Environmental context Base Mine Lake (BML) is a demonstration pilot pit lake for long term oil sands tailings reclamation in Alberta, Canada. This study quantified BML water cap sulfur mass balance and speciation to help understand potential risks to oxygen levels during its early-stage development. Results provide important insights for the adaptive management of water-capped oil sands tailings reclamation. Rationale Sulfur cycling is crucial to the persistence of oxygen in the water cap of Base Mine Lake (BML), the first demonstration oil sands pit lake for water capped tailings technology (WCTT) in Alberta, Canada. Methodology Here, we report on the first investigation of sulfur mass balance and sulfur speciation (SO42−, SO32−, S2O32−, S0 and ƩH2S) over seasonal, annual and spatial scales in BML. Results and discussion High aqueous total sulfur concentration (1.7–2 mM), dominated by sulfate (>75%), decreased over the study period (2015–2021), due to the consolidation of fluid fine tailings (FFT) and operational pump-in and pump-out activities. Expanded BML water cap S biogeochemical cycling occurred after a 2016 alum amendment. Late summer hypolimnetic anoxia emerged post-alum (2017–2021), coincident with detectable total sulfide (ƩH2S) and elemental sulfur (S0) concentrations and expanded sulfur-reducing bacteria activity in anoxic bottom waters. Post spring turnover resuspended FFT and particle settling rates also likely increased post-alum, supported by the observed migration of epilimnetic highest sulfite concentration (pre-alum) to metalimnetic waters (post-alum). These sulfide containing particles are likely the primary reduced S substrate for spring–summer sulfur-oxidising bacteria activity, as winter aqueous reactive S species (ƩH2S, S0, S2O32− and SO32−) were non-detectable across years. Concentrations of reactive S species reached up to 200 µM, posing risks to BML O2 levels (maximum 300–350 µM). Results of this study establish the interactive effects of physical and biogeochemical processes, as well as operational activities in emergent S risks to water cap oxygen levels, a key criterion for success of this reclamation tailings technology.