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Hazardous Materials and Waste Management Division
Bonanza Mining District Restoration Project

Prior to 1993, the United States Environmental Protection Agency (EPA), the Colorado Department of Public Health and Environment, and the U.S. Department of Agriculture - Forest Service began investigating the environmental impacts at the Bonanza Mining District.  Early in 1993, information request letters were sent to identified potentially responsible parties (PRPs) and a Hazard Ranking Score process was initiated by the Colorado Department of Public Health and Environment on behalf of the EPA.  Beginning in the spring of 1993, discussions were held between the potentially responsible parties, the Colorado Department of Public Health and Environment, EPA and the Forest Service to identify an appropriate administrative framework for conducting a voluntary program to implement actions to address the environmental impacts at the Bonanza Mining District.  After the initial meeting in April 1993, several potentially responsible parties, including ASARCO Incorporated (ASARCO), formed the Bonanza Mining District Group.  Subsequent meetings between the agencies and the Bonanza Group led to the formulation of a draft written agreement to suspend the Hazardous Ranking Score process in order to facilitate a voluntary environmental restoration project at the Bonanza Mining District. 
 

Site Summary

Location

The Bonanza Mining District consists of approximately 30 to 35 square miles in the extreme northeastern portions of the San Juan Mountains of south central Colorado in Saguache County. The district is situated approximately 22 miles south of Poncha Springs on Colorado State Highway 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, and fluvially deposited tailings along Kerber Creek.

History

Sporadic production in the Bonanza Mining District occurred between the 1890's into 1970. Ore was reportedly 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 twelve 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 6200 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 were 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.

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Environmental Concerns

The major human health concern was considered to be exposure to heavy metals in the tailings, specifically lead. ASARCO and Colorado Department of Public Health and Environment conducted a residential soil sampling investigation in the town of Bonanza to determine lead concentrations in these soils. The results from this investigation indicated that the concentrations of lead were not significant with respect to human health. The US EPA agreed with these results and no further action was necessary.

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Site Remediation

In September 1994, ASARCO Inc, 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. In this 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. In addition, 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. Colorado Department of Public Health and Environment 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 their riparian restoration effort along the main stem of Kerber Creek. At the same time, Colorado Department of Public Health and Environment 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 in order to eliminate the discharge.

The Bonanza Mining District project would not have not achieved the success it has without the involvement of ASARCO Inc. in organizing the Bonanza Group, their 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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