FOUR DECADES OF RESEARCH ON ENVIRONMENTAL GEOCHEMISTRY AND MINERALOGY IN SUPPORT OF REMEDIATION AT THE IRON MOUNTAIN MINE SUPERFUND SITE, CALIFORNIA

The mining of pyritic massive-sulfide deposits at Iron Mountain (1896–1962) has led to the formation of some of the most concentrated acid mine drainage (AMD) ever recorded. Inside the mine workings, drip waters from stalactites of Fe-sulfate minerals have negative pH values, sulfate concentrations in the range of 120 to 760 grams per liter (g/L), Fe up to 162 g/L, and other metals (Cu, Zn, and Al) in excess of 1 g/L. Water flowing from the Richmond mine has pH from 0.5 to 1.2 with elevated concentrations of metals and sulfate. Prior to remediation and active water treatment by lime neutralization (pre-1994), the mine site discharged thousands of kilograms per year of Cu, Zn, and Cd to Keswick Reservoir and the lower Sacramento River, an ecologically sensitive habitat that hosts several threatened and endangered species of anadromous fish including steelhead and winter-run Chinook salmon. Scientific studies since the 1970s (Nordstrom, 1977, Ph.D. thesis, Stanford Univ.) have provided the basis for on-site remediation activities that have substantially improved water quality. Current loads of Cu, Zn, and Cd have been reduced by more than 95% compared with pre-remediation loads. Before 1994, the water entering Keswick Reservoir had pH from 2 to 3 (mean of 2.8) and median Cu of 1,920 μg/L (interquartile range 1,230 to 3,910 μg/L). In contrast, during 2008–2012, pH ranged from 3.0 to 7.7 (mean of 5.9) with median Cu of 49 μg/L (interquartile range 36 to 92 μg/L). In addition to Cu removal at the treatment plant, higher pH may facilitate Cu sorption on hydrous Fe(III) oxides formed by microbial oxidation of Fe(II), further reducing Cu loads.

We will provide an overview of the mining history and environmental setting at Iron Mountain, describe regulatory and remediation milestones, and summarize research on geochemical and mineralogical characterization in support of remediation by the U.S. Environmental Protection Agency during 1983–2013, including an ongoing investigation of microbially mediated Fe(III) scaling of drainage pipe. Although significant progress has been made in 30 years of remediation under Superfund, the complex biogeochemistry of Iron Mountain continues to provide a challenging and useful laboratory for improving the understanding of dynamic processes that affect the formation and attenuation of AMD.