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Item Acidic microenvironments in waste rock characterized by neutral drainage: Bacteria-mineral interactions at sulfide surfaces(MDPI, 2014-03-21) Dockrey, John W.; Lindsay, Matthew B. J.; Mayer, K. Ulrich; Beckie, Roger D.; Norlund, Kelsey L. I.; Warren, Lesley; Southam, GordonMicrobial populations and microbe-mineral interactions were examined in waste rock characterized by neutral rock drainage (NRD). Samples of three primary sulfide-bearing waste rock types (i.e., marble-hornfels, intrusive, exoskarn) were collected from field-scale experiments at the Antamina Cu–Zn–Mo mine, Peru. Microbial communities within all samples were dominated by neutrophilic thiosulfate oxidizing bacteria. However, acidophilic iron and sulfur oxidizers were present within intrusive waste rock characterized by bulk circumneutral pH drainage. The extensive development of microbially colonized porous Fe(III) (oxy)hydroxide and Fe(III) (oxy)hydroxysulfate precipitates was observed at sulfide-mineral surfaces during examination by field emission-scanning electron microscopy-energy dispersive X-ray spectroscopy (FE-SEM-EDS). Linear combination fitting of bulk extended X-ray absorption fine structure (EXAFS) spectra for these precipitates indicated they were composed of schwertmannite [Fe8O8(OH)6–4.5(SO4)1–1.75], lepidocrocite [γ-FeO(OH)] and K-jarosite [KFe3(OH)6(SO4)2]. The presence of schwertmannite and K-jarosite is indicative of the development of localized acidic microenvironments at sulfide-mineral surfaces. Extensive bacterial colonization of this porous layer and pitting of underlying sulfide-mineral surfaces suggests that acidic microenvironments can play an important role in sulfide-mineral oxidation under bulk circumneutral pH conditions. These findings have important implications for water quality management in NRD settings.Item Adsorption of (Poly)vanadate onto Ferrihydrite and Hematite: An In Situ ATR–FTIR Study(American Chemical Society (ACS) Publications, 2020-03-25) Vessey, Colton; Schmidt, Michael P.; Abdolahnezhad, Mojtaba; Peak, Derek; Lindsay, Matthew B. J.Vanadium (V) has been a useful trace metal in describing Earth’s biogeochemical cycling and development of industrial processes; however, V has recently been recognized as a potential contaminant of concern. Although Fe (oxyhydr)oxides are important sinks for aqueous V in soils and sediments, our understanding of adsorption mechanisms is currently limited to mononuclear species (i.e., HxVO4(3–x)–). Here we use in situ attenuated total reflectance – Fourier transform infrared spectroscopy to examine sorption mechanisms and capacity for (poly)vanadate attenuation by ferrihydrite and hematite from pH 3 to 6. Adsorption isotherms illustrate the low affinity of polyvanadate species for ferrihydrite surfaces compared to hematite. Mononuclear V species (i.e., [HxVO4](3−x)− and VO2+) were present at all experimental conditions. At low surface loadings and pH 5 and 6, H2VO4− adsorption onto ferrihydrite and hematite surfaces results from formation of inner sphere complexes. At [V]T above 250 µM, adsorbed polynuclear V species in this study include H2V2O72− and V4O124−. Whereas, HV10O286−, H3V10O285−, and NaHV10O284− are the predominant adsorbed species at pH 3 and 4 and elevated [V]T. Surface polymers were identified on hematite at all experimental pH values, whereas polymeric adsorption onto ferrihydrite was limited to pH 3 and 4. These results suggest that hematite offers a more suitable substrate for polymer complexation compared to ferrihydrite. Our results demonstrate the pH and concentration dependant removal of (poly)vanadate species by Fe(III) (oxyhydr)oxides, which has implications for understanding V mobility, behaviour, and fate in the environment.Item Alum addition triggers hypoxia in an engineered pit lake(MDPI, 2022-02-26) Jessen, Gerdhard L.; Chen, Lin-Xing; Mori, Jiro F.; Colenbrander Nelson, Tara E.; Slater, Gregory F.; Lindsay, Matthew B. J.; Banfield, Jillian F.; Warren, Lesley A.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.Item Aqueous vanadate removal by iron(II)-bearing phases under anoxic conditions(American Chemical Society (ACS) Publications, 2020-03-06) Vessey, Colton; Lindsay, Matthew B. J.Vanadium contamination is a growing environmental hazard worldwide. Aqueous vanadate (HxVVO4(3−x)− (aq)) concentrations are often controlled by surface complexation with metal (oxyhydr)oxides in oxic environments. However, the geochemical behaviour of this toxic redox sensitive oxyanion in anoxic environments is poorly constrained. Here we describe results of batch experiments to determine kinetics and mechanisms of aqueous H2VVO4− (100 μM) removal under anoxic conditions in suspensions (2.0 g L−1) of magnetite, siderite, pyrite, and mackinawite. We present results of parallel experiments using ferrihydrite (2.0 g L−1) and Fe2+(aq) (200 μM) for comparison. Siderite and mackinawite reached near complete removal (46 µmol g−1) of aqueous vanadate after 3 h and kinetic rates were generally consistent with ferrihydrite. Whereas magnetite removed 18 µmol g−1 of aqueous vanadate after 48 h and uptake by pyrite was limited. Uptake by Fe2+(aq) was observed after 8 h, concomitant with precipitation of secondary Fe phases. X ray absorption spectroscopy revealed V(V) reduction to V(IV) and formation of bidentate corner-sharing surface complexes on magnetite and siderite, and with Fe2+(aq) reaction products. These data also suggest that V(IV) is incorporated into the mackinawite structure. Overall, we demonstrate that Fe(II)-bearing phases can promote aqueous vanadate attenuation and, therefore, limit dissolved V concentrations in anoxic environments.Item Aqueous- and solid-phase molybdenum geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada(Elsevier, 2018-11-12) Robertson, Jared M.; Nesbitt, Jake A.; Lindsay, Matthew B. J.Fluid petroleum coke generated at oil sands operations in the Athabasca Oil Sands Region of northern Alberta, Canada, contains elevated concentrations of molybdenum (Mo) and other metals including nickel (Ni) and vanadium (V). Solid-phase Mo concentrations in fluid petroleum coke are typically 10 to 100 times lower than V and Ni, yet dissolved Mo concentrations in associated pore waters are often comparable with these metals. We collected pore water and solids from fluid petroleum coke deposits in the AOSR to examine geochemical controls on Mo mobility. Dissolved Mo concentrations increased with depth below the water table, reaching maxima of 1.4 to 2.2 mg L-1, within a mixing zone between slightly acidic and oxic meteoric water and mildly alkaline and anoxic oil sands process-affected water (OSPW). Dissolved Mo concentrations decreased slightly with depth below the mixing zone. X-ray absorption spectroscopy revealed that Mo(VI) and Mo(IV) species were present in coke solids. The Mo(VI) occurred as tetrahedrally coordinated MoO42- adsorbed via inner- and outer-sphere complexation, and was coordinated in an environment similar to Fe-(hydr)oxide surface complexes. The OSPW likely promoted desorption of outer-sphere Mo(VI) complexes, resulting in higher dissolved Mo concentrations in the mixing zone. The principal Mo(IV) species was MoS2, which originated as a catalyst added upstream of the fluid coking process. Although MoS2 is likely stable under anoxic conditions below the mixing zone, oxidative weathering in the presence of meteoric water may promote long-term Mo release.Item Benchmarking multiphysics software for mantle convection(Elsevier, 2021-09) Trim, Sean; Butler, Samuel; Spiteri, RaymondNumerical simulations are a highly valuable tool for improving our understanding of mantle dynamics. COMSOL Multiphysics® is a commercial software suite designed to numerically model experiments featuring multiple branches of physics. This modeling approach applies to mantle convection, which can be viewed as a combination of fluid dynamics and thermodynamics. COMSOL® is of interest to the geoscience community due to its ease of use compared to other available codes, and it has been used in previous mantle convection studies. However, COMSOL® has not been extensively benchmarked for mantle convection. In this study, we confirm the accuracy of COMSOL® against several established benchmarks pertaining to a variety mantle convection features and geometries. Overall, we find reasonable agreement between the results from COMSOL® and reported benchmark data. This study may also serve the geoscience community as a guide for using COMSOL® to model mantle convection.Item Chemical mass transport between fluid fine tailings and the overlying water cover of an oil sands end pit lake(American Geophysical Union, 2017-05-17) Dompierre, Kathryn A.; Barbour, S. Lee; North, Rebecca L.; Carey, Sean K.; Lindsay, Matthew B. J.Fluid fine tailings (FFT) are a principal by-product of the bitumen extraction process at oil sands mines. Base Mine Lake (BML)—the first full-scale demonstration oil sands end pit lake (EPL)—contains approximately 1.9 3 108 m^3 of FFT stored under a water cover within a decommissioned mine pit. Chemical mass transfer from the FFT to the water cover can occur via two key processes: (1) advection-dispersion driven by tailings settlement; and (2) FFT disturbance due to fluid movement in the water cover. Dissolved chloride (Cl) was used to evaluate the water cover mass balance and to track mass transport within the underlying FFT based on field sampling and numerical modeling. Results indicated that FFT was the dominant Cl source to the water cover and that the FFT is exhibiting a transient advection-dispersion mass transport regime with intermittent disturbance near the FFT-water interface. The advective pore water flux was estimated by the mass balance to be 0.002 m^3 m^-2 d^-1, which represents 0.73 m of FFT settlement per year. However, the FFT pore water Cl concentrations and corresponding mass transport simulations indicated that advection rates and disturbance depths vary between sample locations. The disturbance depth was estimated to vary with location between 0.75 and 0.95 m. This investigation provides valuable insight for assessing the geochemical evolution of the water cover and performance of EPLs as an oil sands reclamation strategy.Item Dissolved selenium(VI) removal by zero-valent iron under oxic conditions: Influence of sulfate and nitrate(American Chemical Society, 2017-04-17) Das, Soumya; Lindsay, Matthew B. J.; Essilfie-Dughan, Joseph; Hendry, M. JamesDissolved Se(VI) removal by three commercially-available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO3- and SO42-. Environmentally relevant Se(VI) (1 mg/L), NO3- ([NO3-N] = 15 mg/L) and SO42- (1800 mg/L) were employed to simulate mining-impacted waters. Ninety percent Se(VI) removal was achieved within 4 to 8 h in the absence of SO42- and NO3-. Similar Se(VI) removal was observed after 10 to 32 h in the presence of NO3-. Dissolved Se(VI) removal rates exhibited the greatest decrease in the presence of SO42-; 90% Se(VI) removal was measured after 50 to 191 h for SO42- and after 150 to 194 h for SO42- plus NO3- depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe3O4], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used control Se(VI) concentrations in mining impacted waters.Item Ebullition enhances chemical mass transport across the tailings-water interface of oil sands pit lakes(Elsevier, 2021-12-09) Francis, Daniel; Barbour, S. Lee; Lindsay, Matthew B. J.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 fine fluid 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 enhance 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.Item Equilibrium shapes of two and three dimensional two-phase rotating fluid drops with surface tension: effects of inner drop displacement(American Institute of Physics, 2022-11-01) Butler, SamuelThe shapes of rotating fluid drops held together by surface tension is an important field of study in fluid mechanics. Recently, experiments with micron-scale droplets of liquid helium have been undertaken and it has proven useful to compare the shapes of the resultant superfluid droplets with classical analogs. If the helium is a mixture of He3 and He4, two phases are present. In a recent paper, the shapes of rotating two phase fluid droplets were calculated where the inner drop was constrained to stay at the drop center. The outer shapes and dimensionless rotation rate-angular momentum relationships were shown to be similar to single phase drops provided that the density and surface tension scales were chosen appropriately. In the current paper, I investigate models in which the inner drop can displace from the centre. In order to simplify the analyses, two dimensional drops are first investigated. I show that the inner drop is unstable in the centre position if its density is greater than the outer density and that the inner drop will move towards the outer boundary of the drop in these cases. When the inner drop has a higher density than the outer drop, the moment of inertia of displaced inner drops is increased relative to centered drops and hence the kinetic energy is decreased. Shapes of two and three dimensional drops, rotation rate-angular momentum and kinetic and surface energy relationships are investigated for off-axis inner drops with parameters relevant to recent liquid He experiments.Item Forward modeling of magnetotellurics using Comsol Multiphysics(2021-12) Li, Ang; Butler, SamuelMagnetotellurics is an electromagnetic geophysical method that has been widely used to study structures in Earth's subsurface. Numerical modeling of magnetotellurics is important for survey design, inversion, geological interpretation and many other aspects of geophysical studies. For example, modeling a subsurface conductive body in terms of its conductivity, geometry and dipping angle would yield substantial information on the phase response and sensitivity in an MT survey. While there are many different modeling techniques, the finite element method is most commonly used. In this effort, we present magnetotelluric models of layered Earth, uplift structures, auroral electrojets, and geomagnetically induced currents in power-line skywires using the commercial finite-element package Comsol Multiphysics. The AC/DC module in Comsol can be used to solve Maxwell's equations in the quasi-static limit for modeling the magnetotelluric response. One of the advantages of Comsol modeling is its Graphical User Interface (GUI), which allows users to solve complex single or multi-physics problems in a meshed domain. The use of Comsol also reduces the need for sophisticated computer coding when solving partial differential equations such as Maxwell's equations. In the effort presented here, we first discuss model validation for layered Earth geometries. We then present two examples of magnetotellurics modeling in impact crater and geomagnetically induced current studies. Numerical results were compared with analytical solutions or benchmark results whenever possible.Item Geochemical and mineralogical aspects of sulfide mine tailings(Elsevier, 2015-01-30) Lindsay, Matthew B. J.Tailings generated during processing of sulfide ores represent a substantial risk to water resources. The oxidation of sulfide minerals within tailings deposits can generate low-quality water containing elevated concentrations of SO4, Fe, and associated metal(loid)s. Acid generated during the oxidation of pyrite [FeS2], pyrrhotite [Fe(1-x)S] and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate, (oxy)hydroxide, and silicate minerals. The extent of acid neutralization and, therefore, pore-water pH is a principal control on the mobility of sulfide-oxidation products within tailings deposits. Metals including Fe(III), Cu, Zn, and Ni often occur at high concentrations and exhibit greater mobility at low pH characteristic of acid mine drainage (AMD). In contrast, (hydr)oxyanion-forming elements including As, Sb, Se, and Mo commonly exhibit greater mobility at circumneutral pH associated with neutral mine drainage (NMD). These differences in mobility largely result from the pH-dependence of mineral precipitation-dissolution and sorption-desorption reactions. Cemented layers of secondary (oxy)hydroxide and (hydroxy)sulfate minerals, referred to as hardpans, may promote attenuation of sulfide-mineral oxidation products within and below the oxidation zone. Hardpans may also limit oxygen ingress and pore-water migration within sulfide tailings deposits. Reduction-oxidation (redox) processes are another important control on metal(loid) mobility within sulfide tailings deposits. Reductive dissolution or transformation of secondary (oxy)hydroxide phases can enhance Fe, Mn, and As mobility within sulfide tailings. Production of H2S via microbial sulfate reduction may promote attenuation of sulfide-oxidation products, including Fe, Zn, Ni, and Tl, via metal-sulfide precipitation. Understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings.Item Geochemical characteristics of oil sands fluid petroleum coke(Elsevier, 2016-11-30) Nesbitt, Jake A.; Lindsay, Matthew B. J.; chen, ningThe geochemical characteristics of fluid petroleum coke from an oil sands mine in the Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada were investigated. Continuous core samples were collected to 8 m below surface at several locations (n = 12) from three coke deposits at an active oil sands mine. Bulk elemental analyses revealed the coke composition was dominated by C (84.2 ± 2.3 wt. %) and S (6.99 ± 0.26 wt. %). Silicon (9210 ± 3000 mg kg−1), Al (5980 ± 1200 mg kg−1), Fe (4760 ± 1200 mg kg−1), and Ti (1380 ± 430 mg kg−1) were present in lesser amounts. Vanadium (1280 ± 120 mg kg−1) and Ni (230 ± 80 mg kg−1) occurred at the highest concentrations among potentially-hazardous minor and trace elements. Sequential extractions revealed potential for release of these elements under field-relevant conditions. Synchrotron powder X-ray diffraction revealed the presence of Si and Ti oxides, organically-complexed V and hydrated Ni sulfate, and provided information about the aromatic carbon matrix. X-ray absorption near edge structure (XANES) spectroscopy at the V and Ni K-edges revealed that these metals were largely hosted in porphyrins and similar organic complexes throughout coke grains. Slight variations among V and Ni K-edge spectra were largely attributed to slight variations in local coordination of V(IV) and Ni(II) within these organic compounds. However, linear combination fits were improved by including reference spectra for phases with octahedrally-coordinated V(III) and Ni(II). Sulfur and Fe K-edge XANES confirmed that thiophenic coordination and pyritic-ilmenitic coordination are predominant, respectively. These results provide new information on the geochemical and mineralogical composition of oil sands fluid petroleum coke and improve understanding of potential controls on associated water chemistry.Item Geochemical conditions influence vanadium, nickel, and molybdenum release from oil sands fluid petroleum coke(Elsevier, 2022-01-07) Abdolahnezhad, Mojtaba; Lindsay, Matthew B. J.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.Item Influence of As(V) on precipitation and transformation of schwertmannite in acid mine drainage-impacted waters(Schweizerbart science publishers, 2018-12-07) Cruz-Hernandez, Pablo; Carrero, Sergio; Pérez-Lópeza, Rafael; Fernandez-Martinez, Alejandro; Lindsay, Matthew B. J.; Dejoie, Catherine; Nieto, José M.Iron-rich sediments commonly cover riverbeds affected by acid mine drainage (AMD). Initial precipitates are often dominated by schwertmannite, which has an exceptionally high capacity to sequester As and other toxic elements. This poorly crystalline Fe oxyhydroxysulfate rapidly recrystallizes to goethite; however, the influence of trace elements on ageing rates and products is poorly understood. This study examined the influence of As(V) concentrations on the kinetics of schwertmannite precipitation and transformation. Schwertmannite was synthesized in the presence of various initial dissolved As concentrations (i.e., 0–2 mM) and subsequently aged at 40, 60 or 85 °C for 1 h to 300 d. The initial As concentration had a profound impact on schwertmannite precipitation and transformation. Schwertmannite precipitation was inhibited at higher initial As concentrations in favor of pseudo-amorphous Fe-hydroxyarsenate formation. Schwertmannite transformation to goethite was accompanied by sulfate release and, over longer time, As release. Pair distribution function (PDF) analysis of high-energy X-ray diffraction (HEXD) patterns revealed that increasing initial As concentration produced structural defects in associated precipitates. Schwertmannite precipitation exerts an important control on As mobility in AMD-impacted waters; however, this study has demonstrated that the long-term stability of schwertmannite and associated precipitates should be considered when designing AMD remediation strategies and AMD treatment systems.Item Initial geochemical characteristics of fluid fine tailings in an oil sands end pit lake(Elsevier, 2016-06-15) Dompierre, Kathryn; Lindsay, Matthew B. J.; Cruz-Hernández, Pablo; Halferdahl, GeoffreyGeochemical characteristics of fluid fine tailings (FFT) were examined in Base Mine Lake (BML), which is the first full-scale demonstration oil sands end pit lake (EPL) in northern Alberta, Canada. Approximately 186 M m3 of FFT was deposited between 1994 and 2012, before BML was established on December 31, 2012. Bulk FFT samples (n = 588) were collected in July and August 2013 at various depths at 15 sampling sites. Temperature, solids content, electrical conductivity (EC), pH, Eh and alkalinity were measured for all samples. Detailed geochemical analyses were performed on a subset of samples (n = 284). Pore-water pH decreased with depth by approximately 0.5 within the upper 10 m of the FFT. Major pore-water constituents included Na (880 ± 96 mg L−1) and Cl (560 ± 95 mg L-1); Ca (19 ± 4.1 mg L-1), Mg (11 ± 2.0 mg L-1), K (16 ± 2.3 mg L-1) and NH3 (9.9 ± 4.7 mg L−1) were consistently observed. Iron and Mn concentrations were low within FFT pore water, whereas SO4 concentrations decreased sharply across the FFT-water interface. Geochemical modeling indicated that FeS(s) precipitation was favoured under SO4-reducing conditions. Pore water was also under-saturated with respect to gypsum [CaSO4·2H2O], and near saturation with respect to calcite [CaCO3], dolomite [CaMg(CO3)2] and siderite [FeCO3]. X-ray diffraction (XRD) suggested that carbonate-mineral dissolution largely depleted calcite and dolomite. X-ray absorption near edge structure (XANES) spectroscopy revealed the presence of FeS(s), pyrite [FeS2], and siderite. Carbonate-mineral dissolution and secondary mineral precipitation have likely contributed to FFT dewatering and settlement. However, the long-term importance of these processes within EPLs remains unknown. These results provide a reference for assessing the long-term geochemical evolution of oil sands EPLs, and offer insight into the chemistry of pore water released from FFT to the overlying water cover.Item Internal thermal boundary layer stability in phase transition modulated convection(AGU, 1997-02) Butler, S.L.; Peltier, W.R.The stability of a horizontal thermal boundary layer embedded within a very viscous fluid is investigated using the formalism of linear stability analysis. Thin thermal boundary layers in deep fluid regions and in the absence of phase transition and dynamical effects are thereby shown to be unstable at extremely long wavelengths. The stability of the internal thermal boundary layer which may exist at 660 km depth in the Earth's mantle as a consequence of the dynamical influence of the endothermic phase transition from γ spinel to a mixture of perovskite and magnesiowüstite, recently discussed in some detail by Solheim and Peltier [1994a], is investigated in order to better understand the “avalanche effect” observed in this and similar nonlinear, time dependent simulations of the mantle convection process. It is demonstrated that if the stability problem is treated as purely thermal, then the boundary layer is predicted to be extremely unstable and the presence of the 660‐km endothermic phase transition at middepth within the boundary layer is further destabilizing. When the kinematic effect of flow convergence onto the boundary layer and phase transition region is active, however, it is shown that the layer may be strongly stabilized. In the regime of physically realistic velocity convergence, the critical Rayleigh number is predicted to lie in the range suggested by the numerical simulations of Solheim and Peltier [1994a]. A threshold value of the magnitude of the Clapeyron slope of the endothermic phase transition for a given velocity convergence is also shown to exist, beyond which the fastest‐growing mode of instability changes from avalanche type to layered type.Item Isotopic and chemical assessment of the dynamics of methane sources and microbial cycling during early development of an oil sands pit lake(MDPI, 2021-12-03) Slater, Greg; Goad, Corey A.; Lindsay, Matthew B. J.; Mumford, Kevin G.; Colenbrander Nelson, Tara E.; Brady, Allyson; Jessen, Gerdhard L.; Warren, LesleyWater-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.Item Long-term mineralogical and geochemical evolution of sulfide mine tailings under a shallow water cover(Elsevier, 2015-06-01) Moncur, Michael; Ptacek, Carol; Lindsay, Matthew B. J.; Blowes, David; Jambor, JohnThe long-term influence of a shallow water cover limiting sulfide-mineral oxidation was examined in tailings deposited near the end of operation in 1951 of the former Sherritt-Gordon Zn-Cu mine (Sherridon, Manitoba, Canada). Surface-water, pore-water and core samples were collected in 2001 and 2009 from above and within tailings deposited into a natural lake. Mineralogical and geochemical characterization focused on two contrasting areas of this deposit: (i) sub-aerial tailings with the water table positioned at a depth of approximately 50 cm; and (ii) sub-aqueous tailings stored under a 100 cm water cover. Mineralogical analyses of the sub-aerial tailings showed a zone of extensive sulfide-mineral alteration extending 40 cm below the tailings surface. Moderate alteration was observed at depths ranging from 40–60 cm and was limited to depths > 60 cm. In contrast, sulfide-mineral alteration within the submerged tailings was confined to a < 6 cm thick zone located immediately below the water-tailings interface. This narrow zone exhibited minimal sulfide-mineral alteration relative to the sub-aerial tailings. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy showed results that were consistent with the mineralogical investigation. Pore-water within the upper 40 cm of the sub-aerial tailings was characterized by low pH (1.9-4.2), depleted alkalinity, and elevated SO4 and metal concentrations. Most-probable number (MPN) enumerations revealed abundant populations of acidophilic sulfur-oxidizing bacteria within these tailings. Conversely, pore-water in the sub-aqueous tailings was characterized by near-neutral pH, moderate alkalinity, and relatively low concentrations of dissolved SO4 and metals. These tailings exhibited signs of dissimilatory sulfate reduction (DSR) including elevated populations of sulfate reducing bacteria (SRB), elevated pore-water H2S concentrations, and strong δ34S-SO4 and δ13C-DIC fractionation. Additionally, mineralogical investigation revealed the presence of secondary coatings on primary sulfide minerals, which may serve as a control on metal mobility within the sub-aqueous tailings. Results from this study provide critical long-term information on the viability of sub-aqueous tailings disposal as a long-term approach for managing sulfide-mineral oxidation.Item Mineralogy and geochemistry of oil sands froth treatment tailings: Implications for acid generation and metal(loid) release(Elsevier, 2019-02-04) Lindsay, Matthew B. J.; Vessey, Colton; Robertson, Jared M.Froth treatment tailings (FTT) are one of three principal tailings streams generated during bitumen extraction at oil sands mines in northern Alberta, Canada. Unlike the coarse tailings and fluid fine tailings, FTT are enriched in sulfide-minerals content and exhibit the potential for acid generation and metal(loid) leaching. However, the mineralogical and geochemical characteristics of this sulfide-bearing tailings stream remain poorly constrained. We examined samples of fresh FTT (n = 3) and partially-weathered FTT collected from a sub-aerial beach deposit (n = 15). X-ray diffraction revealed that weathering-resistant silicates, phyllosilicates, and oxides dominated (85 ± 7.3 wt. %) the FTT mineral assemblage, while sulfides (6.2 ± 3.6 wt. %) and carbonates (8.9 ± 4.3 wt. %) were relatively minor phases. Pyrite [FeS2] was the principal sulfide in all samples, while minor amounts of marcasite [FeS2] occurred only in beach samples. Sulfide mineral textures were highly variable and included euhedral to subhedral pyrite crystals, discrete and clustered pyrite framboids, and marcasite replacements of pyrite framboids. Siderite [FeCO3] accounted for 55 to 90 % of all carbonates, while dolomite [CaMg(CO3)2], calcite [CaCO3] and ankerite [Ca(Fe,Mg,Mn)(CO3)2] accounted for the remainder. Statistical analysis of bulk geochemical compositions suggested that environmentally-relevant metal(loid)s, including As, Cu, Co, Fe, Mn, Ni, Pb and Zn, were likely associated with sulfides, carbonates and, to a lesser extent, phyllosilicates. Electron probe microanalyses revealed a wide range of As, Cu, Co, Mn, Ni and Zn concentrations in pyrite, with As and Cu concentrations elevated in framboids. Rare earth elements (REEs), Th and U also occurred at elevated concentrations and statistical analyses suggest they are associated with zircon and, potentially, monazite and xenotime. Static acid-base accounting (ABA) tests indicated that all FTT samples are potentially acid generating. Our study describes the mineralogical and geochemical characteristics of oil sands FTT, and indicates that oxidative weathering has the potential to generate acidic drainage containing elevated dissolved concentrations of several metal(loid)s.