Water drainage was through interconnected stopes, drifts, or diamond-drill drainage holes that were used to transport water to the central pump stations. By the 1910s as many as 28 mines were dewatered this way. The Berkeley Pit, an open-pit copper mine, was dewatered through the underground mine dewatering system. The Atlantic Richfield Company bought the Anaconda Company in 1977 and continued to operate the underground pumping system until April 1982, when the decision was made to suspend mining in the Berkeley Pit. Water levels had risen more than 1,300 feet by the end of 1982 and 3,000 feet by the end of 1999.
Pit are part of a Federal Superfund Site.
A long-term monitoring program was put in place to ensure that the Berkeley Pit remains the sink, or terminal pit, for bedrock ground water entering and filling the historic mine workings. This closed system would preclude mine-water discharge to nearby aquifers and surface waters. Both water levels and water quality are monitored. Water levels are monitored monthly in 73 wells and 12 mine shafts. The water in the underground mines and Berkeley Pit is highly acidic and high in concentrations of arsenic, copper, cadmium, cobalt, iron, manganese, zinc and sulfate, plus other inorganic constituents.