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BP said Friday that the amount of crude oil spilled into Lake Michigan earlier this week from the Whiting refinery in Northwest Indiana was at least double the amount that had been estimated.

The company said a faulty distillation unit caused up to 39 barrels or 1,638 gallons of oil into the lake just across the Illinois border.

The earlier estimate was that just 9 to 18 barrels had been spilled.

BP based the estimate on the amount of oil collected by vacuum trucks and absorbent booms.

The U.S. Environmental Protection Agency said the spill appears to pose no risks to Lake Michigan.

About 7 million people in Chicago and the suburbs are supplied drinking water from the lake.

Oil from the spill didn’t reach the main intake about 8 miles north of the spill site.