Initial geochemical characteristics of fluid fine tailings in an oil sands end pit lake

Geochemical characteristics of fluid fine tailings (FFT) were examined in Base Mine Lake (BML), which is the first full-scale demonstration oil sands end pit lake (EPL) in northern Alberta, Canada. Approximately 186Mm3 of FFT was deposited between 1994 and 2012, before BML was established on December 31, 2012. Bulk FFT samples (n=588) were collected in July and August 2013 at various depths at 15 sampling sites. Temperature, solid content, electrical conductivity (EC), pH, Eh and alkalinity were measured for all samples. Detailed geochemical analyses were performed on a subset of samples (n=284). Pore-water pH decreased with depth by approximately 0.5 within the upper 10m of the FFT. Major pore-water constituents included Na (880±96mgL−1) and Cl (560±95mgL−1); Ca (19±4.1mgL−1), Mg (11±2.0mgL−1), K (16±2.3mgL−1) and NH3 (9.9±4.7mgL−1) were consistently observed. Iron and Mn concentrations were low within FFT pore water, whereas SO4 concentrations decreased sharply across the FFT–water interface. Geochemical modeling indicated that FeS(s) precipitation was favoured under SO4-reducing conditions. Pore water was also under-saturated with respect to gypsum [CaSO4·2H2O], and near saturation with respect to calcite [CaCO3], dolomite [CaMg(CO3)2] and siderite [FeCO3]. X-ray diffraction (XRD) suggested that carbonate-mineral dissolution largely depleted calcite and dolomite. X-ray absorption near edge structure (XANES) spectroscopy revealed the presence of FeS(s), pyrite [FeS2], and siderite. Carbonate-mineral dissolution and secondary mineral precipitation have likely contributed to FFT dewatering and settlement. However, the long-term importance of these processes within EPLs remains unknown. These results provide a reference for assessing the long-term geochemical evolution of oil sands EPLs, and offer insight into the chemistry of pore water released from FFT to the overlying water cover. [Display omitted] •First study of fluid fine tailings geochemistry in oil sands end pit lake•Carbonate dissolution has promoted ion exchange reactions.•Microbial iron and sulfate reduction influence pore-water chemistry.•Biogeochemical processes have likely enhanced tailings settlement.•Long-term geochemical evolution of oil sands end pit lakes remains unknown.