The oil spill at the BP Whiting Refinery may be as much as three times as large as first estimated.

BP is now estimating that between 15 and 39 barrels of crude oil were released into Lake Michigan on Monday. The first estimate was that between eight and 12 barrels had been discharged, and that was updated to between nine and 18 barrels Wednesday.

Each barrel contains about 42 gallons of oil, so between 630 and 1,638 gallons may have spilled into the lake.

"The estimate of recovered oil updates a previous estimate of nine to 18 barrels provided at the request of federal authorities before any collected oil had been measured," spokesman Scott Dean said in a statement. "This estimate was based on a visual observation of the water's surface at the outset of the incident."

BP updated the estimate it is providing to the U.S. Environmental Protection Agency and the U.S. Coast Guard based on measurements of oil collected by vacuum trucks and absorbent boom, as well as semi-solid congealed tarballs collected by hand from the shoreline. The estimate is still preliminary, and BP is conducting further analysis to confirm it.

"Crews have recovered the vast majority of oil that had been visible on the surface of a cove-like area of the lake and on the shoreline between the refinery and an adjacent (ArcelorMittal Indiana Harbor) steel mill," Dean said. "BP is continuing to monitor the shoreline and has procedures in place to collect additional oil as discovered." 

A crew moved the boom to a new position to stop the sheen from spreading outside the cove, according to a U.S. Coast Guard news release. Workers found small amounts of oil on the beach and along the east shoreline of the cove.

On Thursday afternoon, the shoreline cleanup team moved further northwest along the rocky wall of the cove to get a better idea of the oiling along the shore, which has been described as minimal. The team will do a comprehensive survey of the path of the discharge to look for any oil that may be submerged.

The spill heightened concerns for an environmental group about Canadian tar sands around the Great Lakes.

The Whiting Refinery recently underwent a $4 billion upgrade to process Canadian tar sands, a move opposed by the Alliance for the Great Lakes. The alliance hosted a webinar Thursday on transportation of tar sands and touched on the BP release.

Jerome Popiel, incident management and preparedness adviser for the U.S. Coast Guard's 9th District in Cleveland, said during the webinar it is still unclear whether any tar sands were included in the oil spill at BP but all indications are they were not involved.

"We haven't seen any tar sands or oil sands that were part of this," Popiel said. "Could there be some other heavy oils? There could, but we have not seen that."

The spill took place Monday after a mechanical glitch discharged cooling water tainted with crude oil into the lake.

Cleanup efforts have been underway since, with private contractors hired by BP working at the scene with representatives from the EPA, Coast Guard and Indiana Department of Environmental Management monitoring the process.

More than 2,000 feet of boom was deployed to help contain the oil to the cove. Workers have been removing tarballs on a half mile of shoreline there. Vacuum trucks and the booms cleared the majority of the visible oil from the lake.

Popiel said the EPA set up air-quality monitors at the scene of the spill to detect benzene, an indicator of heavy oils.

"I have not heard of any elevated levels or unsafe levels," Popiel said.

Lyman Welch, water quality program director for Alliance for the Great Lakes in Chicago, said the oil spill at BP occurred on the 25th anniversary of the Exxon Valdez oil spill in Alaska. Welch said heavy oil, such as that in tar sands, can sink to the bottom of a waterway, making it difficult and expensive to remove.

Popiel said on-scene coordinators heard from surfers in the area of the spill Wednesday.

"Their perception was it looks just like normal," Popiel said.

Two congressmen from Illinois remained concerned about the issue Thursday.

U.S. Sens. Dick Durbin, D-Ill., and Mark Kirk, R-Ill., sent a joint letter to BP CEO John Minge on Thursday saying they are "deeply concerned" about Monday's spill. The senators said they want to talk with Minge about potential public health and environmental threats to surrounding communities.

Popiel said the Coast Guard is trained to approach search and rescue missions for people and vessels lost in the lakes as if guardsmen were looking for their own relatives. The Coast Guard treats oil spills in much the same way, he said.

"We treat it like we're cleaning up our own back yard," Popiel said.

Popiel said the Great Lakes historically have the lowest number of oil spills among U.S. waters.

"When you look at the number of spills and how much has been spilled on the Great Lakes, it's a great success story," Popiel said.

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The Associated Press contributed to this report.