by Bence V. Close, P.E.
Tetra Tech RMC, Inc., Longmont, Colorado
|Photo by Bence Close|
|Kerber Creek above Bonanza before clean-up.|
The historic Bonanza Mining District is located in the upper Kerber Creek watershed and in the vicinity of Bonanza, Colorado. Significant mining activity began in the district in the 1880s when gold, silver, lead, copper and zinc were discovered. Mining activities continued sporadically in the area until the late 1960s with the most intensive production occurring during the 1920s.
While there were numerous smaller mines and mills in the Bonanza District, the Rawley Mine and Mill was the largest producer. Large-scale production in the district was made possible by the construction of a 6,200-foot long adit, referred to as the Rawley 12 adit, in 1911 and 1912. The Rawley 12 adit was driven from Squirrel Creek to intercept and drain the Rawley Vein at the 1200-foot depth level of the mine, and allow transport of ore to the Rawley Mill site, which was constructed in the late 1910s and early 1920s. Along with the mill, a 7.5-mile long aerial tramway was constructed to transport mill concentrates to a railhead at Shirley, Colorado, near Poncha Pass.
|Photo by Bence Close|
|CRA members on the field trip to Kerber Creek.|
A 40-foot high timber crib dam was constructed across Squirrel Creek below the Rawley Mill to contain tailings from the milling process. When this dam filled with tailings, a series of three dams were constructed across Kerber Creek to contain tailings approximately four miles downstream and south of Bonanza. Tailings from the Rawley Mill and other mines and mills washed down Kerber Creek and its tributaries and accumulated behind the three dams, referred to as the Lower Tailings Area. Kerber Creek and its riparian corridor were impacted along the way as tailings were washed overbank or trapped behind beaver dams.
ASARCO and other private parties, including NL Industries, have worked cooperatively with representatives from Local, State and Federal governments and the public to develop and implement proactive and expedited watershed-based solutions to environmental effects from historic mining and milling in the Bonanza Mining District. The U.S. Environmental Protection Agency (EPA), Colorado Department of Public Health and Environment (CDPHE), and the USDA Forest Service (USFS) evaluated the Bonanza District for possible inclusion on the National Priorities List (NPL) under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), or “Superfund.” Based upon those studies and processes initiated by the agencies in the early 1990s, the site was targeted for placement on the NPL.
|Photo by Bence Close|
|Rawley 12 Portal before tunnel rehab and plugging.|
Negotiations between the private parties and the agencies resulted in suspension of the NPL listing process, given implementation of a proactive plan for closing various mining and milling sites in a manner consistent with State and Federal regulations. The project plan, which was performed by the private parties in cooperation with the USFS, EPA, CDPHE, Saguache County and the Town of Bonanza, involved assessment of environmental impacts from inactive mining and milling sites within the Bonanza Mining District, and implementation of remedial measures. The cleanup actions have been performed under innovative non-binding agreements pre-dating the formal State voluntary cleanup program. The innovative process developed for the Bonanza project resulted in on-the-ground earthwork starting less than 16 months after the initial meeting between the agencies and private parties, as opposed to the five or more years typically associated with Superfund sites of similar scale. All work has been funded by the private parties, with the exception of several orphan mine site cleanups performed by the CDPHE with grant funds. As part of the cooperative effort, ASARCO provided assistance to the CDPHE by allowing disposal of wastes from those orphan mines within its site repository.
In order to facilitate the response activities, the private parties worked closely with the Colorado Water Quality Control Commission to establish long-term water quality goals for the watershed. Brewery Creek provides a significant source of dilution to Kerber Creek, which allowed for higher stream use classification goals for Kerber Creek downstream of the Brewery Creek confluence. The long-term goals for the mining district portions of the Kerber Creek watershed above the Brewery Creek confluence were to achieve drinking water and agricultural use standards. Below the Brewery Creek confluence, the long-term goal for Kerber Creek was to also establish water quality suitable for certain trout.
The remedies implemented at the Bonanza District were selected through a series of cooperative arrangements between the CDPHE, the USFS, the Bonanza Group, Saguache County, and the Town of Bonanza. Primary criteria for selected actions were protection of human health and the environment, and achievement of long-term water quality goals.
Primary aspects of the response actions have included:
- Evaluation of mining-related impacts on surface water, groundwater and soils, and evaluation of contaminant pathways and mine hydrology, through sampling and analysis and geologic mapping;
- Assessment of stream biology and riparian habitat impacts;
- Development of an overall site prioritization for addressing impacts from the areas of concern;
- Consolidating and capping of tailings and other metals-bearing and acid-generating mine wastes in an on-site repository;
- In-place closure of tailings in areas of difficult access along Kerber Creek;
- Design and construction of a concrete plug 2200 feet inside in the primary mine drainage feature, the Rawley 12 adit, including design and construction of a pond for surge control and passive water treatment during the mine adit rehabilitation and plugging and installation of a drying bed to stabilize sludge removed from the surge pond;
- Development of a local limestone quarry to supply limestone in various sizes and gradations for repository capping, repository and stream bank erosion protection (boulders and riprap), neutralization of certain waste materials, and reclamation soil amendment;
- Implementation of mine infiltration and storm water controls;
- Stream channel restoration and stabilization, riparian zone enhancements, and sediment control structures associated with waste material removal and consolidation;
- Stream diversions to reduce contact with residual mine wastes;
- Upland and riparian area reclamation;
- Road and bridge improvements to facilitate response actions and public safety; and
- Monitoring of the effectiveness of response actions.
|Photo by Bence Close|
|Kerber Creek above Bonanza after clean-up.|
Specific stream work has included re-establishment of natural meandering channel patterns, channel stabilization with rock barbs, vortex weirs, root wads and willow/alder plantings, and lining of channels to preclude infiltration into mine workings. With the response actions that have been implemented between 1994 and 2003, Kerber Creek has already progressed from supporting virtually no aquatic life over a 10-mile long stretch to primary achievement of the water quality goals. Fish are now present in Kerber Creek below its confluence with Brewery Creek, as well as other areas within the former mining district. Other orphan mine sites continue to be addressed by the USFS and CDPHE, but now that the primary contaminant source areas have been addressed, continued improvement in the quality of Kerber Creek and its tributaries is expected through natural recovery.
A tour of the Kerber Creek project was conducted as part of the 16th Annual Colorado Riparian Association Conference. For more information, please contact Bence Close at 303-772-5282 , or firstname.lastname@example.org.
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