EPA offers the following definition of Operable Unit:

During cleanup, a site can be divided into a number of distinct areas depending on the complexity of the problems associated with the site. These areas called operable units may address geographic areas of a site, specific site problems, or areas where a specific action is required. An example of a typical operable unit could include removal of drums and tanks from the surface of a site.

In the case of western Montana and environmental impacts related to mining and smelting in Butte and Anaconda, the area has been geographically divided into a number of larger sites (Butte, Anaconda, and the Clark Fork River) and smaller Operable Units (or OUs) to organize cleanup tasks, as illustrated in the following map from EPA.

Map of western Montana/Clark Fork Basin Superfund environmental cleanup sites, including the Clark Fork River Operable Unit stretching from roughly the town of Deer Lodge, Montana downstream to Missoula, Montana; the Anaconda Smelter Superfund site, and the Silver Bow Creek/Butte Area Superfund site. All sites, except for the Clark Fork River, are part of the National Priority List (NPL).

Additionally, the Anaconda Smelter and Silver Bow Creek/Butte sites have each been further subdivided into different Operable Units. These OUs sometimes overlap geographically, and are separated based on the types of contaminants present or other factors.

The Silver Bow Creek/Butte Area Superfund site contains these Operable Units:

• Streamside Tailings (Silver Bow Creek) (OU 01): Includes most of Silver Bow Creek from Butte west and north to the Warm Springs Ponds.
• Area 1 (OU 02).
• The Berkeley Pit and Butte Mine Flooding Operable Unit (BMFOU) (OU 03): Includes the historic underground mines, which have been flooded with rising groundwater since the early 1980s, and the associated Berkeley Pit workings.
• The Warm Springs Ponds (OU 04): Includes the historic Warm Springs tailings ponds on Silver Bow Creek, just above the headwaters of the Clark Fork River, that were associated with the Anaconda Smelter.
• The Reduction Works Tailings (Silver Bow Creek) (OU 05): Includes segments of Silver Bow Creek and its floodplain near historic tailings deposits from the Anaconda Smelter.
• The Travona Mine (Butte) (OU 06): Includes historic west camp mines in the Butte urban area and associated groundwater, which is distinct from the groundwater system of the Berkeley Pit and Butte Mine Flooding Operable Unit.
• Rocker (Silver Bow Creek) (OU 07): Includes stretches of Silver Bow Creek west of Butte near the town of Rocker.
• Soils (Butte) (OU 08): Also known as the Butte Priority Soils Operable Unit (BPSOU), this includes most affected areas of historic uptown Butte.
• Butte Residential Soils (OU 10): Includes residential areas in historic uptown Butte that were affected by tailings or other mine waste.
• Lower Area One (LAO) (OU 11): Includes the area of Silver Bow Creek on the western edge of urban Butte, near the city’s sewage treatment plant.
• Warm Springs Ponds/Inactive Area (OU 12): Includes additional segments of the former Warm Springs Ponds tailings treatment complex associated with the Anaconda Smelter.
• West Side Soils (formerly Non-Priority Soils) (OU 13): Includes areas immediately west and north of urban uptown Butte that were primarily impacted by surface mining and associated activities.

For additional details about the Silver Bow Creek/Butte Area site and its Operable Units, visit the EPA Superfund Information System site, or visit the EPA Silver Bow Creek/Butte Area site.

The Anaconda Smelter Superfund site contains these Operable Units:

• Water, Waste and Soils (OU 04)
• Old Works/East Anaconda (OU 07)
• Beryllium Removal (OU 09)
• Flue Dust (OU 11)
• Arbiter Removal (OU 12)
• Smelter Hill (OU 14)
• Mill Creek (OU 15)
• Community Soils (OU 16)

For additional details about the Anaconda Smelter site and its Operable Units, visit the EPA Superfund Information System site, or visit the EPA Anaconda Area site. Additional information can also be found from the Arrowhead Foundation, the EPA-funded TAG organization for Anaconda.

The Clark Fork River/Milltown Superfund site contains these Operable Units:

• Milltown Reservoir Sediments
• Milltown Water Supply
• Clark Fork River

For additional details about the Clark Fork River site and its Operable Units, visit the EPA Superfund Information System site, or visit the EPA Clark Fork River Operable Unit page, or the EPA Milltown page. Additional information can also be found from the Clark Fork River Technical Assistance Committee (CFRTAC), the EPA-funded TAG organization for the Clark Fork.

Note that EPA considers all of these sites and Operable Units to be part of the Superfund cleanup National Priority List (NPL), with the exception of the mainstem Clark Fork River.