Evaluation of boreal plant and microbe communities on tailings solvent recovery unit (TSRU) tailings: A meso-scale greenhouse study

Surface mining and bitumen extraction in Alberta's oil sands generates various tailings waste streams as by-products. Among these tailings, froth treatment tailings (FTT), originating from the froth treatment process, are particularly complex due to high levels of iron sulfide minerals (i.e. pyrite) with acid mine drainage (AMD) potential. Paraffinic FTT that are passed through a Tailings Solvent Recovery Unit (TSRU) to recover residual solvent are referred to as TSRU tailings. This study evaluated the ability of TSRU tailings to support upland and wetland vegetation under various thin capping designs using peat mineral soil mix (PMM) and coarse sand tailings (CST). A 30 cm PMM cover cap successfully supported upland and wetland vegetation for three years, while a thicker, multi-layer cap of 30 cm PMM above 20 cm of CST further improved plant survival and growth. The cap was found to effectively act as a barrier, protecting the vegetation from the adverse effects of pyrite oxidation in the TSRU tailings. While the TSRU tailings acidified (pH ≤ 2) under all capping treatments, the thickest cap (30 cm PMM/20 cm CST) delayed acidification by one year. TSRU and CST had low microbial biomass and diversity, with bacterial communities mostly composed of sulfur oxidizers and acidophilic taxa. In contrast, the PMM layer maintained a higher microbial biomass, diversity, and stability across all treatments, highlighting its potential to enhance plant-microbe interactions and improve the reclamation success of TSRU tailings.