Increases in salinity following a shift in hydrologic regime in a constructed wetland watershed in a post-mining oil sands landscape

Bitumen extraction via surface mining in the Athabasca Oil Sands Region results in permanent alteration of boreal forests and wetlands. As part of their legal requirements, oil companies must reclaim disturbed landscapes into functioning ecosystems. Despite considerable work establishing upland forests, only two pilot wetland-peatland systems integrated within a watershed have been constructed to date. Peatland reclamation is challenging as it requires complete reconstruction with few guidelines or previous work in this region. Furthermore, the variable sub-humid climate and salinity of tailings materials present additional challenges. In 2012, Syncrude Canada Ltd. constructed a 52-ha pilot upland-wetland system, the Sandhill Fen Watershed, which was designed with a pump and underdrain system to provide freshwater and enhance drainage to limit salinization from underlying soft tailings materials that have elevated electrical conductivity (EC) and Na⁺. The objective of this research is to evaluate the hydrochemical response of a constructed wetland to variations in hydrology and water management with respect to water sources, flow pathways and major chemical transformations in the three years following commissioning. Results suggest that active water management practices in 2013 kept EC relatively low, with most wetland sites <1000 μS/cm with Na⁺ concentrations <250 mg/L. With limited management in 2014 and 2015, the EC increased in the wetland to >1000 μS/cm in 2014 and >2000 μS/cm in 2015. The most notable change was the emergence of several Na⁺ enriched zones in the margins. Here, Na⁺ concentrations were two to three times higher than other sites. Stable isotopes of water support that the Na⁺ enriched areas arise from underlying process-affected water in the tailings, providing evidence of its upward transport and seepage under a natural hydrologic regime. In future years, salinity is expected to evolve in its flow pathways and diffusion, yet the timeline and extent of these changes are uncertain.