Chemical succession of naphthenic acid fraction compounds in reclamation landscape mesocosms established on centrifuged and co-mixed fluid fine tailings from the Athabasca oil sands

The Athabasca oil sands region of Alberta, Canada contains one of the world's largest unconventional petroleum deposits. There is concern about residual contaminants where tailings are integrated during reclamation and the related adverse effects this may have. Some of the primary toxic organic contaminants in oilsands tailings are naphthenic acid fraction compounds (NAFCs). To plan for successful reclamation, it is imperative to understand the behaviour and fate of tailings-derived NAFCs over time where these are re-integrated into landforms. In this companion article to Degenhardt et al. (2023), we examine different reclamation integration scenarios using oil sands process-affected materials (OSPMs) at laboratory scale using wetland plants and minimal amounts of caps to explore how cap materials and thickness will affect the of NAFCs in the systems. Specifically, we examine how capping materials and thicknesses affect NAFCs adsorbed to two treated fluid fine tailings, centrifuged (CF) and co-mixed (CM) tailings over three years. Both CF and CM were placed in columns capped with peat mineral mix (PMM) and/or glacial till and planted with graminoid or woody wetland plants. Both CF and CM had considerable NAFC concentrations (CF 3717 mg/kg, CM 321 mg/kg). The O₂ NAFC class was >90 % of the total adsorbed to CF and CM throughout. Relatively thin PMM caps reduced NAFC burdens in CF columns, where 10 cm of PMM resulted in the greatest reduction. Expressed water from substrate self-weight consolidation had significantly lower NAFC concentrations when capping material was present. However, NAFC oxidation (i.e., O₂ to O₃, O₃ to O₄, etc.) was limited in capped CF and CM. These results indicate that capping CF and CM with a thin (5-10 cm) layer of PMM may help reduce NAFC concentrations and limit their mobility into the cap, potentially lowering plant mortality and improving tailings reclamation success rates.