ISOTOPIC SIGNATURES OF DISSOLVED CU, FE, AND ZN IN AN ALPINE WATERSHED IMPACTED BY ACIDIC METAL-RICH DRAINAGE

In hydrologic systems, the stable isotopic signatures of transition metals like Cu, Fe, and Zn may (1) help to fingerprint and track various anthropogenic and natural metal loading sources throughout a watershed, (2) and elucidate the biogeochemical mechanisms that control metal loading, availability and attenuation. Despite this potential, the majority of transition metal isotopic studies have focused on rocks and minerals. The value of these emerging isotopic tools in hydrologic environments is largely untested. In this study, we measure the isotopic ratios of dissolved Cu, Fe, and Zn in an alpine watershed impacted by acidic metal-rich drainage. We combine a tracer-dilution study of the main stream drainage with bulk geochemistry and water quality data to provide a comprehensive surface-hydrological and geochemical framework for interpreting the Cu, Fe, and Zn isotopic signatures of selected water samples within the watershed. Initial isotopic results demonstrate that a number of the inflows (quantified through tracer-dilution) that contribute acidic runoff into the main stream channel possess distinctive Cu, Fe, or Zn isotopic signatures. For example, Zn, Cu, and Fe, isotopic ratios in the stream inflows, which contribute significantly to metal loading in the watershed, vary from +0.3 to +0.5 per mil δ66Zn (relative to JMC), -1.4 to 0.2 per mil δ65Cu (relative to SRM 976), and -0.5 to 0.35 per mil δ56Fe (relative to IRMM14). Moreover, inflows that flow through abandoned mine tailings (i.e., anthropogenic metal sources) appear to be isotopically distinct from inflows that flow through un-mined and weakly mineralized country rock (i.e., natural metal sources). By combining isotopic and traditional bulk geochemical data, it is also evident that some of the isotopic variation in the stream channel itself is directly attributable to geochemical processes like mineral precipitation and adsorption. This investigation demonstrates that the isotopic ratios of dissolved transition metals like Cu, Fe, and Zn, can be a useful tool for investigating hydrogeochemical fluxes on the watershed scale. The application of Cu, Fe, and Zn isotopes to aqueous systems holds a great deal of promise and warrants further investigation on a variety of temporal and spatial scales.