LOUISANNA, United States — After securing a new, tight-fitting cap on top of the leaking well in the Gulf of Mexico, BP prepared yesterday to begin tests to see if it will hold and stop fresh oil from polluting the waters for the first time in nearly three months.

The oil giant expects to know within 48 hours if the new cap, which was affixed Monday after almost three days of painstaking, around-the-clock work a mile (1,500 metres) below the Gulf’s surface, can stanch the flow. The solution is only temporary, but it offers the best hope yet for cutting off the gush of billowing brown oil.

The cap’s installation was good news to weary Gulf Coast residents who have warily waited for BP to make good on its promise to clean up the mess. Still, they warned that even if the oil is stopped, the consequences are far from over.

“I think we’re going to see oil out in the Gulf of Mexico, roaming around, taking shots at us, for the next year, maybe two,” Billy Nungesser, president of Louisiana’s oil-stained Plaquemines Parish, said Monday. “If you told me today no more oil was coming ashore, we’ve still got a massive cleanup ahead.”

Starting Tuesday, the cap will be tested and monitored to see if it can withstand pressure from the gushing oil and gas. The tests could last anywhere between six to 48 hours, according to National Incident Commander Thad Allen.

Kent Wells, a senior vice president at the oil giant, made no promises in a Tuesday morning news briefing about whether the cap will work.

“We need to wait and see what the test actually tells us,” Wells said. “It’s not simple stuff. What we don’t want to do is speculate around it.”

The cap will be tested by closing off three separate valves that fit together snugly, choking off the oil from entering the Gulf. BP expects no oil will be released into the ocean during the tests, but remained cautious about the success of the system.

Pipes can be hooked to the cap to funnel oil to collection ships if BP decides the cap can’t take the pressure of the gusher, or if low pressure readings indicate oil is leaking from elsewhere in the well.

“The sealing cap system never before has been deployed at these depths or under these conditions, and its efficiency and ability to contain the oil and gas cannot be assured,” the company said in a statement.

BP will be watching pressure readings. High pressure is good, because it would mean the leak has been contained inside the wellhead machinery. But if readings are lower than expected, that could mean there is another leak elsewhere in the well.

Even if the cap works, the blown-out well must still be plugged. A permanent fix will have to wait until one of two relief wells being drilled reaches the broken well, which will then be plugged up with drilling mud and cement. That may not happen until mid-August.

Even if the flow of oil is choked off while BP works on a permanent fix, the spill has already damaged everything from beach tourism to the fishing industry.

Tony Wood, director of the National Spill Control School at Texas A&M-Corpus Christi said the sloppiest of the oil — mousse-like brown stuff that has not yet broken down — will keep washing ashore for several months, with the volume slowly decreasing over time.

He added that hardened tar balls could keep hitting beaches and marshes each time a major storm rolls through for a year or more. Those tar balls are likely trapped for now in the surf zone, gathering behind sand bars just like sea shells.

“It will still be getting on people’s feet on the beaches probably a year or two from now,” Wood said.

Meanwhile, the Obama administration issued a revised moratorium on deepwater offshore drilling Monday to replace the one that was struck down by the courts as heavy-handed. The new ban, in effect until Nov. 30, does not appear to deviate much from the original moratorium, as it still targets deep-water drilling operators while defining them in a different way.

As of Monday, the 83rd day of the disaster, between 89 million gallons (337 million liters) and 176 million gallons (666 million litres) of oil had poured into the Gulf, according to government estimates. The spill started April 20 when the Deepwater Horizon rig, leased by BP from Transocean Ltd, exploded and burned, killing 11 workers. It sank two days later.