Superfund Information Systems: Site Profile

Superfund Site:

CARPENTER SNOW CREEK MINING DISTRICT
NEIHART, MT

Cleanup Activities

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Background

The 9,000-acre Carpenter Snow Creek Mining District site is located in the Little Belt Mountains of southern Cascade County, Montana. The site includes mine tailings, waste rock and mine-influenced waters due to many inactive and abandoned mines. EPA has identified about 96 abandoned mines at the site; at least 21 of these are probable sources of contamination. Historic mining operations contaminated soil, groundwater, surface water and sediment with metals and other hazardous chemicals. Investigation and cleanup activities are ongoing.

Mining began in the area in the 1880s when silver deposits were discovered near the future Neihart town site. Mines yielded primarily silver, lead and zinc ores. During the 1920s, lead and zinc were produced in large quantities. The mining district has been largely inactive since the 1940s, although some mines have reported mine development work and some sporadic production.

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What Has Been Done to Clean Up the Site?

The site is being addressed through federal and state actions. EPA is the lead agency under the Superfund program for site cleanup and works in cooperation with Montana Department of Environmental Quality (DEQ), the U.S. Forest Service (USFS) and other state and federal agencies.

Cleanup at this site uses two approaches. Removal actions are used to expedite cleanup of the most immediate threats to human health and the environment. A longer term remedial program is being implemented to assist in determining the nature and extent of contamination and selecting cleanup alternatives.

Several adits at the site have the space and are easily accessible throughout the year to allow for passive treatment of mine influenced waters using a sulfate reducing biochemical reactor or a passive hydrated lime system and several pilot tests have been completed.

In late 2016/early 2017, EPA, DEQ and USFS pilot tested a hydrated lime system at the Silver Dyke Mine and will conduct a bench scale test of the Haystack Creek Mine adit discharge during 2017/2018. Data collected from these bench scale and pilot tests will support the development of upcoming feasibility studies that will assist with choosing appropriate response actions for addressing contaminated groundwater and surface water.

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What Is the Current Site Status?

The site consists of three areas, referred to by EPA as operable units (OUs).

  1. OU1 is the Neihart Community Soils Area.
    • As an institutional control, the Property Owner’s Soils Management (POSM) plan was implemented to support the proper handling and disposal of contaminated and potentially contaminated soils from residential and town of Neihart properties during construction projects until the Remedial Action begins. EPA, under Cooperative Agreement with Cascade County, is working to establish the Institutional Controls plan for all elements of OU1.
    • The long-term remedy, selected in 2009, included excavation and replacement of contaminated residential soils and roadway material, and the excavation of the Belt Creek Tailings pile, followed by transport of the waste materials to a secure repository for disposal.
    • The remedy also included institutional controls. Repository location selection is in the process of being finalized, which will allow for the transition of the Remedial design from draft to final. Remedial action is anticipated to commence pending availability of federal funding.
  2. OU2 includes abandoned mines, mills and associated wastes along Snow Creek and the Neihart Slope as well as all streamside tailing along Carpenter Creek below Snow Creek and Belt Creek down to the town of Monarch.
    • These areas contain over 500,000 cubic yards of mine-related waste.
    • The remedial investigation (RI) for OU2 is ongoing.
      • EPA plans to issue an RI report for the mines and watershed portions of the site in 2018.
      • The RI report will provide information concerning metals concentration in tailings, soil, sediment, surface water and groundwater throughout the mining district, for the purposes of characterizing and identifying the extent of material that may require a response action.
      • The RI report will also identify risks that the mine waste pose to human health and the environment.
  3. OU3 addresses waste associated with the Silver Dyke Mining Complex located in Carpenter Creek and any wastes in the Carpenter Creek below the Silver Dyke tailings impoundment down to the confluence with Snow Creek.
    • The Silver Dyke Mining Complex was the largest mine in the district and includes the Silver Dyke Glory Hole, associated underground workings, mill facilities, tailings piles, and eroded tailings in the floodplain areas of Carpenter Creek.
      • The ore in this complex was mined between 1921 and 1929 and is characterized by a wide body of low grade ore containing zinc and lead and a high proportion of copper that also resulted in a large quantity of tailings and refuse along Carpenter Creek.
      • In 1925, a tailings dam next to the Silver Dyke Mill was damaged by an earthquake and resulted in a flood of tailings into the valley below. In 1926, two impoundments were built in Carpenter Creek (upper and lower) for collection of mine tailings. These tailings, now known as the Carpenter Creek tailings, were placed into the impoundments by slurry from the upstream Silver Dyke Mill and are thought to have originally spanned the entire valley.
      • It is estimated that over 500,000 cubic yards of mine-related waste is present within the Carpenter Creek floodplain between the Silver Dyke tailings impoundment and the Snow Creek confluence.
    • The investigation of OU3 is ongoing with a proposed plan for cleanup expected to be issued in late 2018/early 2019.

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Activity and Use Limitations

At this site, activity and use limitations that EPA calls institutional controls are in place. Institutional controls play an important role in site remedies because they reduce exposure to contamination by limiting land or resource use. They also guide human behavior. For instance, zoning restrictions prevent land uses – such as residential uses – that are not consistent with the level of cleanup.

For more background, see Institutional Controls.

EPA will develop options for specific Institutional Controls (ICs) for this site, as needed, in the Feasibility Study for OUs 2 & 3. ICs will be documented in the Record of Decision as a component of the remedy.

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Sampling and Monitoring

EPA conducts semi-annual sampling of surface water, groundwater, mine adit discharges and sediment in June and September. These sampling events have been ongoing since 2009 and have allowed EPA to collect baseline data prior to future remedial actions that will address the mine waste.

Continuous monitors have been in place at several of the worst discharging adits since 2012 to determine seasonal variations in flows and metal concentrations.

Sampling conducted since 2009 in the Carpenter Creek, Snow Creek and Neihart slope drainages north of Neihart revealed elevated levels of lead, cadmium, copper and zinc in the surface water.

Elevated levels of lead and zinc were also found in sediments and fluvial (adjacent stream) soils along Carpenter Creek and Belt Creek adjacent and below the various waste rock and tailings piles present in these watersheds.

Sampling in 2012 and 2013 indicates that contaminated soils that were deposited in the floodplain during historic Belt Creek flood events, such as the 1953 and 1981 floods, extend all the way to Monarch.

Several adits that are discharging from abandoned mines continue to contribute to the degradation of the water quality throughout the mining district. These adits have been monitored continuously since 2011.

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Emergency Response and Removal

In September 2017, EPA initiated response activities in coordination with Malmstrom Airforce Base Explosive Ordinance Disposal (EOD) team and Monarch and Neihart Volunteer Fire Department on the Big Seven Mine Complex located in OU2. Additional information about the response actions is available on the EPA On-Scene Coordinator Site Profile page.

A removal action was initiated at OU3 in September 2014 to prevent continued releases from the estimated 35,000 cubic yards of tailings at the Silver Dyke tailings impoundment within the No Name Creek drainage area. The action included the following elements: (1) removing the tailings from the hillside slopes and staging for disposal; (2) constructing an onsite repository, pending the repository location decision; (3) placing the tailings in an onsite repository; (4) reclaiming/restoring removal area slopes. More information about the removal action is available on the EPA On-Scene Coordinator Site Profile page.

In 2013, the U.S. Forest Service (USFS) completed a removal action at OU3. It included interim response actions at the Silver Dyke tailings and upper and lower Carpenter Creek tailings impoundments to stabilize these tailings until a permanent remedial action is taken. Activities at various locations within the Carpenter Creek and Silver Dyke tailings impoundments included construction of lined surface run-on and runoff ditches on the upper and lower Carpenter Creek tailings to reduce storm flows and snowmelt from eroding tailings, installation of diversion channels to route clean water around all three tailings impoundments, installation of erosion check dams at all three tailings impoundments in areas where deep rills have formed in past erosion events, installation and maintenance of a certified weed-free straw bale erosion berm at the base of the Silver Dyke tailings, application of wood straw mulch over tailings piles as an erosion control measure, placing of riprap to stabilize the access road to Silver Dyke tailings, and engineering and informational controls, including fencing and signage on the tailings piles in Carpenter Creek, to eliminate trespass with recreational vehicles and to educate the public on potential dangers.

In 2004, EPA conducted a short-term cleanup, or removal action and excavated about 5,300 cubic yards of contaminated soil and moved it to the Belt Creek Tailings pile for temporary storage. Upon completion, EPA capped the surface of the Belt Creek Tailings with cover soil, re-engineered it for greater stability and better drainage, and revegetated the area.

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