A disabled barge that had already spilled hundreds of thousands of gallons of oil into the Mississippi River leaked again Wednesday, forcing authorities to close a two-mile stretch to ship traffic for six hours.

Traffic had just returned to normal on a 100-mile section that was closed for six days after a tanker hit the barge, which was carrying 419,000 gallons of heavy fuel oil. The tanker broke the middle of three oil tanks and bent the barge into a twisted wreck.

The forward tank, which is now braced against a bridge pier, leaked about 2,500 gallons of oil Wednesday when it settled as the river dropped a bit. Similar small leaks can be expected until crews secure the tank to a massive concrete pier, said Capt. Lincoln Stroh, Coast Guard captain of the Port of New Orleans.

Once that happens, possibly in a few days, crews will pump as much oil as possible out of the bow and stern tanks before a crane hauls the barge out of the river.

Divers have found the stern tank, which is lying along the bottom, is apparently still full, meaning the spill could be one-third less than the original worst-case estimate of 419,000 gallons.

The current is so swift that divers haven’t been able to stay in position long enough to figure out how much oil remains in the tank that leaked Wednesday.

A new worry also surfaced for environmentalists who learned that oily muck had been dredged from the mouth of Mississippi, at the other end of the 100-mile stretch affected by the original spill.

On Tuesday, the Corps of Engineers stopped dredging Southwest Pass because it found oily muck topping the sediment that is regularly vacuumed into two hopper barges to keep the channel clear.

Although oil floats on water, the heavy bunker oil can pick up so much sediment that it falls to the bottom. Stroh and others involved in the cleanup had said they expected the swiftness of the river to keep the globs of oil from falling.

For environmentalists, the question was how much oil is piling up at the river’s mouth, and what to do with it, said Carlton Dufrechou, executive director of the Lake Pontchartrain Basin Foundation.

“We don’t want to take a problem from one area and put it somewhere else,” Dufrechou said.

The corps had another big question: Whether environmental agencies will approve a safe place to dump the contaminated sediment in time to keep a major river entry from getting too shallow for big ships.

Southwest Pass is dredged regularly to keep the channel clear. The sediment was being pumped into open water near the Delta National Wildlife Reserve to rebuild wetlands — as it was in 1998, 2003 and last year, said Ed Creef, environmental resources specialist for the corps.

Wilma Subra, who advises the Louisiana Environmental Action Network, the “riverkeeper” for the lower Mississippi, said she is concerned the cleanup will leave too much oil behind. Officials have said the cleanup will go on until it would do more environmental damage than leaving whatever oil remains.

That could leave a considerable amount coating material eaten by creatures that are eaten by fish, which in turn may be eaten by people, Subra said.

“There are a large number of people who subsistence fish along there, and a large number of people who make their living fishing along there,” she said. “The long-term impact could be huge.”