Controls on arsenic speciation in mine tailings and co-existing pore water in freshwater and marine settings from the Lower Seal Harbour Gold District, Nova Scotia, Canada.

摘要

Environmental impacts from historical mining and milling of lode gold deposits in eastern Nova Scotia include elevated arsenic (As) and mercury (Hg) concentrations within the tailings and in downstream receiving environments. The Lower Seal Harbour Gold District (LSH), operated between 1904 and the mid-1940s, produced more than 435,000 tons of tailings. Examination and understanding of the speciation of As in the tailings and associated pore water is needed to better understand the processes that control the mobility of As from these mine wastes.; Within LSH, three geographically and geochemically distinct sample locations were chosen to collect tailings and pore water samples, including: Site 1 where the tailings are sub-aerially exposed; Site 2 where the tailings are shallowly submerged beneath approximately 20 cm of slow-moving water, and Site 3 where tailings have been transported approximately 2 km downstream, with eventual deposition into the Atlantic Ocean in Seal Harbour. Several techniques were employed to examine and characterize the tailings and marine sediments, including chemical analyses via ICP-MS, organic carbon analysis, petrography, electron microprobe analysis, x-ray diffraction and sequential extraction analyses. Arsenic is present in the tailings at Sites 1, 2 and 3 at maximum concentrations of 9,200 mg/kg, 12,000 mg/kg and 1,040 mg/kg, respectively. Elevated concentrations of As correspond to arsenopyrite-bearing sections of the tailings cores, or intervals with abundant As-bearing secondary phases. Oxidation of near-surface tailings at Sites 1 and 2 has resulted in the formation of Ca-Fe arsenates rimming arsenopyrite and As-bearing iron oxyhydroxides. Arsenopyrite grains analysed from Site 3 show no evidence of oxidation. Sequential extraction analyses indicate As is preferentially partitioned into amorphous and crystalline Fe oxides in the near-surface samples at Sites 1 and 2, and to sulphide phases at depth. Arsenic in the tailings samples collected from Site 3 is partitioned mainly to the sulphide phases throughout most of the core.; Tailings pore waters were sampled using dialysis arrays and analyzed for a range of cations, inorganic arsenate and arsenite, pH, ferrous iron, sulphide and sulphate. Pore water analyses indicate oxidizing conditions persisted to depths of 10--30 cm, 5 cm and 10 cm below the sediment-water interface at each of the three sample sites. Measured pH values ranged from 6.24 to 8.03.; The results from this study will help to clarify the potential ecological and human health risks surrounding historical gold mines, and will support better informed land-management decisions.