An operable unit is a subsection of a larger EPA Federal Superfund site. There are four Operable Units (OUs) in the Butte mining district.
- The Butte Mine Flooding Operable Unit, which includes the Berkeley Pit, the hydraulically-connected underground mine workings associated with the historic East Camp and West Camp tunnel systems, associated bedrock, and alluvial aquifers. The area covers approximately 23 square miles.
- Butte Priority Soils is a five square mile area that includes the town of Walkerville, along with the part of the Butte Hill that is north of Silver Bow Creek, west of the Berkeley Pit, and east of Big Butte. It also includes a section of land extending south from Silver Bow Creek to Timber Butte. This Operable Unit includes residential yards, mine dumps, contaminated railroad beds, and stormwater drainages on the Butte Hill and in Walkerville.
- Silver Bow Creek/Streamside Tailings Operable Unit, which follows Silver Bow Creek from just below the historic Colorado Tailings deposit in Butte 25 miles downstream to the Warm Springs Ponds. The area includes Silver Bow Creek itself, as well as the adjacent mining wastes deposited along the creek bank and nearby floodplain, and railroad embankments adjacent to the stream that contain mining wastes.
- West Side Soils Operable Unit includes lands that fall outside of the Butte Priority Soils OU. There are approximately 1,500 mine waste dumps located north and west of Butte. The dumps have not been sampled. A schedule for investigating this OU and selecting a remedy has not yet been set.
There are numerous additional operable units in the greater Clark Fork River Superfund site, which stretches from Butte 120 miles downstream to the Milltown Dam near Missoula.
For more information on non-Berkeley Pit Superfund sites in Butte and the Clark Fork Basin on western Montana, visit the Citizens Technical Environmental Committee (Butte CTEC) website, or the Clark Fork Watershed Education Program (CFWEP) website.