Pore-gas dynamics in overburden and reclamation soil covers

Abstract

Pore-gas dynamics in single-layered and multi-layered soil covers were characterised in a mine cover testing programme for the reclamation of lean oil sand overburden. Pore-gas concentrations of oxygen and carbon dioxide in the upper 1·5 m of soil covers and overburden did not reach the threshold that poses a risk for plant growth. Below 1·5 m in the overburden, however, oxygen dropped to 0% and carbon dioxide rose to >16%. While methane concentrations were typically <1·7%, there was a peak in methane at one sampling location to concentrations >35%. Novel subsurface flux chambers were designed and fabricated to measure directly oxygen ingress through soil covers and carbon dioxide efflux from overburden into soil covers. Oxygen fluxes peaked at 18·0 kg/(m2 a) and carbon dioxide fluxes peaked at 2·3 kg/(m2 a). Soil cover material and placement thickness affected gas flux rates. Diffusive fluxes were calculated from soil gas profiles by estimating diffusion coefficients from position-dependent soil moisture and temperature. Advective fluxes were calculated from pressure gradients and in situ air conductivity. Advection dominated over diffusion in soil covers except for one location that was either a localised zone of microbial activity (hydrocarbon degradation or methane oxidation) or the overburden was coarser textured and responded quickly to barometric pressure fluctuations.