Geophysical Infrastructure Study: Brooklyn Mine Superfund Site

Brooklyn Mine Superfund Site, Bonita Peak Mining District, Silverton, Colorado

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The Bonita Peak Mining District (BPMD) was added to the National Priority List in September 2016 and includes 48 sites where mining-related contamination exists in the drainages of Mineral Creek, Cement Creek and Upper Animas River in San Juan County, Colorado. The regional mineralization and ore emplacement is associated with the San Juan and Silverton caldera complex in the western San Juan Mountains. A once active gold mining district for decades, the presence of these mines and associated waste rock and tailings piles contribute to high heavy metal loads and acidity in surface and groundwater in the region.

The USGS and U.S. Army Corps of Engineers (USACE) Sacramento District collaborated on a pilot study at Brooklyn Mine, situated on U.S. Forest Service (USFS) land, to demonstrate the utility of selected ground-based geophysical methods to aid in characterizing abandoned mine lands (AML). This information will be used to inform remediation design strategies by USFS and to help prioritize strategies for other AML sites.  Specific priorities for geophysical characterization at this site include: 

• (A) identification of shallow groundwater seepage from settling ponds and through waste rock piles, 
• (B) mapping of mine adits from the surface and determination of whether air- or water-filled, and 
• (C) determination of the volume of waste rock that has accumulated and been reworked over time outside of the adit entrances.

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The field campaign was completed in August 2018 with site characterization priorities being grouped into three study areas.

1. Settling Ponds. There are two settling ponds with a pH range of 3-5 surrounded by berms created from scrap material and waste rock.

   Area objective: Can we identify seepage areas and pathways and thickness of berm material?

   Methodology: 
   - 2D electrical resistivity tomography (ERT) profiles,
   - Frequency-domain electromagnetic induction (EMI) grid, and
   - Self-potential (SP).

2. Waste Rock Pile. The waste rock generated from all four levels of adits has been deposited on the steep slopes downslope from the adit entrances. Due to the steepness and unstable nature of the waste rock, geophysical data were not acquired across the middle of the pile. Profiles were acquired along areas more accessible by road.

   Area objective: Can we determine thickness and volume of waste rock for estimating acid mine drainage contribution and for cost estimate for removal?

   Methodology: 
   - 2D electrical resistivity tomography (ERT) profiles,
   - 2D P-wave seismic refraction/reflection profile,
   - Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic, and
   - Self-potential (SP)

3. Air Shaft and Glory Hole. Two collapse features that extend from Level 1 adit to surface. The air shaft was intentionally collapsed by the State of Colorado over safety concerns, and the glory hole is an unintentionally collapsed stope. The collapsed area of the glory hole was reworked into a waste rock repository in 2001. Waste rock from the level 2 adit pile was transferred into the repository lined with a clay liner and overlaid with rip rap, sand, and topsoil.

   Area objective: Can we map the level 1 adit or associated collapse features?

   Methodology: 
   - 2D electrical resistivity tomography (ERT) profiles,
   - 3D ERT grid,
   - 2D P-wave seismic refraction/reflection profile,
   - Frequency-domain electromagnetic induction (EMI) grid oriented northwest-southeast with nominal 5m line spacing,
   - Vertical-gradiometer magnetics over same grid as EMI, and
   - Microgravity.

This project was funded by the USACE Albuquerque District Restoration of Abandoned Mine Sites (RAMS) Program. Additional funding to study Bandora Mine, also located within the BPMD, is anticipated in FY2020.

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

Sources/Usage: Public Domain. View Media Details

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