Isotopic and Chemical Assessment of the Dynamics of Methane Sources and Microbial Cycling during Early Development of an Oil Sands Pit Lake

Abstract

Water-capped tailings technology (WCTT) is a key component of the reclamation strategies in the Athabasca oil sands region (AOSR) of northeastern Alberta, Canada. The release of microbial methane from tailings emplaced within oil sands pit lakes, and its subsequent microbial oxidation, could inhibit the development of persistent oxygen concentrations within the water column, which are critical to the success of this reclamation approach. Here, we describe the results of a four-year (2015–2018) chemical and isotopic (δ13C) investigation into the dynamics of microbial methane cycling within Base Mine Lake (BML), the first full-scale pit lake commissioned in the AOSR. Overall, the water-column methane concentrations decreased over the course of the study, though this was dynamic both seasonally and annually. Phospholipid fatty acid (PLFA) distributions and δ13C demonstrated that dissolved methane, primarily input via fluid fine tailings (FFT) porewater advection, was oxidized by the water column microbial community at all sampling times. Modeling and under-ice observations indicated that the dissolution of methane from bubbles during ebullition, or when trapped beneath ice, was also an important source of dissolved methane. The addition of alum to BML in the fall of 2016 impacted the microbial cycling in BML, leading to decreased methane oxidation rates, the short-term dominance of a phototrophic community, and longer-term shifts in the microbial community metabolism. Overall, our results highlight a need to understand the dynamic nature of these microbial communities and the impact of perturbations on the associated biogeochemical cycling within oil sands pit lakes.

Alum Addition Triggers Hypoxia in an Engineered Pit Lake

“…Changes in phospholipid fatty acid (PLFA) concentrations and profiles consistent with this scenario also occurred, as described in more detail in Slater et al [ 56 ]. Specifically higher PLFA concentrations and increased autotrophic PLFA abundance (i.e., C18:1 unsaturated, polyunsaturated), as well as δ 13 C values of PLFA consistent with autotrophy, collectively indicate the establishment of an epilimnetic phototrophic microbial community post alum.…”

“…Branched PLFA are generally associated with heterotrophic microbial communities, suggesting the greater establishment of heterotrophy in the bottom waters of BML by 2018. Stable isotopic compositions (δ 13 C) of PLFA in 2018 showed some of the largest changes over the four-year interval studied in Slater et al [ 56 ]. Most notably, there was a convergence of epilimnetic δ 13 C of PLFA in August 2018 to that observed in May 2017, consistent with the establishment of a phototrophic community two years post alum addition.…”

Ebullition Regulated by Pressure Variations in a Boreal Pit Lake

“…Bubbles have been observed to rise through the water column using echo sounding (Lawrence et al, 2016;Zhao et al, 2021). In their modelling of methane dynamics in Base Mine Lake, Slater et al (2021) have assumed that the rising bubbles were: "composed either entirely of methane, or of 75% methane and 25% nitrogen (by volume), based on the results of the preliminary gas analyses (unpublished data)".…”