Alum Addition Triggers Hypoxia in an Engineered Pit Lake

Abstract

Here, we examine the geobiological response to a whole-lake alum (aluminum sulfate) treatment (2016) of Base Mine Lake (BML), the first pilot-scale pit lake established in the Alberta oil sands region. The rationale for trialing this management amendment was based on its successful use to reduce internal phosphorus loading to eutrophying lakes. Modest increases in water cap epilimnetic oxygen concentrations, associated with increased Secchi depths and chlorophyll-a concentrations, were co-incident with anoxic waters immediately above the fluid fine tailings (FFT) layer post alum. Decreased water cap nitrate and detectable sulfide concentrations, as well as increased hypolimnetic phospholipid fatty acid abundances, signaled greater anaerobic heterotrophic activity. Shifts in microbial community to groups associated with greater organic carbon degradation (i.e., SAR11-LD12 subclade) and the SRB group Desulfuromonodales emerged post alum and the loss of specialist groups associated with carbon-limited, ammonia-rich restricted niches (i.e., MBAE14) also occurred. Alum treatment resulted in additional oxygen consumption associated with increased autochthonous carbon production, watercap anoxia and sulfide generation, which further exacerbate oxygen consumption associated with on-going FFT mobilized reductants. The results illustrate the importance of understanding the broader biogeochemical implications of adaptive management interventions to avoid unanticipated outcomes that pose greater risks and improve tailings reclamation for oil sands operations and, more broadly, the global mining sector.

Anaerobic Microbial Activity May Affect Development and Sustainability of End-Pit Lakes: A Laboratory Study of Biogeochemical Aspects of Oil Sands Mine Tailings

“…However, several key issues with EPLs need investigation to understand their environmental implications. , The first full-scale demonstration of EPL (∼8 km 2 surface area) was commissioned in 2013 after FFT was transferred from the active Mildred Lake Settling Basin (tailings pond) into the closed West In-Pit mine site to create Base Mine Lake (BML) as a potential FFT reclamation strategy . Since then, several aspects of BML development and progression into environmentally functional aquatic ecosystem have been under investigation, particularly in the cap water zone of BML, at the laboratory and full scale including dissolved oxygen consumption in response to biogeochemical changes; alum addition , and role of biofilms and carbonate mineral dissolution in turbidity reduction; , wind action effects on physical processes (mixing, oxygenation, and turbidity); nitrogen cycling; and the effect of chemical fractions on fish reproduction . The sustainability of these ecological processes in cap water is strongly influenced by cap water chemistry which, in turn, is linked to FFT porewater chemistry via advective and diffusive chemical flux from FFT to cap water; the latter process will dominate as FFT settlement slows with time. ,, While investigating the effect of cap water chemistry on ecological processes, it is important to understand the driving forces so as to predict longevity of chemical flux/partitioning and project EPL evolution to a functional aquatic ecosystem.…”

Evaluating the long-term behaviour and viability of an oil sands tailings management and reclamation strategy

“…Previous studies have reported extensive sulfate reduction in similarly sulfate-amended FFT deposits, resulting in the generation of aqueous and gaseous hydrogen sulfide (H2S) (Reid & Warren 2016;Warren et al 2016). Additionally, alum was added to the BML water cap in 2016, resulting in the generation of aqueous sulfide species near the FFT-water interface and a shift in microbial communities, including the appearance of sulfate-reducing bacteria (Jessen et al 2022). As such, the implications of PASS treatment on biogeochemical cycling processes, particularly sulfur cycling, in EPLs must be evaluated.…”