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  Final Report, Characterization and Eh/pH-Based Leaching Tests of Mercury-Containing Mining Wastes From the Sulfur Bank Mercury Mine, Lake County, California (98 pp, 3.01 MB) (EPA/600/R-02/032) September 2001

Clear Lake, in northern California, received inputs of mercury mining wastes from the Sulfur Bank Mercury Mine (SBMM) for nearly 100 years. About 1.2 million tons of mercury-contaminated overburden and mine tailings were distributed over a 50-hectare surface area due to mining operations from 1865 to 1957.

The objective of this study was to evaluate a range of different pH and Eh values in order to evaluate the potential of SBMM waste ore to leach mercury.

The SBMM includes an open, unlined mine pit (Herman Pit) that covers about 23 acres and is 750 feet upgradient of Clear Lake. Water samples collected from Herman Pit and Clear Lake were analyzed and reported; pH values at those locations were 3 and 8, respectively. The SBMM was placed on the final National Priorities List in 1990. The site has been under investigation as a Comprehensive Environmental Response, Compensation, and Liability Act site and has experienced some minor corrective actions, but Clear Lake remains under a fish advisory due to the mercury contamination.

Mercury in contaminated soils is a unique pollutant that requires innovative remediation solutions. Conventional stabilization and solidification treatments cannot effectively reduce the leachability of mercury. As part of the remediation effort at the SBMM site, EPA is assisting in the development of treatment alternatives for waste materials from the site. Waste materials consist of waste ore, waste rock, and roaster tailings.

To support this work, leaching profiles of waste ore over a range of different pH and oxidation-reduction (Eh) conditions were performed. Chemical and biological processes affecting the mobility of metals may be initiated by altering the physicochemical environment (i.e., pH and Eh conditions). Important processes influencing the chemistry and availability of trace and toxic metals include:

  • Precipitation as insoluble sulfides under highly reduced conditions
  • Formation of discrete metal oxides and hydroxides of low solubility
  • Adsorption of colloidal hydrous oxides of iron and manganese, primarily in aerobic, neutral, or alkaline environments
  • Complex formation with soluble and insoluble organic matter

Each experiment was designed to evaluate leachability of mercury from the waste materials under controlled conditions in order to assess conditions that may contribute to the destabilization of mercury in the waste ore.


Paul Randall

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