Less than a year after BP started up a new unit to process Canadian tar sands at its Whiting refinery, the company reported today that a malfunction allowed a slug of crude oil into Lake Michigan a few miles away from the Chicago city limits.

It remains unclear how much oil spilled into the lake or how long the discharge continued. Workers at the refinery reported an oil sheen on the water about 4:30 p.m. Monday, and an official from the U.S. Environmental Protection Agency said the leak was plugged by the time he arrived at 9 p.m.

Mike Beslow, the EPA’s emergency response coordinator, said there appeared to be no negative effects on Lake Michigan, the source of drinking water for 7 million people in Chicago and the suburbs. The 68th Street water intake crib is about eight miles northwest of the spill site, but there were no signs of oil drifting in that direction.

Initial reports suggest that strong winds pushed most of the oil toward a sandy cove on BP’s property between the refinery and an Arcelor Mittal steel mill. A flyover Tuesday afternoon revealed no visible oil beyond booms laid on the water to prevent the oil from spreading, Beslow said.

“There is no known impact to wildlife or human health at this time,” Beslow said.

Frigid temperatures caused some of the oil to harden into a waxy consistency that made it easier to collect, said Scott Dean, a BP spokesman. Crews used vacuum trucks to suck up any liquid oil that washed ashore.

The malfunction occurred at the refinery’s largest crude distillation unit, the centerpiece of a nearly $4 billion overhaul that allowed BP to process more heavy Canadian oil from the tar sands region of Alberta. The unit, which Dean said has resumed normal operations, performs one of the first steps in the refining of crude oil into gasoline and other fuels.

BP said a mixture of heavy and light oil somehow leaked into a sealed cooling system that circulates water between Lake Michigan and the refinery. Oil is never supposed to come in direct contact with the cooling water.

The spill comes amid years of legal challenges from federal officials and environmental groups that have forced BP to take extra steps to curb air and water pollution at the nation’s seventh-largest oil refinery. The Whiting plant remains one of the largest sources of industrial pollution discharged into Lake Michigan, according to federal records.

In a joint statement, U.S. Sens. Dick Durbin and Mark Kirk of Illinois said they are concerned that BP’s move to increase production could lead to more oil spills.

“We plan to hold BP accountable for this spill,” the senators said, “and will ask for a thorough report about the cause of this spill … and steps are being taken to prevent any future spill.”