TRANSPORT AND FATE OF METALS IN EXTREMELY ACIDIC MINE WATER AT IRON MOUNTAIN MINE, CALIFORNIA, AND DOWNSTREAM ENVIRONMENTS (Invited Presentation)

The massive sulfide deposits at Iron Mountain Mine (IMM), California, are notorious for producing some of the world’s most acidic mine drainage (AMD). Previous work has documented negative pH in drip waters within the Richmond mine associated with metal-sulfate salts that form at temperatures up to 60°C caused by exothermic pyrite oxidation. From Aug. 2012 to July 2016, weekly data were collected for pH, Cd, Cu, Fe, and Zn in effluent from four sources of AMD at IMM. Median values of pH were 1.0 at the Richmond portal, 1.9 at the Lawson portal, 2.6 at the Old-#8 Mine area, and 2.9 at the Slickrock Creek Retention Reservoir (SCRR). Systematic seasonal variations in Zn/Cu and Fe/Cu ratios were observed at the Richmond and Lawson portals, but not in the two higher-pH AMD sources, Old-#8 and SCRR. Lime-neutralization treatment removes more than 95% of the Cd, Cu, and Zn from the AMD draining IMM. Nonpoint sources of AMD in the Boulder Creek drainage (east side of Iron Mtn.) are not currently treated and are the origin of most metals downstream in Lower Spring Creek (LSC). From Sep. 2016 to Mar. 2018, pH in Boulder Creek ranged from 3.3 to 4.6 (median 3.6), with median Cu of about 0.5 mg/L and median Zn of about 2 mg/L. LSC flows through Spring Creek Reservoir (SCR) and then empties into Keswick Reservoir (Sacramento River system). Water quality in LSC and SCR improved substantially after water treatment began at IMM in 1994. Another major improvement occurred in 2004, when SCRR was completed, allowing treatment of nonpoint sources of AMD in the Slickrock Creek drainage (west side of Iron Mtn.). Prior to 1994, filtered (0.45 µm pore size) Cu concentration below SCR ranged from about 0.1 to 15 mg/L; between 1994 and 2004 the range was about 0.01 to 1.5 mg/L. Between 2004 and 2009, Cu concentration below SCR was typically < 0.1 mg/L and pH was 5 to 7 when SCR water elevation was > 720 ft above sea level; however, during periods when SCR water elevation was < 720 ft, Cu below LSC was typically above 0.1 mg/L and pH was < 5. Since 2010, water elevation has been kept above 720 ft except for occasional maintenance, resulting in improved water quality. Ongoing research is investigating whether acidity and higher Cu at lower water levels are caused by oxidation of sulfide minerals in sediment, by oxidation and hydrolysis of ferrous iron in sediment pore water, or by other processes.