BIOGEOCHEMICAL STUDIES AND GEOCHEMICAL MODELING IN SUPPORT OF REMEDIATION AT THE IRON MOUNTAIN MINE, CALIFORNIA

Historic mining of pyritic massive sulfide deposits at Iron Mountain Mine, near Redding, California, has led to formation of some of the most acidic and metal-rich mine drainage ever recorded. Investigation of the water and mineralogy inside the mine workings has led to insights into waters with negative pH values and the geochemistry of efflorescent iron-sulfate minerals and their influence on seasonal variations of metal ratios in mine effluent. Water that drains from mine adits poses site management challenges, given the low pH (~0.5–2.8) and high metal content that is expected to persist for hundreds to thousands of years, under current conditions. A high-density sludge treatment plant and other site remediation activities, under the auspices of the Superfund Program of the U.S. Environmental Protection Agency (EPA), have reduced the efflux of metals from the site by more than 95%. However, pipelines that carry the acid mine drainage from the adits to the lime-neutralization treatment plant form an iron (Fe)-rich precipitate (scale) due to microbial Fe(II) oxidation, which will occlude the pipeline without removal. We developed a geochemical model based on coupled biotic-abiotic processes measured in field- and laboratory-based studies to evaluate various possible remediation strategies to prevent or decrease pipe scale. Downstream of the treatment plant, the mobility of copper, zinc, and other metals has been investigated with seasonal sampling of stream water, streambed sediment, streambed algae, reservoir water, and reservoir sediment. The role of metal adsorption onto Fe-rich colloids and biomass such as algae are important factors in seasonal attenuation of metals. Using metal load calculations of the stream and a geochemical modeling approach, we assessed where metal attenuation is greatest to help EPA’s site management decisions and planning efforts.