New Orleans ? It could take hours or it could take months to stop a 42,000-gallon-a-day oil leak polluting the Gulf of Mexico at the site of a wrecked drilling platform. Whether the environmental threat grows many times bigger depends on whether the oil company can turn the well completely off.

Crews are using robot submarines to activate valves at the well head in hopes of cutting off the leak, which threatens the Gulf Coast’s fragile ecosystem of shrimp, fish, birds and coral. If the effort fails, they’ll have to start drilling again.

The submarine work will take 24 to 36 hours, Doug Suttles, chief operating officer for BP Exploration and Production, said Sunday afternoon.

“I should emphasize this is a highly complex operation being performed at 5,000 feet below the surface and it may not be successful,” he said.

Oil continued to leak nearly a mile underwater Sunday at the site where the Deepwater Horizon rig exploded on Tuesday. Eleven workers are missing and presumed dead.

Rough weather

For the second consecutive day, high waves prevented boats and equipment from going out to clean the spill. Airplanes sprayed chemicals to break up the oil.

The spill initially appeared to be easily manageable after the oil rig sank Thursday about 50 miles off the Louisiana coast, but it has turned into a more serious environmental problem. Officials on Saturday discovered the leak, which is spewing as much as 1,000 barrels — or 42,000 gallons — of oil each day.

The oil spill has been growing — officials said the oily sheen on the surface of the gulf covered about 600 square miles Sunday. The environmental damage would be especially serious if it reaches land.

The spill was still about 70 miles from the mainland, but only about 30 miles from an important chain of barrier islands known as the Chandeleurs.

The islands, part of a national wildlife refuge, are an important nesting ground for pelicans and other sea birds. They have been under serious threat since Hurricane Katrina washed out much of the sand there.

“Katrina did kick it pretty good, but they have been growing back,” said Greg Thornton, the 52-year-old owner of Horn Island and Due South Charters in Biloxi. He takes fishing parties out to the islands.

Looking at wind patterns on his computer, which showed favorable conditions until Thursday, Thornton held out hope that the oil could be contained.

“We might have some trouble if they don’t get the boom around it and stop it from spreading,” he said.

Containing the spill

The spill so far appears to be small relative to some major oil accidents. The Exxon Valdez spilled 11 million gallons in Alaska’s Prince William Sound in 1989 — the worst oil spill in U.S. history.

“It has the potential to be pretty serious, but at 1,000 barrels a day, if it comes to the surface they’ll probably be able to contain it and vacuum it up,” said James Cowan, an oceanography and coastal sciences professor at Louisiana State University in Baton Rouge.

The company is planning to collect leaking oil on the ocean bottom by lowering a large dome to capture the oil and using pipes and hoses to pump it into a vessel on the surface, said Suttles, the BP executive.

“That system has been deployed in shallower water,” he said, “but it has never been deployed at 5,000 feet of water, so we have to be careful.”

The robot submarines are attempting to close off the flow of oil by activating a shutoff device at the well head known as a blowout preventer.

In case that doesn’t work, BP PLC, which leased the Deepwater, moved another deepwater rig, the DD3, toward the explosion site. If necessary, the new rig would drill relief wells into the damaged well underneath the ocean floor. That could take several months.

Benton F. Baugh, who holds patents for blowout preventer parts, said the subs should be able to do the job.

“If they can’t get it closed off, something really unusual happened,” said Baugh, president of Radoil Inc. in Houston and a National Academy of Engineering member.

Coast Guard officials said weather conditions for the next three days would help keep the Gulf spill away from the coast.

Mark Schexnayder, a regional coastal adviser at the Louisiana Sea Grant, said the oil spill had the potential to do long-term damage to the coastal environment.