Illinois’ U.S. senators asked Thursday for a meeting with BP after oil from the company’s Whiting refinery spilled Monday into Lake Michigan and the company increased its estimate of the amount of oil that was released.

U.S. Sens. Dick Durbin and Mark Kirk sent a letter to BP CEO John Minge saying they’re “deeply concerned” about the spill. They want to talk with him about potential public health and environmental threats to surrounding communities.

Company estimates now indicate that as many as 39 barrels — or 1,638 gallons — of crude oil may have been released Monday afternoon when the BP Whiting refinery experienced a malfunction in its oil processing unit, officials said Thursday.

BP spokesman Scott Dean said the company estimates that between 15 and 39 barrels of oil have been recovered in the cleanup thus far through vacuum trucks, oil-absorbent boom and solid material on the shore. That’s an increase from the previous estimate of nine to 18 barrels provided to the U.S. Environmental Protection Agency and the U.S. Coast Guard before any collected oil had been measured. The initial estimate was based on a visual observation of the water’s surface.

Inclement weather meant boats carrying the Shoreline Cleanup Assessment Team could not be deployed on Thursday, but they expect to be on site on Friday if the weather cooperates.

The EPA said the spill isn’t expected to affect municipal water supplies that draw on the lake.