The Diggings East area is a former unpermitted municipal landfill that contains a combination of municipal waste,
construction debris, tailings, and other waste. The proposed remedy for this area would remove tailings, mine wastes, and contaminated soils at Diggings East to the high groundwater elevation and would construct a lined stormwater basin in the excavated area to reduce the amount of contaminated stormwater and sediment that reach Silver Bow Creek and improve the quality of water in the creek. The proposed remedial activities would be performed by Atlantic Richfield Company and would include:
Excavation and Disposal – Approximately 345,000 cubic yards of tailings, wastes, and contaminated soils, including municipal wastes, would be excavated from the site and hauled to a repository. Some excavated materials, including uncontaminated soil and small concrete, may be reused to complete reconstruction of the site.
Stormwater Basin – A lined stormwater basin would be constructed at Diggings East to capture stormwater and sediments from upper Silver Bow Creek and improve water quality before it enters Silver Bow Creek. The stormwater basin will be designed to cover approximately 10 acres (about eight football fields), which is a sufficient size to manage stormwater and accumulated contaminated sediments. Maintenance of the basin would include removal and disposal of captured sediments at a repository.
End Land Use – The Diggings East area would be reconstructed in a manner that enables future community use of the site and land around the stormwater basin. Atlantic Richfield Company, Butte-Silver Bow, EPA, and MDEQ would work with the residents of Butte to develop an end land use plan that is compatible with the proposed remedy.
Tailings, waste, and contaminated soils, including municipal wastes, would be removed to the depth of the most recently observed 3-year maximum groundwater elevation.
Tailings, waste, and contaminated soils would be disposed of at a repository outside of the Silver Bow Creek and Blacktail Creek corridors.
Municipal waste, household waste, timbers, and other construction debris would be disposed of at an appropriately permitted facility.
The stormwater detention/retention basin would be sized to capture a 10 year, 24-hour Type I Soil Conservation Service (SCS) design storm. This would allow for capture and treatment of a minimum of 32 acre-feet (more than 10 million gallons) of stormwater.
Additional storage volume would be provided to support management of accumulated sediments.