Berkeley Pit
Updated: January 21, 2026
The Berkeley Pit is located in southwestern Montana near the city of Butte, within the Butte Mining District in the upper Silver Bow Creek drainage. When mining ceased in 1982, its bottom sat at about 4,265 feet above sea level (USGS datum), providing a total rim-to-bottom depth of approximately 1,780 feet. The pit spans roughly 675 acres (1.06 square miles) and initially held an estimated volume of 1.18 × 1010 cubic feet at a base elevation of 5,543 feet above sea level (USGS, 1991).
Mining operations, launched by the Anaconda Copper Mining Company in 1955 and later continued by Atlantic Richfield (ARCO), ended in 1982. Over its 27-year run, approximately 1.5 billion tons of material were extracted-290 million tons of which were copper ore-earning Butte the moniker "Richest Hill on Earth." The Pit features prominently in the local and regional heritage and remains viewable today from a public platform.
Once mining halted, deep level dewatering pumps were shut off, allowing groundwater to rebound to its natural levels. A comprehensive monitoring program was set up to track the rising water levels and evaluate changes in groundwater quality in the deep bedrock and overlying alluvium.
From 1982 through the late 1990s, inflows amounted to about six million gallons per day, resulting in an approximate rise of 21 vertical feet every nine months-split equally between underground aquifer input and surface water via the Horseshoe Bend "waterfall." Around 1996, ARCO and Montana Resources began diverting surface inflow into Montana Resources' active tailings systems, which slowed the fill rate to about three million gallons per day, or roughly 10 vertical feet every nine months.
As of early 2025, the water level in the Pit is approximately 5,355.7 feet above sea level, well below the designated protective water level of 5,410 feet. After initiating pumping and treatment in October 2019, Montana Resources and ARCO have pumped around 9 billion gallons of treated water into Silver Bow Creek by April 2025 . That has successfully halted any rise in the Pit's water level; previously, it had been rising six to seven feet annually. Treatment occurs at the Horseshoe Bend Water Treatment Plant (capacity seven million gallons per day), with a polishing facility added in 2019. Daily sampling-over 14,000 checks-shows discharge quality meets or exceeds Montana's environmental standards.
PitWatch remains the official source for up-to-date Pit information, including the protective water level (5,410 feet), water quality, hydrology, and remediation efforts.
Water Quality & Treatment
The water is strongly acidic (pH ~4.1-4.5) and rich in heavy metals-copper, arsenic, cadmium, zinc, iron, and more-due to acid-rock drainage. Since 2020, WSP has demonstrated that ~10 million gallons per day of water can be treated to EPA-declared discharge standards using multimedia filtration and reverse osmosis, adaptable depending on water quality dynamics. Compliance is maintained with 7 MGD treatment capacity, including the polishing plant completed in 2019-years ahead of original projections.
Protective Water Level & Monitoring
The protective water level -formerly termed the critical water level-is defined at 5,410 feet above sea level. Surpassing this threshold risks contaminating Silver Bow Creek or the alluvial aquifer. Monthly monitoring via 23 wells and 14 shafts, managed by the Montana Bureau of Mines & Geology, ensures the water remains well below that level.
Construction, Discharge & Enforcement
The 1994 Record of Decision stipulates that pumping and treatment must begin before the water reaches 5,410 feet; failure to comply could result in daily fines-still applicable today. With pumping ongoing since 2019 and the level stable, compliance has been sustained.
Ecosystem Management
Bird protection is actively managed-scare devices and observed hazing prevent waterfowl from landing on the toxic waters. The water quality and migratory bird programs remain continuous components of Pit management.
Recent & Emerging Developments
Rare Earth Extraction Pilot: In 2024, Montana lawmakers petitioned Congress to support extracting rare earth elements (e.g. yttrium, cerium) from pit water. Pilot initiatives, partnered with Montana Tech and West Virginia University, aim to extract ~40 tons per year, with commercial efforts ongoing. Federal Support: Under Interior's 2025 minerals initiative, the Pit was highlighted as a potential source of critical minerals like rare earths, uranium, and zirconium.
Mining East of Butte
Mining activities continue at the Continental Pit, directly east of Butte. In April 2025, Montana Resources filed an amendment to extend the Yankee Doodle Tailings Storage Facility's capacity and prolong mining operations through 2056.
Visiting the Pit
The Berkeley Pit Viewing Stand is open free to the public from March through November, weather permitting. Additionally, a small visitor center and educational tours are available (donation suggested). While admission is generally free, some sources indicate a nominal fee (~$3) can apply during peak months-check current Butte Chamber listings.
Updated: January 21, 2026
