Mother Nature appears poised to deliver the output of a massive Gulf Coast oil spill to the Louisiana coastline a day later than originally expected, giving authorities some extra time to prepare for its unwelcome arrival.

On Wednesday, oil-rig operator BP PLC said it managed to cap one of the three leaks in the underwater well that has been spewing an estimated 5,000 barrels of oil per day into the Gulf of Mexico.

BP said the move would make it easier to deal with the leak, but would not reduce the volume of oil spewing from the blown-out well.

The problem is that the other two leaks are the dominant sources of contamination, CTV's Lisa LaFlamme reported Wednesday morning.

"They have managed to seal it off with a slip valve, but it is the smallest of the three leaks," LaFlamme told CTV News Channel from Dauphin Island, Louisiana.

"So, the other two are still gushing oil. In fact, BP said early today they do not predict that even capping that one small leak has slowed the flow at all."

The hope is that BP -- which U.S. President Barack Obama said will be held responsible for paying the cost of the cleanup -- will lower specially-engineered, concrete-and-steel containment domes into the sea to siphon off the gushing oil.

"The big effort today is to get one of those massive, 74-ton containment boxes onto that flat-bed truck and driven from Louisiana and shipped out to the site," LaFlamme said. "That though, is a 12-hour operation just to get it to the site."

Such technology has been used before, but never at the depth of the current disaster -- more than 1.5 kilometres below the surface.

BP spokesperson Bill Salvin said the company doesn't "know for sure" whether the plan will work.

"What we do know is that we have done extensive engineering and modelling and we believe this gives us the best chance to contain the oil," he said. "And that's very important to us."

By Wednesday afternoon, workers were nearly finished preparing one of the enormous contraptions for transport to the Gulf of Mexico, on board an 85-metre supply boat.

It may leave Port Fourchon late Wednesday afternoon, proceeding to the Mississippi Delta in 10 or 11 hours. The site of the spill lies about 80 kilometres off the coast.

BP spokesperson John Curry said the company expects to have the containment dome in place on the seabed by Thursday.

The oil spill problem began when a BP-operated oil rig exploded April 20. Eleven rig workers died and both BP and rig-owner Transocean Ltd. are now facing multiple lawsuits from survivors and the families of the dead.

The growing oil slick threatens the sensitive wetlands and wildlife of the Gulf Coast, along with all of the businesses that depend on its health.

The weather has co-operated so far, with oil coming ashore in only a few spots in Louisiana. Local forecasts said it would likely reach the shore no earlier than Thursday.

"It's a gift of a little bit of time," said U.S. Coast Guard Rear Adm. Mary Landry. "I'm not resting."

Some 7,900 people have been working to protect Louisiana's coastal wetlands and they returned to the ocean waters Wednesday, helping to tow an absorbent boom out to the mouth of the Mississippi River.

In concert with the containment dome plan, BP has used chemical dispersants to reduce the amount of oil that is surfacing. It has also used controlled burns and skimmer vessels to try to reduce the oil residue on the ocean's surface.

LaFlamme said another concern is that the oil slick could reach the Current Loop of the Gulf of Mexico in the coming days, which could transport the oil down the Florida coast.

"Every single day, they are waiting for those satellite images to show how far the slick has spread and how far it's travelled. At this point, they say it's about 40 kilometres away from that Current Loop and still about 40 or 50 kilometres away from shore here at least," she said.

With files from The Associated Press