Historical mining of a massive sulfide deposit at Iron Mountain (CA, USA) has produced an environmental legacy including some of the world’s most acidic and metal-rich acid mine drainage (AMD). Prior to initial remediation in 1994, the site was the source of thousands of kilograms per year of Cu, Zn, Cd, and other metals to tributaries of the Sacramento River, an ecologically sensitive water resource and drinking-water supply. On-going remediation includes water treatment by lime neutralization producing high-density sludge. Water is conveyed to the treatment plant through a series of pipelines from the most contaminated drainages including underground mine workings and a retention reservoir for surface-water containment. Microbial Fe(II) oxidation of AMD (pH 2.5-3.1) in one of the pipelines results in precipitation of hydrous Fe(III) oxides and oxy-sulfates (primarily schwertmannite with minor goethite), requiring expensive maintenance. The pipeline precipitates host a diverse group of acidophilic microorganisms and contain elevated concentrations of metals and metalloids. Laboratory experiments and geochemical modeling suggest that decreasing the pH of the AMD through mixing of different source waters, including AMD with pH as low as 0.5, could be a viable option to prevent or retard precipitation in the pipeline. Ongoing water treatment has reduced loading of Cu, Zn, and Cd to the Sacramento River system from Spring Creek by more than 95%. Understanding the biogeochemical and mineralogical fate of the remaining Cu, Zn, Cd, Al, and Fe in Spring Creek is key to evaluating the attenuation mechanisms, seasonality, and potential bioavailability of metals. In addition to substantial metal removal at the treatment plant, the neutral pH of the effluent causes metal adsorption on colloidal hydrous Fe(III) oxides formed in the effluent and on bed sediments in Spring Creek, further reducing metal loads to the Sacramento River system.

Understanding coupled biogeochemical and mineralogical processes that control trace-metal fate in acidic mine water supports water treatment at Iron Mountain Mine, CA

255th National Meeting (2018)

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