Active Mining in Butte and the Berkeley Pit


PitWatch Issue Volume 3, Number 1 (1998)

We can explain some, but not all, aspects of the relationship between the Berkeley Pit and Montana Resources (MR), the corporation actively mining here in Butte. As you may recall, last year MR and ARCO hammered out an agreement on Berkeley Pit clean-up responsibilities, but the details were not made public. Here’s what we do know:

The Present

Horseshoe Bend. For the past two years, MR and ARCO have been diverting and treating the stream of water that once flowed directly into the Berkeley Pit from the northeast. The Horseshoe Bend diversion project has reduced by about half the amount of water entering the Pit. MR reuses this water in its concentrating operations. MR and ARCO share the project’s annual operating cost of about $2 million. Around $1.6 million of that goes toward buying and transporting lime, which is used to treat the water.
Continental Pit Dewatering. From March through October, MR pumps groundwater out of the south end of the Continental Pit to keep the area dry for future mining. Starting this summer, the company may begin pumping additional water out of the north end of the pit, with all water routed to the MR concentrator. This pumping diverts water that could otherwise flow into the Berkeley Pit from the east.

The Future

Monitoring Program Expansion. Over time, the monitoring well system will be expanded south and east to cover the entire Berkeley Pit/Continental Pit Complex, rather than just the area around the Berkeley Pit. This expansion will be necessary because groundwater pumping at the Continental Pit will eventually take place at an elevation lower than the water level of the Berkeley Pit. Pumping now occurs about 90 feet above the Berkeley Pit water level, but over the next 20 years, MR plans to expand the Continental Pit eastward and southward and mine it down to 4,986 feet—166 feet below the current Berkeley Pit water level (all USGS elevations).

When this new low spot is created, some water that would otherwise flow west toward the Berkeley may instead start flowing east toward the Continental. Water levels in the monitoring wells between the two pits (such as Well H) will be affected by this change, although exactly how they will react is unknown. Most importantly, however, all water will still be confined to the mining area with flow going toward the two pits.

Central Zone Mining. MR may also expand active mining westward into the Central Zone area between the two pits. Five bedrock monitoring wells (H, C, D1, D2, and DDH-2) lie within this zone and would be mined out. The Continental Fault, which runs along the Continental Pit’s current west border, would also be mined out, eliminating what is now a partial groundwater barrier between the two pits. MR officials said the Central Zone is rich in copper, but they also admitted that a great deal of research must be done to determine the feasibility of mining there.

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