The Coast Guard has stopped 59 ships from traversing a closed stretch of the Mississippi River from New Orleans to the Gulf of Mexico Thursday while hundreds of workers tackled the difficult task of cleaning up about 400,000 gallons of heavy oil that spilled when a barge and tanker collided.

Besides those stopped along the major waterway, 33 more vessels were hung up in canals that enter the river at New Orleans, the Coast Guard said.

A sheen of oil coated the water from New Orleans almost to the Gulf, about 97 miles away, Petty Officer Jaclyn Young said. About 50,000 feet of containment boom had been placed on the river, and workers planned to put down another 30,000 feet and use vacuum skimmers to pick up the oil.

It could take days to open the river and weeks to clean the spill, Young said

Meanwhile, authorities were investigating why the tugboat towing the barge in Wednesday morning’s crash did not have a properly licensed pilot, the Coast Guard said. The person operating the boat had an apprentice mate’s license, but no one on the vessel was properly documented to guide it, said Lt. Cmdr. Cheri Ben-Iesau from the Coast Guard in New Orleans.

The barge held more than 419,000 gallons of heavy fuel oil in three tanks. Divers were examining the wreckage to see how much more oil was in the tanks.

By midday, about 6,900 gallons had been removed, said Paul Book, a vice president with American Commercial Lines Inc., of Jeffersonville, Ind., which owns the barge. About 350 people were working on the cleanup, using 45 boats, he said.

“This is a very large, very fast-moving river. It makes the job very difficult to contain that oil,” said Charlie Henry, a scientific coordinator for the National Oceanic and Atmospheric Administration.

The barge owner brought 250 people on cleanup crews and 11,500 feet of boom to keep oil away from water intakes and environmentally fragile areas, said Paul Book, the company’s vice president of operations facilities.

His company is responsible for the cleanup, but legal blame for the accident will be set after an investigation by the Coast Guard and the National Transportation Safety Board, said Capt. Lincoln Stroh, the Coast Guard captain for the Port of New Orleans.

Officials in New Orleans and surrounding areas say tapwater is safe to drink. Authorities urged residents of Algiers and Gretna and in Plaquemines Parish to conserve because their system intakes have been shut down. St. Bernard Parish President Craig Tafffaro rescinded his conservation order Thursday but expected reduced pressure in water distribution lines over the next day until reserves are restored.

The city of New Orleans, where an estimated 55,000 households have been affected, hired an independent contractor to test the water, besides the Sewage and Water Board, out of an “abundance of caution,” Mayor Ray Nagin said late Wednesday.

The double-hulled tanker Tintomara, which was headed downriver, was loaded with about 4.2 million gallons of biodiesel and nearly 1.3 million gallons of styrene, but did not leak, said Michael Wilson, president of ship management company Laurin Maritime (America) Inc. in Houston, the U.S. subsidiary of Laurin Maritime AB of Goteborg, Sweden. The tanker is owned by Whitefin Shipping Co. Ltd. of Gibraltar.