Published: May 10, 2010 5:38 PM EDT

ON THE GULF OF MEXICO - A remote-controlled submarine shot a chemical dispersant into the maw of a massive undersea oil leak Monday, further evidence that authorities expect the gusher to keep erupting into the Gulf of Mexico for weeks or more.

Crews using the deep-sea robot attempted to thin the oil - which is rushing up from the seabed at a pace of about 210,000 gallons per day - after getting approval from the Environmental Protection Agency, BP spokesman Mark Proegler told The Associated Press.

The agency had halted two previous rounds of the dispersant to test its potential impact on the environment, and approved a third round of spraying that began early Monday, Proegler said. An EPA spokeswoman didn't immediately return messages seeking comment.

BP engineers, casting about after an ice buildup thwarted their plan to siphon off most of the leak using a 100-ton containment box, pushed ahead with other potential short-term solutions, including using a smaller box and injecting the leak with junk to plug it. None of these have been tried so deep -- about a mile -- however.

Workers were simultaneously drilling a relief well, the solution considered most permanent, but that was expected take up to three months. At least 3.5 million gallons were believed to have leaked since an April 20 drilling rig blast killed 11. If the gusher continues unabated, it would surpass the Exxon Valdez disaster as the nation's worst spill by Father's Day.

The engineers appear to be "trying anything people can think of" to stop the leak, said Ed Overton, a LSU professor of environmental studies. Back on land, authorities in Louisiana deployed helicopters to drop sandbags the size of elephants along barrier islands and marshes already being lapped at by a sheen of oil. Authorities also planned to use south Louisiana's system of locks and levees to release water to help keep the worst of the oil at sea.

BP -- which is responsible for the cleanup -- said Monday the spill has cost it $350 million so far for immediate response, containment efforts, commitments to the Gulf Coast states, and settlements and federal costs. The company did not speculate on the final bill, which most analysts expect to run into tens of billions of dollars.

Among plans under consideration for the gusher, BP is looking at cutting the riser pipe, which extends from the well, undersea and using larger piping to bring the gushing oil to a drill ship on the surface, a tactic considered difficult and less desirable because it will increase the flow of oil. Above the oil leak, waves of dark brown and black sludge crashed into the support ship Joe Griffin. The fumes there were so intense that a crew member and an AP photographer on board had to wear respirators while on deck.

Philip Johnson, a petroleum engineering professor at the University of Alabama, said cutting the riser pipe and slipping a larger pipe over the cut end could conceivably divert the flow of oil to the surface. "That's a very tempting option," he said. "The risk is when you cut the pipe, the flow is going to increase. ... That's a scary option, but there's still a reasonable chance they could pull this off." Dime- to golfball-sized balls of tar washed up over the weekend on Dauphin Island, three miles off the Alabama mainland at the mouth of Mobile Bay and much farther east than the thin, rainbow sheens that have arrived sporadically in the Louisiana marshes.

The containment box plan had been designed to divert up to 85 percent of the leaking oil to a tanker at the surface. Icelike hydrates, a slushy mixture of gas and water, clogged the opening in the top of the peaked box like sand in a funnel, only upside-down.

The blowout aboard the rig, which was being leased by BP, was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP's internal investigation.

Deep sea oil drillers often encounter pockets of methane crystals as they dig into the earth. Lane Zirlott, 32, a commercial fisherman from Irvington, Ala., said he's not frustrated about BP failing so far to cap the leak because he understands how difficult the job is. "When they said they were going to put this little cap over this thing, I laughed and said there's no way," he said. "I said there's no way they're going to do that. And then sure enough, it didn't happen."