The efficacy of oil sands water capped tailings technology is currently being assessed in the first pilot pit lake in the Athabasca Oil Sands region (AOSR). Base Mine Lake (BML, Syncrude Canada Mildred Lake mine, Fort McMurray AB Canada) was commissioned in December of 2012 and consists of circa 40m depth of fluid fine tailings (FFT) overlain by circa 10m of a freshwater cap. As this is the first oil sands pit lake, it is unknown to what extent known oxygen consuming constituents (OCC) such as methane, hydrogen sulfide and ammonia will mobilize from the underlying FFT layer in BML and impact oxygen concentrations within the water cap. Thus, the field objectives here were to characterize the physico-chemistry and geochemistry of the BML water cap from the FFT water interface (FWI) to the surface over two summers (2015 and 2016). Results identify that the similar to 10m water cap thermally stratifies and that oxygen persists, albeit at low levels (i.e. <5% saturation) to the FWI during the summer season for both years. Consistent with the FFT acting as an OCC source, aqueous CH4, Sigma H2S and NH4+ concentrations were highest closer to the FWI, decreasing upwards into the water cap. As O-2 persists to the FWI, Sigma H2S was rapidly removed within this region, with little mobilized to the overlying water as evidenced by nondetectable Sigma H2S within 1 m of the FWI. In contrast, CH4 and NH4+ concentrations were detectable higher up into the water cap, indicating incomplete oxidation at the FWI most likely due to oxygen limitation. In 2015, CH4 was the only identified variable significantly negatively related to BML water cap oxygen concentrations. However in 2016, NH4+ emerged as an important OCC negatively related to water cap oxygen concentrations in addition to CH4. Mass balance of nitrogen redox species throughout the water column are consistent with active nitrification occurring in 2016; which was not evident in 2015 results. Thus, within 4 years of commissioning, nitrification has become an active and important process affecting water column oxygen concentrations within this pit lake. This is in contrast to an absence of detectable microbial nitrification in active oil sands tailing ponds, despite high levels of ammonia. While the absence of nitrification occurring in oil sands tailings ponds has previously been suggested to reflect naphthenic acids toxicity, results here identify that some lower limit of oxygen may also be required for this metabolism to flourish and that at very low oxygen levels (i.e. <5% saturation), nitrifying microbes appear to be outcompeted by methanotrophs, at least during early establishment stages within a pit lake.
