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 (SO
4
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 (O
2
, CO
2
, CH
4
) 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 O
2
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 SO
4
2−
production rates and weathering depths were consistent with numerical simulations of the diffusion and subsequent consumption of atmospheric O
2
in the overburden. The rate of SO
4
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
.