ARSENIC AND LEAD TRANSLOCATION AND MOLECULAR SPECIATION CHANGE IN MINE TAILING SITES ACROSS THE WESTERN CLIMATES OF THE UNITED STATES

Ore processing solid-phase residuals known as mine tailings can include a range of hazardous metal(loid)s (e.g., arsenic [As] and lead [Pb]) that present serious dangers to human health and the environment. It is imperative that we deepen our understanding of the processes and factors that control the release, transformation, and accumulation of these elements at different strata of the tailings weathering profile. Reactive transport results in the formation of distinct elemental distribution trends as tailings undergo climate-driven weathering. By combining total elemental analysis from tailings digests subject to inductively coupled plasma mass spectrometry (ICP-MS) with As, Fe and S speciation analysis from X-ray absorption spectroscopy, we assessed the relationships between concentration patterns and molecular speciation in abandoned sulfide ore tailings sites. We hypothesized that climatic conditions, weathering period, and mineral composition govern the types of reaction products and the rate of tailings weathering. Thirteen sulfide ore tailings sites with elevated As concentrations were sampled across the climate gradient of western U.S. Samples were collected across the oxidative weathering reaction front (0-200 cm), at discrete 10 cm depth intervals. The results show that As was enriched throughout the weathering profile of arid sites, with the highest enrichment occurring between 20 and 150 cm. In contrast, in humid climate sites, As was primarily depleted in the horizons above 50 cm, while deeper profiles showed As enrichment. This implies that the extent of As mobilization is influenced by the through-flux of H₂O and dissolved O₂, whereas As deposition is via co-precipitation/adsorption processes at depth. Furthermore, Pb showed patterns similar to As, but with more enrichment in the top layers for humid climates, indicating significant retention and restricted mobility of Pb as compared to As. These climatic and elemental effects are shown to be directly related to variation in molecular speciation of As, Pb, Fe, and S as revealed through X-ray absorption spectroscopic (XANES and EXAFS) analyses. The results have implications for human health risk assessment associated with sulfide ore mine tailings sites, and suggest that variation in climatic forcing of weathering trajectories is an important consideration for assessment.