Butte, Montana stands at the headwaters of the Clark Fork River and has been a center of extensive underground mining since about the 1870s. Open-pit mining began at the Berkeley Pit in 1955 and continues at the Continental Pit. As part of the underground mining activities, ground-water levels had to be continually lowered as mining went deeper. When Berkley Pit Then and Nowunderground mining finally ended (1975), the workings included over 49 miles of vertical shafts and over 5,600 miles of horizontal workways. The Anaconda Company estimated there were over 10,000 miles of workings when all other passages were included. As early as 1901, groundwater encountered in Anaconda Copper Mining Company properties was drained to a common mine level for pumping to the surface. 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 pumped mine water contained sufficient quantities of dissolved copper sulfate that it was directed to precipitation plants for copper recovery.
The Berkeley Pit was also dewatered through the underground mine dewatering system.
The Atlantic Richfield Company (ARCO) 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. The pump station,
located on the 3,900 level (3,600 feet below ground surface) of the Kelley Mine,
was shut down, and ground-water levels began to rise immediately. Levels had risen
more than 1,300 feet by the Berkeley pit water levelsend of 1982 and 3,180 feet
by the end of 2012. The maximum elevation (5,410 feet) that water levels will be
allowed to reach in the underground workings and Berkeley Pit has been established
by the U.S. Environmental Protection Agency (EPA) and the Montana Department of
Environmental Quality (DEQ).
To date, the Butte Mine-Flooding monitoring network maintained by the Montana Bureau of Mines and Geology (MBMG), Montana Resources (MR), and ARCO consists of:
78 ground-water monitoring sites (wells and mine shafts), 51 of which are equipped with continuous water-level recorders;
two surface-water gauging stations;
database that contains over 8,000 complete water-quality analyses that were collected from surface-water, groundwater, and mine-shaft locations;
and 35 years of monitoring data, i.e., water levels and flow.
The water in the Berkeley Pit is highly acidic and high in concentrations of arsenic, copper, cadmium, cobalt, iron, manganese, zinc, and sulfate, plus other inorganic constituents. Water-quality samples are collected semi-annually at selected locations. Water quality in the underground mines was very acidic and high in metals concentrations when flooding began; however, as water levels have risen, pH has increased and metals concentrations have decreased.
Berkeley Pit Facts
|Pit opened — 1955||Material mined c Approximately 1 billion tons|
|Dimensions — 1.5 miles east/west by 1 mile north/south in diameter||Mining suspended — 1982|
|Pit lake elevation — July 2017 5,337'ft containing 46.8 billion gallons of water (pH ~3.5)||Inflow of water into the Pit — Approximately 1,800 gallons per minute, 2.50 million gallons per day (2016 average)|
Current Conditions (Berkeley PIT CWL Projections,based on Anselmo Mine Water-levels (July 2023))
Berkeley PIT CWL Projections, based on Anselmo Mine Water-levels (July 2023)
Berkeley Pit water level January 5th, 2017, was at an elevation of 5,337, which is 73 feet below the critical water level (CWL)
2016 Berkeley Pit infilling model predicts water level will reach action level at Anselmo Mine compliance point in June 2023
2016 groundwater flow into pit averaged 2.20 million gallons per day (mgd)
The Butte Mine-Flooding Operable Unit (OU) is one of four OUs within the Butte Mining district (BMD) and consists of approximately 23 square miles. The OU is divided into two systems: 1) East Camp/Berkeley Pit, and 2) West Camp. Other OUs in the BMD are the Butte Priority Soils, Silver Bow Creek/Streamside Tailings, and Non-Priority Soils. Major features of the OU are 1) underground mine workings, 2) Berkeley Pit, and 3) alluvial and bedrock aquifers contributing flow to the system.Berkeley Pit is the most prominent feature of the OU; it is 1,780 feet deep (from the north rim) and occupies 675 acres. It is approximately 1½ miles across, East to West, and one mile across, North to South.
Record of Decision specifies the following:
*Note See September 29, 1994 Record of Decision for complete description of selected remedy. Conditions are those described in ROD
** Elevation datum is USGS