Fluid Fine Tailings Research Articles

Crystalline iron oxide mineral (magnetite) accelerates methane production from petroleum hydrocarbon biodegradation

Methane (CH4) emissions are a factor in climate change; in addition, CH4 production may affect reclamation of fluid fine tailings (FFT) in tailings ponds, and end-pit lakes (EPLs). In laboratory cultures, we investigated the effect of crystalline iron mineral (magnetite) on CH4 production from the biodegradation of hydrocarbons added to FFT collected from methanogenically more and less active sites in a demonstration EPL. Magnetite enhanced CH4 production from both sites, having a greater effect in more active FFT, where it increased the CH4 production rate as much as 48% (from 6.67 μmol d−1 to 9.87 μmol d−1) compared to FFT without magnetite. Correspondingly, magnetite hastened biodegradation of hydrocarbons (monoaromatics, n-alkanes and iso-alkanes), with a pronounced effect on o-xylene, ethylbenzene, m/p-xylenes, n-octane, n-nonane, and 2-methyloctane, where biodegradation rates increased by 46, 117, 11, 45, 28 and 37%, respectively, compared to FFT without magnetite. Little FeII was produced, suggesting that magnetite is not being used as an electron acceptor but rather functions as a conduit for electron transfer. Thus, magnetite may be a suitable amendment to enhance bioremediation of anaerobic environments contaminated with hydrocarbons. Importantly, our observations imply that magnetite may increase CH4 emissions from terrestrial ecosystems, thus affecting carbon budget estimations.

Simulation-Based Integration of Thermal Drying of Fluid Fine Tailings for Tailings Management and Freshwater Conservation in Oil Sands Mining

Simulation-Based Integration of Thermal Drying of Fluid Fine Tailings for Tailings Management and Freshwater Conservation in Oil Sands Mining

Cascading sulfur cycling in simulated oil sands pit lake water cap mesocosms transitioning from oxic to euxinic conditions

Base Mine Lake (BML), the first full-scale demonstration of oil sands tailings pit lake reclamation technology, is experiencing expansive, episodic hypolimnetic euxinia resulting in greater sulfur biogeochemical cycling within the water cap. Here, Fluid Fine Tailings (FFT)-water mesocosm experiments simulating the in situ BML summer hypolimnetic oxic-euxinic transition determined sulfur biogeochemical processes and their controlling factors. While mesocosm water caps without FFT amendments experienced limited geochemical and microbial changes during the experimental period, FFT-amended mesocosm water caps evidenced three successive stages of S speciation in ∼30 days: (S1) rising expansion of water cap euxinia from FFT to water surface; enabling (S2) rapid sulfate (SO42−) reduction and sulfide production directly within the water column; fostering (S3) generation and subsequent consumption of sulfur oxidation intermediate compounds (SOI). Identified key SOI, elemental S and thiosulfate, support subsequent SOI oxidation, reduction, and/or disproportionation processes in the system. Dominant water cap microbes shifted from methanotrophs and denitrifying/iron-reducing bacteria to functionally versatile sulfur-reducing bacteria (SRB) comprising sulfate-reducing bacteria (Desulfovibrionales) and SOI-reducing/disproportionating bacteria (Campylobacterales and Desulfobulbales). The observed microbial shift is driven by decreasing [SO42−] and organic aromaticity, with putative hydrocarbon-degrading bacteria providing electron donors for SRB. Comparison between unsterile and sterile water treatments further underscores the biogeochemical readiness of the in situ water cap to enhance oxidant depletion, euxinia expansion and establishment of water cap SRB communities aided by FFT migration of anaerobes. Results here identify the collective influence of FFT and water cap microbial communities on water cap euxinia expansion associated with sequential S reactions that are controlled by concentrations of oxidants, labile organic substrates and S species. This emphasizes the necessity of understanding this complex S cycling in assessing BML water cap O2 persistence.

Development of a small-scale geotechnical centrifuge

Oil sand fluid fine tailings in northern Alberta have very poor water-release characteristics and require many decades to fully consolidate under their own weight. Generally, physical and/or chemical treatments are necessary to improve the consolidation behaviour and manage the tailings in an economical and environmentally acceptable manner. The effect of each treatment method is commonly evaluated, in part, by measuring the short- and long-term consolidation behaviour of treated samples. Geotechnical beam centrifuges, large-strain consolidation apparatus and geocolumns are typically used to measure the consolidation properties. However, these methods require many months to complete or are expensive and difficult to deploy in the field. A small-scale geotechnical centrifuge suitable for use in both research environments and laboratories at industrial sites was developed to study and/or monitor various treatment and disposal options in a short time. This paper presents the motivation for developing a small-scale centrifuge, the components and features of the centrifuge and the assortment of tests completed to validate the apparatus and testing methods. The validation tests proved that the small-scale centrifuge produces comparable results with existing methods while reducing the cost and time required to evaluate the consolidation performance of different tailings treatment options.

Effect of flyash addition to flocculation and freezing and thawing treatment on consolidation of oil sands fluid fine tailings

The total volume of fluid fine tailings (FFT), reached 1,270 Mm3 in 2019. Extensive research is underway by a number of operators to develop dewatering technologies for oil sand tailings reclamation to comply with Directive 085 issued by the Alberta Energy Regulator. A promising technology for the disposal of FFT is to add flocculents and then use thickeners or centrifuges to decrease the water content. Following this treatment, freezing/thawing processes can then be utilized to further dewater the tailings. The effect of flocculation/flyash addition and thickening coupled with freezing/thawing treatments on FFT was investigated by performing large-strain consolidation and shear strength tests on the treated flocculated TTs. It was found that flocculation and thickening treatment increases the hydraulic conductivity of the treated TT which will result in the TT consolidating much faster than the untreated TT. The most important benefit of the flyash addition is the increase in shear strength and hydraulic conductivity of the flyash-treated TTs. The benefit of the freezing/thawing treatment processes coupled with flyash treatment is the increase in the compressibility and hydraulic conductivity at effective stresses lower than 100 kPa and void ratios greater than 1.2, respectively. This will facilitate earlier progressive reclamation required to support hydraulic sand capping.

Aerobic biodegradation of cycloalkanes in non-aqueous extracted oil sands tailings

Management of growing volumes of fluid fine tailings (FFT) is a significant challenge for oil sands industry. A potential alternative non-aqueous solvent extraction (NAE) process uses cycloalkane solvent such as cyclohexane or cyclopentane with very little water and generates smaller volumes of ‘dry’ solids (NAES) with residual solvent. Here we investigate remediation of NAES in a simulated bench-scale upland reclamation scenario. In the first study, microcosms with nutrient medium plus FFT as inoculum were amended with cyclohexane and incubated for ∼1 year, monitoring for cyclohexane biodegradation under aerobic conditions. Biodegradation of cyclohexane occurred under aerobic conditions with no metabolic intermediates detected. A second study using NAES mixed with FFT spiked with cyclohexane and cyclopentane, with or without additional nutrients (nitrogen and phosphorus), showed complete and rapid aerobic biodegradation of both cycloalkanes in NAES inoculated with FFT and supplemented with nutrients. 16S rRNA gene sequencing revealed dominance of Rhodoferax and members of Burkholderiaceae during aerobic cyclohexane biodegradation in FFT, and Hydrogenophaga, Acidovorax, Defluviimonas and members of Porticoccaceae during aerobic biodegradation of cyclohexane and cyclopentane in NAES inoculated with FFT and supplemented with nutrients. The findings indicate that biodegradation of cycloalkanes from NAES is possible under aerobic condition, which will contribute to the successful reclamation of oil sands tailings for land closure.

Compressive Stress Dewaterability Limit in Fluid Fine Tailings

Reclamation of fluid fine tailing (FFT) storage facilities to their pre-disturbance equivalent landforms is hampered because micrometer size fines, whose surface-area-to-volume ratio is remarkably high, are occupied with siloxane and hydroxy groups, which bind water strongly. The purpose of this study is to differentiate the forms of water physically distributions in FFT and determine their propensities for dewaterability under compressive stresses. Two thermal and two mechanical methods were used to analyze water distributions in FFT. Dynamic and isothermal thermogravimetric analyses of FFT gave a transition from predominately bulk water to coevolution with water of higher enthalpy of vaporization at 81% (w/w) solids. Differential scanning calorimeter studies were used to determine the non-freezable water amount, with the premise that water that does not freeze by −30 °C is also unlikely to be removable by compressive stresses encountered in tailing treatment processes. The solid weight percent of FFTs with the non-freezable water was 79.6%. A 1D finite-strain model simulation using the constitutive relations of void ratio and effective stress, void ratio, and hydraulic conductivity show that deep-pits filled with such clayey-silt FFT will consolidate to a maximum solids content of 74% (w/w). For separation by centrifugation, the solids content plateaued to a mean of 74% (w/w) for total centrifugal force of ≥30 mega Newtons. These solid contents represent upper thresholds and demonstrate dewatering limit property of an FFT under compressive stresses.

Accelerating the Consolidation of Thickened Tailings Using Sand Co-Disposal

Only grams or kilograms of valuable resources are extracted for each tonne of ore processed, generating immense amounts of tailings. Returning tailings-containment dams to their equivalent pre-disturbance landscape state is many stakeholders’ acceptable management strategy. The depositing of thickened fluid fine tailings (FFT), which the industry has relied upon to develop geotechnically stable dry landscapes, has not materialised due to the low hydraulic conductivity of such placements. The addition of sand to thickened FFT is proposed to accelerate the consolidation of thickened FFT in a co-disposal scheme, and this study evaluates its impact. In the continuum of solid wastes from mining operations, sand is the next-larger-sized grade of solids after “fine” that is produced in high proportions. Blends of highly-plastic FFT and fine-grade sand tailings at varying sand-to-fine ratio (SFR) values were tested. Sand lowered the liquid limit but did not significantly affect the plastic limit of FFT. The changes in liquid and plastic limits for SFR ≥ 1 blends were under 5% of water content. The maximum unit weight for the co-disposal mixes was obtained at SFR 3. Despite increases in unit weights, hydraulic conductivity increased with increasing SFR, while the trend for compression rates was in the reverse order. Void ratio—effective pressure—hydraulic conductivity power law relationships at varying SFR were obtained from the consolidation measurements. The constitutive constants of these relationships were used to model settlement of 30 m deep pits over a 25-year period. For all the deposits studied, the dewatering converged to a maximum solids content characteristic of each SFR in the order 74, 85, 88, and 94% (w/w) for thickened FFT and SFRs 1, 2, and 3, respectively. The numerically calculated average solid contents of the deep-pit placements grew closer with increasing SFR becoming 59, 77, 82, and 86% (w/w), respectively. Balancing the consolidation-rate gains and the land footprint required for storage with SFR increase, the optimal composition for co-disposal for this material and similar tailings is SFR 1.

Biodegradation of 2-methylpentane in fluid fine tailings amended with a mixture of iso-alkanes under sulfate-reducing conditions.

Anaerobic microorganisms in Canada Natural Upgrading Limited (CNUL) fluid fine tailings (FFT) are sustained by residual solvent hydrocarbons. Although FFT are methanogenic in nature, sulfate-reducing microorganisms represent a significant portion of FFT bacterial community. In this study, we examined biodegradation of three iso-alkanes (2-methylbutane, 2-methylpentane, and 3-methylpentane), representing major iso-alkanes in paraffinic solvent, in CNUL FFT under sulfate-reducing conditions. During ∼1100 days of incubation, only 2-methylpentane was degraded partially, whereas 2-methylbutane and 3-methylpentane were not degraded. During active degradation of 2-methylpentane, the bacterial community was dominated by Anaerolineaceae followed by Syntrophaceae, Peptococcaceae, Desulfobacteraceae, and Desulfobulbaceae. The archaeal community was co-dominated by acetoclastic (Methanosaetaceae) and hydrogenotrophic (Methanobacteriaceae) methanogens. This study underlines the limited capability of the microbial community indigenous to CNUL FFT in degrading recalcitrant iso-alkanes under sulfate-reducing conditions.

Effect of wetting–drying cycles on the desiccation of tailings pre-dewatered with super absorbent polymer

The main objective of this investigation is to understand the desiccation behaviour of fluid fine tailings (FFT) pre-dewatered with a superabsorbent polymer (SAP) with respect to wetting–drying cycles. Raw FFT (control) and pre-dewatered FFT with SAP were subjected to five cycles of wetting and six cycles of drying. Evaporation was found to be the dominant mechanism in the dewatering of raw FFT, as well as FFT pre-dewatered with SAP. In both cases, the suction developed in each drying cycle was eliminated temporarily by the succeeding wetting cycle. The minimum solids content within the column was found to be around 70%, which corresponded to a vane shear strength of 4.5 kPa. Based on the digital images of the surface, the crack intensity factors of the raw FFT and pre-dewatered FFT were calculated to be 26 and 14%, respectively. Scanning electron microscopy and mercury intrusion porosimetry analyses indicated that the FFT pre-dewatered with SAP after 25 days of evaporation in the second lift exhibited the most compacted texture. The solids content of the FFT pre-dewatered with SAP increased to 98% from an initial 50%. The SAP pre-dewatering method can significantly increase the shear strength of FFT subjected to wetting–drying cycles.

Anaerobic Microbial Activity May Affect Development and Sustainability of End-Pit Lakes: A Laboratory Study of Biogeochemical Aspects of Oil Sands Mine Tailings

Reclamation of fluid fine tailings (FFT) produced by mined oil sands ore processing is an environmental and technical challenge. End-pit lakes (EPLs) are a prospective reclamation strategy needing comprehensive evaluation. We investigated biogeochemical changes in water quality and sediments by using 140 L columns containing 50 L of FFT, either unamended or amended with hydrocarbons, and capped with 20 L of process water to simulate an EPL. The columns were incubated up to 3 years anaerobically at 10, 20, or 30 °C. Microbial metabolism of hydrocarbons in FFT produced methane (CH4) and carbon dioxide (CO2), accelerated FFT settling by expressing more porewater to the surface, and increased turbidity in cap water. Gas ebullition caused bitumen release and chemical flux from the underlying FFT to cap water. Dissolution and biotransformation of carbonate and Fe-bearing minerals in FFT during incubation led to the positive flux of major cations (Ca2+ and Mg2+), anions (HCO3–), and some trace elements (primarily Ba and Sr), thereby influencing the chemistry of overlying cap water. No discernible trend in flux of dissolved organic carbon and naphthenic acids was observed. The results suggest that microbial anaerobic activity stimulated by residual hydrocarbons in FFT may influence the progression of EPLs into viable aquatic ecosystems.

Application of Tempe Cell to Measure Soil Water Characteristic Curve along with Geotechnical Properties of Oil Sands Tailings

The traditional Tempe cell can be used to adequately determine the soil water characteristic curve (SWCC) for soils that do not undergo significant volume change as matric suction is increased, such as coarse-grained material such as sand with a low air entry value (AEV) (<500 kPa). When soils undergo substantial volume change as soil suction increases, such as fine-grained silts, clays, and oil sands tailings material, the soils need to be tested with distinctly different methods involving two apparatuses when using the Tempe cell. A single-step Tempe cell technique was developed and tested to measure the geotechnical and unsaturated properties of oil sands tailings samples. A series of nine Tempe cells were simultaneously used to measure the geotechnical and unsaturated soil properties of untreated fluid fine tailings (FFT) and treated flocculated centrifuged tailings (FCT). The results of the single-step Tempe cell technique provide several useful engineering functions relating matric suction to water content (SWCC), void ratio (volume change), solids content, and undrained shear strength. Both the traditional and single-step Tempe cell techniques yield comparable SWCC results, but the single-step Tempe cell yields result about three times faster than the traditional Tempe cell. The geotechnical results indicate that both the solids content and undrained shear strength of the FCT are greater than those of the untreated FFT and this indicates that flocculation and centrifugation increase solids content and undrained shear strength of the treated samples. Furthermore, the results indicate that the FFT starts at higher fine void ratio than the FCT and loses more water (volume change) at matric suctions lower than 7 kPa. Beyond 7 kPa, the compressions of both samples become the same. The single-step Tempe cell technique is, however, labor-intensive. The number of Tempe cell can be reduced to six depending on the starting load of the test. The use of the single-step Tempe cell technique in providing fast estimates of SWCC and geotechnical properties for oil sands tailings will be attractive to practitioners who intend to incorporate matric suction in oil sands geotechnical engineering problems.

Characterization and Dynamics of Residual Organics in Oil Sands Fluid Fine Tailings

Characterization and Dynamics of Residual Organics in Oil Sands Fluid Fine Tailings

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.

Geotechnical Properties Determination of Thickened Fluid Fine Tailings

AbstractFluid fine tailings (FFT) comprising clayey-silt solids pose environmental and financial challenges. Currently, mining operators are depositing thickened FFT in deep-pits counting on self-weight consolidation to form stable ground. The motivation of this study is to model the long-term prospects of such deposits utilizing consolidation and direct shear strength measurements. The tests were conducted using scroll decanter centrifuge separated FFT sediment referred to as cake. Drained direct shear tests of the cake gave a linear Mohr–Coulomb failure envelope of 1.2 kPa cohesion intercept and 9° internal friction angle for normal stresses up to 1 MPa. Hydraulic conductivity of the cake was non-linear with normal stress decreasing to 1.7 × 10–11 m/s at 300 kPa. Consolidation results confirmed that the cake exhibits properties similar to those of active clay minerals. The cake compression index is governed by the same relationship as for active clays. The coefficient of consolidation for the cake was nearly constant and had a mean value of 2.25 × 10–3 m2/y, also similar to that of active clays. The void ratio—effective stress—hydraulic conductivity power law empirical relations were used to simulate settlement with a finite-strain model. Numerical results show that the top portion of an FFT deep-pit deposit remains in the liquid state while the lower portion whose maximum solids content converges to 74% is in plastic state. These mean that options that improve hydraulic conductivity and increase the shear strength of the thickened FFT should be integrated prior to final placement in order to create stable ground.

Enzyme-assisted dewatering of oil sands tailings: Significance of water chemistry and biological activity

Remediation of oil sands tailings is a crucial environmental aspect of the Canadian oil sands industry. Because of the poor released water quality and slow consolidation of tailings, engineered non-natural strategies to improve the consolidation of tailings are warranted. Here, we investigated enzymatic treatment (using cellulase, protease, and lysozyme) to accelerate the dewatering of fluid fine tailings (FFT). Our findings illustrated that lysozyme (0.5% and 1%) significantly improved FFT dewatering by increasing the water recovery (WR) up to 20% compared with the other enzymes (up to 12%) or the control (2%). Moreover, lysozyme treatment resulted in the highest increase in ionic strength (0.038 to 0.1 mol L−1), decrease in diffuse double layer (DDL) thickness (1.54 × 10−7 to 9.40 × 10−8 cm), and increase in zeta potential (−34.7 to −14.8 mV). Increased methane production was observed for cellulase (0.5% and 1%), lysozyme (0.5% and 1%), and protease (0.5%). The enhanced dewatering could be linked to the ebullition of methane gas resulting from the methanogenic activity, which created pathways for the trapped water release. In addition, the dissolution of carbonate minerals during the release of methane gas increased ionic strength and decreased the DDL of the FFT. Lysozyme 1% treatment was also the most effective in reducing naphthenic acid fractions (1934.6 to 243 ng mL−1); however, the released water had high toxicity toward Vibrio fischeri and had a slight decrease in microbial populations. This study provides a fundamental insight into enzymatic treatment for oil sand tailings.

A comparison study between bioflocculants and PAM for dewatering of ultrafine phyllosilicate clay minerals

Several environmental problems have been reported when synthetic organic and inorganic flocculants are used to treat fluid fine tailings (FFT), especially phyllosilicate clay minerals, generated in the mining plants. These hazardous issues have increased the importance of using natural flocculants from renewable sources (biopolymers). This investigation has explored the impact of proteins (protamine and lysozyme) and polysaccharides (chitosan, pectin, lignin, and alginic acid) as natural flocculants and compared their results in the same conditions by using Polyacrylamide (PAM) as a traditional organic polymer. Ultrafine kaolinite and serpentine particles (−10 μm) were chosen as the challenging gangue minerals for chalcopyrite copper ore (CuFeS2) and pentlandite nickel ore ((Fe, Ni)9S8)), respectively. Capillary suction timer (CST) and filtration tests were considered for comparative assessments. Assessments indicated that PAM in shear rate ~ 20 (1/s) has a viscosity ~90 times higher than the examined biopolymers. This high viscosity limited PAM distribution as a flocculant through the suspensions. Results highlighted that PAM had the highest flocculation activity in low dosages (0.05 and 0.1 wt%) while in dosages above 0.25 wt%, cationic biopolymers; protamine, and lysozyme for kaolinite and anionic pectin for serpentine showed a higher performance than the other polymers. The addition of Ca2+ could increase the dewatering of the clay minerals and promote the flocculation strength of the biopolymers. Generally, results indicated that the flocculation function of protamine and lysozyme (cationic proteins) at the optimum dosage (0.5 wt%) was higher than other tested biopolymers, while pectin (an anionic polysaccharide) improved the dewatering of serpentine particles. Outcomes of this study proved that commercially available bioflocculants have a significant potential to use in the treatment of FFTs and decrease environmental problems that occur by using inorganic and organic flocculants.

Sulfur mass balance and speciation in the water cap during early-stage development in the first pilot pit lake in the Alberta Oil Sands

Environmental context Base Mine Lake (BML) is a demonstration pilot pit lake for long term oil sands tailings reclamation in Alberta, Canada. This study quantified BML water cap sulfur mass balance and speciation to help understand potential risks to oxygen levels during its early-stage development. Results provide important insights for the adaptive management of water-capped oil sands tailings reclamation. Rationale Sulfur cycling is crucial to the persistence of oxygen in the water cap of Base Mine Lake (BML), the first demonstration oil sands pit lake for water capped tailings technology (WCTT) in Alberta, Canada. Methodology Here, we report on the first investigation of sulfur mass balance and sulfur speciation (SO4 2−, SO3 2−, S2O3 2−, S0 and ƩH2S) over seasonal, annual and spatial scales in BML. Results and discussion High aqueous total sulfur concentration (1.7–2 mM), dominated by sulfate (>75%), decreased over the study period (2015–2021), due to the consolidation of fluid fine tailings (FFT) and operational pump-in and pump-out activities. Expanded BML water cap S biogeochemical cycling occurred after a 2016 alum amendment. Late summer hypolimnetic anoxia emerged post-alum (2017–2021), coincident with detectable total sulfide (ƩH2S) and elemental sulfur (S0) concentrations and expanded sulfur-reducing bacteria activity in anoxic bottom waters. Post spring turnover resuspended FFT and particle settling rates also likely increased post-alum, supported by the observed migration of epilimnetic highest sulfite concentration (pre-alum) to metalimnetic waters (post-alum). These sulfide containing particles are likely the primary reduced S substrate for spring–summer sulfur-oxidising bacteria activity, as winter aqueous reactive S species (ƩH2S, S0, S2O3 2 − and SO3 2−) were non-detectable across years. Concentrations of reactive S species reached up to 200 µM, posing risks to BML O2 levels (maximum 300–350 µM). Results of this study establish the interactive effects of physical and biogeochemical processes, as well as operational activities in emergent S risks to water cap oxygen levels, a key criterion for success of this reclamation tailings technology.

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 [CH4(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 [CH4(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 CH4(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.

Isotopic and Chemical Assessment of the Dynamics of Methane Sources and Microbial Cycling during Early Development of an Oil Sands Pit Lake.

Water-capped tailings technology (WCTT) is a key component of the reclamation strategies in the Athabasca oil sands region (AOSR) of northeastern Alberta, Canada. The release of microbial methane from tailings emplaced within oil sands pit lakes, and its subsequent microbial oxidation, could inhibit the development of persistent oxygen concentrations within the water column, which are critical to the success of this reclamation approach. Here, we describe the results of a four-year (2015–2018) chemical and isotopic (δ13C) investigation into the dynamics of microbial methane cycling within Base Mine Lake (BML), the first full-scale pit lake commissioned in the AOSR. Overall, the water-column methane concentrations decreased over the course of the study, though this was dynamic both seasonally and annually. Phospholipid fatty acid (PLFA) distributions and δ13C demonstrated that dissolved methane, primarily input via fluid fine tailings (FFT) porewater advection, was oxidized by the water column microbial community at all sampling times. Modeling and under-ice observations indicated that the dissolution of methane from bubbles during ebullition, or when trapped beneath ice, was also an important source of dissolved methane. The addition of alum to BML in the fall of 2016 impacted the microbial cycling in BML, leading to decreased methane oxidation rates, the short-term dominance of a phototrophic community, and longer-term shifts in the microbial community metabolism. Overall, our results highlight a need to understand the dynamic nature of these microbial communities and the impact of perturbations on the associated biogeochemical cycling within oil sands pit lakes.