

The US Coast Guard has set fire to part of a big oil slick in the Gulf of Mexico, in an attempt to save environmentally fragile wetlands. The "test burn" is taking place in an area about 30 miles (50km) east of the Mississippi river delta, officials say. About 1,000 barrels (42,000 gallons) of oil a day have been leaking into the sea from a well since an explosion on a drilling rig off Louisiana last week. Officials fear the leak could cause one of the worst oil spills in US history. An investigation has been ordered into the incident, which is believed to have left 11 of the rig's workers dead. They are still missing. Changing winds Coast Guard Rear Admiral Mary Landry, who is in charge of the government clean-up effort, warned on Tuesday that work on sealing the leaking well using several robotic submersibles could take months. With the spill moving towards Louisiana's coastline, which contains some 40% of the nation's wetlands and spawning grounds for countless fish and birds, she said a "controlled burn" of petroleum contained by special booms could limit the impact. How the oil has spread

Approximate oil locations 22 April - 15 May Controlled burns had been done and tested before, and had been shown to be "effective in burning 50 to 95% of oil collected in a fire boom", she said. Environmental experts say animals nearby might be affected by toxic fumes, but perhaps not as much as if they were coated in oil. On Wednesday, two vessels dispatched by the Coast Guard and the British oil company BP - which had hired the sunken rig - swept the thickest concentrations of oil on the surface into a 500ft (150m) fire-resistant boom. They then towed it to a five-mile "burn zone" set up inside the slick, where it was set alight shortly before nightfall. It will be allowed to burn for an hour. FROM BBC WORLD SERVICE Please turn on JavaScript. Media requires JavaScript to play.

More from BBC World Service If the test is deemed successful, BP is expected to continue the controlled burns as long as the weather conditions are favourable. The decision to start the test burn came after the US National Oceanic and Atmospheric Administration (NOAA) warned that winds in the area were about to shift and possibly push the oil onto the coast by Friday night. The oil slick currently has a circumference of about 600 miles (970km) and covers about 28,600 sq miles (74,100 sq km). The leaks causing it - about 5,000ft (1,525m) under the surface - were found on Saturday, four days after the Deepwater Horizon platform, to which the pipe was attached, exploded and sank. Possible solutions Workers on a nearby oil platform were evacuated by the US authorities on Monday after the oil slick came dangerously close. British oil company BP has not been able to activate a device known as a blow-out preventer, designed to stop oil flow in an emergency. OIL SPILL DISASTERS 1991: 520m gallons were deliberately released from Iraqi oil tankers during the first Gulf War to impede the US invasion 1979: 140m gallons were spilt over nine months after a well blow-out in the Bay of Campeche off Mexico's coast 1979: 90m gallons leaked from a Greek oil tanker after it collided with another ship off the coast of Trinidad 1983: 80m gallons leaked into the Gulf over several months after a tanker collided with a drilling platform 1989: 11m gallons were spilt into Alaska's Prince William Sound in the Exxon Valdez disaster

Richard Black's blog In pictures: Efforts to contain leak Doug Suttles, the chief operating officer for exploration and production at BP, said it had not yet given up on engaging the valve, but was considering other possible solutions. These include placing a dome directly over the leaks to catch the oil and send it up to the surface, where it could be collected by ships. This has only been done in shallow water before and is still two to four weeks from being operational. BP will also begin drilling a "relief well" intersecting the original well, but it is also experimental and could take two to three months to stop the flow. Forty-nine vessels - oil skimmers, tugboats barges and special recovery boats that separate oil from water - were working to round up oil, BP said. An investigation has been ordered into the cause of the leak by the interior and homeland security departments. It will have the power to compel witnesses to testify, and will look into possible violations by the operators of the rig, Transocean.





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