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Berkeley Pit stumps experts

The Berkeley Pit partially froze this winter for the first time in what appears to be a long time. Pit experts are stumped as to why. This winter's temperatures have been about average, hovering around 19 degrees, according to the National Weather Service in Missoula.

Susan Dunlap, The Montana Standard

The Berkeley Pit partially froze over this winter and experts are stumped as to why.

Although no one could say exactly how long it’s been since the last time the Berkeley Pit froze over, Mark Thompson, Montana Resources vice president for environmental affairs, said that by the late 1990s it became “hit or miss.”

But this winter, the pit partially froze. Thompson estimated that perhaps as much as half the lake, which is about a mile wide, iced over.

A considerable amount of ice on the surface of the water was still visible late last week.

The pit holds approximately 50 billion gallons of contaminated water. Nobody knows for sure why it partially froze this year. Montana Bureau of Mines and Geology hydrogeologist Ted Dwaime said he doesn’t have any theories as to the cause.

“I really don’t know,” Dwaime said.

The bureau monitors the pit’s water quality.

Missoula-based National Weather Service meteorologist Bob Nester said that with the average winter temperature in Butte from Dec. 1 to Feb. 28 hovering around 19 degrees, this winter was pretty average.

Last winter Butte had a much colder winter. The average temperature during the 2016-2017 winter lingered around 15 degrees. It hasn’t been that cold in Butte since the 1992-1993 winter, Nester said.

But last winter, despite the more extreme cold, Thompson said there was very little freezing on the pit's surface.

Thompson did not have any theories as to why the pit partially froze this winter.

No one The Montana Standard spoke with could remember the last time the pit’s water froze completely or significantly. Thompson said that up to the mid-1990s, the pit froze every winter. 

It hasn’t frozen like this in the last three years, Thompson said.