ENVIRONMENTAL LEGACY OF THE CALIFORNIA GOLD RUSH: 2. ACID MINE DRAINAGE FROM ABANDONED PLACER-GOLD MINES

A reconnaissance of water quality at abandoned placer-gold mines in the Bear River and Yuba River watersheds, northern Sierra Nevada, California, during 1999–2001 indicates acid mine drainage (AMD) in several areas. AMD (pH 3.0 to 5.5) was observed in several ground sluices (n=3) and tunnels (n=7) draining hydraulic mine pits, and in pit lakes (n=5) generally upgradient from clogged drain tunnels. Trace-metal concentrations were elevated in AMD from selected placer-gold deposits (filtered; in micrograms per liter): Al, 3 to 5,800; Cd, 0.04 to 2.6; Cu, 0.5 to 48; Fe, 2 to 54,000; Ni, 2 to 20,000; Zn, 10 to 560. Some of these concentrations exceed state and federal water-quality criteria for protection of aquatic life. Clasts of pyrite, pyrrhotite and base-metal sulfides in quartz and chlorite were partially replaced and overprinted by iron monosulfides (griegite, mackinawite, and poorly crystalline material with a zoned, framboidal texture) that in turn were replaced by pyrite. Concentrations of trace elements (including As, Au, Co, Cu, Hg, Ni, Pt, and Zn) determined by laser-ablation ICP-MS were higher in iron monosulfides and poorly crystalline material than in pyrite. Sulfate concentrations in AMD ranged from 26 to 1,700 milligrams per liter. Diagenetic sulfides are a likely source of sulfate and trace metals to acidic waters, especially in areas where mining has lowered the water table by tens to hundreds of meters. Preliminary isotope studies indicate a relatively wide range of d³⁴S values in AMD sulfate (–25.9 to –0.6 ‰; n=15), a wide range of values in iron-sulfide minerals from the placer-gold deposits (–10.1 to +28 ‰, n=6) and a relatively narrow range of values in sulfate from downstream rivers (+0.9 to +5.6 ‰, n=6). Additional sulfur and oxygen isotope studies are needed to evaluate potential sources of sulfate, including urban runoff and oxidation of sulfide minerals in the placer deposits and metamorphic bedrock, and to evaluate sulfide oxidation mechanisms and possible management scenarios for reducing sulfate concentrations and loads (mass/time). Because of the likely role of sulfate-reducing bacteria in mercury methylation, lowering sulfate loads, rather than mercury loads, may be a more feasible and cost-effective alternative for reducing mercury bioaccumulation in these watersheds.