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Berkeley Pit

The Berkeley Pit is a former open pit copper mine located in Butte, Montana, USA, about a mile and a half wide and about 1,780 feet deep. It contains about 900 feet of water that is heavily acidic, with a pH level of 2.5, and laden with heavy metals and dangerous chemicals such as arsenic, cadmium, zinc, and sulfuric acid.

The mine was opened in 1955 and operated by the Anaconda Mining Company and later by the Atlantic Richfield Company (ARCO), until its closure in 1982. When the pit was closed, the water pumps at the bottom were removed, and groundwater sourced from the surrounding aquifers soon filled the pit to the natural groundwater level.

This has presented an environmental problem in that the water, with dissolved oxygen, allows pyrite and sulphide minerals in the ore and wall rocks to decay, releasing acid. The acidic water in the pit can carry a heavy load of dissolved heavy metals. The water contains so much dissolved metal (up to 187 ppm Cu) that "mining" of the water is actually done.

In the 1990s were plans devised for solving the groundwater problem. The Berkeley Pit has since become one of the largest Superfund sites.


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 The open-pit style of mining superseded underground operations because it was far more economical: even very low-grade ore could be recovered using shovels and trucks, which were also less dangerous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, meaning that about 127 tons of copper was produced from the ore. Ultimately, about 1,000,000,000 tons of material was mined from the Berkeley Pit [1]. Copper was the principal metal produced, together with subsidiary amounts of silver and gold.

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 and McQueen, east of the pit site. Many of these homes were either destroyed, buried, or moved to the southern end of Butte.

Environmental effects

In 1995, a large flock of migrating snow geese landed on the Berkeley Pit water and were killed. 342 carcasses were recovered.[1] 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. These findings were disputed by the State of Montana on the basis of its own lab tests. [1]

Nearby residents are also concerned about the fog produced by the pit and are wondering what it is doing to their health. The most recent development in the clean-up was the construction of a treatment plant on Horseshoe Bend. This facility is intended to treat and divert water coming from the Horseshoe Bend flow. In addition, it will be able to treat the existing Berkeley Pit water in 2018, or whenever the water level hits the critical point of 5,410 feet above sea level. This number was set by Federal order and is intended to protect the ground water from being contaminated by the water in the pit.


New fungal and bacterial species have been found to have adapted to the harsh conditions inside the pit. 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 [2] and Berkeley acid [3] have been isolated from these organisms which show selective activity against cancer cell lines.

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.


  • 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, p. K-1-K-4.. 
  • Shovers, B.; Fiege, M., Martin, D., and Quivik, F. [1991]. Butte and Anaconda revisited, Special Pub. 99. Montana: Montana Bureau of Mines and Geology, 64 p.. 
  • Weed, W.H. [1912]. Geology and ore deposits of the Butte District, Professional Paper 74. Montana: U.S. Geological Survey, 262 p.. 
  1. ^ a b Adams, Duncan (1995-12-11). Did toxic stew cook the goose?. High County News. Retrieved on 2006-05-07.
  2. ^ doi:10.1021/ol049852k
  3. ^ doi:10.1021/jo060018d

Coordinates: 46°01′02.38″N, 112°30′36.60″W

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Berkeley_Pit". A list of authors is available in Wikipedia.
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