GEOCHEMICAL CONTROLS ON ARSENIC AND ANTIMONY MOBILITY IN SEDIMENT AND PORE-WATER IN A MINE-IMPACTED LACUSTRINE ENVIRONMENT

Elevated levels of As and Sb in water and sediment are downstream residues from gold-mining activities at the Giant Mine in Yellowknife, Northwest Territories, Canada. The fate and transport of As and Sb are determined by collecting samples of sediment and pore-water downstream of the mine. Pore-water and sediment samples are analyzed for bulk chemistry, aqueous and solid-state speciation. The distribution of elements and species with depth at three different sites were compared. The distribution of As and Sb in sediments are similar at all sites, yet their distribution in the corresponding pore-waters is mostly dissimilar. The site densely populated by horsetails has the highest concentrations of As and Sb in sediment, yet their concentrations in pore-water are low. Both elements, most notably As, appear to remobilize in the root zone as indicated by an increase in pore-water concentrations over the root depth interval. At the site where As and Sb sediment concentrations are low relative to other sites, both elements are well attenuated at the sediment-water interface. Where As and Sb concentrations are elevated in sediment, competition for sorption sites is the most likely cause of low As and elevated Sb concentrations in relatively oxidized pore-water and surface water. Below the oxic sediment-water interface As concentrations in pore-water are elevated and Sb concentrations are low. The increase of As concentrations in suboxic pore-water is associated with reduction to As(III) and subsequent remobilization, as evidenced by the dominance of As(V) in shallow, oxic pore-water and As(III) in suboxic pore-water. Consistent with pore-water observations, the As(V)-O solid phase becomes depleted with depth below the sediment-water interface. In contrast, the Sb(V)-O solid phase persisted and the Sb(V) species dominated in all but one pore-water sample, even under reducing conditions. Antimony(III) appears to preferentially precipitate or adsorb onto sulfides as evidenced by the prevalence of an Sb(III)-S secondary solid-phase and the lack of Sb(III)(aq) in the deeper zones. The co-existence of reduced and oxidized As and Sb species and instability of some phases under changing redox conditions pose challenges for remediation efforts at the mine.