Geochemistry of arsenic in low sulfide-high carbonate coal waste rock, Elk Valley, British Columbia, Canada

This study investigated the geochemistry of arsenic (As) in low sulfide-high carbonate coal waste rock of the Elk Valley, British Columbia, Canada. Its abundance and mineralogical associations in waste rock of different placement periods were determined in addition to its mobilization into porewater and rock-drain effluent. The mean (5.34 mg/kg; 95% confidence interval: 4.95–5.73 mg/kg) As concentration in the waste rock was typical of sedimentary rock. Electron microprobe and As K-edge X-ray absorption near-edge spectroscopic analyses showed the As is predominantly associated with primary pyrites in both source and freshly blasted waste rock. However, in aged waste rock the As is associated with both primary pyrites and secondary Fe oxyhydroxides. Oxidation of pyrite in waste rock dumps was reflected by the presence of high concentrations of SO₄²⁻ in porewater and oxidation rims of Fe oxyhydroxides around pyrite grains. Acid released from pyrite oxidation to Fe oxyhydroxides is neutralized by carbonate mineral dissolution that buffers the pH in the waste rock to circumneutral values. Adsorption of As onto secondary Fe oxyhydroxides provides an internal geochemical control on As release during pyrite oxidation and porewater flushing from the dump, resulting in the low As concentrations observed in porewater (median: 9.91 μg/L) and rock-drain effluent (median: 0.31 μg/L). Secondary Fe oxyhydroxides act as a long-term sink for As under present day hydrologic settings in waste rock dumps in the Elk Valley. - Highlights: • Geochemistry of As in low sulfide-high carbonate coal waste rock is discussed. • The major As-containing mineral phases are primary pyrite and secondary Fe oxides. • Carbonate dissolution facilitates Fe oxide precipitation during pyrite oxidation. • Sorption to Fe oxides is a geochemical control on As release during pyrite oxidation. • Fe oxides are a long-term sink for As in waste rock dumps under oxic conditions.