Field determination of sulphide oxidation rates in mine tailings

A field study was conducted to assess the rates of oxygen consumption in the vadose zone of sulphide mine tailings. Oxygen uptake rates were measured directly at the ground surface and were compared to rates of sulphate production in the tailings profile. A nonreactive tracer gas was also used to assess effective diffusion coefficients in the shallow subsurface. The 26 measurement sites were located within a 0.1 km2 area in tailings with a water table depth ranging from 0.6 m to ponding at the surface. Ambient oxygen consumption rates in the 8‐year‐old tailings were compared to measurements through 0.2‐m‐thick covers of fine sand and 0.2 m layers of fresh tailings replacing oxidized tailings at ground surface. The oxygen flux across the tailings surface ranged from 0.1 to 250 mol O2 m−2 yr−1 for ambient oxidation conditions. Fresh tailings at the surface produced rates that were as much as twice the ambient rates. The 0.2 m fine sand cover lowered rates by a factor of 100 below ambient rates. Sulphate production rates agreed well with oxygen consumption measurements. Independent diffusion flux measurements with a nonreactive gas exhibited an excellent correlation with oxygen uptake fluxes, indicating the dominance of oxygen diffusion in the control of oxidation rates in the tailings. Spatial trends in oxidation rates were consistent with depth to the water table, and temporal changes by as much as a factor of 2 were attributed to a 0.16 m variation in the depth to the water table over a 2‐week period. The results of this study support the use of simple Fickian models to evaluate oxygen diffusion and oxidation rates in sulphide tailings.