Pyrite Weathering in Reclaimed Shale Overburden at an Oil Sands Mine near Fort McMurray, Canada

Abstract

Saline-sodic shale overburden associated with oil sand mining is a potential source of salt release to surface water and groundwater and can lead to salinization and/or sodification of reclamation covers. Weathering of shale overburden due to oxidation of sulphide minerals within the shale leads to sulphate (SO4 2−) production and increased salinity. The controls on the rates of weathering of a shale overburden dump in the oil sands region of northern Alberta were determined from soil chemistry sampling and in situ monitoring of pore gases (O2, CO2, CH4) in three shallow profiles (1.9–4.45 m deep) and one deep (25 m deep) profile under reclamation covers of varying thickness. Oxidation, defined by the depth over which O2 concentrations were depleted, reached depths of approximately 1.1 m under the reclamation soil covers over a 6 year period after dump placement. Calculations of SO4 2− production rates and weathering depths were consistent with numerical simulations of the diffusion and subsequent consumption of atmospheric O2 in the overburden. The rate of SO4 2− production during the 6 year weathering period estimated from direct measurements of solids chemistry ranged from 0.70 to 8.3 g m−2 day−1. The rates calculated from the oxygen diffusion models were within that same range, between 1.6 and 4.1 g m−2 day−1.