BP today more than doubled its maximum estimate of how much crude oil spilled into Lake Michigan earlier this week from its Whiting refinery in Northwest Indiana.

In a statement, the company said a malfunction in a new distillation unit forced up to 39 barrels or 1,638 gallons of oil into the lake just across the Illinois border. A day earlier, the company had estimated that 18 barrels at most had been spilled.

BP said it based its latest estimate on the amount of oil collected by vacuum trucks and absorbent booms, along with an inventory of the waxy balls of oil that cleanup crews scoured from a beach on BP’s property. Strong winds appear to have pushed most of the oil toward a shallow cove between the refinery and an ArcelorMittal steel mill.

Officials from the U.S. Environmental Protection Agency have said the spill likely poses no long-term risks to Lake Michigan, the source of drinking water for 7 million people in Chicago and the suburbs. The 68th Street water intake crib is about 8 miles northwest of the spill site, but there have been no signs of oil drifting in that direction.

At a minimum, 15 barrels of oil ended up in the lake after crude leaked into a sealed cooling system, BP said. The low estimate the previous day was 9 barrels.

The EPA declined to comment on BP’s updated spill estimates. The agency is conducting its own investigation into possible violations of the federal Clean Water Act.

Workers at the refinery reported an oil sheen on the water about 4:30 p.m. Monday, and the EPA said the leak was plugged by 9 p.m. The distillation unit, which came online in July 2013, is the centerpiece of a nearly $4 billion overhaul that enabled the nation’s seventh-largest refinery to process more heavy Canadian oil from the tar sands region of Alberta.

The unit performs one of the first steps in the refining of crude oil into gasoline and other fuels. It has since resumed normal operations, BP said.

Indiana officials have been silent about the spill. But a bipartisan group of politicians from Illinois and Michigan, including Mayor Rahm Emanuel and U.S. Sens. Dick Durbin and Mark Kirk, are demanding more aggressive action from BP to prevent additional spills.

“This spill raises questions about the long-term safety and reliability of BP’s new, expanded production at Whiting,” Durbin and Kirk wrote in a letter asking for a meeting with John Minge, the top U.S. official for the London-based oil company. “It is in all of our best interests … to ensure that this greater processing capacity will do no harm to Lake Michigan.”