Pore-gas Dynamics in Overburden and Reclamation Soil Covers

Abstract

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. Pore-gasses 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. Pore-gasses 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.