At the Nikanotee Fen Watershed, a pioneering reclamation project in the Athabasca Oil Sands Region, elevated sodium (Na+) in the porewater of mine-waste materials has been shown to migrate to the fen through groundwater, likely influencing fen vegetation health. Given the potential of Na+ to steer the ecological development of the fen, the goal of this research is to quantify the spatial distribution of Na+ and to characterize the Na+ mass balance of the surface and shallow subsurface of the fen. For a given time, the highest Na+ concentrations were generally found in the wettest part of the fen in the southwestern corner, and the lowest in the relatively dry northeast corner near the drainage outlet. Na+ concentrations in ponded surface water were responsive to rainfall-induced dilution and evapoconcentration, whereas porewater salinity in shallow groundwater was insensitive to meteorological conditions. Surface discharge controlled the mass efflux of Na+ from the system at an estimated average rate of 4 kg•day 1 between June and August. Given the relatively small proportion of salt being flushed annually, and the greater rate of mass inflow relative to export, elevated salinity will likely be sustained for several decades. Since elevated Na+ concentrations are impacting the rooting zone of vegetation and have exceeded the salinity stress-threshold of mosses, targeting salt-tolerant vegetation will be important to maintain carbon accumulation in constructed systems.