SULFUR AND ZINC ISOTOPES AS INDICATORS OF GROUNDWATER FLOW PATHWAYS IN ALPINE WATERSHEDS IMPACTED BY ACID ROCK DRAINAGE

Complex flow pathways, large seasonal fluctuations, and a lack of water monitoring infrastructure make metal loading and transport pathways difficult to define in mountain watersheds impacted by acid rock drainage. In this study, we used S and Zn isotopes to investigate groundwater flow sources and water-rock interaction in several Alpine watersheds affected by acid rock drainage in the San Juan Mountains of Southwest Colorado. In summer 2009, we collected water samples from 10 locations where water was exiting abandoned mine tunnels or adits. The δ³⁴S of dissolved sulfate in these acidic, metal-rich waters varied over a wide range from -2.5 to +6.0 ‰ (relative to V-CDT) indicating mixing between sulfate from two end member sources, dissolution of sulfate minerals (heavier S isotopes) and the oxidation of sulfide minerals (lighter S isotopes). Gypsum (CaSO₄ · 2H₂O), anhydrite (CaSO₄), and sulfide minerals (mainly pyrite, FeS₂) formed during hydrothermal alteration in the mining district. The δ⁶⁶Zn varied over a smaller range of +0.10 to +0.53 ‰ (relative to JMC 3-0749L) and the dissolved Zn component was generally isotopically heavier than Zn adsorbed onto suspended particles by 0.1 to 0.2 ‰. We found a positive correlation between δ³⁴S and δ⁶⁶Zn, indicating that Zn associated with the sulfate alteration zones in the district may be isotopically heavier than Zn in sulfides. This relationship suggests considerable mixing of acidic waters derived from the oxidative weathering of sulfides with water flowing through acid sulfate-altered bedrock. Zn isotopes can be additionally impacted by geochemical reactions like adsorption and precipitation; however, many of these samples were collected from water with low pH values, suggesting adsorption in not a primary control. If acid-sulfate alteration zones can be spatially identified, the S and Zn isotopic signatures might be used to distinguish flow paths and metal loading sources in these mountain watersheds.