THE DIRT: CIA sludge pond capped
The Central Impoundment Area (CIA) has been a notable feature in the Silver Valley for nearly a century. Constructed in 1928, the CIA was designed to contain vast amounts of mine tailings and waste generated from decades of milling and smelting operations at the Bunker Hill Mining and Metallurgical Complex. Before its creation, tailings were routinely discharged directly into the South Fork of the Coeur d’Alene River, contributing to widespread contamination throughout the Basin. Over time, the CIA grew into one of the largest repositories in the region, ultimately holding an estimated 24 million cubic yards of mine waste.
The Central Treatment Plant (CTP) treats groundwater and acid mine water to remove the metals before the water enters the South Fork of the Coeur d’Alene River. The process of removing the metals creates a waste product called sludge that needs to be properly stored. The sludge pond was developed by the mine for this purpose.
In June of 2023, after 30 years of service, the historic sludge pond reached its capacity and could no longer accept waste. Placement of sludge from the CTP was shifted to the new Sludge Impoundment Area (SIA). The sludge pond was the only area on top of the CIA where a cap was not installed, due to active sludge placement from the CTP. The new SIA was installed in conjunction with upgrades to the CTP. The SIA was completed in Fall 2021 and has a design capacity of 30 years.
In the spring of 2024, the design for the cap of the sludge pond was completed. The design integrated the use of contaminated materials and material from the former sludge pond access road to fill the historic sludge pond and create a final grade that would promote drainage off the pond area once capping was complete. A large amount of effort was put into planning and communication for this project, as a portion of the former sludge pond access road will continue to be used for the ongoing operation of the CTP. In addition, access to this area was required and had to be maintained. With planning complete, the construction work began in May of 2025. The first phase of the project utilized materials onsite to build the grade of the designed cap. With the final capping completed, a liner system was installed and tied into the existing CIA cap. Clean fill material was brought in to cover the newly capped area prior to hydroseeding. Work has finished at the site, but inspections will continue into 2026 to ensure that seeding in the area is successful.
The Dirt is a series of informative articles focused on all aspects of cleanup efforts associated with the Bunker Hill Superfund Site. Our goal is to promote community awareness of contamination issues, to provide tools for protecting public health, and to keep the community informed of current and future cleanup projects. The Dirt is a group of committed and local experts from multiple agencies, including the Basin Environmental Improvement Project Commission, Panhandle Health District, Shoshone County, Silver Valley Economic Development Corporation, and the Idaho Department of Environmental Quality.
