The main features of the Butte Mine Flooding Operable Unit and the Berkeley Pit. Map from the EPA Five Year Review Report for the site (2011). Click on the map to view a larger version.
Together, the Berkeley Pit and the historic underground mines of Butte, which have been flooded since groundwater pumps were turned off by ARCO in 1982, form the Butte Mine Flooding Operable Unit (BMFOU). EPA and the state of Montana are concerned about rising levels of contaminated mine water in the abandoned Berkeley Pit and connected underground mine workings that could eventually migrate into the shallow aquifer under urban Butte and into Silver Bow Creek. Berkeley Pit water is acidic and contains high concentrations of heavy metals. As of 2009, the Pit contained over 40 billion gallons of contaminated water.
The Berkeley Pit and connected underground mines have been filling with toxic water since 1982.
The investigation of the Berkeley Pit has been completed and the remedy calls for:
- Permanent control of surface inflow into the Pit
- Maintenance of the water level in the Berkeley Pit system via a pump-and-treat system
- Continued control of the West Camp/Travona System, which is connected to an aquifer separate from the Berkeley Pit
- Implementation of an extensive compliance monitoring program
- Implementation of institutional controls to restrict access to contaminated bedrock aquifer waters, as well as increased public education efforts
The complete Record of Decision (ROD) for the site can be downloaded at the link below, or via the Superfund Library.
The Potentially Responsible Parties (PRPs) started inflow control (average 3.8 million gallons per day) in April 1996. Controlling inflow has cut the rise rate in the Berkeley Pit by 50 percent. This inflow is treated with lime and used as makeup water in the present mining operation.
A treatment plant, known as the Horseshoe Bend Water Treatment Plant, was constructed in 2003 near the Pit. Currently, it is used to capture and treat inflow, but does not treat groundwater or water present in the toxic Berkeley lake. Prior to the water reaching the critical level where it could spread, the plant will be expanded to handle additional capacity, and then used to maintain the water level below the critical point. Currently, it is projected that water levels will approach the critical point around 2022 or 2023.
Groundwater movement and features in the Butte Mine Flooding Operable Unit and under the Butte Priority Soils Operable Unit. Map from the EPA Record of Decision (ROD) for the site (2006).
Monitoring wells for the alluvial (shallow) aquifer in the Butte Mine Flooding Operable Unit and around the Berkeley Pit. Map from the EPA Five Year Review Report for the site (2011).
Monitoring wells for the bedrock (deep) aquifer in the Butte Mine Flooding Operable Unit and around the Berkeley Pit. Map from the EPA Five Year Review Report for the site (2011).
Five Year Review (2010-2011)
The EPA Five Year Review Report for the site and associated appendices can be downloaded using the links below, or via the Superfund Library.
Vol3_ButteMineFloodingOU.pdf
3AppB_ResponsivenessSummary.pdf
Further Exploration
- Much more information about the Berkeley Pit can be found at the PitWatch website.
- A list of past cleanup activities at the site, as well as other area sites, can be found at the EPA website.
- Visit the Superfund Library or use this link to download the EPA Record of Decision for the BMFOU, completed in 1994.
