The Berkeley Pit and Surrounding Mine Waters of Butte
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THE BERKELEY PIT AND SURROUNDING MINE WATERS OF BUTTE Christopher H. Gammons1 and Terence E. Duaime2 1Department of Geological Engineering, Montana Technological University, Butte, Montana 2Montana Bureau of Mines and Geology, Butte, Montana INTRODUCTION to the north and west, is an extensive complex of aban- doned underground mines, once serviced by over a The Butte mining district is famous for its prodi- hundred vertical shafts, many of which exceeded 1 km gious mineral wealth (see Reed and Dilles, 2020). It is in depth. Although many of the bigger shafts still have also widely known as an example of environmental headframes today, underground mining at Butte ceased damage caused by over 150 years of hard rock min- in 1975 and the shafts and associated mine workings ing, milling, and smelting (Moore and Luoma, 1990; began fl ooding with groundwater when dewatering Dobb, 1996). These impacts extend from Butte to pumps were shut off in April 1982. Most of the shafts Missoula via the upper Clark Fork River corridor, and labeled in fi gure 1 are monitored for water levels by include over 100 km of streamside tailings washed the MBMG, with the Anselmo, Steward, and Kelley downstream during floods, as well as the major mines sampled annually for water quality. The MBMG smelting operations near the town of Anaconda. The also samples a set of bedrock monitoring wells to the severity of the environmental damage is episodically east and south of the Berkeley Pit (labeled A, B, C, brought into focus by events in the news, such as the D-2, E, F, G, and J in fi g. 1). recent (November, 2016) death of over 3,000 snow geese that landed on the Berkeley Pit during their Approximately 2 km east of the Berkeley Pit, annual southward migration (Robbins, 2016). Despite the Continental Pit has been mined by MR for low- the massive scale of the contamination on the Butte grade copper and molybdenum from early 1986 to Hill, there have been significant reductions in risk to the present day (with a suspension in production from human and ecosystem health through ongoing clean- July 2000 to November 2003). Ore from the Continen- up eff orts led by the U.S. Environmental Protection tal Pit is hauled to the mill where the rock is crushed Agency (EPA), British Petroleum-Atlantic Richfield and mineral concentrates containing chalcopyrite (BP-ARCO), Montana Resources (MR), and the Mon- and molybdenite are recovered. Most of the volume tana Department of Environmental Quality (MDEQ). of rock (>99%) that enters the mill is waste, and is pumped as a tailings slurry to the large Yankee Doodle This chapter gives an overview of the Butte Mine impoundment, several kilometers north and uphill of Flooding Operable Unit (BMFOU), which includes the Berkeley Pit. Prior to 2019, MR’s operation was a the inundated Berkeley Pit and surrounding flooding zero-discharge facility, meaning that all water on the underground mine workings. The paper draws on over mine property was recycled and reused. Beginning 30 years of water-level and water-quality data collect- in October 2019, water from the northeast end of the ed and archived by the Montana Bureau of Mines and active tailings impoundment has been sent to a new Geology (MBMG). polishing plant and discharged into the headwaters of Site Overview Silver Bow Creek near its confl uence with Blacktail The general layout of major features on the Butte Creek. As of this writing, the treated water meets all Hill is shown in fi gure 1. The Berkeley Pit, which State and Federal regulations for human and aquatic mined copper in the upper and central part of the Butte life. ore body between 1955 and 1982, is now fi lled with Two other features in fi gure 1 include the precip- acidic and metal-rich water. Surrounding and underly- itation (cementation) plant and the lime-treatment ing the Berkeley Pit, and extending many kilometers plant. The former, located near the base of the tail- 1 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics Figure 1. Map of the Butte district showing geology and mining features. ings dam, is used to extract copper by passing acidic is stored in two large silos, and is slowly added to water over bins of scrap iron. The dissolved copper the infl uent water to raise the pH and precipitate out precipitates, or “cements,” at the expense of the iron, dissolved metals. The metal precipitates, referred and is later recovered for profi t. The chemical reac- to as “sludge,” are discharged back to the Berkeley tion involves electron exchange, and can be written: Pit, where many of the solids re-dissolve, eff ectively Cu2+ + Fe(s) = Cu(s) + Fe2+. When fi rst built in the neutralizing some of the acidity of the pit lake. Fifteen early 20th century, the precipitation plant was used to years of sludge disposal has had a signifi cant impact recover copper from water pumped from the under- on the total acidity and pH of the lake (Duaime and ground mines. After the Berkeley Pit opened, the plant others, 2019a; Gammons and Icopini, 2019). Since received acidic, Cu-rich water from heap leach pads September 2019, HSB water has been pumped di- and also from seeps near the base of the tailings dam rectly to the Yankee Doodle tailings impoundment [referred to as Horseshoe Bend (HSB) springs], along for co-disposal with mill tailings, and the lime plant with the pumped underground mine water. After the switched to treating Berkeley Pit water full time. Berkeley Pit closed, water from the pit lake has inter- Sludge from the lime treatment is still being dis- mittently been passed through the precipitation plant charged to the pit, and the treated water is used by the to recover copper, along with water from HSB springs active mine and mill. (see timeline, below). Previous Studies The Horseshoe Bend lime-treatment plant, located Beginning in 1987 (Sonderegger and others, near the east rim of the pit lake, is used to treat acidic 1987), and annually since 1998 (Duaime and others, water for use in the mine operations. From November 1998), the MBMG publishes an update and interpre- 2003 to May 2019, this facility collected and treated tative report on water levels and water chemistry in acidic drainage from HSB. Hydrated lime (Ca(OH)2) 2 Gammons and Duaime: The Berkeley Pit and Surrounding Mine Waters the BMFOU (Duaime and others, 2019a, and refer- HISTORY OF MINE FLOODING ences therein). Most of these reports are available on the internet through the MBMG Publications Cata- In order to mine the Butte mineral deposit, enor- log (http://www.mbmg.mtech.edu/). Raw data can mous volumes of water (roughly 5,000 gallons per be found at the MBMG’s Groundwater Information minute, gpm) had to be continuously pumped out of Center (GWIC) website. In addition, a list of journal the workings. After consolidation of the underground papers and student theses dealing with the Berkeley mines by the Anaconda Company, mine workings Pit lake, the Continental Pit lake, the Yankee Doo- were interconnected to drain water to central pump dle tailings pond, and the fl ooded underground mine stations to improve effi ciency (Daly and Berrien, workings is given in table 1. Selected publications by 1923). The High Ore mine served as the central pump the US-EPA are also listed in table 1, including the station from 1901 to 1967, when the pump station was 1994 Record of Decision (ROD), the 2002 Consent moved to the Kelley mine. Once the water reached Decree, and two of the more recent “Five-Year Re- the surface, it was routed to the precipitation plant for views.” copper recovery and then discharged to Silver Bow Creek (SBC). The practice of discharging untreated, The low pH and unusually high concentrations of acidic, metal-laden water to SBC continued until the dissolved metals in the Butte mine waters were rec- late 1950s, at which time the Anaconda Company ognized and scientifi cally investigated as early as the began adding lime to the water to raise the pH and beginning of the 20th century. Stone (1909) and Febles reduce the mineral content of the water (Spindler, (1913) fi rst described the cementation process at Butte 1977). On April 22, 1982, the mine-dewatering pumps whereby copper could be extracted by passing the were turned off , and the vast complex of underground mine water over scrap iron. Weed (1912) and Hodge workings began to fl ood (see timeline in table 2). The (1915) gave additional descriptions of the chemistry of fi rst 2 yr of mine fl ooding were especially complex the mine waters being pumped out of the underground (Metesh, 2006). In the fi rst few months, water levels workings. It would be more than 70 years before the in the underground mine shafts rose hundreds of feet. next generation of scientifi c reports on the mine waters Also, beginning in the summer of 1983, acidic seepage of Butte was published (table 1). from the Horseshoe Bend area, South East Berkeley, and leach pads was diverted 7DEOH6XPPDU\RIOLWHUDWXUHRQ%XWWHPLQHZDWHUV into the Berkeley Pit, which 0%0*5HSRUWV )ORRGHG0LQH6KDIWV contributed to the rise in water 'XDLPHDQGRWKHUV :HHG DQGUHIHUHQFHVWKHUHLQLQFOXGLQJ +RGJH level. In November 1983, the 6RQGHUHJJHUDQGRWKHUV 0HWHVK water level in the Kelley mine 'XDLPHDQG0HWHVKD 3HOOLFRULDQGRWKHUV reached the same elevation 'XDLPHDQGRWKHUV *DPPRQVDQGRWKHUVD as the bottom of the open pit, %HUNHOH\3LW/DNH 5RHVOHUDQGRWKHUV 6Q\GHU and the Berkeley Pit lake was 'DYLVDQG$VKHQEHUJ born. Sometime in early to 5RELQVDQGRWKHUV *DPPRQVDQGRWKHUV -RQDV 3HWULW]DQGRWKHUV mid-1984, the pit became the 1HZEURXJKDQG*DPPRQV 7KRUQWRQDQGRWKHUV lowest point in the fl ooded 6FKPLGW 0DGLVRQDQGRWKHUV mine system, allowing water *DPPRQVDQGRWKHUV 0DHVWDQGRWKHUV <DQNHH'RRGOH7DLOLQJV3RQG from the underground mines 3HOOLFRULDQGRWKHUV %HU]HO in all directions to fl ow to- *DPPRQVDQG'XDLPH wards the pit (Duaime and 7ZLGZHOODQGRWKHUV (3$'RFXPHQWV McGrath, 2019).