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Have there been any major rock slides along the Pit walls?

Yes. Wet weather may have also played a role in the 1998 Pit wall slough that sent about 3 million tons of rock and dirt into the water. Rising groundwater saturated and gradually weakened that section of the southeast wall, eventually causing it to break away.

Montana Resources, Inc. (MR) is taking steps to stabilize the piles of waste rock that form sections of the Pit walls. Two options are available: The first is to remove material from the tops of the dumps (the crests) to relieve pressure, and the second is to add material to the bottoms of the dumps (the toes) to bolster their foundations.

MR is employing both strategies to minimize future problems. Using a bulldozer, crews shaved the crest of what is called the “Bird Watch Dump” along the Pit’s south wall. And to shore up the underwater toe of another dump in the Pit’s southeast corner, crews pushed in material from the Bird Watch Dump, plus additional waste rock and dirt from the active Continental Pit.

The Berkeley Pit in 1982. The water seen here is surface runoff flowing into the Leonard mine shaft to the right at the Pit bottom.

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.

Record of Decision (ROD) for the Berkeley Pit

The Record of Decision (ROD) is a public document that explains which cleanup alternatives will be used to clean up a Superfund site. The ROD for sites listed on the NPL (NPL Site Listing Process) is created from information generated during the Remedial Investigation/Feasibilty Study (RI/FS). The ROD for the Butte Mine Flooding Operable Unit, which includes the Berkeley Pit, was published in 1994 and can be downloaded below.

1994 Record of Decision Download

A ROD contains site history, site description, site characteristics, community participation, enforcement activities, past and present activities, contaminated media, the contaminants present, scope and role of response action and the remedy selected for cleanup.

  • You can search for RODs in EPA’s Record of Decision System. It contains full-text Records of Decision (RODs), ROD Abstracts, ROD Amendments (AMDs) and Explanations of Significant Differences (ESDs).
  • Examples of well written RODs are available on the EPA ROD of the Year Contest Web page. The purpose of the ROD-of-the-Year Contest is to foster increased awareness of decision document quality.

Records of Decision for other Butte/Silver Bow Creek area Superfund Operable Units are available from the EPA Superfund Information Systems.

The Montana Bureau of Mines & Geology also offers a summary of the ROD on their website.

The site is also managed through a Consent Decree (2002) that resulted from a lawsuit filed by the State of Montana against the Potentially Responsible Parties (PRPs) for the site.