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The Bonita Peak Mining District site consists of 48 historic mines or mining-related sources where ongoing releases of metal-laden water and sediments are occurring within the Mineral Creek, Cement Creek and Upper Animas River drainages in San Juan County, Colorado. Historic mining operations have contaminated soil, groundwater and surface water with heavy metals. A remedial investigation to determine the nature and extent of contamination is underway.

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What Has Been Done to Clean Up the Site?


EPA and the Colorado Department of Public Health and Environment (CDPHE) conducted a Superfund Site Assessment of the area in the 1990s. This assessment identified the severe impacts to aquatic life in the Upper Animas River and its tributaries from naturally occurring and mining-related heavy metals. It also acknowledged the community-based collaborative effort that was under way at that time to address those impacts. In recognition of the community-based collaborative effort, EPA agreed to postpone listing of a site to the Superfund National Priorities List (NPL), as long as progress was being made to improve the water quality of the Animas River.


Until approximately 2005, water quality in the Animas River was improving. However, since 2005, water quality in the Animas River has not improved, and for at least 20 miles below the confluence with Cement Creek, has declined significantly.

In 2008, because of this declining water quality in the Animas River, EPA’s Superfund Site Assessment program began investigations in upper Cement Creek focused on evaluating whether the upper Cement Creek area alone would qualify for inclusion on the NPL. This evaluation indicated that the area would qualify, although after receiving additional community input, EPA again postponed efforts to include the area on the NPL and continued to participate in the community-based collaborative efforts.

In support of the collaborative effort, EPA’s Superfund Remedial program contributed resources for water quality sampling, ecological risk assessment and data analysis. In addition, the Superfund Removal program contributed resources for the investigation and closure (bulkheading) of the Red & Bonita Mine tunnel.

EPA, through its Ecosystem Protection program, also provides the Colorado Water Quality Control Division of CDPHE with Nonpoint Source Management program (Section 319) grant funds. The Animas River Stakeholders Group (ARSG) and others have received grants under that program, as well as contributing other resources, for investigation and cleanup efforts in the Upper Animas watershed.

Sunnyside Gold Corporation (SGC), the U.S. Bureau of Land Management and the State of Colorado also have conducted reclamation activities in the Upper Animas watershed. These efforts have included diverting runoff away from and capping mine waste piles, moving mine waste piles away from drainages, consolidating mine waste piles, placing bulkheads in draining adits and re-vegetating mine waste piles.


Under the Superfund Removal program, an engineered concrete bulkhead was installed in the adit of the Red & Bonita Mine during summer 2015.

On August 5, 2015, EPA’s Superfund Removal program was conducting an investigation and assessment of the Gold King Mine to:

  • assess the on-going water releases from the mine,
  • treat mine water, and
  • assess the feasibility of further mine remediation.

While investigating the adit, pressurized water began leaking above the mine tunnel, spilling about three million gallons of water stored behind the collapsed material into Cement Creek. Since October 2015, EPA has been treating the discharge from the Gold King Mine at the Interim Water Treatment Plant (IWTP) at Gladstone, Colorado.

This event prompted renewed interest in addressing the long-term, mining-related impacts to water quality in the Animas River. On February 29, 2016, EPA received a letter from Colorado Governor John Hickenlooper expressing support for the addition of the BPMD to the NPL. In his letter, he noted that the Town of Silverton, San Juan County, City of Durango, La Plata County, local tribes and other interested stakeholders requested that the site be added to the NPL.

EPA proposed the BPMD for addition to the NPL on April 7, 2016. A 68-day public comment period, during which EPA accepted comments from the public on the NPL proposal, closed on June 13, 2016. After carefully considering and responding to all comments in a responsiveness summary, EPA officially added the BPMD to the NPL on September 9, 2016.

In October 2016, the U.S. Forest Service and EPA completed an early action at the Brooklyn Mine in the Mineral Creek Drainage. An existing pipeline that drains mining impacted water from the mine adit to a passive treatment trench was cleaned and repaired. Due to a blockage in the pipe, 15 gallons per minute of drainage had been flowing across the mine tailings and bypassing the passive treatment system. By cleaning out and repairing the pipeline, the flow of drainage across the tailings pile was reduced significantly. Additional work inside the adit is planned for a later date.


The site team, including EPA, U.S. Forest Service, Bureau of Land Management and Colorado Department of Public Health and Environment conducted a Remedial Investigation/Feasibility Study (RI/FS) at the site. This includes a Human Health Risk Assessment, Ecological Risk Assessment and a hydrologic study of the Bonita Peak groundwater system. These investigations provide information that will be used in determining cleanup options for the site.

  • Remedial Investigation
    • The portal of the Natalie/Occidental Mine was cleared and a flume was installed.
    • Monitoring wells were installed at the Howardsville tailings to better understand groundwater movement and contaminant loading in that area.
    • Sunnyside Gold Corporation conducted investigations of the Mayflower Mill Tailing under an administrative order on consent with EPA.
    • Colorado Department of Public Health and Environment inspected all accessible bulkheads at the site to assess stability.
    • U.S. Forest Service collected LiDAR data from the site.
    • Stream gauges were installed throughout the site to help the site team better understand the hydrology of the BPMD.
  • Ecological Risk Assessment
    • High- and low-flow sampling of surface water and sediments was conducted.
    • XRF (X-ray fluorescence) screening was done on select waste rock piles.
  • Human Health Risk Assessment
    • Activity-based air monitoring was conducted.
    • Deer and grouse were harvested for tissue samples.
    • Plant tissue sampling was conducted.
  • Bonita Peak groundwater system
    • Seeps and springs sampling is being conducted to help define the hydrology of the Bonita Peak groundwater system.
    • Precipitation data was collected.
    • Groundwater sampling will take place in November.

The Interim Water Treatment Plant (IWTP) at Gladstone continued to treat ongoing acid mine drainage being discharged from the GKM. The GKM was stabilized, and additional actions were taken to reduce the potential of an unplanned discharge from the mine, including installation of a flow control structure, drilling a horizontal well into the historic level 7 adit and construction of an earthen berm.

The valve in the bulkhead of the Red & Bonita mine has not yet been closed. EPA continued collecting data and evaluating plans to temporarily close the valve in the engineered concrete bulkhead in the Red & Bonita Mine to determine any impacts to the Bonita Peak groundwater system from incrementally increasing the water level behind the bulkhead. In preparation for a temporary closure of the Red & Bonita bulkhead valve, the following actions were taken:

  • A new valve on the Red & Bonita pipeline was installed to direct adit flow away from the waste rock pile and reduce contamination loading from the mine to Cement Creek.
  • A pressure gauge was installed at the Mogul Mine.
  • A flume has been installed at the Black Hawk Mine and the adit has been stabilized.
  • The American Tunnel was cleaned out, the adit stabilized and the bulkhead inspected. Due to safety concerns related to weather late in the 2017 field season, the drilling of the planned monitoring well into the American Tunnel was postponed.

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What Is the Current Site Status?

For information about ongoing investigative/cleanup activities and public participation opportunities, view our site calendar.

The Interim Water Treatment Plant (IWTP), installed at Gladstone as part of the emergency removal action at the Gold King Mine (GKM) in October 2015, is treating ongoing acid mine drainage being discharged from the GKM. All the sludge storage capacity at that location has now been utilized, and in cooperation with the property owner, EPA established a new interim sludge management location at the Kittimac Tailings source area, approximately 6.5 miles northeast of Silverton, along County Road 2. EPA is mixing the non-hazardous sludge with mine tailings located at Kittimac. This reduces the high water content of the sludge and allows it to be more efficiently managed. A secondary benefit is to immobilize heavy metals found in the tailings. Sludge will be transferred to the interim sludge management location by truck using the County Road 110 bypass. EPA plans to complete the transfer of sludge by the end of August 2018.

The site team is continuing a Remedial Investigation/Feasibility Study (RI/FS) at the site. The Remedial Investigation includes a Human Health Risk Assessment and an Ecological Risk Assessment. In addition, the site team is investigating the Bonita Peak groundwater system to better understand the hydrogeology of the site.

On June 14, 2018, EPA released for public comment a Proposed Plan for Interim Response Actions to address environmental cleanup at 26 mining-impacted locations at the site. EPA is considering public comments and will respond to them in the Interim Record of Decision.

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Sampling and Monitoring

Water sampling has shown that there was a marked increase in the levels of several heavy metals in the Animas River below the confluence with Cement Creek after active and passive treatment ceased in the Cement Creek drainage late in 2004.

Modeling conducted by U.S. Geological Survey (USGS) using the One-Dimensional Transport with Equilibrium Chemistry (OTEQ) model identified the need for additional information about pollutant loads from sources other than Cement Creek to the Upper Animas River.

Sampling activities conducted by EPA in 2015, 2016 and 2017 are reported in the yearly Sampling Activities Reports.

Detailed interpretation of the data presented in the SARs will be completed as part of the planned remedial investigation (RI) at the Bonita Peak Mining District Superfund site.

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Emergency Response and Removal

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Enforcement Information

Sunnyside Gold Corporation is conducting investigations of the Mayflower Mill Tailing under an administrative order on consent with EPA. EPA is also working with other private property owners who are interested in conducting investigative and cleanup activities on their properties.

On March 15, 2018, EPA issued a unilateral administrative order to Sunnyside Gold Corporation to conduct groundwater investigation activities at the BPMD. Sunnyside Gold Corporation is a current owner and past operator of the Sunnyside Mine in the BPMD.

EPA issued the order to Sunnyside Gold Corporation to conduct a remedial investigation of the Bonita Peak Groundwater System, designated as Operable Unit 3, within the larger BPMD. EPA is ordering the company to complete this work so the agency can identify surface water impacts from the groundwater system, assess the condition of existing bulkheads associated with the groundwater system, determine the hydrological interconnection of the various underground mine workings, and evaluate potential cleanup options at this portion of the site.

View the Unilateral Administrative Order (PDF) (44 pp, 1.2 MB, About PDF)

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