SPATIAL AND TEMPORAL VARIATIONS IN MINE WATER GEOCHEMISTRY AND CHALLENGES FOR WATER TREATMENT AT IRON MOUNTAIN, CALIFORNIA

The inactive copper-zinc mines at Iron Mountain, California, are known for producing drainage that is among the world’s most acidic and metal-rich. On-site water treatment by lime neutralization began in 1988 with a short-term plant that was replaced by a long-term, high-density-sludge plant in 1994. These plants have treated the site’s most acidic drainage from the Richmond tunnel (median pH 1.0, range 0.8–1.5) and Lawson tunnel (median pH 1.9, range 1.6–2.3). Since 2004, treatment has included drainage from the Slickrock Creek Retention Reservoir (SCRR; median pH 2.9, range 2.4–3.2) and water pumped from underground workings in the Old-#8 mine area (median pH 2.6, range 2.3–3.1). Weekly sampling of these sources, with field measurement of pH, oxidation-reduction potential, temperature, and specific conductance, and lab analysis of Cu, Zn, Cd, and Fe, was done from August 2012 to July 2016 to assess temporal variability. Comprehensive analyses of major and trace elements plus Fe(II) were done several times during the study period. Monitoring results are being used to improve understanding of biogeochemical and hydrodynamic factors affecting Fe-rich scale formation in a pipeline that conveys acidic drainage from SCRR and Old-#8 mine to the treatment plant. The operators of the treatment plant currently address scale buildup in reaches of the Slickrock Creek pipeline by shutting down the pipeline periodically and soaking it with Richmond mine drainage to loosen the scale; at lower pH conditions (< 2), the scale (predominantly schwertmannite with minor goethite and trace jarosite) is more soluble and is loosened for easier cleanout. Monitoring results indicate systematic seasonal variability in pH and metals in the Richmond and Lawson tunnels, but not in the SCRR or Old-#8 mine area. The monitoring results make it possible to construct geochemical models of scale formation and treatment that span the range of expected field conditions.