By Ed Curran

(CBS) — Lake Michigan is the source of drinking water for 7 million people in Chicago and the suburbs.

So, when 39 barrels of oil spilled, it raised a lot of concern.

Still, as CBS 2’s Ed Curran reports, the impact could have been much worse.

At the Shedd Aquarium, senior research biologist Philip Willink says BP was very lucky.

“For example, the winds were blowing onshore so that a lot of the oil washed up onto the beach. If the wind had been in the other direction it could have gone into the middle of the lake,” he says.

That would have sent the oil to the offshore cribs that collect Chicago’s drinking water. Oil can break down over a very long period, but it can also affect fish DNA. Another lucky break: the temperature.

“If this spill had happened a month ago and there was ice cover on Lake Michigan, that would have made cleaning it up a lot more complicated,” Willink says.

Also, the cold lake means fish are in deep water and away from the spill, which is now estimated to be 39 barrels of oil.

The spill affected a half mile of Lake Michigan shoreline.

On Friday, demonstrators rallied in front of BP’s Chicago offices on South Wacker.

BP more than doubled its estimate of how much crude oil spilled into the lake on Monday – now estimating as much as 1,600 gallons.

U.S. Sens. Mark Kirk and Dick Durbin want some answers from the company.

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