The Berkeley Pit Lake, located in Butte, Montana, is a part of the largest Federal Superfund site in the United States.  Selective underground mining in the Butte area began in the 1860s.  The Berkeley Pit mine was eventually opened in 1955, when the area surrounding the mine was surface-stripped by the Anaconda Copper Mining Company to allow miners access to the depths of the mountain. The mines were worked until 1982, when the operation was abandoned¹.  Over the twenty-seven years it was active, the Berkeley Pit Mine produced over one billion tons of ore, including copper, lead, zinc, gold, and manganese, leading to the nickname “the Richest Hill on Earth.”
 
 

Early photos of Butte, Montana¹⁸
 
 

     The diameter of the entire mine complex is roughly 1.5 miles east to west by 1 mile north to south. The sum of all the Butte mines contains over 42 vertical miles of mineshafts connected by about 2,700 miles of other passageways. Pumping stations were established at the mines to prevent groundwater from leaching in and filling the shafts. At the Berkeley Pit, groundwater and runoff were pumped continuously from the Kelly Shaft by a pumping station located at a depth of 3,800 feet.  The Atlantic Richfield Company (ARCO) purchased the Berkeley Pit mine from Anaconda in 1977.  ARCO maintained the mine site until 1982, when the site was closed and the pumping station abandoned, allowing water to flow into the mineshafts.  By 1983, the rising water level had filled the shafts and had reached the bottom of the pit.  The pit is currently over 900 feet deep, and contains over 30 billion gallons of water².  Water continuously enters the pit via surface run-off, ground water leaching, infiltration, and from the interconnected underground mine workings.  The current fill rate of the pit is about 17-25 feet per year.
 

Following the flooding of the mines, a Butte Mine Flooding monitoring network was soon established, and currently consists of the Montana Bureau of Mines and Geology, Montana Resources, and ARCO.  The purpose of the network is to prevent the mine-contaminated waters from reaching nearby aquifers and surface waters. The consortium uses 85 groundwater-monitoring sites, nine continuous water-level recorders, and 3 surface water-gauging stations to keep track of changes at the pit.  All readings are recorded in a database maintained by the network.  In 2000, ARCO also abandoned the nearby Continental Pit, which allowed the inflow of an additional 733 million gallons of water per year into the Berkeley Pit Lake site.  As a result, the Environmental Protection Agency and the Montana Department of Environmental Quality set a maximum water level for the Berkeley Pit Lake to maintain its classification as a “Terminal Pit.”  ARCO and Montana Resources were notified that the Pit Lake cannot exceed a depth of 1,147 feet (or 5,410 feet from the bottom of the mineshafts).  Also, a treatment plan must be in place at least 8 years before the depth is predicted to reach the Safe Water Level.  Currently, the projected date is roughly 2021, when the lake will hold about 63 billion gallons of water1.

Berkeley  Pit before and after flooding2.