been inconclusive. The new study employed an autonomous underwater vehicle (AUV) called Sentry which detected the plume and zig-zagged through it on a 10-day reconnaissance mission.

Bristling with sophisticated instruments, the submersible recorded information about chemical compounds and biological activity at various depths. Unlike polluted surface water, samples taken from the plume were clear and had no oil odour. However, this does not mean it is not harmful to the environment, the scientists point out.

The analysis revealed a family of hydrocarbons called BTEX, consisting of benzene, toluene, ethylbenzene and xylenes. These chemicals were present at concentrations of more than 50 micrograms per litre, but are thought to make up only a small fraction of the compounds in the oil. It will take months of further work to identify the full list of chemicals in the plume.

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