South Ponds Remediation Project
Columbia Falls Aluminum Company (CFAC), is located two miles northeast of Columbia Falls in Flathead County, Montana. An electrolytic aluminum reduction facility, originally constructed by Anaconda Aluminum Company, was previously located at the site. The industrial portion of the property covers approximately 960 acres adjacent to the Flathead River and was in operation between 1955–2009. In 2016, the CFAC site was listed on the National Priorities List under the Environmental Protection Agency’s (EPA) Superfund program.
Stormwater and wastewater from a portion of the plant was discharged into three cascading infiltration ponds constructed adjacent to the Flathead River. These ponds, referred to as the South Ponds, were originally constructed with an earthen dam at the east end of an old seasonal side channel to block active river flow. The water discharged to the South Ponds and included suspended solids in the wastewater that deposited a sediment layer in the bottom.
Two of the initial tasks in the EPA Superfund process are Remedial Investigation (RI) and Feasibility Study (FS). Through the RI/FS, materials were identified in the upper pond sediment layer that were found to be above ecological risk thresholds.
When the South Ponds were initially constructed, the main channel of the Flathead River was along the south riverbank. Over the years, natural river migration and high flows had required occasional maintenance of the earthen dam retaining the South Ponds. However, the pace of natural river migration had increased over the past several years to the point that the main channel of the river had shifted from the south bank to the north bank. This natural river migration resulted in constant hydraulic attack and was eroding the earthen dam and the island area that separated the river’s main channel from the South Ponds. The accelerating river migration increased the risk of erosion and transport of the sediment layer in the South Ponds into the Flathead River system.
In 2016 and 2017, Morrison-Maierle assisted CFAC with two projects to stabilize the earthen dam and mitigate river erosion and migration into the South Ponds. These projects fixed the natural migration of the Flathead River but didn’t eliminate the possibility that the river migration – and the erosion that comes with it – could extend beyond the stabilization and transport sediments from the South Ponds.
In early 2020, CFAC initiated an Early Action within the EPA Superfund process to address the need to remove impacted sediments from the Flathead River’s migration zone and return the river to its natural condition. CFAC worked with Morrison-Maierle and environmental consultant, ROUX Inc., to develop a plan to remove the impacted sediments from the South Ponds. The team engaged local, state, and federal stakeholders for their input and agreement. The plan was developed on an accelerated schedule with the goal of removing the impacted South Ponds’ sediment layer prior to the spring runoff in 2021.
The project was designed and constructed in two phases. The first phase included:
- Removing the impacted South Ponds sediments to a suitable landfill.
- Removing earthen berms that separated the South Ponds.
- Decommissioning and removing CFAC facilities in the South Ponds island area.
The second phase included:
- Removing the stabilization features previously installed to mitigate erosion risk to the South Ponds.
- Removing the access road embankment across the historic northern side channel.
- Revegetating disturbed ground and access roads in the island area above the anticipated high-water mark of the Flathead River.
In addition to mitigating the risk of erosion and transporting impacted sediments from the South Ponds, CFAC also removed all man-made features in the South Ponds island area, which returned the site to its natural condition and reduced the need for future operation and maintenance tasks in the Flathead River floodplain.
The project was constructed between October 2020 and April 2021 on an accelerated pace during the river’s seasonal low flow. Completion prior to spring runoff also ensured that impacted sediments would not be incorporated into the Flathead River system.