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Mining leases in the Athabasca oil sands (AOS; near Ft. McMurray, Canada) produce large volumes of oil sands process-affected water (OSPW) that contain potentially problematic constituents requiring treatment prior to surface water discharge into receiving aquatic systems. The aim of this research was to identify constituents of concern (COCs) in OSPW sourced from an AOS external tailings facility and design a hybrid pilot-scale constructed wetland treatment systems (CWTS) to decrease concentrations of COCs and subsequently mitigate risks. COCs were identified based on comparisons to ambient water quality thresholds for the protection of aquatic life (i.e. Canadian Environmental Quality Guidelines [CEQGs], Alberta Environment Water Quality Guidelines [Alberta WQGs], and United States Environmental Protection Agency Water Quality Criteria [USEPA WQC]) and toxicity endpoints. Performance of the hybrid pilot-scale CWTS was evaluated by rate and extent of COC removal and change in toxicity as measured by an aquatic invertebrate Ceriodaphnia dubia . Following characterization of OSPW, specific COCs were identified as: naphthenic acids (NAs), oil and grease (O/G), metals/metalloids (Al, B, Cu, Ni, Se, and Zn), chloride, and total suspended solids (TSS). A hybrid pilot-scale CWTS was designed to promote treatment processes to alter (transfer and transform) COCs using sequential reducing and oxidizing wetland reactors and a solar photocatalytic treatment reactor using fixed film titanium dioxide (TiO 2 ). Performance criteria were achieved as the CWTS decreased concentrations of NAs, O/G, and metals below protective thresholds and decreased toxicity to C . dubia . Results from this study provide proof-of-concept data to inform hybrid passive or semi-passive treatment approaches (i.e. constructed wetlands) that could be used to mitigate COCs contained in OSPWs. [ABSTRACT FROM AUTHOR]