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Bonanza Mining District Restoration Project

Site summary

Location

  • Approximately 30 to 35 square miles in the extreme northeastern portions of the San Juan Mountains of south-central Colorado in Saguache County.
  • Situated approximately 22 miles south of Poncha Springs on U.S. 285 and 14 miles west on Saguache County Road LL56.
  • The town of Bonanza (population 16) is located within the district.
  • Kerber Creek, which has been impacted by mining/milling related activities, is the main drainage within the district.
  • The quality of water in Squirrel Creek and Rawley Gulch, tributaries to Kerber Creek, are also impacted where these streams flow through the district.
  • The environmental impacts in the Bonanza Mining District identified for remedial action under this voluntary restoration include:
    • The Rawley 12 tailings and the Rawley 12 mine discharge located along Squirrel Creek.
    • The Superior Mill tailings also located along Squirrel Creek.
    • The Cocomongo/Bonanza Mill tailings in Kerber Creek.
    • Tailings impoundments along Kerber Creek
    • Fluvially deposited tailings along Kerber Creek.

History

  • Sporadic production in the district occurred from the 1890s until 1970. Ore reportedly was discovered at the outcrop of the Rawley Vein, located in Rawley Gulch, in 1880 and significant production of ore from the Rawley Mine began in 1902.
  • The mine consists of 12 levels, including the Rawley 1200 level, which was completed as a drainage and ore haulage tunnel. Ore from the Rawley vein consisted primarily of copper, lead, zinc and silver.
  • Production from the Rawley Mine was intermittent due to flooding of the mine workings and difficulties in obtaining a sufficient volume of water to operate a mill in Rawley Gulch.
    • In 1910 the Rawley 12 drainage tunnel was driven for a distance of 6,200 feet from Squirrel Creek to the Rawley Mine in Rawley Gulch.
    • In 1916, planning began for a 300 ton/day mill along Squirrel Creek at the Rawley 12 portal.
  • The Rawley Mill was completed in 1923.
  • An estimated 476,000 tons of ore was produced from the Rawley Mine from 1923 to 1930.
  • To accommodate the tailings resulting from the processing of this volume of ore, a dam was built across Squirrel Creek below the mill. Also, three tailings dams were constructed on Kerber Creek below the town of Bonanza between 1925 and 1930.

Environmental concerns

The major human health concern was considered to be exposure to heavy metals in the tailings, specifically lead. We and ASARCO Inc. conducted a residential soil sampling investigation in the town of Bonanza to determine lead concentrations in these soils. The results from this investigation indicated the concentrations of lead weren't significant with respect to human health. The U.S. Environmental Protection Agency (EPA) agreed with these results and no further action was deemed necessary.

Site remediation

  • In September 1994 ASARCO initiated the removal of the Rawley Mill tailings from Squirrel Creek. An estimated 31,000 cubic yards of tailings were excavated from Squirrel Creek and disposed of at an on-site engineered repository at the middle tailings dam below the town of Bonanza.
  • In 1995, ASARCO excavated an additional 85,000 cubic yards of tailings from the upper and lower tailings dams and consolidated them at the repository.
    • The same year, ASARCO reclaimed the area incorporating the upper tailings dam. In 1996 ASARCO reconstructed and reclaimed the stream reach of the Squirrel Creek channel that was impacted by the Rawley Mill tailings.
    • ASARCO successfully opened the collapse at the Rawley 12 portal, diverted the discharge to an oxidation-sedimentation pond and began treatment of the discharge.
  • Other members of the Bonanza Group completed the in-place closure of the Superior Mill tailings in 1996. This successfully eliminated any future environmental impacts from this mill site.
  • In 1997, ASARCO continued to conduct restoration activities along the riparian corridor of Kerber Creek.
    • ASARCO has worked cooperatively with the state to facilitate the cleanup of orphan shares at the site by agreeing to accept tailings to be excavated under a separate removal action from an "orphan" (outside the area of responsibility of the Bonanza Group) mill site located within the town of Bonanza adjacent to Kerber Creek.
    • We performed the removal of 4500 cubic yards of tailings from the mill site in July 1997.
    • This project was funded through a Clean Water Act (CWA) Section 319(h) grant.
  • In 1998, ASARCO completed its riparian restoration effort along the main stem of Kerber Creek.
  • At the same time, we performed a second removal action at three waste source piles located within the watershed. This project was funded through an EPA grant.
  • In 1999, ASARCO constructed a plug in the Rawley 12 adit (side entrance or tunnel) in order to eliminate the discharge.
The Bonanza Mining District project would not have achieved the success it has without the involvement of ASARCO Inc. in organizing the Bonanza Group, its consideration and response to agency issues, and the overall desire of ASARCO and the Bonanza Group to achieve the goals of the project and to improve the water quality of the lower Kerber Creek watershed.
 

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