RESOLVING THE NEED FOR GROUNDWATER RECHARGE VERSUS FOREST PRODUCTIVITY IN A RECLAIMED WATERSHED USING NUMERICAL MODELLING

Oil sands mining disturbs vast regions of Alberta’s boreal forest, necessitating reclamation at unprecedented scales. Reclamation requires the reconstruction of groundwater flow systems that provide adequate water to down-gradient ecosystems and water bodies (e.g. end pit lakes). However, this is challenging due to the region’s sub-humid climate, salinity of available reclamation materials, and need to cap with soil reclamation covers that promote forest growth. In 2012, Syncrude Canada Ltd. constructed an experimental 50 ha watershed, Sandhill Fen Watershed, on a coarse-textured, soft tailings deposit. In the watershed, basin-scale upland hummocks were constructed to function as sources of water to a fen wetland. This desired hydrologic function necessitates adequate recharge in these landscape units, while simultaneously providing a suitable substrate for forest growth. Thus, we estimated groundwater recharge on mineral hummocks having varying heights, textures, and soil reclamation cover depths. One-dimensional and two-dimensional numerical models were calibrated and validated using the HYDRUS software package against field observations of soil moisture and water levels over a four year period. Recharge was primarily dependent on the texture of prescribed soil reclamation covers, water table depth, and vegetation characteristics (e.g. leaf area index and rooting depth). Soil reclamation covers comprised of peat-mineral mixtures and fine-textured soils, even when only 0.15 m thick, exhibited lower recharge (~60 mm yr^-1) relative to coarse-textured covers (~100 mm yr^-1). Two-dimensional simulations of vertical cross sections indicate the importance of integrating recharge across upland hummocks, highlighting how variability in root water uptake and the proximity of the water table determine their overall recharge function. Long-term scenario testing indicated a likely decline in recharge due to succession and vegetation development; however, climate cycles lead to substantial variability in recharge. Overall, reclamation managers should recognize the tradeoff between having productive forest areas, which primarily occupy fine-textured reclamation covers, versus having areas conducive to groundwater recharge, which are facilitated by coarse-textured reclamation covers.