The Hydrogeochemistry of a Constructed Fen Peatland in the Post-Mined Landscape in the Athabasca Oil Sands Region, Alberta, Canada
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Large areas of land within the Athabasca Oil Sands Region (AOSR) have been subjected to open pit mining to recover bitumenous oil sands. Following extraction, oil sands require the addition of solvents to optimize the solubility and separation of bitumen from sand. Consequently, the tailings sands, a byproduct of oil sands production, contain elevated residual concentrations of sodium (Na+), amongst other constituents. In an attempt to reclaim post-mined landscape in the AOSR, a fen peatland system has been constructed, in which groundwater inputs are received from an upland aquifer constructed of tailings sands. The intent is to mimic the function of naturally occurring fens in which groundwater supplements precipitation inputs, especially important during periods of limited water availability, which are common in the AOSR. However, given the elevated concentrations of Na+ in tailings materials, there is a concern that it will leach from the tailings sand and accumulate in the fen at concentrations toxic for the recently planted fen species. The purpose of this study was to determine the spatial distribution of Na+ generated throughout the upland – fen system and to evaluate the hydrological processes controlling its transport from the upland to the fen. Na+ concentrations were highest within the tailings sand materials and petroleum coke underdrain and lowest within the fen peat deposit in all three years (2013 – 2015); however, there was considerable temporal variation in concentrations within all materials. Concentrations within the tailings sand upland and petroleum coke underdrain were generally the highest after the winter months which appeared to allow for elevated Na+ concentrations in the absence of freshwater recharge. Following rainfall, Na+ concentrations decreased within all construction materials. Upland recharge basins were important for detaining overland flow and encouraging upland freshwater recharge, which ultimately determined the trajectory for Na+ re-distribution along the eastern region of the upland. High precipitation and recharge in 2013 and 2014 resulted in the highest fluxes towards the fen; however, allowed for dilution of upland groundwater which resulted in relatively low loads of Na+ being received at the fen. A Na+ plume was observed migrating from the upland into the petroleum coke underdrain and beneath the fen. Little upward migration of the Na+ plume into the fen peat occurred in 2013 and 2014. In 2015, a lack of precipitation resulted in less groundwater dilution, which despite lower fluxes, resulted in greater loads of Na+ being received at the fen. The Na+ plume advanced throughout the petroleum coke underdrain. Considerable upwards migration of Na+ into the peat deposit was observed, resulting in an increase in concentration, but mainly at depth. Evapo-concentration resulted in elevated concentrations at the fen surface by the end of the study. This study documented the distribution and transport of Na+ from a constructed tailings sand upland to the adjacent constructed fen within the first three years post-construction, and provides insight into the variability of salinity with location within reclamation materials, and the effects of weather conditions typical of the AOSR. This study also makes important fen construction design modifications and recommendations to minimize OSPW generation while simultaneously ensuring sub-surface storage. Success in reclaiming fen peatlands in the AOSR must account for the management and transport of solutes generated from the tailings sand materials.