Application of iron and zinc isotopes to track the sources and mechanisms of metal loading in a mountain watershed

Here the hydrogeochemical constraints of a tracer dilution study are combined with Fe and Zn isotopic measurements to pinpoint metal loading sources and attenuation mechanisms in an alpine watershed impacted by acid mine drainage. In the tested mountain catchment, delta(56) Fe and delta(66) Zn isotopic signatures of filtered stream water samples varied by similar to 3.5 parts per thousand. and 0.4 parts per thousand, respectively. The inherent differences in the aqueous geochemistry of Fe and Zn provided complimentary isotopic information. For example, variations in delta(56)Fe were linked to redox and precipitation reactions occurring in the stream, while changes in delta(66)Zn were indicative of conservative mixing of different Zn sources. Fen environments contributed distinctively light dissolved Fe (<-2.0 parts per thousand) and isotopically heavy suspended Fe precipitates to the watershed, while Zn from the fen was isotopically heavy (>+0.4 parts per thousand.). Acidic drainage from mine wastes contributed heavier dissolved Fe (similar to+0.5 parts per thousand) and lighter Zn (similar to+0.2 parts per thousand) isotopes relative to the fen. Upwelling of Fe-rich groundwater near the mouth of the catchment was the major source of Fe (delta(56)Fe similar to 0 parts per thousand.) leaving the watershed in surface flow, while runoff from mining wastes was the major source of Zn. The results suggest that given a strong framework for interpretation, Fe and Zn isotopes are useful tools for identifying and tracking metal sources and attenuation mechanisms in mountain watersheds. (C) 2009 Elsevier Ltd. All rights reserved.