In 1994, EPA and the Montana Department of Environmental Quality (DEQ) issued the Record of Decision (ROD) for the Butte Mine Flooding Operable Unit, which includes the Berkeley Pit. The ROD, according to the EPA, is a technical and legal document that:
- summarizes the science behind the cleanup decision;
- demonstrates how the remedy selection process was carried out; and
- provides a thorough site history and includes public concerns, comments, and EPA responses.
The complete ROD document can be downloaded here:
Further details of Berkeley Pit management were set in a 2002 Consent Decree (CD) between the U.S., the State of Montana, and the companies liable for the site. The complete CD document can be downloaded here:
Butte Mine Flooding Consent Decree (2002) (575.6 KiB)
In 1996, as directed in the ROD, water flows from Horseshoe Bend, a drainage to the north of the Berkeley Pit, were diverted away from the Berkeley and instead pumped up to the Yankee Doodle Tailings Pond that sits along the west slope of Rampart Mountain. At this facility, the ground-up rock (also known as tailings) that is generated as a byproduct of the milling process is deposited in order to settle out the solids and recycle decanted water back into the mill system. The Horseshoe Bend diversion reduced the rate at which the Berkeley was filling with water, keeping up to 3,000 gallons per minute of surface water from flowing into the Berkeley.
In 1998, Montana Resources began pumping water out of the Berkeley Pit for copper recovery, a technique known as precipitation used at the active mine until a 2013 slough of material from the Pit wall into the water knocked out the needed pump. In precipitation, the copper-rich water is pumped over scrap iron, and, in a replacement reaction, the copper solidifies as sludge, while iron takes its place in the water, which was returned to the Pit by gravity flow, thus not increasing or decreasing the total volume of Pit water.
The diversion of water from Horseshoe Bend was discontinued on June 30, 2000, when increased costs led Montana Resources’ Continental Pit mine to shut down. As a result of this shutdown, conditions contained in the 1994 ROD required Montana Resources and ARCO to begin design and construction of a water treatment plant. But, with copper prices rising, the mine again reopened in 2003, and it has continued to operate since that time.