Geochemistry and geomicrobiology of fluid fine tailings in an oil sands end pit lake

Please join us for the final seminar of the year – a graduate student seminar this Friday April 6 at 3:30 pm in rm 155 Geology presented by Sarah Rudderham, MSc Candidate:

Geochemistry and geomicrobiology of fluid fine tailings in an oil sands end pit lake

Oil sands end pit lakes (EPLs) are a proposed mine closure landscape for fluid fine tailings (FFT), where FFT are transferred to a mined-out pit and capped with fresh and process water. Base Mine Lake (BML) is the first full-scale demonstration EPL and is located at Syncrude Canada Limited’s Mildred Lake mine. The geochemical development of BML has important implications for the viability of EPLs as a reclamation strategy within the Alberta oil sands. Anaerobic microbial metabolisms can control pore-water pH, as well as the abundance and distribution of reduced chemical species within FFT and across the tailings-water interface. Samples were collected from BML in 2016 and 2017, extending from 0.5 m above the tailings-water interface to 40 m below the interface. High throughput amplicon sequencing and a detailed analysis of FFT pore-water chemistry was conducted to identify the spatial distribution of microbial populations and associated geochemical gradients. Preliminary sequencing results indicated the presence of microbes with a diverse range of metabolisms, notably sulfate reduction and methanogenesis. Sulfate-reducing genera of interest included Desulfocapsa, Desulfuromonas and Desulfoprunum. Methane-producing genera included Methanoregula and Methanosaeta. Pore-water pH decreased sharply below the tailings-water interface from above 8.1 to below 7.8 and fluctuated with depth, reaching a minimum of 6.9. Concentrations of dissolved NH₃-N, Fe, H₂S and CH₄ identified zones of nitrogen cycling, iron reduction, sulfate reduction and methanogenesis. Specifically, H₂S concentrations fluctuated within the first 15 m below the tailings-water interface and reached a maximum of over 1 mg L^-1 within the first 3 m. Dissolved SO₄ concentrations decreased correspondingly across the interface from above 150 mg L^-1 to below 50 mg L^-1 throughout the depth profile. Ongoing research will further assess the influences of microbial processes on FFT pore-water chemistry and the implications for the biogeochemical development of EPLs.