Berkeley Pit

The Berkeley Pit is a former open pit copper mine located in Butte, Montana, United States. It is one mile long by half a mile wide with an approximate depth of 1,780 feet (540 m). It is filled to a depth of about 900 feet (270 m) with water that is heavily acidic (2.5 pH level), about the acidity of cola or lemon juice.[1] As a result, the pit is laden with heavy metals and dangerous chemicals that leach from the rock, including copper, arsenic, cadmium, zinc, and sulfuric acid.[1]

Silver Bow Creek/Butte Area
Superfund site
Berkeley Pit (center) and Yankee Doodle Tailings Pond (upper left) with terraced levels/access roadways. The town of Butte is in the lower right corner.
CountySilver Bow County
Coordinates46°01′02.38″N 112°30′36.60″W
Silver Bow Creek/Butte Area
Berkeley Pit's location in Montana
ContaminantsArsenic, cadmium, copper, zinc, lead
List of Superfund sites

The mine was opened in 1955 and operated by the Anaconda Copper Mining Company, and later by the Atlantic Richfield Company (ARCO), until its closure on Earth Day in 1982. When the pit was closed, the water pumps in the nearby Kelley Mine, 3,800 feet below the surface, were turned off, and groundwater from the surrounding aquifers began to slowly fill the Berkeley Pit, rising at about the rate of one foot a month.[1] Since its closure in 1982, the water level in the pit has risen to within 150 feet of the natural water table.

The pit and its water present a serious environmental problem because the water, with dissolved oxygen, allows pyrite and sulfide minerals in the ore and wall rocks to decay, releasing acid. When the pit water level eventually reaches the natural water table, estimated to occur by around 2020, the pit water will reverse flow back into surrounding groundwater, polluting Silver Bow Creek, which is the headwaters of Clark Fork of the Columbia River.[1] The acidic water in the pit carries a heavy load of dissolved heavy metals. The pit's water contains so much metal that at one point one of its owners, Montana Resources, mined copper directly from the water.[2]

The first plans for solving the groundwater problem were devised in the 1990s. Water flowing into the pit has been diverted by the Horseshoe Bend Water Treatment Plant to slow the rise of the water level. Construction started on a water treatment facility in September 2018, with the goal to have the facility fully operational five years before the pit's water level rises high enough to contaminate the groundwater of Butte.[3] The Berkeley Pit is currently one of the largest Superfund sites.

The Berkeley Pit is currently a tourist attraction, with an adjacent gift shop. An admission fee is charged to go out on the viewing platform.

Early history and development

The underground Berkeley Mine was located on a prominent vein extending to the southeast from the main Anaconda vein system.When open pit mining operations began in July 1955, near the Berkeley Mine shaft, the older mine gave its name to the pit. The open-pit style of mining superseded underground operations because it was far more economical and much less dangerous than underground mining.

Within the first year of operation, the pit extracted 17,000 tons of ore per day at a grade of 0.75% copper. Ultimately, about 1,000,000,000 tons of material were mined from the Berkeley Pit. Copper was the principal metal produced, although other metals were also extracted, including silver and gold.[4]

Two communities and much of Butte's previously crowded east side were consumed by land purchases to expand the pit. The Anaconda Company bought the homes, businesses and schools of the working-class communities of Meaderville, East Butte, and McQueen, east of the pit site.Many of these homes were either destroyed, buried, or moved to the southern end of Butte. Residents were compensated at market value for their acquired property.


Mining of sulfide minerals began in the Butte mining district in 1864.Placer deposits were mined out by 1867.Silver vein lodes were then the most productive until copper was discovered in 1888.Open-pit mining started in 1955.Copper has historically been the main metal produced, though lead, zinc, manganese, silver and gold have been produced at various times.[5]

The district is characterized by the Late Cretaceous Boulder batholith which metamorphosed surrounding rocks during the Laramide orogeny.Ore formation occurred with the intrusion of the Butte quartz monzonite pluton.[5]

Environmental effects

The Berkeley Pit is a part of the Silver Bow Creek/Butte Area Environmental Protection Agency Superfund site,[6] considered one of the largest in America,[7] and the pit itself was added to the federal Superfund site list in 1987. It is one of sixteen Superfund sites in Montana, and the Berkeley Pit is the final unrestored section of the Silver Bow Creek/Butte Area site.

In 1995, a flock of migrating geese landed in the Berkeley Pit and died. A total of 342 carcasses were recovered.[8] ARCO, the custodian of the pit, denied that the toxic water caused the death of the geese, attributing the deaths to an acute aspergillosis infection that may have been caused by a grain fungus, as substantiated by Colorado State University necropsy findings.[8] These findings were disputed by the State of Montana on the basis of its own lab tests.[8] Necropsies showed their insides were lined with burns and festering sores from exposure to high concentrations of copper, cadmium, and arsenic.[1]

The Berkeley Pit currently utilizes Phoenix Wailers, which have been effective in deterring birds from landing at or staying in Berkeley Pit for an extended period of time.[9][10] Studies have shown that the Wailers have been effective in limiting bird deaths in most cases,[11] but they have not worked as well when faced with a large number of birds.

On November 28, 2016 several thousand snow geese died after a large flock landed in the pit's water to avoid a snow storm.[12] Immediately after the event, officials made efforts to scare birds away and prevent more from landing in the area. An official report issued in 2017 by the U.S. Fish and Wildlife Service found that the 3,000 to 4,000 snow geese that died at the Berkeley Pit were killed by exposure to sulfuric acid and heavy metals.[13]

Construction began on a treatment plant at the Berkeley Pit in September 2018. The facility will be able to treat the existing waters of the Pit before the water level hits the critical point of 5,410 feet (1,650 m) above sea level. This number was set by federal order from the EPA and is intended to protect groundwater from being contaminated by the water in the pit. The treatment facility will be able to treat ten million gallons of water per day. The companies working jointly to build the plant, Montana Resources and Atlantic Richfield, aim to have the facility open by mid-2019.[3]


A protozoan species, Euglena mutabilis, was found to reside in the pit by Andrea A. Stierle and Donald B. Stierle, and the protozoans have been found to have adapted to the harsh conditions of the water.[1] Intense competition for the limited resources caused these species to evolve the production of highly toxic compounds to improve survivability; natural products such as Berkeleydione, berkeleytrione,[14] and Berkelic acid[15] have been isolated from these organisms which show selective activity against cancer cell lines. Some of these species ingest metals and are being investigated as an alternative means of cleaning the water.[1]

Important dates

  • 1994 – September, EPA/DEQ issue Record of Decision (ROD) for Butte Mine Flooding Operable Unit.
  • 1996 – April, Montana Resources (MR) and ARCO divert Horseshoe Bend (HSB) drainage water away from Berkeley Pit to slow filling rate, per ROD.
  • 2000 – July, MR suspends mining operations due to high energy costs; HSB water allowed to flow back into pit, increasing pit filling rate.
  • 2002 – March, USEPA and Montana Department of Environmental Quality (MDEQ) enter into a Consent Decree with BP/ARCO and the Montana Resources Group (known as the Settling Defendants) for settlement of past and future costs for this site.
  • 2002 – Fall, USEPA and MDEQ issue order for Settling Defendants to begin design of water treatment plant for HSB water. Settling Defendants issue contract and begin construction of treatment plant.
  • 2003 – November, MR resumes mining operations.
  • 2003 – November 17, HSB water treatment plant comes on line slowing pit filling rate.


The mine is at 46°00′56″N 112°30′37″W, at an altitude of 4698 feet (1432 m) above mean sea level.

See also


  1. Edwin Dobb. "New Life in a Death Trap". Discover, 2000.
  2. "Montana Resources mines the water". July 5, 2009. Retrieved November 18, 2018.
  3. Saks, Nora (September 25, 2018). "Montana's Berkeley Pit Water Treatment Facility Begins Construction". Montana Public Radio. Retrieved November 18, 2018.
  4. "Berkeley Pit History". Colorado State University, Department of Biology. Archived from the original on 2009-04-28. Retrieved 2009-02-21.
  5. Edwin W. Tooker (1990). Gold in the Butte District, Montana in USGS Bulletin 1857 Gold in Copper Porphyry Copper Systems. United States Government Printing Office. p. E17-E27.
  6. "NASA - Berkeley Pit: Butte, Montana". Retrieved October 9, 2009.
  7. Robbins, Jim (June 25, 2018). "Let the Stream Run Through It". New York Times. Retrieved November 18, 2018.
  8. Adams, Duncan (December 11, 1995). "Did toxic stew cook the goose?". High Country News. Retrieved November 18, 2018.
  9. Guarino, Ben (December 7, 2016). "Thousands of Montana snow geese die after landing in toxic, acidic mine pit". The Washington Post. Retrieved November 18, 2018.
  10. "Migratory birds shooed away by drone, fireworks, lasers". The Spokesman-Review. Associated Press. September 16, 2017. Retrieved November 18, 2018.
  11. Daley, Jason (December 8, 2016). "Thousands of Snow Geese Die at Abandoned Pit Mine". Retrieved November 18, 2018.
  12. "Thousands of snow geese die in Montana after landing on contaminated water". The Guardian. Associated Press. November 18, 2018. Retrieved 7 December 2016.
  13. Dunlap, Susan (April 18, 2017). "Metals, acid in Berkeley Pit water killed geese, report confirms". Montana Standard. Retrieved November 18, 2018.
  14. "Berkeleydione and Berkeleytrione, New Bioactive Metabolites from an Acid Mine Organism". Organic Letters. 6: 1049–1052. doi:10.1021/ol049852k.
  15. "Berkelic Acid, A Novel Spiroketal with Selective Anticancer Activity from an Acid Mine Waste Fungal Extremophile". The Journal of Organic Chemistry. 71: 5357–5360. doi:10.1021/jo060018d.
  • McClave, M. A. (1973). Control and distribution of supergene enrichment in the Berkeley Pit. in Guidebook. Butte District, Montana: Butte Field Meeting of Society of Economic Geologists. pp. K–1–K–4.
  • Shovers, B.; Fiege, M.; Martin, D.; Quivik, F. (1991). Butte and Anaconda revisited. Special Pub. 99. Montana: Montana Bureau of Mines and Geology.
  • Weed, W. H. (1912). Geology and ore deposits of the Butte District. Professional Paper 74. Montana: U.S. Geological Survey.

Further reading

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