Methane production dynamics in fluid fine tailings of an oil sand pit lake

The reclamation of oil sands mining sites is a regulatory requirement for oil sands mine operators, ensuring the restoration of post-mining landscapes to locally common, functional ecosystems. Water-capped tailings technology (WCTT), which involves placing a layer of water over tailings deposited in a mined-out pit to create a pit lake. This is a tailings reclamation concept being tested. The Base Mine Lake Demonstration (BML), commissioned by Syncrude Canada Ltd in 2012, serves as the first full-scale demonstration of water-capped tailings technology. Microbial methane production in fluid fine tailings (FFT) influences BML development because aerobic methane oxidation in the water column can deplete oxygen levels, methane ebullition transports organic compounds from FFT to the water column, and methane emission from the lake is a source of greenhouse gas. This study investigated methane production dynamics in BML by measuring methane concentrations, stable carbon isotope composition (δ 13 C), and methane production rates across five sampling locations. Dissolved methane concentrations showed consistency with previous years. Methane production rates in BML FFT ranged from 1 to 28 μmol/L FFT/day, comparable to rates observed for petroleum hydrocarbon-derived microbial methane production. Methane production rates exhibit spatial variability across depths and locations and were strongly correlated with naphtha content, suggesting that the low molecular weight hydrocarbons in naphtha serve as a primary carbon source for microbial methanogenesis in FFT. These findings on methane production dynamics have direct implications for understanding the BML development and optimizing oil sands reclamation strategies. • Methane production rates were systematically quantified for the first time in a commercial-scale oil sands pit lake. • Methane production rates in Base Mine Lake fluid tailings ranged from 1 to 28 μmol/L FFT/day, comparable to petroleum hydrocarbon-derived microbial methane production. • Methane production exhibited spatial variability with depth and location and was strongly correlated with naphtha content, indicating that low-molecular-weight hydrocarbons in naphtha are a key carbon source for methanogenesis in FFT. • Dissolved methane concentrations monitored in 2021 in Base Mine Lake fluid tailings showed consistency with data in previous years.