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Berkeley Pit

Updated: August 17, 2020

The Berkeley Pit is located in southwestern Montana near the city of Butte. It is within the Butte Mining District in the upper Silverbow Creek drainage. When mining was discontinued, the bottom of the pit was at an elevation of 4,265 feet above sea level (USGS datum). The total depth of the pit from the bottom to the highest rim is 1,780 feet. It encompasses approximately 675 acres (1.06 square miles) and has a volume of approximately 1.18x10^10 cubic feet from the base elevation of 5,543 feet above sea level. (Butte Mine Flooding Operable Unit RIFS., 1991)

Mining by the Anaconda Company in the Berkeley Pit began in 1955 and was discontinued in 1982. One and one-half billion tons of material were taken out of the pit; 290 million of that was copper ore. The pit enabled Butte to claim the title "Richest Hill on Earth," as it became the backbone of the local and regional economy. Present-day visitors can view the mine from a platform above it.

When the mining stopped, dewatering was no longer necessary, and the deep level pumps were shut down. This allowed the groundwater to rise toward its natural, pre-mining levels. At the same time, a water level monitoring program was established to track that rise and to measure changes in deep groundwater quality.

Since 1982, water from the bedrock and overlying alluvium adjacent to the mines has been entering the abandoned Berkeley Pit and adjacent, interconnecting underground mines. Previously, that flow amounted to approximately 6,000,000 gallons a day or 21 vertical feet of rise every nine months. Approximately 3,000,000 gallons flowed into the pit from underground aquifers, and another three million gallons of surface water flowed via the Horshoebend "waterfall" which was visible from the Pit viewing stand. In April 1996, however, ARCO and Montana Resources started diverting this surface water into the active Montana Resources' tailings pond system. Therefore, the pace of the rising water in the pit has slowed to approximately 3,000,000 gallons per day or 10 vertical feet of rise every nine months.

The Official Record of Decision (ROD) issued by the Environmental Protection Agency (EPA) and Montana Department of Environmental Quality in September of 1994, established the critical water level at 5,410 feet. This represents the point at which the agencies feel the Pit water could potentially seep into Silverbow Creek or into the alluvial aquifer.

The ROD stipulates that ARCO and Montana Resources must begin pumping and treating the pit water before the water reaches this critical level. If they don't, they will be fined $25,000 per day.(Pitwatch, Vol.1,1996) For current information on the Berkeley Pit and the surrounding mine-flooding areas see PitWatch.

The Berkeley Pit Viewing Stand, open from March until November, can be used for free. Mining has resumed just east of Butte in the Continental Pit.

Visit's History and Prehistory section to read more about gold mining.

Updated: August 17, 2020

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