CERCLA Time-Critical Removal Action

The NPL Site is within the Neihart Mining District, approximately 50 miles southeast of Great Falls, Montana.  The mine district was a relatively steady producer of silver, lead and zinc from its discovery in 1881 to the 1940s.  Historic mining and milling operations generated substantial volumes of waste rock and mill tailings that remain within the Site and continue to release hazardous substance into the environment. In 1925, the Silver Dyke tailings dam failed due to an earthquake resulting in tailings being deposited along the length of Carpenter Creek. Erosion and subsequent re-deposition of the tailings along Carpenter Creek and erosion from the upper and lower tailings impoundments moved waste into the floodplains of Carpenter and Belt creeks at least as far as Monarch, MT, approximately 14 miles downstream.

 
The Site has been divided into three operable units (OU). This removal action included activities in OU 3, which contains the Silver Dyke mining complex including upper mine sites and tailings in Carpenter Creek to the confluence with Belt Creek. The three main areas focused on in the Removal Action were the upper and lower Carpenter Creek tailings, and the Silver Dyke tailings pile

The Mackay Gulch Repository site was investigated as part of the Remedial Investigation and identified as one of two locations to store mine waste from the Site.  The Mackay Gulch repository is proposed to have a capacity of approximately 600,000 cubic yards.  As part of the Removal Action a small portion of the total area has been identified to receive waste from the Silver Dyke Tailings impoundment area.

1.1.2.1 Location

The Site is located on the northern flank of the Little Belt Mountains at elevations from approximately 5,100 feet Above Mean Sea Level (AMSL) along Belt Creek to 8,621 feet AMSL on Long Mountain. It is located in the southeastern end of Cascade County, Montana and primarily along Carpenter and Snow Creek and then along Belt Creek from the Town of Neihart extending downstream to Monarch, MT.

Aquatic life surveys conducted in Carpenter Creek below the confluence with Sih-mem Creek, indicate populations of benthic invertebrates are severely impaired, and fish populations are absent.  Investigations conducted by Montana Fish Wildlife and Parks and the EPA in 2010 and 2011 (FWP, 2011, 2012, and TechLaw, 2011, 2012) clearly indicate that metals contamination associated with ongoing contaminant releases impairs water quality and severely inhibits aquatic life in Carpenter Creek, and suggests that they are contributing to the impairment of aquatic life in Belt Creek below the confluence of Carpenter Creek.

Water quality in Carpenter Creek is poor throughout the drainage due to degrading influences from the Carpenter Snow Creek Site. Creek waters are subject to active tailings erosion as well as metals contaminants from mine adits. There are general water quality standards exceedences for metals including arsenic, cadmium, copper, lead and zinc, that can be attributed in part to the erosion. Much higher exceedences of metals have been measured during storm events which are directly a result of erosion of the tailings. These metals inhibit aquatic life in the drainage.

The Site investigations by the State and MT Operations office determined that approximately 35,000 cubic yards of tailings remain on the slopes of the No Name Creek valley.  This was the location of the former tailings impoundment that failed and dispersed tailings throughout the Carpenter Creek floodplain.  Analyses of samples collected at the Site indicate the presence of high concentrations of heavy metals including zinc, cadmium and lead in waste and sediment. Routine run-off and high flows during spring snow melt continue to cause migration of the tailings materials from the Carpenter Creek and Silver Dyke tailings impoundment into the environment

The objective of this removal action is to prevent continued releases from the estimated 35,000 cubic yards of tailings at the Silver Dyke tailings impoundment within No Name Creek drainage area. The action includes the following elements: (1) Removing the tailings from the hillside slopes and staging for disposal; (2) constructing an onsite repository, pending the repository decision; (3) placing the tailings in an onsite repository; (4) reclaiming/restoring removal area slopes. Designs have been developed for the repository construction and waste placement, and preliminary plans are developed for the site restoration and revegetation.

The Proposed Plan public comment comment period ended on August 30, 2014 and the the Mackay Gulch Repository site was approved in the Action Memorandum Amendment approved and signed on September 2, 2014.

 2.1.3 Progress Metrics

• Receive one additional haul truck to facilitate a 6-truck hauling operation.
• Conduct compaction testing at the repository to ensure proper design strength and stability.
• Procure revegetation and restoration services at the site to focus ERRS efforts on tailings and repository.
• Continue consolidation of the tailings near the access road at the tailings area.
• Develop access roads for the excavator along the west slope of the tailings area to facilitate removal.
• Continue hauling tailings from the tailings area to the repository.