Initial geochemical characteristics of fluid fine tailings in an oil sands end pit lake

Aerobic degradation of anionic polyacrylamide in oil sands tailings: Impact factor, degradation effect, and mechanism

An investigation was carried out to study the degradation of anionic polyacrylamide (A-PAM) under different temperature and microorganism conditions as well as to assess its effects on water chemistry and toxicity in oil sands tailings. The maximum removal efficiency of A-PAM was 41.0 % in tailings water with augmented microorganisms at 20 °C. No acrylamide (AMD) monomer was released during the A-PAM degradation, while residual AMD, from the manufacturing process to make A-PAM, was completely removed within 4 weeks. Both temperature and microorganisms showed significant effects (p < 0.05) on the degradation of A-PAM and residual AMD. Gel permeation chromatography (GPC) and Fourier transform infrared (FT-IR) analyses showed that biodegradation could be the active pathway for A-PAM degradation in oil sands tailings. These analyses also indicated that macromolecular A-PAM was degraded into lower molecular weight organic compounds. No remarkable changes of the total concentration of naphthenic acids (NAs) were observed in A-PAM treated tailings water. However, low concentrations of fatty acids (<2.5 mg/L), which fit the NAs formula, were detected in pure polymer solution, indicating that A-PAM degradation would not affect the total concentration of NAs in tailings water but affect their distribution. Our results also showed that total organic carbon (TOC) and chemical oxygen demand (COD) could be used as indicators of A-PAM degradation in tailings water due to their strong linear correlations (R² > 0.90). Only slight increases in zeta potential and pH were found during A-PAM degradation. Limited effect on acute toxicity and no genotoxicity were found in A-PAM treated tailings water. Furthermore, the results suggest that A-PAM undergoes hydrolysis of amide groups by amidase enzymes, releasing ammonia and smaller molecules like organic acids. This research provides valuable information regarding the stability and impacts of A-PAM and thus will be beneficial for the management of oil sands tailings in long period of time.

Alum Addition Triggers Hypoxia in an Engineered Pit Lake

Here, we examine the geobiological response to a whole-lake alum (aluminum sulfate) treatment (2016) of Base Mine Lake (BML), the first pilot-scale pit lake established in the Alberta oil sands region. The rationale for trialing this management amendment was based on its successful use to reduce internal phosphorus loading to eutrophying lakes. Modest increases in water cap epilimnetic oxygen concentrations, associated with increased Secchi depths and chlorophyll-a concentrations, were co-incident with anoxic waters immediately above the fluid fine tailings (FFT) layer post alum. Decreased water cap nitrate and detectable sulfide concentrations, as well as increased hypolimnetic phospholipid fatty acid abundances, signaled greater anaerobic heterotrophic activity. Shifts in microbial community to groups associated with greater organic carbon degradation (i.e., SAR11-LD12 subclade) and the SRB group Desulfuromonodales emerged post alum and the loss of specialist groups associated with carbon-limited, ammonia-rich restricted niches (i.e., MBAE14) also occurred. Alum treatment resulted in additional oxygen consumption associated with increased autochthonous carbon production, watercap anoxia and sulfide generation, which further exacerbate oxygen consumption associated with on-going FFT mobilized reductants. The results illustrate the importance of understanding the broader biogeochemical implications of adaptive management interventions to avoid unanticipated outcomes that pose greater risks and improve tailings reclamation for oil sands operations and, more broadly, the global mining sector.

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.

Ebullition enhances chemical mass transport across the tailings-water interface of oil sands pit lakes

Base Mine Lake (BML) was the first commercial-scale demonstration oil sands pit lake established in northern Alberta, Canada. Recent studies indicate that ebullition enhances internal mass loading of dissolved constituents during settlement and dewatering of methanogenic fluid fine tailings (FFT) below the overlying water cap. Here, we describe results of integrated field measurements and numerical modelling to (i) determine potential for ebullition and enhanced mixing within BML, and (ii) assess impacts on chemical mass transport across the tailings-water interface. We observed sharp increases in [CH_4(aq)] with depth from <0.1 mg L^-1 immediately above the interface to >60 mg L^-1 over the upper 1.5 to 3.0 m of FTT. Thermodynamic modelling revealed that maximum [CH_4(aq)] values represent 60 to 80% of theoretical saturation, and corresponding total dissolved gas pressures approach or exceed fluid pressures. These findings supported integration of enhanced mixing into one-dimensional (1-D) advective-dispersive transport models, which substantially improved upon previous simulations of conservative tracer (i.e., Cl^-) profiles and chemical mass fluxes. The models revealed a positive relationship between CH_4(aq) saturation and enhanced mixing, showing that ebullition enhances internal mass loading. This information has potential to inform ongoing assessments of pit lake performance and support improved closure and reclamation planning at oil sands mines.

Seasonal dynamics of methanotrophic bacteria in a boreal oil sands end-pit lake

Base Mine Lake (BML) is the first full-scale demonstration end pit lake for the oil sands mining industry in Canada. We examined aerobic methanotrophic bacteria over all seasons for five years in this dimictic lake. Methanotrophs comprised up to 58% of all bacterial reads in 16S rRNA gene amplicon sequencing analyses (median 2.8%), and up to 2.7 × 10⁴ cells mL^-1 of water (median 0.5 × 10³) based on qPCR of pmoA genes. Methanotrophic activity and populations in the lake water were highest during fall turnover, and remained high through the winter ice-covered period into spring turnover. They declined during summer stratification, especially in the epilimnion. Three methanotroph genera (Methylobacter, Methylovulum, and Methyloparacoccus) cycled seasonally, based on both relative and absolute abundance measurements. Methylobacter and Methylovulum populations peaked in winter/spring, when methane oxidation activity was psychrophilic. Methyloparacoccus populations increased in the water column through summer and fall, when methane oxidation was mesophilic, and also predominated in the underlying tailings sediment. Other, less abundant genera grew primarily during summer, possibly due to distinct CH₄/O₂ microniches created during thermal stratification. These data are consistent with temporal and spatial niche differentiation based on temperature, CH₄ and O₂. This pit lake displays methane cycling and methanotroph population dynamics similar to natural boreal lakes. Importance statement: The study examined methanotrophic bacteria in an industrial end pit lake, combining molecular DNA methods (both quantitative and descriptive) with biogeochemical measurements. The lake was sampled over 5 years, in all four seasons, as often as weekly, and included sub-ice samples. The resulting multi-season and multi-year dataset is unique in its size and intensity, and allowed us to document clear and consistent seasonal patterns of growth and decline of three methanotroph genera (Methylobacter, Methylovulum, and Methyloparacoccus). Laboratory experiments suggested that one major control of this succession was niche partitioning based on temperature. The study helps to understand microbial dynamics in engineered end-pit lakes, but we propose that the dynamics are typical of boreal stratified lakes, and widely applicable in microbial ecology and limnology. Methane oxidising bacteria are important model organisms in microbial ecology, and have implications for global climate change.

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.

Toward sustainable remediation of oil sands fine Tailings-A review

Significant quantities of tailings are produced during the extraction of bitumen from oil sands. Tailings are stored in tailings ponds for several years before they can be appropriately managed. Current fine tailings management approaches include mechanical and/or chemical amendments of fine tailings (FT) to speed up tailings reclamation. However, complex structures of fine tailings, stringent tailings management regulations, failure in production of trafficable deposits with current FT reclamation technologies and biogenic gas (e.g., CH₄) generations have prompted the re-evaluation of current FT remediation technologies and exploration of alternative biological treatments (e.g., bioaugmentation and biostimulation). Biological treatments have proven to effectively remediate environmental pollutants by creating favourable environments for the desire microorganisms. Thus their effects on FT reclamation have been increasingly investigated in the last two decades. Many of these studies confirmed that biological treatments can improve FT dewatering and densification. However, other studies found that not all biological treatments can effectively suppress CH₄ generations or they may lead to the generation of other biogenic gases (e.g., H₂S, N₂O, NO). Therefore, it is critical to identify potential environmental risks associated with the biological treatments before their full-scale applications. This review revolved around two questions. First, whether bioaugmentation and biostimulation methods can improve FT reclamation. Secondly, what are the potential environmental issues that may arise from the applications of biological treatments. To address these questions, the existing peer-reviewed documents on fine tailings management were carefully reviewed to provide an introduction to the currently practiced FT reclamation technologies. Further discussions on biological treatments and their potentials and limitations were also presented. Finally, the review highlighted the knowledge gap in the area of biological treatments of FT and provided recommendations for future research.

A Deep Look into the Microbiology and Chemistry of Froth Treatment Tailings: A Review

In Alberta’s Athabasca oil sands region (AOSR), over 1.25 billion m³ of tailings waste from the bitumen extraction process are stored in tailings ponds. Fugitive emissions associated with residual hydrocarbons in tailings ponds pose an environmental concern and include greenhouse gases (GHGs), reduced sulphur compounds (RSCs), and volatile organic compounds (VOCs). Froth treatment tailings (FTT) are a specific type of tailings waste stream from the bitumen froth treatment process that contains bioavailable diluent: either naphtha or paraffins. Tailings ponds that receive FTT are associated with the highest levels of biogenic gas production, as diverse microbial communities biodegrade the residual diluent. In this review, current literature regarding the composition, chemical analysis, and microbial degradation of FTT and its constituents is presented in order to provide a more complete understanding of the complex chemistry and biological processes related to fugitive emissions from tailings ponds receiving FTT. Characterizing the composition and biodegradation of FTT is important from an environmental perspective to better predict emissions from tailings ponds and guide tailings pond management decisions.

The role of carbonate mineral dissolution in turbidity reduction in an oil sands end pit lake

Surface water turbidity from dispersed clay particles can hinder the development of aquatic ecosystems. One of the primary objectives for proposed oil sands end pit lakes is that they support ecological functions and lake-specific wildlife habitat. However, high surface water turbidity has been observed in the Base Mine Lake cap water, the first full-scale demonstration oil sands end pit lake. Our previous study showed that adjusting the solution pH through carbon dioxide (CO₂) addition reduced surface water turbidity in oil sands tailings. Carbonate minerals such as calcite and dolomite were also previously identified in tailings, and thus the goal of this study was to determine the effect of calcite and dolomite dissolution through CO₂-mediated pH reduction on turbidity and the stability of suspended clay particles. Calcite dissolution resulted in ∼99% reduction of turbidity. The suspended clay particle stability was analyzed using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory with water chemistry data from this column study. An inverse correlation was observed between the amount of dolomite and the energy barrier values on day 42 of the experiment. These results suggest CO₂-mediated calcite dissolution changes the water chemistry and is the most promising treatment condition for the settlement of suspended tailings particles.

Energy and carbon fluxes from an oil sands pit lake

Currently, post-mining landscape plans in the Athabasca Oil Sand Region include large watersheds terminating in pit lakes. In 2012, Base Mine Lake (BML), was constructed with the aim of demonstrating technologies associated with lake reclamation in the region. This paper examines the first 6.5 years of lake-atmosphere energy and carbon exchange. Energetically, BML behaved similar to other northern lakes, storing large quantities of heat in the spring and releasing it in the fall as sensible and latent heat fluxes. At various times a hydrocarbon sheen formed on the lake, which may have suppressed evaporation. However, simple linear relationships failed to statistically quantify the impacts and more comprehensive modelling of the variability may be required. At daily scales, variability in evaporation was well explained by the product of vapour pressure deficit and wind speed as well as the available energy (R² = 0.74), while sensible heat was explained by the product of wind speed and the difference in air and surface temperature as well as available energy (R² = 0.85). Spring CH₄ fluxes were high, particularly around ice melt, with a maximum flux of 3.3 g m^-2 day^-1. Otherwise fluxes were low, except during irregular periods. The peak flux of these periods occurred following ~58 h of continuously falling pressure, relating cyclone activity to these large periods of methane emissions. Annually, CO₂ and CH₄ fluxes were initially high, with median fluxes of 231 mg CO₂ m^-2 h^-1 and 23 mg CH₄ m^-2 h^-1 in 2014. However, the median fluxes reduced quickly and over the least three years of the study (2017 through 2019) the median fluxes declined to 36 mg CO₂ m^-2 h^-1 and 10 mg CH₄ m^-2 h^-1. Overall, BML behaves similar to other boreal lake ecosystems with above average carbon fluxes compared to other constructed reservoirs.

Isomer-specific monitoring of naphthenic acids at an oil sands pit lake by comprehensive two-dimensional gas chromatography-mass spectrometry

Naphthenic acids (NAs) are persistent, toxic contaminants that are found to accumulate in oil sands process-affected water (OSPW) and tailings after bitumen extraction. A number of strategies for the reclamation of oil sands tailings are currently being tested, including the development of the first demonstration pit lake by Syncrude Canada, Base Mine Lake (BML). An important component of reclamation activities is understanding the source and cycling of NAs in such reclamation systems. However, NAs exist as a highly complex mixture of thousands of compounds which makes their analysis an ongoing challenge. Herein, comprehensive two-dimensional gas chromatography coupled to time of flight mass spectrometry (GC × GC/TOFMS) was used to analyze the methylated extracts of water samples from the water cap and fluid fine tailings (FFT) deposit of BML to characterize the variations in NA distributions between geochemical zones. A collection of (alkylated) monocyclic-, bicyclic-, adamantane-, and thiophene-type carboxylic acids were identified. Total relative abundances were calculated for each NA class (by summation of peak areas of all detected isomers) and minimal variability was detected in the water cap. Total relative abundances for each NA class were either similar or higher in the FFT, relative to the water cap. Examination of isomer distributions indicated that differences in abundance values were generally driven by variations in only one or two isomers of a given NA class. Furthermore, GC × GC revealed distinct isomer profiles were observed between two FFT samples and between the FFT and water cap. While it is not yet clear whether these differences are due to differences in sources of NAs or in their environmental processing, these results illustrate the capability of GC × GC to investigate these questions and thus contribute to the management of these compounds within reclamation or environmental systems.

Chronic Toxicity of Surface Water from a Canadian Oil Sands End Pit Lake to the Freshwater Invertebrates Chironomus dilutus and Ceriodaphnia dubia

Permanent reclamation of tailings generated by surface mining in the Canadian oil sands may be achieved through the creation of end pit lakes (EPLs) in which tailings are stored in mined-out pits and capped with water. However, these tailings contain high concentrations of dissolved organics, metals, and salts, and thus surface water quality of EPLs is a significant concern. This is the first study to investigate the chronic toxicity of surface water from Base Mine Lake (BML), the Canadian oil sands first large-scale EPL, to aquatic invertebrates that play a vital role in the early development of aquatic ecosystems (Chironomus dilutus and Ceriodaphnia dubia). After exposure of C. dilutus larvae for 23 days and C. dubia neonates for 8 days, no mortality was observed in any treatment with whole BML surface water. However, the emergence of C. dilutus adults was delayed by nearly 1 week, and their survival was significantly reduced (36%) compared with the controls. Reproduction (fecundity) of C. dubia was reduced by 20% after exposure to 2014 BML surface water; however, the effect was not observed after exposure to BML surface water collected a year later in 2015. Despite some adverse effects, the results of this study indicate that BML surface water quality is improving over time and is able to support certain salt-tolerant aquatic organisms. Because salinity within BML will persist for decades without manual intervention, the ecological development of the lake will likely resemble that of a brackish or estuarine ecosystem with reduced diversity.

Model naphthenic acids removal by microalgae and Base Mine Lake cap water microbial inoculum

Naphthenic acids (NAs) originate from bitumen and are considered a major contributor to acute toxicity in oil sands process-affected water (OSPW) produced from bitumen extraction processes. To reclaim oil sands tailings and remediate OSPW, in-pit fluid fine tailings can be water-capped as end pit lakes (EPL). Addressing NAs present in OSPW, either through removal, dilution or degradation, is an objective for oil sands reclamation. EPLs can remediate NAs through degradation or dilution or both. To assess and understand degradation potential, Chlorella kessleri and Botryococcus braunii were tested for their tolerance to, and ability to biodegrade, three model NAs (cyclohexanecarboxylic acid, cyclohexaneacetic acid, and cyclohexanebutyric acid). Water sourced from the industry's first EPL, the Base Mine Lake (BML), was used alone as an inoculum or co-cultured with C. kessleri to biodegrade cyclohexanecarboxylic acid and cyclohexanebutyric acid. All cultures metabolized the model compounds via β-oxidation. Biodegradation by the co-culture of C. kessleri and BML inoculum was most effective and rapid: the cyclohexaneacetic acid generated from cyclohexanebutyric acid could be further degraded by the co-culture, while the cyclohexaneacetic acid generated could not be consumed by pure algal cultures or BML inoculum alone. Adding C. kessleri greatly increased the diversity of the microbial community in the BML inoculum; many known hydrocarbon and NA degraders were identified from the 16S rRNA gene sequencing from this co-culture. This more diverse microbial community could have potential for EPL remediation.

Putative Mixotrophic Nitrifying-Denitrifying Gammaproteobacteria Implicated in Nitrogen Cycling Within the Ammonia/Oxygen Transition Zone of an Oil Sands Pit Lake

Anthropogenically-impacted environments offer the opportunity to discover novel microbial species and metabolisms, which may be undetectable in natural systems. Here, a combined metagenomic and geochemical study in Base Mine Lake, Alberta, Canada, which is the only oil sands end pit lake to date, revealed that nitrification was performed by members from Nitrosomonadaceae, Chloroflexi and unclassified Gammaproteobacteria “MBAE14.” While Nitrosomonadaceae and Chloroflexi groups were relatively abundant in the upper oxygenated zones, MBAE14 dominated the hypoxic hypolimnetic zones (approximately 30% of total microbial communities); MBAE14 was not detected in the underlying anoxic tailings. Replication rate analyses indicate that MBAE14 grew in metalimnetic and hypolimnetic water cap regions, most actively at the metalimnetic, ammonia/oxygen transition zone consistent with it putatively conducting nitrification. Detailed genomic analyses of MBAE14 evidenced both ammonia oxidation and denitrification into dinitrogen capabilities. However, the absence of known CO₂-fixation genes suggests a heterotrophic denitrifying metabolism. Functional marker genes of ammonia oxidation (amo and hao) in the MBAE14 genome are homologous with those conserved in autotrophic nitrifiers, but not with those of known heterotrophic nitrifiers. We propose that this novel MBAE14 inhabits the specific ammonia-rich, oxygen and labile organic matter-limited conditions occurring in Base Mine Lake which selectively favors mixotrophic coupled nitrifier denitrification metabolism. Our results highlight the opportunities to better constrain biogeochemical cycles from the application of metagenomics to engineered systems associated with extractive resource sectors.

Current knowledge of seepage from oil sands tailings ponds and its environmental influence in northeastern Alberta

Seepage of oil sand process-affected waters (OSPW) from tailings ponds into surface waters is a common concern in the minable oil sands region of northeast Alberta. Research on seepage has been extensive, but few comprehensive treatments evaluating all aspects relevant to the phenomenon are available. In this work, the current information relevant for understanding the state of seepage from tailings ponds was reviewed. The information suggests the infiltration of OSPW into groundwater occurs near some ponds. OSPW may also be present in sediments beneath the Athabasca River adjacent to one pond, but there are no clear observations of OSPW in the river water. Similarly, most water samples from tributaries also show no evidence of OSPW, but these observations are limited by the lack of systematic, systemic, and repeated surveys, missing baseline data, standard analytical approaches, and reference materials. Waters naturally influenced by bitumen, discharge of saline groundwaters, and dilution also potentially affect the consolidation of information and certainty of any conclusions. Despite these challenges, some data suggest OSPW may be present in two tributaries of the Athabasca River adjacent to tailings ponds: McLean Creek and Lower Beaver River. Irrespective of the possible source(s), constituents of OSPW often affect organisms exposed in laboratories, but research in all but one study suggests the concentrations of organics in the surface water bodies assessed are below the standard toxicological effect thresholds for these compounds. In contrast, many samples of groundwater, irrespective of source, likely affect biota. Biomonitoring of surface waters suggests generic responses to stressors, but the influence of natural phenomena and occasionally nutrient enrichment are often suggested by data. In summary, valuable research has been done on seepage. The data suggest infiltration into groundwater is common, seepage into surface waters is not, and anthropogenic biological impacts are not likely.

Modelling the potential effects of Oil-Sands tailings pond breach on the water and sediment quality of the Lower Athabasca River

Within the Oil-Sands industry in Alberta, Canada, tailings ponds are used as water recycling and tailings storage facilities (TSF) for mining activities. However, there could be possible circumstances under which a sudden breach of an embankment confining one of the TSFs may occur. Such a tailings pond breach would result in a sudden release of a huge volume of Oil Sands process-affected water (OSPW) and sediment slurry containing substantial amount of chemical constituents that would follow the downstream drainage paths and subsequently enter into the Lower Athabasca River (LAR). This study investigates the implications of OS tailings release on the water and sediment quality of the LAR by simulating the fate of sediment and associated chemicals corresponding to a hypothetical breach and release scenarios from a select set of tailings ponds using a two-dimensional hydrodynamic and constituent transport model. After predicting the total volume, time evolution and concentration of sediment and associated chemicals (metals, polycyclic aromatic hydrocarbons (PAHs) and naphthenic acids (NAs)) reaching the LAR, the transport and deposition of these materials within the study reach is simulated. The results show that, depending on tailings release locations, between 40 and 70% of the sediment and associated chemicals get deposited onto the river bed of the 160 km study reach while the rest leaves the study domain during the first three days following the release event. These sediment/chemicals deposited during the initial spill may also have long-term effects on the water quality and aquatic ecosystem of the river and the downstream delta. However, care has to be taken in interpreting the results as further analysis has shown that the outcomes of such model simulations are very sensitive to the various underlying assumptions as well as the values assigned to some model parameters representing the physical properties of the tailings material.

Engineered river flow-through to improve mine pit lake and river values

Mine pit lakes may develop at mine closure when mining voids extend below groundwater levels and fill with water. Acid and metalliferous drainage (AMD) and salinity are common problems for pit lake water quality. Contaminated pit lake waters can directly present significant risk to both surrounding and regional communities and natural environmental values and limit beneficial end use opportunities. Pit lake waters can also discharge into surface and groundwater; or directly present risks to wildlife, stock and human end users. Riverine flow-through is increasingly proposed to mitigate or remediate pit lake water contamination using catchment scale processes. This paper presents the motivation and key processes and considerations for a flow-through pit lake closure strategy. International case studies as precedent and lessons for future application are described from pit lakes that use or propose flow-through as a key component of their mine closure design. Chemical and biological processes including dilution, absorption and flocculation and sedimentation can sustainably reduce pit lake contaminant concentrations to acceptable levels for risk and enable end use opportunities to be realised. Flow-through may be a valid mine closure strategy for pit lakes with poor water quality. However, maintenance of existing riverine system values must be foremost. We further suggest that decant river water quality may, in some circumstances, be improved; notably in examples of meso-eutrophic river waters flowing through slightly acidic pit lakes. Flow-through closure strategies must be scientifically justifiable and risk-based for both lake and receptors potentially affected by surface and groundwater transport. Due to the high-uncertainty associated with this complex strategy, biotic and physico-chemical attributes of both inflow and decant river reaches as well as lake should be well monitored. Monitoring should directly feed into an adaptive management framework discussed with key stakeholders with validation of flow-through as a sustainable strategy prior to mine relinquishment.

Early chemical and toxicological risk characterization of inorganic constituents in surface water from the Canadian oil sands first large-scale end pit lake

End pit lakes (EPLs) have been proposed as a method for the long-term reclamation of process water and fluid fine tailings (FFT) produced from surface mining within the Canadian oil sands. These waste products contain elevated concentrations of dissolved organics, metals, and salts which reduce surface water quality and are toxic to aquatic organisms. This study measured the concentrations of inorganic constituents in surface water from the industry's first large-scale EPL over the course of a three-year period (2014-2016). The toxicological risk was subsequently assessed to identify constituents of concern that may impair surface water quality necessary for the development of a functional aquatic ecosystem or for release to the surrounding environment. Changes in surface water concentrations over the three-year period were strongly correlated with hydrological processes occurring within the lake: advective-diffusive chemical influx from FFT pore water to the overlying surface water was offset by efflux via continuous manual pumping (freshwater in, process water out). These processes resulted in a net dilution effect of approximately 5-10% per year, however, a significant chemical mass is expected to persist within the underlying FFT. Elevated salinity (as Na⁺, Cl^-, HCO₃^-) and concentrations of boron and nickel were predicted to pose very high toxicological risk to aquatic organisms. Despite these risks, the discovery of wild Daphnia pulex in the August 2016 sample suggested that surface water quality was sufficient to support populations of certain salt-tolerant zooplankton and primary producers. However, the time required for development into a robust aquatic ecosystem remains unknown.

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 microbiology of oil sands tailings: past, present, future

Surface mining of enormous oil sands deposits in northeastern Alberta, Canada since 1967 has contributed greatly to Canada's economy but has also received negative international attention due largely to environmental concerns and challenges. Not only have microbes profoundly affected the composition and behavior of this petroleum resource over geological time, they currently influence the management of semi-solid tailings in oil sands tailings ponds (OSTPs) and tailings reclamation. Historically, microbial impacts on OSTPs were generally discounted, but next-generation sequencing and biogeochemical studies have revealed unexpectedly diverse indigenous communities and expanded our fundamental understanding of anaerobic microbial functions. OSTPs that experienced different processing and management histories have developed distinct microbial communities that influence the behavior and reclamation of the tailings stored therein. In particular, the interactions of Deltaproteobacteria and Firmicutes with methanogenic archaea impact greenhouse gas emissions, sulfur cycling, pore water toxicity, sediment biogeochemistry and densification, water usage and the trajectory of long-term mine waste reclamation. This review summarizes historical data; synthesizes current understanding of microbial diversity and activities in situ and in vitro; predicts microbial effects on tailings remediation and reclamation; and highlights knowledge gaps for future research.

Characterization of physical mass transport through oil sands fluid fine tailings in an end pit lake: a multi-tracer study

Soft tailings pose substantial challenges for mine reclamation due to their high void ratios and low shear strengths, particularly for conventional terrestrial reclamation practices. Oil sands mine operators have proposed the development of end pit lakes to contain the soft tailings, called fluid fine tailings (FFT), generated when bitumen is removed from oil sands ore. End pit lakes would be constructed within mined-out pits with FFT placed below the lake water. However, the feasibility of isolating the underlying FFT has yet to be fully evaluated. Chemical constituents of interest may move from the FFT into the lake water via two key processes: (1) advective-diffusive mass transport with upward pore water flow caused by settling of the FFT; and (2) mixing created by wind events or unstable density profiles through the lake water and upper portion of the FFT. In 2013 and 2014, temperature and stable isotopes of water profiles were measured through the FFT and lake water in the first end pit lake developed by Syncrude Canada Ltd. Numerical modelling was undertaken to simulate these profiles to identify the key mechanisms controlling conservative mass transport in the FFT. Shallow mixing of the upper 1.1 m of FFT with lake water was required to explain the observed temperature and isotopic profiles. Following mixing, the re-establishment of both the temperature and isotope profiles required an upward advective flux of approximately 1.5 m/year, consistent with average FFT settling rates measured at the study site. These findings provide important insight on the ability to sequester soft tailings in an end pit lake, and offer a foundation for future research on the development of end pit lakes as an oil sands reclamation strategy.