In Northern Alberta, Canada, large volumes of low-grade ‘lean’ oil sand (LOS) overburden are translocated during the surface mining of oil sands and remain in future reclaimed landscapes. The objectives addressed in this paper are to (a) characterise the on-site petroleum hydrocarbon (PHC) content of LOS; (b) evaluate the effect of LOS temperature on rates of carbon dioxide (CO2) flux and PHC biodegradation and (c) evaluate the potential for PHC to leach from LOS into groundwater. The results show that LOS is predominantly composed of heavier F3 and F4 PHC fractions, the temperature appears to affect carbon dioxide fluxes and PHC degradation rates and it is unlikely that the presence of LOS in reclamation soils will release significant quantities of PHC into groundwater.

The storage and transportation of pore gasses in overburden and reclamation soil covers were evaluated using statistical analyses and finite difference numerical modelling to guide mine operators regarding practical issues surrounding the construction of overburden landforms, design of soil cover systems and management of reclamation sites. Factors that were found to impact gas transfer were soil moisture, soil temperature, differential pressures and dry bulk density of the overburden landform. Furthermore, the construction of the overburden landform appears to be more impactful to pore-gas dynamics than the design of the soil covers. Practicable recommendations can therefore be inferred to facilitate simultaneously methane oxidation in the uppermost horizon of the overburden while maintaining sufficient pore-gas oxygen in the plant-rooting zone of the soil covers to facilitate growth and survivability of reclamation vegetation. It is recommended that overburden be placed to approximately 1·6 Mg/m3. Mine operators should also recognise and manage extreme moisture conditions in the soil covers and uppermost overburden to mitigate restrictions in gas exchange and methane oxidation.

Large above-grade overburden landforms comprised of low-grade lean oil sands (LOS) are amassed during the surface mining of oil sands in northern Alberta, Canada. Reclamation soil covers consisting of locally-salvaged soils are subsequently placed above the LOS landforms in single and multi-layered configurations. The soil covers are intended to 1) provide sufficient pore-gas O2 to facilitate the growth of native boreal forest vegetation and to 2) oxidize methane (CH4) produced in the anaerobic zones of the LOS before being exhausted to the atmosphere as a greenhouse gas.Prior to covering the LOS with soil covers, rates of CO2 efflux from the surface of the LOS to the atmosphere ranged from 0.1-7.1 kg/m2/a. Pore-gas concentrations within the uncovered LOS ranged from 0-18% for oxygen (O2), 3-21% for carbon dioxide (CO2), and 0-12% for CH4. Following placement of soil covers, peak rates of CO2 efflux were 2.3 kg/m2/a from the surface of the soil covers to the atmosphere and 1.8 kg/m2/a from the LOS into the soil covers.Peak rates of O2 influx from the atmosphere through the soil covers was 18 kg/m2/a. Poregasses within the overlying soil covers and uppermost LOS were typically below the threshold that poses a risk to the survivability of reclamation vegetation, >10% O2 and <15% CO2. Poregasses deeper than 2 m within the LOS surpassed this threshold with O2 falling to 0% and CO2 rising to >16%.Rates of CH4 oxidation were quantified in batch soil column experiments for the soil cover materials and LOS in single and multi-layered configurations. Oxidation rates were sensitive to variations in temperature, moisture content, and bulk density. The results of the column experiments indicate that CH4 generated deeper than 2 m within the LOS landform will be partially oxidized in both the soil covers and uppermost LOS horizon.Statistical analyses and finite difference numerical modelling were conducted to guide mine operators regarding practical issues involving the construction of LOS landforms, design of soil cover systems, and management of reclamation sites. Based on these exercises, it appears that the characteristics of the LOS landform are more important to pore-gas dynamics than the design of the soil cover systems.iii ACKNOWLEDGEMENTS