Solute pools in Nikanotee Fen watershed in the Athabasca oil sands region

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.

Characterizing the hydraulic and transport properties of a constructed coarse tailings sand aquifer

Millions of tonnes of coarse tailings sand are produced every year as a byproduct of the bitumen extraction process in the Athabasca Oil Sands Region. These tailings materials contain residual quantities of mobile solutes, which can be transported through groundwater to downgradient terrestrial and aquatic ecosystems. The anticipated ubiquity of coarse tailings sand on the post-mined landscape necessitates the characterization of its hydraulic and transport properties. Hydraulic conductivity and dispersivity was evaluated at multiple scales, and included the first field-scale tracer test conducted in a tailings sand aquifer. Average hydraulic conductivity derived using laboratory cores, single-well response tests, and the tracer test were 3.2 m d^-1, 2.9 m d^-1, and 3.4 m d^-1, respectively. These measurements demonstrated close agreement and were consistent with expectations of a material that experiences some grain-size segregation and homogenization due to the oil sands process and the nature of deposition. The field-scale tracer test appeared to obtain the asymptotic dispersivity of the coarse tailings sand aquifer, reaching a maximum value of 0.5 m after 18 m of displacement. Coarse tailings in the oil sands that experience similar processes of segregation, settling, and deposition on the reclamation landscape could be expected to have similar hydraulic properties.

Geochemical conditions influence vanadium, nickel, and molybdenum release from oil sands fluid petroleum coke

Petroleum coke is a potential source of vanadium (V), nickel (Ni), and molybdenum (Mo) to water resources in Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada. Large stockpiles of this bitumen upgrading byproduct will be incorporated into mine closure landscapes and understanding the processes and conditions controlling the release and transport of these transition metals is critical for effective reclamation. We performed a series of laboratory column experiments to quantify V, Ni, and Mo release from fluid petroleum coke receiving meteoric water (MW), oil sands process-affected water (OSPW), and acid rock drainage (ARD) influents. We found that influent water chemistry strongly influences metal release, with variations among metals largely attributed to pH-dependent aqueous speciation and surface reactions. Cumulative V, Ni, and Mo mass release was greatest for columns receiving the low-pH ARD influent. Additionally, cumulative V and Mo mass release were greater in columns receiving OSPW compared to MW influent, whereas cumulative Ni mass release was greater in columns receiving MW compared to OSPW influent. Nevertheless, only a small proportion of total V, Ni, and Mo was released during the experiments, with the majority occurring during the first 10 pore volumes (PVs). This study offers insight into geochemical controls on V, Ni, and Mo release from fluid petroleum coke that supports ongoing development of oil sands mine reclamation strategies for landscapes that contain petroleum coke.

Optical properties of dissolved organic matter highlight peatland-like properties in a constructed wetland

Constructing novel peatland ecosystems can help to restore the long-term carbon accumulating properties of northern soil systems that have been lost through resource extraction. Although mining companies are legally required to restore landscapes following extraction, there are limited tools to evaluate the effectiveness of restoring peat accumulating landscapes. This study analyzed the spatial patterns of the first seven years (n = 575) of dissolved organic matter (DOM) optical characteristics from a pilot watershed built to restore boreal plains peatlands on a former open pit oil sands mine. A principal component analysis (PCA) indicated a very high degree of redundancy in absorption-florescence DOM properties (PARAFAC, HIX, FI, freshness index, SUVA, and peak A, B, C, T, wavelength, and intensity ratios) at this site. The leading principal component indicated a gradient of fresh protein rich inputs, which are highest near the upland region, to older highly degraded DOM, which is highest in the lowland closest to the outlet. Two functionally different reference peatlands, a poor-fen and bog system and a moderate-rich fen, had relatively similar optical DOM characteristics indicating a high level of decomposition at these sites. Over the first seven years, in some regions of the reconstructed lowland the DOM characteristics are becoming increasingly similar to the highly decomposed DOM observed at the reference sites. When combined with carbon flux measurements these findings indicate the potential for long term organic matter accumulation at this reconstructed site.

High sulfate concentrations maintain low methane emissions at a constructed fen over the first seven years of ecosystem development

Wetlands comprise a large expanse of the pre-disturbance landscape in the Athabasca Oil Sands Region (AOSR) and have become a focus of reclamation in recent years. An important aspect of wetland reclamation is understanding the biogeochemical functioning and carbon exchange, including methane (CH4) emissions, in the developing ecosystem. This study investigates the drivers of CH4 emissions over the first seven years of ecosystem development at a constructed fen in the AOSR and looks towards future CH4 emissions from this site. Specifically, the objectives were to: 1) investigate the environmental controls on CH4 emissions measured using manual static chambers between 2013 and 2019 and 2) investigate the relationship between water table depth, sulfate (SO42-) concentrations and CH4 emissions during the 2019 growing season. Methane emissions remained low throughout the majority of the measurement period; however, in later years, a small but significant increase became apparent. High levels of SO42- are likely the cause of the low CH4 emissions, despite the high-water tables and dominance of vegetation with aerenchyma such as Carex aquatilis and Typha latifolia in later years. Although low CH4 emissions may be beneficial from a climate warming perspective, the results also suggest that this constructed peatland is not functioning similarly to regional reference fens. Future climate scenarios across Western Boreal Canada could lead to higher air temperatures and changing precipitation patterns, influencing the direction of future CH4 emissions from this site. However, given the likelihood of this site maintaining extremely high SO42- concentrations over the next decade, it is expected that CH4 emissions will remain low.

Geochemical Stability of Oil Sands Tailings in Mine Closure Landforms

Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard to the long-term geochemical stability of tailings landforms are identified, including salt fluxes and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the biogeochemical implications of various tailings treatment methods meant to improve geotechnical properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition.

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.

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.

Nickel geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada

Nickel (Ni) leaching from oil sands petroleum coke can have toxicological effects on aquatic organisms. However, geochemical controls on Ni release, transport, and attenuation within coke deposits remains limited. We examined the geochemistry of fluid coke and associated pore waters from two deposits at an oil sands mine near Fort McMurray, Alberta, Canada. Synchrotron-based micro-X-ray fluorescence (μXRF) and micro-X-ray absorption near edge structure (μXANES) spectroscopy show that Ni(II)-porphyrin complexes dominate, but inorganic phases including Ni(II)-sulfide and Ni(II)-oxide comprise a minor component of fluid coke. Sequential chemical extractions suggested that sorption–desorption reactions may influence Ni mobility within fluid coke deposits. Although only a small proportion of total Ni (<4%) is susceptible to leaching under environmentally relevant concentrations, dissolved Ni concentrations ( n = 65) range from 2 to 120 μg·L−1 (median 7.8 μg·L−1) within the two deposits and generally decrease with depth below the water table. Pore water Ni concentrations are negatively correlated with pH, but not with dissolved sulfate, bicarbonate, or chloride. Overall, our findings suggest that pore water pH and sorption–desorption reactions are principal controls on dissolved Ni concentrations within oil sands fluid petroleum coke deposits.

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.