Projecting the hydrochemical trajectory of a constructed fen watershed: Implications for long-term wetland function

Surface mining operations for bitumen have fundamentally altered large areas of boreal forest and fen peatland in the Athabasca Oil Sands Region (AOSR) of Alberta, Canada. Pilot projects intended to assess the feasibility of fen construction as a reclamation option have been designed, built, and are currently undergoing monitoring. Initial assessments of ecohydrologic function have been conducted for these systems but offer limited insight into their evolution and likely successional pathway. Thus, this study projects the hydrologic and geochemical behaviour of a constructed fen watershed to understand whether the system will be capable of supporting peatland processes into the future. A numerical groundwater flow and sodium transport model was calibrated and validated with 7 years of hydraulic head, water flux, and water chemistry data. Based on Monte Carlo simulations, the projected fen water table would be stable and remain close to the surface (<15 cm), indicating that the design of the system can generate sufficient water quantity to meet evaporative demand and maintain surface water discharge. However, water quality was more sensitive to climatic variability, which induced a large range in potential sodium concentrations at the fen surface (450-850 mg L^-1). Evapoconcentration of salts across the surface of the fen will likely limit moss establishment for decades following construction. Yet stress-thresholds of salt-tolerant vegetation like sedges will not be exceeded. Ultimately, these projections support the original design principles and philosophy that guided the creation of the watershed. Nonetheless, this work indicates that increasing the area of the fen relative to the upland would not have a detrimental impact on the ability of the system to maintain a high water table. This could allow for the proportion of peatlands on the reclamation landscape to reflect the pre-disturbance environment more faithfully.

Modelling the hydrologic effects of vegetation growth on the long-term trajectory of a reclamation watershed

Reclamation watersheds that integrate fen peatlands into the design require the inclusion of uplands that are capable of supporting forest development while concurrently supplying sufficient groundwater recharge to downgradient wetland ecosystems. This necessitates selecting materials with suitable soil hydraulic properties and identifying the appropriate thickness and layering to fulfill the dual function of uplands as water storage, and water conveyance features. Currently, these systems incorporate tailings sand - a mine waste material - overlain by a cover soil of fine forest-floor material. The developmental pathway of these uplands is currently unknown, and it is unclear whether these landforms will provide enough groundwater recharge once a climax vegetation community establishes. Therefore, this research attempts to estimate the maximum density of vegetation, and associated water balance fluxes of a constructed upland integrated into a peatland watershed. The numerical modelling software HYDRUS-1D simulated soil moisture dynamics using a 65-year meteorological record, and a plant water stress algorithm was used to estimate the maximum sustainable leaf area index that the upland could support. Based on the thickness of the cover soil, the upland could support an average leaf area index of 1.2. Under this vegetation density, average annual groundwater recharge was 83 mm, and predominantly supplied by snowmelt (64%). Given this quantity of recharge, the model indicates that the upland will continue to provide enough groundwater to offset the anticipated water deficit in the downgradient fen ecosystem. However, by altering the design of the upland, specifically the spatial arrangement and thickness of cover soil, the same recharge could be supplied while also allowing for a higher average vegetation density. Such a design could allow for the creation of watersheds with a higher proportion of peatland.

The distribution and migration of sodium from a reclaimed upland to a constructed fen peatland in a post-mined oil sands landscape

Post-mine landscape reclamation of the Athabasca Oil Sands Region requires the use of tailings sand, an abundant mine-waste material that often contains large amounts of sodium (Na⁺). Due to the mobility of Na⁺ in groundwater and its effects on vegetation, water quality is a concern when incorporating mine waste materials, especially when attempting to construct groundwater-fed peatlands. This research is the first published account of Na⁺ redistribution in groundwater from a constructed tailings sand upland to an adjacent constructed fen peat deposit (Nikanotee Fen). A permeable petroleum coke layer underlying the fen, extending partway into the upland, was important in directing flow and Na⁺ beneath the peat, as designed. Initially, Na⁺ concentration was highest in the tailings sand (average of 232mgL^-1) and lowest in fen peat (96mgL^-1). Precipitation-driven recharge to the upland controlled the mass flux of Na from upland to fen, which ranged from 2 to 13tons Na⁺ per year. The mass flux was highest in the driest summer, in part from dry-period flowpaths that direct groundwater with higher concentrations of Na⁺ into the coke layer, and in part because of the high evapotranspiration loss from the fen in dry periods, which induces upward water flow. With the estimated flux rates of 336mmyr^-1, the Na⁺ arrival time to the fen surface was estimated to be between 4 and 11years. Over the four-year study, average Na⁺ concentrations within the fen rooting zone increased from 87 to 200mgL^-1, and in the tailings sand decreased to 196mgL^-1. The planting of more salt-tolerant vegetation in the fen is recommended, given the potential for Na⁺ accumulation. This study shows reclamation designs can use layered flow system to control the rate, pattern, and timing of solute interactions with surface soil systems.