Alum Addition Triggers Hypoxia in an Engineered Pit Lake.

Area of Science:

• Geobiology
• Environmental chemistry
• Microbial ecology

Background:

• Base Mine Lake (BML) is the first pilot-scale pit lake in the Alberta oil sands region.
• Alum (aluminum sulfate) treatment is a common method to reduce phosphorus loading in eutrophic lakes.

Purpose of the Study:

• To investigate the geobiological impacts of a whole-lake alum treatment on BML.
• To assess the effectiveness and potential risks of alum treatment for oil sands tailings reclamation.

Main Methods:

• Whole-lake alum treatment applied in 2016.
• Monitoring of water cap epilimnetic oxygen, Secchi depths, chlorophyll-a, nitrate, and sulfide concentrations.
• Analysis of hypolimnetic phospholipid fatty acid abundances and microbial community shifts.

Main Results:

• Modest increases in epilimnetic oxygen, Secchi depths, and chlorophyll-a.
• Anoxic conditions developed above the fluid fine tailings (FFT) layer.
• Decreased nitrate, detectable sulfide, and increased anaerobic heterotrophic activity indicated by phospholipid fatty acids.
• Microbial community shifts favored organic carbon degradation and sulfate-reducing bacteria (SRB).
• Loss of specialist microbial groups adapted to specific niches.

Conclusions:

• Alum treatment led to increased autochthonous carbon production, exacerbating oxygen consumption.
• Anoxia and sulfide generation were unintended consequences, increasing risks associated with FFT reductants.
• Understanding broader biogeochemical implications is crucial for adaptive management in tailings reclamation.
• Results emphasize the need for careful consideration of interventions to improve oil sands operations and mining practices globally.