Mobility and chemical fate of antimony and arsenic in historic mining environments of the Kantishna Hills district, Denali National Park and Preserve, Alaska

The Kantishna Hills mining district of interior Alaska, USA, located within Denali National Park and Preserve, contains a number of antimony lode deposits, including Alaskaʹs historically largest antimony producer, the Stampede mine. Oxidative weathering of sulfidic tailings and waste rock associated with historic mining operations has impacted water quality in the region. In the Stampede and Slate Creek watersheds, antimony and arsenic concentrations in stream waters were as high as 720 μg/L and 239 μg/L, respectively. Antimony in all water samples is predominantly present as Sb(V), whereas arsenic was detected in varying ratios of As(III) and As(V). Based on X-ray absorption spectroscopy (XAS) measurements reduced As(III) and Sb(III) were identified in mine waste materials, whereas predominantly oxidized forms, As(V) and Sb(V), were found in downstream sediments. Elevated antimony concentrations extend for more than 8 km downstream from the antimony lodes, whereas arsenic quickly attenuates within 1.5 km. The difference between antimony and arsenic aqueous phase speciation suggests that antimony oxidation is more rapid than arsenic within this system. A high correlation is observed between antimony, arsenic, and iron concentrations in fine-fraction streambed sediments downstream of the source lodes. This suggests that sorption and co-precipitation with iron (hydr)oxides are important pathways for the attenuation of antimony and arsenic in these interior Alaska watersheds. Further XAS characterization of the downstream sediments corroborates these observations and indicates that antimony is adsorbed to Fe-oxide phases as inner-sphere bi-dentate edge and corner sharing complexes. The trace element redox states, as well as downstream partitioning, are mainly controlled by iron speciation based on the strong correlation between redox potentials calculated from iron (Fe(II)/Fe(III)) and arsenic (As(III)/As(V)).