WEATHERING OF SULFIDE MINE TAILINGS IN A SEMI-ARID CLIMATE, PART 2: TOXIC METAL BEHAVIOR

Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metals (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion when unvegetated, which in semi-arid environments can be decades without intervention. Metals speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing the risk associated with mine sites. At the semi-arid Iron King Mine and Humbolt Smelter Superfund Site in Arizona, the mineral assemblage has been previously characterized in the top 2 m of tailings. A distinct redox gradient was observed in the top 0.5 m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weathers to schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3 to 2.3).

Within this geochemical context, we examined enriched toxic metals As, Pb, and Zn with surficial concentrations 3080, 2200, and 2570 mg kg^-1, respectively. However, the highest bulk concentrations of As and Zn occur at the redox boundary, and represent a 1.7 and 4.2 fold enrichment, relative to surficial concentrations, respectively, indicating the downward translocation of toxic elements from the gossan zone to the redox boundary. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180 cm) contains sulfides (e.g., arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25-54 cm), contain a mixture of sulfides, carbonates (cerrusite and smithsonite), and metals sorbed to neoformed secondary iron phases, principally ferrihydrite. In surface samples (0-35 cm), metals are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as jarosites. Interestingly, metal-bearing efflorescent salts (e.g., ZnSO₄Ÿ·nH₂O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metals are altered by mineral weathering and result in both the downward migration of metals to the redox boundary and the precipitation of metal salts at the surface.