Relationship between Nutrient Availability, Microbial Activities, Mineral Transformations and Partitioning of Constituents of Potential Concern Under End Pit Lake Scenario.

Fluid fine tailings (FFT) are a by-product of bitumen extraction from surface-mined oil sands ore (in Alberta, Canada) and comprised of oil sands process-affected water (OSPW), fine particles, unrecovered bitumen and residual solvents. End pit lakes (EPLs) with FFT below a water cap in mined out pits are one of the potential strategies to reclaim the large amount of FFT and OSPW produced (>1 million m³ daily). A potential concern with EPLs is the flux of constituents of potential concern (COPCs) from the underlying FFT into the overlying cap water contributing to on-going sources of possible turbidity and toxicity. In this study, chemical, mineralogical and microbiological approaches were used to investigate how biogeochemical processes in the underlying FFT would affect COPC transport across the mudline in EPLs. Columns (10L) were filled with FFT (7 L) and cap water (1.4L), sealed anaerobically and incubated in the dark at room temperature. FFT were amended with labile hydrocarbons endogenous to FFT, nitrogen and phosphorus (at a C: N: P ratio of 100:10:1) to accelerate biogeochemical processes. Our results demonstrated higher microbial activity in amended columns, resulting in decreased pH, increased concentrations of Ca²⁺, Mg²⁺, HCO₃^- and trace metals (Sr, Ba, V and Ni), faster FFT settling and higher chemical flux rates compared to un-amended columns. Fe ^III to Fe ^II minerals transformation was observed in amended columns through iron fractionation. Trace metal host phases in FFT were detected by sequential extraction to characterize the mechanisms behind partitioning of trace metals in EPLs. These results can help in assessing the water quality in EPL and understanding the role that nutrients (N, P) will play in the COPC flux from the underlying FFT into the overlying cap water.