The Berkeley Pit is a gigantic former open pit copper mine located in Butte, Montana, and is the largest Superfund site. It was opened in 1955 and operated by the Anaconda Mining Company and later by ARCO, until its closure in 1982. When the pit was closed, the water pumps at the bottom were also turned off, which caused groundwater from the surrounding basin to leak into the pit. The water seeped through metal deposits, causing it to become heavily acidic and laden with heavy metals and dangerous chemicals such as arsenic, cadmium, zinc, and sulfuric acid. The water was filling up the pit at a very fast rate, but it was not until the 1990s that serious plans were devised for solving this problem.
In 1995, a large flock of migrating snow geese landed on the Berkeley Pit water and were killed, most likely by the high concentration of acid. 342 carcasses were recovered. The most recent development in the clean-up was the construction of a treatment plant on Horseshoe Bend. This facility is intended to treat and divert water coming from the Horseshoe Bend flow. In addition, it will be able to treat the existing Berkeley Pit water in 2018, or whenever the water level hits the critical point of 5,410 feet above sea level.
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