Zinc Isotope Investigation of Surface and Pore Waters in a Mountain Watershed Impacted by Acid Rock Drainage


The pollution of natural waters with metals derived from the oxidation of sulfide minerals like pyrite is a global environmental problem. However, the metal loading pathways and transport mechanisms associated with acid rock drainage reactions are often difficult to characterize using bulk chemical data alone. In this study, we evaluated the use of zinc (Zn) isotopes to complement traditional geochemical tools in the investigation of contaminated waters at the former Waldorf mining site in the Rocky Mountains, Colorado, U.S.A. Geochemical signatures and statistical analysis helped in identifying two primary metal loading pathways at the Waldorf site. The first was characterized by a circumneutral pH, high alkalinity, and high Zn/Cd ratios. The second was characterized by acidic pHs and low Zn/Cd ratios. Zinc isotope signatures in surface water samples collected across the site were remarkably similar (the δ66Zn, relative to JMC 3-0749-L, for most samples ranged from 0.20 to 0.30‰ ± 0.09‰ 2σ). This probably suggests that the ultimate source of Zn is consistent across the Waldorf site, regardless of the metal loading pathway. The δ66Zn of pore water samples collected within a nearby metal-impacted wetland area, however, were more variable, ranging from 0.20 to 0.80‰ ± 0.09‰ 2σ. Here the Zn isotopes seemed to reflect differences in groundwater flow pathways. However, a host of secondary processes might also have impacted Zn isotopes, including adsorption of Zn onto soil components, complexation of Zn with dissolved organic matter, uptake of Zn into plants, and the precipitation of Zn during the formation of reduced sulfur species. Zinc isotope analysis proved useful in this study; however, the utility of this isotopic tool would improve considerably with the addition of a comprehensive experimental foundation for interpreting the complex isotopic relationships found in soil pore waters.


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Acid rock drainage; AMD; ARD; Bulk chemicals; Contaminated water; Dissolved organic matters; Geochemical signatures; Global environmental problems; Metal loadings; Mining sites; Natural waters; Pore waters; Reduced sulfur species; Rocky Mountains; Secondary process; Soil components; Soil pore waters; Transport mechanism; Wetland area; Zinc isotopes; Adsorption; Alkalinity; Analytical geochemistry; Groundwater flow; Isotopes; Landforms; Loading; Metals; Sulfur; Surface waters; Water; Zinc; Zinc sulfide; bicarbonate; calcium; ground water; lead; magnesium; manganese; pore water; surface water; unclassified drug; dissolved organic matter; oxidation; pH; porewater; watershed; acid rock drainage; anion exchange chromatography; geochemical analysis; hydrology; isotope analysis; priority journal; rock; soil degradation; soil pollution; spring; summer; water pollution; water quality; water sampling; weathering; wetland; Colorado; Environmental Remediation; Geography; Groundwater; Models; Theoretical; Soil; Water Movements; Water Pollutants; United States; Isotope; Metal

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Article - Journal

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© 2012 Elsevier, All rights reserved.

Publication Date

01 Mar 2012