Internal loading of antimony from mining contaminated sediments under oxic conditions

Abstract

The environmental fate of antimony (Sb) in aquatic ecosystems has been less studied compared to other metal(loid)s released by mining. This study investigated Sb flux from lake sediments of Yellowknife Bay (Great Slave Lake, Northwest Territories, Canada), which were contaminated by gold mining operations. Sediment Sb fluxes were measured in the field by short term (2–6 day) incubations of intact sediment cores and in the laboratory by incubating bulk sediment over a longer 30-day period. Antimony diffusion from sediment to overlying water was observed in 17 of 28 intact cores (61%), with flux rates ranging from 10 to 279 μg/m²/day. Overlying water and surface sediment remained oxic during the field measurements. Sediment Sb concentration (0.7–47 μg/g) was positively correlated with Sb flux, and the mineralogy of the sediment, characterized in a companion study, likely influenced flux spatial patterns. Other environmental factors, specifically season, temperature, organic matter content, and iron or manganese concentrations of sediment did not explain Sb flux. Porewater Sb concentrations were low (0.2–9.6 μg/L), and porewater depth profiles were not related to solid-phase Sb concentration, suggesting limited post-depositional mobility within sediments. Laboratory incubation of mixed bulk sediments showed higher Sb fluxes of 185–1555 μg/m²/day over the course of a 30-day experiment. Temperature warming from 7 to 22 °C did not enhance the Sb flux. Higher Sb fluxes in the laboratory versus field measurements may have been due to (i) more Sb in the laboratory sediments (∼160 μg/g), and (ii) oxidative dissolution of stibnite and Sb sulfosalt minerals that were previously stable in deeper anoxic sediments but disturbed and exposed to oxygen during laboratory manipulation. This study demonstrated that Sb can diffuse from mining-contaminated sediments into overlying water under oxic conditions, with fluxes influenced by sediment Sb concentrations and mineralogy.