Earth and Environmental Sciences

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Characterizing and Modelling Tailings Hydrogeochemistry Under a Composite Cover at the Abandoned Kam Kotia Mine, ON
(University of Waterloo, 2023-01-09) Zhang, Aria; Blowes, David; Ptacek, Carol
A principal environmental concern associated with mining is the weathering of mine waste in the presence of oxygen and water that generates acid mine drainage (AMD) concentrated in toxic elements. Installation of composite covers on mine waste is a remedial option to prevent the generation of AMD. Consisting of layers of materials of contrasting grain sizes, composite covers can inhibit oxygen and water ingress. Despite extensive studies on the design of composite covers, there is limited information available on the field performance of composite covers on legacy tailings. This thesis investigates the hydrogeochemistry of a legacy tailings impoundment remediated with a five-layer composite cover in 2008 and evaluates the long-term field performance of the cover. To investigate the hydrogeology and gas transport processes in the cover and tailings, groundwater flow, vadose zone hydrology, stable water isotopes, and pore-gas concentrations were monitored over multiple years, and numerical modelling of the variably saturated flow and calculations of gas flux were conducted. To characterize the geochemistry and mineralogy of the tailings under the composite cover, pore-water and groundwater sampling and geochemical analysis, selective chemical extractions, optical and electron microscopic study of thin sections, and synchrotron-based X-ray absorption spectroscopy experiments were conducted. To quantitatively assess the hydrogeochemical processes in the tailings from deposition to remediation by a composite cover, 1-D reactive transport simulations were implemented using the multi-component reactive transport code MIN3P. Characterization and modelling of the cover hydrogeology and gas transport indicate that during the summer, fall, and spring, the capillary barrier effect of the cover functioned well. The clay maintained saturation except for temporary desiccation during summer droughts. Potential local cover defects enabled oxygen advection into the tailings at one out of three monitoring locations. A seasonal change in the hydrology of the cover system occurred in the spring when snowmelt infiltrated deep into the tailings and reduced both oxygen diffusion and advection via cover defects, resulting in depleted pore-gas oxygen concentrations. Overall, the cover was effective in reducing infiltration and oxygen flux into the tailings. Mineralogical studies suggest that due to the high pyrite content (up to 74 wt%), sulfide oxidation depleted pyrites to a depth of 0.1-0.25 m. Aqueous geochemical results suggest that at one monitoring location, the cover reduced AMD generation and improved pore-water quality. The presence of a peat layer rich in organic carbon below the tailings facilitated sulfate reduction, albeit limited in capacity. In contrast, at another monitoring location, potential cover imperfections facilitated continued sulfide oxidation, resulting in low pore-water pH and elevated aqueous concentrations of sulfate, Fe, Zn, Cu, and As. Crystalline oxyhydroxides are a major sink for trace elements and may release them through desorption, acidic dissolution, and potential reductive dissolution under strongly reducing conditions. The complex geochemical processes indicate that sulfate, As, Zn, and Mn may remain long-term contaminants of concern. 1-D reactive transport simulations that incorporated transient infiltration, post-cover hydrogeochemical changes, a dynamic temperature regime, sulfide-mineral oxidation, acid neutralization, secondary-mineral precipitation, and sorption processes resulted in good agreement with the field results. Simulated temporal changes in reaction rates and geochemical parameters indicate that where intact, the cover lessened sulfide oxidation by both oxygen and Fe(III) and improved pore-water quality over time. Sulfate, Zn, and As persisted regardless of cover performance, whereas Cu and Al were the most sensitive to cover imperfections. The combined results from hydrogeochemical characterization and modelling suggest that even though the composite cover was effective in controlling infiltration and oxygen transport a decade after placement, complex geochemical processes in legacy tailings resulted in the persistence of deteriorated water quality and several elements may remain of long-term concern. This study can provide insight into cover design and mine remediation.
Copper Isotope Fractionation and the Evolution of Sulfide Alteration in a Sudbury Tailings Impoundment
(University of Waterloo, 2016-10-18) Buis, Justin Robert; Blowes, David
The fate of dissolved Cu in the Copper Cliff Central Tailings Impoundment was investigated to better understand the extent of Cu stable isotope fractionation accompanying the oxidation of Cu-bearing sulfide minerals and the retention of Cu through secondary mineral formation. Stable isotope ratios (δ65Cu) were measured in samples of pore water extracted from the oxidized zone of the tailings. Samples of the tailings solids were analyzed using a wide range of analytical techniques including transmitted and reflected microscopy, X-ray fluorescence, selective extractions and synchrotron based X-ray absorption spectroscopy (XAS). Observations were compared to the results from a study conducted at the same location on the tailings at the Copper Cliff Central tailings area, which was completed in 1990, allowing for an evaluation of the physical and chemical effects of sulfide alteration over a prolonged period. Prolonged sulfide alteration has led to an expansion in the depth of oxidation from 0.8 m below the ground surface (m bgs) in the previous study to a depth of 1.6 m bgs in the current study, sulfide oxidation was modelled using PYROX and results were compared to measurements of gas-phase O2. Analysis of current pore-water chemistry shows the maximum concentration of dissolved Cu (700 mg L-1) occurs near the boundary between the oxidized and unoxidized zones at 1.6 m bgs, with a sharp decline to lower Cu concentrations at greater depths. Analysis of the tailings solids shows an accumulation of Cu (3,000 ppm) at 1.8 m bgs immediately below this sharp decrease in aqueous Cu. Comparison of the aqueous and solid phase concentrations, selective extraction measurements and mineralogical observations indicate that formation and dissolution of covellite (CuS) at the base of the oxidized zone is the main geochemical control on the mobility of Cu. The dissolution of previously precipitated covellite has led to depletion of aqueous phase δ65Cu (-3.93±0.03‰) above a zone of declining Cu concentrations and an enrichment of δ65Cu (12.01 ±0.50‰) that is attributed to the precipitation of covellite. These observations demonstrate the value of integrating aqueous water chemistry and isotope measurements of field samples to assess the oxidation of Cu-rich sulfide tailings. The Cu isotope fraction observed during covellite formation has the potential for use as an indicator of the natural attenuation of dissolved Cu concentrations by sulfide precipitation as a result of changing redox conditions.
Evaluating Controls on Arsenic Geochemistry at the Long Lake Gold Mine in Sudbury, ON
(University of Waterloo, 2020-06-01) Verbuyst, Brent; Blowes, David
The release of As from old mine sites can persist long after cessation of mining activities. This project combines field and laboratory research components at the Long Lake Gold Mine site, near Sudbury, Ontario. The mine was discovered in 1908 and operated intermittently from 1909 until 1939; the mine was later abandoned and is now the responsibility of the Ontario Ministry of Energy, Northern Development and Mines. Arsenic-bearing sulfide-rich tailings were deposited in three topographic depressions near the mill, named TA-01, TA-02 and TA-03. The purpose of this project is to evaluate controls on As biogeochemistry in the Long Lake tailings areas and to provide a detailed geochemical and mineralogical investigation of aqueous- and solid-phase As. During the past 100 years, extensive sulfide oxidation of sulfide minerals in the Long Lake tailings has resulted in acidic conditions and high concentrations of dissolved metals and SO4 in the tailings pore water. Four nests of monitoring equipment were installed within TA-01, to assist in the understanding of the biogeochemical behaviour of As in the tailings and groundwater. Core samples of the sand cap, tailings, and underlying soils were collected for geochemical, mineralogical, and microbiological characterization. Mineralogical and geochemical characterization of the TA-01 tailings showed a zone of sulfide oxidation extending ~0.3-1.0 m below the tailings surface. Arsenic K-edge X-ray absorption near edge structure (XANES) and bulk As K-edge high energy resolution fluorescence detection X-ray spectroscopy (HERFD-XAS) produced results consistent with the mineralogical investigation. Pore water within the near surface tailings was characterized by low pH (2.0-3.9) and elevated concentrations of dissolved metals and SO4. Groundwater was characterized by circumneutral pH values and low concentrations of dissolved metals and SO4. Arsenic concentrations of up to 500 mg L-1 were measured in the tailings pore water and 70 mg L-1 in the underlying aquifer materials. The highest dissolved As concentrations were measured at shallow depths in the tailings corresponding with the lowest pH values and at the depth of the tailings profile near the organic layer interface. The tailings pore water and groundwater were characterized by δ34S-SO4 and δ13C-DIC fractionation indicating the likelihood of dissimilatory sulfate reduction (DSR). Results of this study will be used to inform and complement remediation efforts being undertaken by the Ministry of Energy, Northern Development and Mines. This study will provide information on the nature of mechanisms that affect the release and attenuation of As in over 100 year old sub-aerially deposited sulfide tailings.
Evaluation of the Potential Impacts of Submerging the Oxidized Tailings and Adding a Cover Layer at Mine Principale
(University of Waterloo, 2024-04-24) Agau, Majak; Ptacek, Carol; Blowes, David
Oxidation of sulfide minerals generates acid and forms various secondary phases including Fe(III) (oxy)hydroxide phases, jarosite, and gypsum, which can attenuate hazardous metal(loid)s by precipitation, co-precipitation, and adsorption reactions within tailings. The abandoned Cu-Ag mine at Mine Principale, Chibougamau, Québec, has three tailings impoundments (Parcs, A, B, and C) that have oxidized for more than 50 years. This project focused on Parc B, where tailings are stored to an average depth of 8.0 meters below the ground surface (mbgs). The study involved mineralogical, chemical, and microbiological analyses to evaluate the potential impacts of the proposed elevated water table (EWT) technique on the partially oxidized tailings. The EWT involves raising the water table and maintaining it above reactive tailings. Increasing the water content limits sulfide-mineral oxidation by minimizing O2 ingress into the tailings, owing to its low diffusivity in saturated media. However, the EWT can cause direct dissolution of soluble secondary phases, and also can potentially induce reducing conditions that promote microbially catalyzed reductive dissolution of Fe(III) (oxy)hydroxide phases fueled by organic C compounds. In this study, mineralogical and selective chemical extraction techniques indicate the presence of various secondary phases including Fe(III) (oxy)hydroxides that are associated with significant amounts of hazardous metal(loid)s including Cu, Ni, Co, Zn, As and Pb in the oxidized zone, which is limited to a depth of about 1.5 mbgs in Parc B. Results of amplicon sequencing of 16S rRNA genes show significant abundances of Fe- and S-oxidizing bacteria and S-reducing bacteria but no presence of Fe-reducing bacteria. Solid-phase analyses indicate a low amount of total C in the oxidized tailings. These findings suggest that EWT technique can be an effective reclamation method for Mine Principale tailings if use in conjunction with another suitable method to address potential low-quality drainage resulting from the inducement of reducing conditions post EWT application.
Geochemical and Microbiological Characterization of the Historic Waste Rock Piles at the Detour Lake Gold Mine
(University of Waterloo, 2016-09-16) McNeill, Brayden; Blowes, David; Blowes, David
Four of the historic waste rock stockpiles (WRS #1-#4) at the Detour Lake mine site were studied to determine the potential for generation of acid rock drainage (ARD). The stockpiles were constructed during the original mine operations (1983 - 1999) and were covered with 1 - 1.5 m of local overburden in 2000 to provide a reclamation cover. Waste rock was composed primarily of plagioclase, horneblende, quartz and clinochlore, with small amounts of biotite. The principal sulfide minerals identified were pyrite and pyrrhotite, with small amounts of chalcopyrite and covellite. Measurements of sulfur content ranged from 0 - 2.2 wt. %, whereas the carbon content ranged from 0 - 2.5 wt. %. The neutralization potential ratios (NPR) of WRS#1 and WRS#2 ranged from 0 - 61.1 with an average of 1.6 and 0.7 in profile excavation samples. Over 50 % of samples from WRS#1 and WRS#2 were potential acid generating (PAG). WRS#3 and WRS#4 were slightly less sulfidic resulting in average NPR of 43 and 10, respectively. None of the samples from WRS#3 were PAG, and 45 % of WRS#4 samples were PAG. The hydrology of the piles is typical of waste rock piles, with a large unsaturated zone. The water tables at WRS#3 and WRS#4 are approximately 16 and 22 mBGS, respectively. The waste rock is usually near residual saturation (5 vol. %), but the passage of wetting fronts commonly increased moisture content to near matrix saturation (~25 vol. %). The cover material retains more moisture than the waste rock, and usually 10 - 20 vol. %. Thermal profiles indicate that both stockpiles remain > 0 ˚C throughout the year, except within the cover. Seasonal fluctuations in temperature are dampened and delayed with greater depth in the stockpile, except near the edge of WRS#4 where the cover was damaged suggesting the cover plays a role in regulating the temperature of the stockpiles. Air-permability testing of the cover material and waste rock indicates that the cover material impedes advective gas and heat flow. Waste rock at WRS#3 and WRS#4 had air-permeability coefficients of 10-9 - 10-10 m2, whereas the cover material had air-permeability coefficients of approximately 10-11 m2 indicating that air flow through the cover is primarily by diffusion. This observation is in agreement with pore-gas trends at WRS#3 which show O2 depletion and CO2 enrichment with depth. Pore gas at WRS#4 is at atmospheric concentrations throughout, since the destruction of the cover material has removed the barrier to advective gas flow. The results of pore-gas monitoring indicate that the installation of a simple, unengineered cover made from local material may be a cost-effective tool in the management of sulfide oxidation and potential ARD generation at this site. Pore-water quality at WRS#3 and WRS#4 is characterized as neutral mine drainage, and compares favourably to other neutral mine drainage sites. The pore-wate throughout WRS#3 and WRS#4 is neutral pH. Concentrations of SO42- between 200 and 1500 mg/L are caused by sulfide oxidation. Circumneutral pH and depletion of alkalinity in the unsaturated zone indicate that acidity released through sulfide oxidation is neutralized through carbonate dissolution. Pore-water at both piles was saturated with respect to calcite and dolomite. Metal concentrations (i.e. Al, Cu, Fe, Mn, Ni, Zn) in the unsaturated zone were usually < 100 µg/L. Pore-water at WRS#3 and WRS#4 are oversaturated with respect to several secondary Fe and Al minerals including Fe(OH)3(a), goethite, gibbsite and diaspore, which provide controls on the concentrations of dissolved Fe and Al. Secondary covellite (CuS) was observed throughout the stockpiles, suggesting that covellite formation may constrain Cu concentrations. Pore-water samples were undersaturated with respect to secondary Cu, Ni and Zn minerals, indicating that these metals are likely removed from the aqueous phase through adsorption or complexation to Fe-hydroxides. Elevated concentrations of Fe (> 1000 µg/L) and Mn (> 500 µg/L) below the water table at WRS#4 indicate that these metals are released by reductive dissolution of Fe and Mn oxides. Overall the concentration of dissolved metals in the pore-water of WRS#3 and WRS#4 is much lower than concentrations measured at other sites characterized by neutral mine drainage. Several genera of fungi were identified in the waste rock of WRS#1 using 18S rRNA analysis, including Pycnopeziza, Leptosphaeria, Tetracladium and Cucurbitaria. None of the encountered fungi were ubiquitous or have been shown to impact pore-water geochemistry through sulfide oxidation. Bacterial enumerations were performed on samples from WRS#1 to evaluate the presence of iron and sulfur oxidizing organisms. The enumerated species are common waste rock bacteria including acidophilic sulfur oxidizers (Thiobacillus thiooxidans and related species; SOBa), neutrophilic sulfur oxidizers (Thiobacillus thiparus and related species; SOBn) and acidophilic iron oxidizers (Acidithiobacillus ferrooxidans and related species; FeO) oxidizers. The most numerous were FeO with an average abundance of 9.0x105 bacteria/g. The average abundance of SOBn was 5.5x105 bacteria/g. The SOBa were much less numerous with an average abundance of 1.2x103 bacteria/g. A 16S rRNA analysis confirmed the presence of Thiobacillus and Acidithiobacillus species. Bacterial diversity was greatest in samples of the cover material. Unoxidized waste rock samples were usually characterized by a dominant iron or sulfur oxidizing genera even at neutral pH (i.e. Thiobacillus), whereas oxidized and acidic waste rock samples saw a shift to a dominant acidophilic genera (i.e. Acidithiobacillus).
Hydrogeochemistry and Trace Element Mobility in an Acidic High-Sulfide Tailings Impoundment After 40 Years of Oxidation
(University of Waterloo, 2024-01-25) Starzynski, Hannah Lucy; Ptacek, Carol; Blowes, David
Abandoned mine sites can create a legacy environmental contamination issue when the generation of acid mine drainage is allowed to continue with insufficient or absent remediation measures. The South Bay mine, a former underground Cu-Zn mine located in northwestern Ontario, is once such site with historical contamination. The mine wastes at South Bay contain high concentrations of sulfide minerals which continue to oxidize decades following mine closure, leading to acidic seepage with high concentrations of dissolved metals impacting the surrounding lakes. This aim of this study is to provide a characterization of the current hydrogeology, geochemistry, mineralogy, and microbiology of the South Bay tailings so that this information can inform future remediation work. Instrument installation and collection of core samples of the tailings was performed at five locations within the tailings impoundment area. Pore-water samples were collected from piezometer and soil water sampler nests. Sub-samples of tailings cores were collected and analysed using optical microscopy, scanning electron microscopy, selective extractions, total carbon/sulfur, X-ray diffraction, X-ray fluorescence, synchrotron, and DNA sequencing techniques. Mineralogical analysis indicated that pyrite was the main sulfide mineral in the tailings, with lesser amounts of sphalerite and chalcopyrite and trace amounts of pyrrhotite, galena, and arsenopyrite. The oxidation zone in which sulfide minerals are depleted is restricted to the upper 0-15 cm of tailings. The moisture content within the tailings is relatively high, contributing to a low O2 diffusion rate into the tailings. High proportions of acidophilic microorganisms capable of catalyzing Fe and S oxidation reactions were found in the shallow tailings. Sulfide oxidation modelling has indicated that oxidation of sulfide minerals in the South Bay tailings may continue for decades to millennia before all sulfide minerals are depleted in the vadose zone. Prolonged sulfide mineral oxidation has led to acidic pore waters with pH as low as 1.26 with high concentrations of dissolved metals, including Fe, Zn, Cu, As, Pb, and Co. The lowest pH and highest concentrations of dissolved metals tends to occur in the shallow tailings near the region of active sulfide-mineral oxidation. High concentrations of dissolved rare earth elements (REEs), up to 9.45 mg/L total REEs, were also found within the shallow acidic pore-waters. Dissolution of gangue minerals and secondary minerals contributes to acid neutralization, with pH increasing to circumneutral values below the water table. Metals and metalloids may be attenuated through adsorption or co-precipitation with secondary mineral phases. Copper was found to be attenuated through covellite precipitation, Pb was attenuated through anglesite precipitation, and As was attenuated by adsorption or co-precipitation with Fe(III) (oxy)hydroxides. Metal(loid)s sequestered within Fe(III) (oxy)hydroxides may be susceptible to remobilization through reductive dissolution should environmental conditions imposed by future remediation efforts induce strong reductive conditions.
Impacts of Dredging and Brush Cutting of Paired Agricultural Drainage Ditches on GHG Emissions and Nutrient Filtration Capacity
(University of Waterloo, 2023-01-04) Schietzsch, Andrew; Blowes, David; Ptacek, Carol
Agriculture provides many beneficial and essential ecosystem services. Along with these beneficial services, the conversion of natural ecosystems into heavily modified agricultural ecosystems is also a source of disservice, including being a major source of global greenhouse gas (GHG) emissions and pollution of downstream waterways due to increased nutrient runoff. Carbon (C), Nitrogen (N) and Phosphorus (P) applied to agricultural fields as fertilizer are a source of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions. Nutrient runoff can lead to excess P in surface water bodies causing algae blooms, and excess N can lead to excess nitrate (NO3) in rural groundwater (GW) wells. There is a need to establish beneficial management practices (BMPs) to take into account agriculture-derived pollution with all agricultural practices. This thesis informs the development of BMPs by examining the environmental pollution aspects of both: 1) GHG emissions; and 2) nutrient export; resulting from the common practices of brush cutting and dredging of ditches to enhance drainage. Riparian vegetation in agricultural drainage ditches has been shown to decrease insolation, which decreases soil and water temperatures. This vegetation also hinders drainage by restricting flow, thus raising water levels, which decreases CO2 emissions and increases CH4 and N2O emissions. However, no previous studies have examined in detail the effects of removing drainage ditch vegetation. This study examines the GHG emissions from four ditch microplots in the South Nation Watershed in southern Ontario, Canada following the removal of riparian vegetation from two microplots. The trials took place over three field seasons, and the intervention methods were selected to observe the effects of brush cutting and of dredging on GHG emissions between years. The Control Shrub and Control Tree microplot sites were left unaltered. The Brush Cut Shrub and Brush Cut Tree sites were brushed in Spring 2018 and Dredged in Fall of 2018, with observations at all sites taking place over 2018-2020 growing seasons. Brushing increased CO2 emissions at the treed site but had little effect on the shrub site. Dredging decreased CH4 emissions. Riparian vegetation has also been shown to obstruct the path for water flow, decreasing water velocities and raising water levels, which increases the ability of ditches to filter and retain nutrients. Simultaneously with the GHG research above, this study also examines the N, P, and C v export from two adjacent watersheds within the South Nation Watershed following the removal of riparian vegetation from one of them. The trials took place over two field seasons and the intervention methods were selected to observe the effects of brush cutting and dredging on N and P export over two years. The southern watershed (Brush Cut) was brushed + dredged in 2018 and the northern watershed (Control) was left intact before flow monitoring took place in 2019 and the Fall of 2020. Tile drain discharge containing DOC, N and P, occurred during the Spring and Fall when the water table is higher, but was not observed during the summer. Brush cutting and dredging increased hydraulic outflow and reduced or eliminated NO3 retention capacity of agricultural drainage ditches by 320% in 2019 and 68% in Fall 2020. This increase in NO3 export may negatively affect rural water supplies. Lack of O2 and increased retention of DOC and SO4 in the Control watershed suggests that significant NO3 reduction occurred. Differences in P export between Brush Cut and Control in 2019 were small. There are more signs of P transformation in the Control watershed, but brush cutting and dredging may not significantly affect eutrophication. This thesis will help inform stakeholders about the environmental geochemical costs and benefits of brushing and dredging so that they can develop BMPs that minimize GHG production and maximize nutrient filtration. Future research is needed to determine how many years the effects of these intervention methods remain, and also determine other environmental impacts such as their effects on biodiversity.
Integration of non-traditional stable isotopes and synchrotron measurements
(University of Waterloo, 2022-08-31) Shrimpton, Heather; Blowes, David
Selenium is well named after the moon, because statements about this element must often be qualified, making it feel ever changing; for instance, Se is a nutrient, but an excess of 400 µg per day is toxic for humans. High Se concentrations can cause reproductive decrease or complete failure in fish, aquatic birds, amphibians, and reptiles. These animals can also bioconcentrate Se, so high aqueous Se concentrations are not required to lead to toxic consequences. The problem of Se is not limited to wetlands; plants that uptake Se can cause selenosis to animals that forage on land, such as sheep and cattle. Remediating Se before it can reach a receptor is important in preventing the loss of the next generation of wildlife. When treating Se in ground water, or measuring Se concentrations at contaminated sites, it can be difficult to determine where the Se is going unless extensive measurements are made, including solid sampling and speciation measurements. Non-traditional stable isotopes are an emerging tool in the remediation of groundwater contamination caused by anthropogenic activities. Taking Se stable isotope measurements in conjunction with information about removal mechanisms could produce a powerful predictive system to solve remediation problems. A combination of theoretical calculations, laboratory experiments, and field measurements were used to evaluate Se stable isotopes as a remediation tool. Molecules for SeO4^2-, SeO3^2-, HSeO3^-, CaSeO4, and CaSeO3 were all modeled using Gaussian 09 (Frisch et al. 2009). The vibrational energies from these models were then used to calculate the equilibrium fractionation between each pair of molecules. These equilibrium fractionation factors were found to be within 0.21 ‰ of other values from the literature, where available. Calculations were of the same magnitude as laboratory studies: 13.4 ‰ for reduction of SeO4^2- to SeO3^2-, and 0.8 ‰ for SeO3^2- to HSeO3^-, which is similar to the range for adsorption of 0 – 1.24 ‰. These theoretical values can be used to establish baseline values when no data is available from laboratory experiments, as is the case for the formation of CaSeO4 and CaSeO3. Calcium plays an important role in the sequestration of Se at pH above 7 (Goldberg and Glaubig 1988), making it relevant to the Se isotope literature. Three separate laboratory experiments were conducted. Abiotic reduction of Se(IV) by Na2S(aq) was investigated to determine whether isotopic fractionation can differentiate reduction by H2S(g) and direct respiration or enzymatic reduction due to sulfur reducing bacteria (SRB) in the environment. A solid precipitate formed rapidly and was collected for PXRD analysis. The precipitate was either yellow, orange, or red depending on the starting pH and the Se:S ratio in solution. All three precipitate colors had different powder X-ray diffraction (PXRD) ring patterns. The yellow precipitate had no pattern, and may have been amorphous S(0), the orange precipitates were most similar to selenium sulfide, and the red precipitate was most similar to a mixture of Se(0) and S(0). The fractionation factor when samples were filtered at 3-4 hours was 7.9 ‰. When the solid was left in contact with solution for a longer duration, the fractionation factor increased to 10.9 ‰. An experiment on biotic reduction of Se(IV) by a natural SRB consortium, including a comparison of results to the work of others, provides contrasting data for the abiotic experiment. Batch vessels were loaded with a mixture of lucerne hay (alfalfa), silica sand, and a small amount of zero valent iron (ZVI) before being transferred to an anaerobic chamber. A solution containing MgSO4, Na-lactate, and SRB culture was then added to each vessel, and they were crimp sealed. Samples were taken along a time series. Scanning electron microscope (SEM) images show microbes colonizing the sand crevices and much of the organic matter, but no Se precipitates are obvious. Initial fractionation in the reduction of Se(IV) was positive (10‰ < ε ≤ 19 ‰), followed by a decrease in δ82Se in solution. Because a negative fractionation factor is unlikely, it is probable that multiple pools of reduced and organic Se species are entering solution, causing a final δ82Se of -13.2 ‰ after three days, with the lowest δ82Se of -16.7 ‰ seen at two days. The final experiment used ZVI to reduce Se(VI) in a flow through cell system, while simultaneously collecting XANES data and isotope samples. A column with a transparent window packed with ZVI was placed in the hutch at a synchrotron (Sector 13, Advanced Photon Source (APS), Argonne, IL, USA), and a Na2SeO4, CaCO3 solution was pumped through. The Na2SeO4 concentration was increased at 8 hour increments, and removed from influent solution at the end of the experiment to observe the effect of rinsing the column with a CaCO3 only solution. The linear combination fit (LCF) shows progressively reduced species of Se accumulating on the solid over time, with more Se present near the input of the cell than the output. The Se(VI) component decreased rapidly in the rinse phase of the LCF, suggesting most Se in solution was Se(VI). The δ82Se could be fit with a straight line, yielding an isotopic discrimination of 9.6 ‰. The δ82Se of the rinse solution could be fit with a Rayleigh type curve, with a fractionation factor of 2.4 ‰. This fractionation factor is between adsorption of Se onto iron minerals, and reduction of Se by ZVI in the presence of CaCO3, as measured in an earlier batch experiment. Simultaneously obtaining isotope and solid phase data helps link removal mechanisms to fractionation factors. Isotope results from all three laboratory experiments suggest reductive processes are of the same magnitude as theoretical calculations, and Se reduction experiments conducted by others. Samples for isotope and cation analysis were collected from along the length of an Se-bearing groundwater plume. Laboratory obtained Se isotope fractionation factors were then used to model the processes occurring in the plume. Geochemical and redox data were used to support the isotope modeling results. These modeling results, supported by redox data, allowed us to infer the processes occurring in the subsurface. The main processes within the plume include adsorption and dispersive dilution, as indicated by only small changes in δ82Se values (δ82Se = 1.5 – 3.4 ‰) and a large decrease in Se concentration (from 9770 to 774 µg L-1). Reduction is occurring within the source area (δ82Se = 1.3 – 3.8 ‰), and is the cause of the sharp increase in δ82Se (8.7 ‰) under the wetland complex. Very low δ82Se values behind the source area (-26 ‰) and at the distal end of the plume (-16 ‰) are likely due to the oxidation of low δ82Se from local shales. Low concentration (< 900 µg L-1), low δ82Se values (-0.2 – 0.9 ‰) at the plume’s edge are either the result of desorption/oxidation of previously reduced plume Se, or mixing of plume Se with background Se with a low δ82Se. The results from this field study demonstrate the potential use for Se stable isotope measurements in environmental samples.
Investigation of Gas Transport Rates Through a Covered Waste Rock Pile and Synchrotron Studies on the Sulfide Oxidation Reaction
(University of Waterloo, 2017-01-19) Steinepreis, Mark; Blowes, David
This thesis presents a field based investigation into gas transport mechanisms and rates through a waste-rock pile with a low permeable cover, and a synchrotron based study into intermediate sulfidic species that are produced as iron-sulfide grains oxidise to sulfate. The two studies are at different size scales, however both improve the understanding of the processes that affect the production of acid rock drainage and the release of metals to the hydrosphere at mine sites. The study site for the gas transport investigation was waste-rock stockpile #3 at Detour Gold Corporation mine (Detour), operating in northern Ontario, Canada. Field monitoring was carried out during late 2014 and through 2015. Wind vector, air pressure and temperature were recorded around the exterior of the pile; pore-gas pressure, pore-gas concentration of O2 and CO2 and temperature were recorded within the pile. Correlations between external and internal pressure indicated that transport through the cover and waste-rock was laminar and therefore followed Darcy’s law. Fluctuations in ambient temperature were dampened through the cover and within the waste-rock; internal pile temperatures were higher than average daily ambient temperature during the winter and lower in the summer. The O2 concentrations in the pore-gas were higher and more variable in the summer (5-15% v/v at approximately 2.5m into the pile) than in the winter (consistently below 3% v/v at 2.5m depth). Design efforts to reduce the O2 level within the pile should therefore be optimised for summer time conditions. The CO2 concentrations were higher and more variable in the summer (0.5-6% v/v at approximately 2.5m into the pile) than in the winter (between 7-8% v/v at 2.5m depth). Numerical simulations were carried out in COMSOL version 5.1 to prepare a calibrated model for gas transport through the pile. Field measured parameters were used as boundary conditions for the exterior of the pile, and field measured parameters for the interior of the pile were compared with model outputs to consider calibration. Numerical simulations indicated that the advective flux of O2 through the cover and into the interior of the pile is approximately 100 times higher than diffusive fluxes during the summer. Increasing the thickness of the cover and using a cover material that has a lower permeability would further reduce the O2 concentration within the pile. Two sulfide grains (one chalcopyrite and one pyrrhotite) that were collected from crushed waste-rock samples from Detour were analysed at the synchrotron at the Advanced Photon Source at Argonne, Illinois, USA. It is understood that sulfate is the ultimate oxidation product of sulfides, however less is known about intermediate sulfidic species that are produced. Linear combination analysis of the X-ray absorption near edge spectra (XANES) for the grains and standard sulfidic species indicated that variable combinations of chalcopyrite (CuFeS2), pyrrhotite/troilite (FexS1-x/FeS), marcasite (FeS2), elemental sulfur (S0) thiosulfate (S2O32-), tetrathionate (S4O62-), sulfite (SO32-) and sulfate (SO42- ) are present over the grains. It is not currently known if the inclusion of these species in predictive simulation of sulfide oxidation rates is warranted.
An Investigation of Heterogeneity and the Impact of Acidic Regions on Bulk Effluent from a Deconstructed Low Sulfide Waste-Rock Pile
(University of Waterloo, 2017-05-15) Atherton, Colleen; Blowes, David
Waste rock is a potential source of low quality drainage resulting from oxidation of naturally occurring sulfide minerals. Sulfide oxidation may result in the generation of effluent with elevated concentrations of SO4 and dissolved metals and low pH. The sulfide content of waste rock is typically much lower than that of tailings; however, the large volumes of waste rock produced during mining may create a large environmental liability. Improving the understanding of the processes affecting the generation of acid mine drainage (AMD) from waste rock will facilitate improved prediction, mitigation, and remediation strategies. Three waste-rock test piles were constructed at the Diavik Diamond Mine, Northwest Territories to investigate the potential for AMD generation in a permafrost environment. The test piles were constructed in 2006 and consisted of low sulfide (0.035 wt. % S), high sulfide (0.53 wt. % S), and covered test piles. The covered test pile was constructed to model the mine closure plan and consisted of a high sulfide (0.082 wt. %) core, covered by a low-permeability layer, and a low-sulfide thermal insulation layer. In 2014, the low-sulfide Type I test pile was systematically deconstructed to investigate the geochemical, hydrogeological, and geotechnical evolution of the waste rock. Samples were collected for microbial community analysis, mineralogical characterization, pore-water extraction, ice distribution, volumetric moisture content, and particle-size distribution. The geochemical evolution of the test pile was investigated using mineral saturation index calculations, neutralization potential ratios, aqueous geochemistry, most probable number enumeration, adsorption isotherm modeling within the test pile, and mass loading calculations at the basal drain. Regions of low pH with elevated dissolved metal and SO4 concentration developed within the test pile as a result of the heterogeneity inherent in the waste rock. Sulfide oxidation rates were depressed in regions that remained frozen for a larger part of the year. Depression of sulfide oxidation rates allowed neutralization reactions within the waste rock to maintain circumneutral pH in these regions. Saturation index calculations indicate circumneutral pH regions were conducive to precipitation of Fe (oxy)hydroxides which have a large capacity to adsorb cations. Adsorption isotherm modeling indicates that adsorption of Cu, Zn, Co, and Ni on ferrihydrite can account for the observed attenuation of these metals with increasing pH. Attenuation reactions resulted in reduced mass loading of metals and SO4 in effluent compared to the higher sulfide waste-rock pile.
Long-term evaluation of an organic-carbon permeable reactive barrier remediating mine-impacted groundwater and the potential of emulsified vegetable oil to increase treatment performance
(University of Waterloo, 2024-10-17) Miller, Austin; Blowes, David; Carol, Ptacek
Degradation of water resources by acid mine drainage (AMD) and metal contaminants from the oxidation of mine wastes that are improperly managed is a global environmental concern. An established economical and passive method for managing the transport of oxidation products in groundwater is the installation of an organic-carbon permeable reactive barrier (PRB) to promote the growth of sulfate reducing bacteria (SRB) and dissimilatory sulfate reduction (DSR) in situ. A detailed biogeochemical evaluation was conducted on a PRB remediating AMD from an abandoned Ni-Cu mine after 26 years of treatment, providing the first long-term evaluation of this technology. Pore-water concentrations of Ni decreased from 97µg L⁻1 to < 25 µg L⁻¹ while Fe decreased by 402 mg L⁻¹ (77% of mean influent) through the PRB. Pore-water SO4 decreased by 1,243 mg L⁻¹ (70% of mean influent) coinciding with an increase in alkalinity and pore-water ẟ34Sₛₒ₄, suggesting DSR is actively occurring. There are distinctly different populations of putative SRB present within the sampled PRB material compared to the surrounding aquifer material. Low abundances of S and Fe oxidizing prokaryotes were detected, which may oxidize Fe-sulfide phases; re-mobilizing Fe and S or result in the formation of Fe(III) (oxy)hydroxide phases. A preponderance of S immobilized within the PRB is in the form of acid volatile sulfur with mineralogical investigations identifying FeS phases often replacing organic carbon in plant cellular material and framboidal pyrite. These results demonstrate that the PRB is still operating as designed with complex organic carbon compounds supporting a diverse microbial community that sustain rates of DSR to effectively precipitate Fe sulfides, decrease the acid potential of groundwater and immobilize contaminants. Column experiments were designed to evaluate the incorporation of emulsified vegetable oil (EVO) into solid-phase organic carbon through soaking and injection to promote and sustain treatment performance. Column effluent compositions demonstrate soaking organic-carbon in EVO resulted in high levels of Fe removal for the duration of observation (315 days). Moreover, an EVO injection re-established treatment after removal rates declined, providing a viable alternative to PRB replacement to maintain effective treatment system performance and extend PRB lifespan.
THE MEASUREMENT OF INTER-PARTICLE DIFFUSION COEFFICIENTS IN MINE WASTES
(University of Waterloo, 2021-05-04) Van Eck, Peter; Blowes, David
Acid mine drainage (AMD) is the release of acidic waters from mines, waste-rock piles and mine tailings impoundments containing high concentrations of SO4, Fe(II) and other metal(oid)s. The bulk of AMD generation occurs in the vadose zone where water and O2(g) can react with the gangue sulfide minerals present in mine waste. The ability to quantify and model the limiting reactants of water and O2(g) is vital to understanding and controlling the generation of AMD at a mine site. Characterizing the unsaturated properties of mine wastes is important for determining the rate of sulfide oxidation and the extent of AMD. Soil-water characteristic curves (SWCC) are a tool used to describe the unsaturated conditions present in mine waste. Soil-water characteristic curves were measured using matrix-matrix material (<4.75 mm) from three mine sites: Faro Mine Complex, YT; the Detour Lake Gold Mine, ON; and Diavik Diamond Mine, NT, to examine the differences in matrix-material particle-size distribution among mine sites and the effect of particle-size distribution on SWCC morphology. Estimations of effective diffusion coefficients and sulfide oxidation rates were calculated using parameters derived from these measurements. The results indicate that mine waste-rock matrix material can exhibit a high degree of hysteresis, that can result in differences between oxygen diffusion coefficients and sulfide oxidation rates during wetting and drying stages. The results contribute to the characterization of the unsaturated properties of mine wastes and provide estimates of the variation in inter-particle diffusion coefficients of mine waste in response to changing moisture content reflecting hysteresis. Laboratory column experiments were conducted to measure O2(g) diffusion rates through variably saturated waste-rock matrix material. Effective diffusion coefficients were calculated using the numerical model MIN3P for both wetting and drying phases of waste-rock matrix material to assess the impact of hysteresis on O2(g) diffusion. The calculated effective diffusion coefficients were used to estimate sulfide oxidation rates to observe potential variability in oxidation rate caused by hysteresis. The results from the SWCC measurements indicate the potential for waste-rock matrix material to exhibit a high degree of hysteresis. The results from the laboratory column experiments indicate that the effect of hysteresis on effective diffusion coefficients and sulfide oxidation rates was negligible for high values of negative matric suction. Potential concern for the impact of hysteresis on sulfide oxidation rates occurs at near zero-values of matric suction where estimated effective diffusion coefficients and sulfide-oxidation rates vary by orders of magnitude.
A mechanistic approach to assessment of the geochemical evolution of low sulfide mine-waste rock
(University of Waterloo, 2019-01-04) Wilson, David; Amos, Richard T.; Blowes, David
The potential for mine wastes to generate elevated concentrations of solutes including metals, sulfate, and reduced pH exists wherever mine-waste rock is stockpiled at the Earth’s surface representing one of the world’s largest environmental problems. The assessment of the long-term geochemical evolution of mine wastes is of critical importance in the process of mine-life planning because of the potential for adverse impacts of released solutes and low pH effluent to receiving environments. The Diavik Waste Rock Project included laboratory and field experiments investigating the geochemical evolution of low-sulfide mine-waste rock at different scales. The experiments included small-scale humidity cells (0.1 m high; laboratory), medium-scale lysimeters (2 m high; field), and large-scale test piles (15 m high; field) to facilitate development of a mechanistic approach to scaling results of the laboratory experiments to make assessments regarding the geochemical evolution at the larger field experiments. This process, generally referred to scale-up, often involves the use of humidity cell experiment results coupled with empirical scale factors to make predictions about the long term geochemistry of effluent released form mine-waste stockpiles. The empirical factors used typically include parameters known to influence rates of sulfide oxidation including mineral content, particle-size distribution, temperature, moisture content, and oxygen availability. These scale-up factors often fail to account for site specific heterogeneities in physical and chemical properties that can strongly influence the prediction process. Mechanistic approaches (i.e., the use of geochemical models including reactive transport models) have the potential to include complex heterogeneities that facilitate a quantitative assessment of the long-term geochemical evolution of mine wastes. A conceptual model of the geochemical evolution of low-sulfide waste rock was developed to facilitate numerical simulations of the small-scale experiments and then was used to simulate the geochemical evolution in the larger scale field experiments. The conceptual model, based on oxidation of sulfide minerals coupled with the geochemical weathering of host minerals present in waste rock produced at the Diavik Diamond Mine (NT, Canada), was implemented using the reactive transport code MIN3P. The 1-D model was calibrated to capture the effluent concentrations from the laboratory-scale experiments then used to simulate the geochemical evolution at the larger scale field experiments, without further calibration, to assess the efficacy of the mechanistic scale-up approach. Geostatistical analyses of mineralogical and particle-size distribution samples were conducted to assess the heterogeneity of S, C, and saturated hydraulic conductivity. The results of the geostatistical analyses were used to inform spatial distributions of S, C, and saturated hydraulic conductivity as input to reactive transport simulations of the large-scale field experiment. The 2-D simulations were conducted to assess the influence of heterogeneity in S, C, and saturated hydraulic conductivity on the geochemical evolution of the waste rock. The results of the humidity cell simulations indicate that the conceptual model represents the primary geochemical processes of the low-sulfide waste rock weathering. The simulated effluent concentrations compares well with the measured solute concentrations from the humidity cells, although some divergence for specific parameters was observed. Mineral surface area, mineral content, temperature, and pH were identified as important factors controlling the geochemical evolution of the waste rock. The results of the model developed and calibrated at the humidity-cell scale suggested that the conceptual model could be representative of the DWRP waste rock weathering in general and the implemented model could be used to simulate waste rock weathering for the field scale experiments. The implementation of the conceptual model at the medium-scale field experiments involved inclusion of measured temporally dynamic temperature and infiltration to better represent the physical conditions at the field experiments. Implementation at the large-scale test pile experiments included providing spatially dynamic temperature. Inclusion of these parameters as model input facilitated completion of multi-year simulations essential to making long-term assessments of the geochemical evolution of waste rock. Scaling the humidity cell conceptual model to simulate the geochemical evolution at the field-experiment scales resulted in good visual agreement between measured and simulated concentrations and mass flux of most parameters. The pH was generally over estimated in the medium- and large-scale field simulations. Supplemental simulations indicate that calcite availability was lower for the field experiments (approximately 20% of measured content). The field experiment simulations did not rely on geochemical data for calibration; however, these simulations did rely on site-specific physical data, including mineralogy-related parameters such as volume fraction, hydrology-related parameters including hydraulic conductivity, grain-size distribution, porosity, and water-retention curve values, and environmental parameters including temperature, precipitation, and O2 concentration (the field systems were not O2 limited); to facilitate an assessment of the geochemical evolution of waste rock. The reactive transport simulations demonstrated that a comprehensive, integrated conceptual model representing the geochemical evolution of low-sulfide waste rock, implemented and calibrated at the humidity-cell scale can be applied to field-scale experiments using a small number of measurable parameters to constrain the simulations. Parameters should include mineral content, bulk mineral surface area, and particle-size distribution, water flow and infiltration characteristics as well as general climatic conditions (specifically temperature and precipitation). The reliance on readily available, measurable parameters suggests that this approach could be implemented at other sites using the appropriate site specific parameters. This mechanistic approach provides the basis for predictive scale-up. Consideration of the influence of temperature on the geochemical reactions was a major factor facilitating the scale-up of the model. The humidity cell experiments were conducted at temperatures 5 °C and 22 °C to allow calibration for the influence of temperature, which was a critical component in the scale-up process because of the varied temperatures at which surface-stored waste rock is exposed. Measurement of temperature at the field scale would be an important component of any scale-up program. The results of the geostatistical analyses indicate the spatial distributions of S, C, and saturated hydraulic conductivity in the test-pile experiments could be approximated using a log normal distribution with mean and standard deviation calculated from samples collected during test pile construction for each parameter. A lack of spatial dependence for matrix hydraulic conductivity was significant because the matrix material exerted strong control over the flow of water through the test-pile experiments. The spatial distributions of S, C, and saturated hydraulic conductivity in the test piles experiments provides a foundation from which full-scale waste-rock piles could be characterized using the geostatistical methods described. The spatial dependence of saturated hydraulic conductivity in larger piles may also depend on the influence of features which were not present in the test-pile experiments (e.g., traffic surfaces). The investigation of the influence of mineralogical and physical heterogeneity on the geochemical evolution in the Type III test-pile suggested that heterogeneous distributions of S and C mineralogy and saturated hydraulic conductivity field resulted in variations of effluent concentrations that were at times, consistent with the measured variation. Analysis of the results of the heterogeneous simulations indicate that the distribution of solute mass fluxes from the test-pile experiment for most parameters could be best approximated with a log normal probability density function.
Metal Isotope Fractionation Associated with Cu and Zn Attenuation by Zero-Valent Iron in Anaerobic Flow-Through Cell Experiments
(University of Waterloo, 2018-01-12) Leon, Jeffrey; Ptacek, Carol; Blowes, David
Metal stable isotope fractionation is an emerging tool in the environmental sciences for studying biogeochemical pathways for cycling of metals in the environment. Copper and zinc are essential elements for many biological functions, but can accumulate in high concentrations toxic to living organisms through anthropogenic activities such as mining. Permeable reactive barrier (PRB) technologies have been implemented at many field sites to remove inorganic contaminants, such as Cu and Zn, from groundwater. A common reactive material used to construct PRBs is zero-valent iron (ZVI), which is useful for its propensity to create strongly reducing conditions that favour the reduction and co-precipitation of metals. Flow-through cells (FTCs) were used to examine the isotope composition of Cu and Zn while these metals were removed from solution by ZVI. Attenuation of dissolved Cu resulted in the enrichment of the heavy isotope (65Cu) in solution, where δ65Cu initially peaked at 1.30 ‰ and decreased towards the mean influent isotope value (δ65Cuinput = 0.27 ‰). Copper isotope ratios in the FTC effluent displayed Rayleigh-type behavior (ɛ = -0.31 ‰). X-ray absorption near edge structure (XANES) spectroscopy was used to identify the solid phases present in the FTCs. In linear combination fitting (LCF), reduced Cu and ZnO were found to primarily contribute to the solid species in respective Cu and Zn FTCs. Reduced Cu0 consistently contributed to the fit for Cu XANES sample spectra present at the central position of the Cu FTC. Similar portions of Cu0 and Cu2O were identified at bottom and top positions of the Cu FTC, indicating that Cu isotope fractionation could be attributed to reduction of Cu during immobilization by ZVI. As Zn breakthrough in the FTC effluent occurred, an increase in δ66Zn values was observed, increasing initially from -0.59 ‰ towards the mean δ66Zninput value of -0.19 ‰ throughout the experiment. The fractionation values (ɛ) for the Zn FTC II was 0.32 ‰. Linear combination fitting indicated contributions from ZnO and Zn(OH)2 in the fitting of four Zn sample spectra. A standard for Zn adsorbed to ferrihydrite also contributed to the fit for the Zn XANES spectra from the bottom location of the FTC. These XAS measurements suggest a combination of adsorption and precipitation mechanisms contributed to the majority of the depletion of the heavy Zn isotope associated with Zn attenuation by ZVI. Characterizing the isotope fractionation linked to Cu and Zn removal by ZVI in PRB systems may contribute to an enhanced knowledge of how isotopes can be correlated to geochemical processes. This insight may be influential for understanding the controls on metal mobility at contaminated sites, and how to devise more efficient and economical groundwater monitoring programs.
Nickel Isotope Geochemistry in Mine Waste System
(University of Waterloo, 2021-01-13) Parigi, Roberta; Blowes, David
Metal stable isotope analyses have proven to make valuable contributions to the study of the biogeochemical cycling of metals and metalloids in the environment. Processes that control the release, mobility and fate of metals in natural systems often lead to stable isotope fractionation. Before the isotopic signatures of natural samples can be applied as environmental tracers, laboratory-scale studies, focused on the measurement of the magnitude of the isotope fractionation associated with specific processes, are needed. Despite recent studies, Ni stable isotope systematics is still in its infancy, thus further research is required to decipher Ni isotope signatures in complex, natural systems. Nickel, similar to other metals, is a nutritionally essential trace element for several organisms and plants, however the exposure to highly Ni-polluted environments can result in a variety of pathological effects in living organisms. Some of the Ni compounds are also classified as carcinogenic, thus, the investigation of Ni attenuation processes is of great importance to protect living beings and the environment. Laboratory batch experiments were conducted to characterize the isotope fractionation during the precipitation of Ni secondary mineral phases: Ni hydroxide, Ni hydroxycarbonate and Ni sulfide. Data were best represented by Rayleigh-type curves, showing fractionation factors εNi hydroxide = −0.40‰, εNi-hydroxycarbonate = −0.50‰ and, εNi-sulfide = −0.73‰. These values indicate a preferential retention of lighter Ni isotopes by the solid phase in all three systems. Synchrotron-based Powder X-ray diffraction (PXRD), and X-ray absorption spectroscopy (XAS) analyses were performed to characterize the precipitates. The interpretation of isotope results suggests that equilibrium effects are the main mechanisms responsible for the measured Ni isotopic signatures. As bacterial reduction of sulfate under anaerobic conditions has been successfully applied to the treatment of waste streams contaminated by heavy metals, laboratory batch experiments were performed to evaluate isotopic fractionation of Ni during microbially-mediated Ni sulfide precipitation. The final solid product was characterized by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Synchrotron-based Powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). The measured isotope data relative to the fraction of Ni in solution and the fraction of Ni associated with the solid phase followed a linear trend which yielded a Δ60solid-solution = −1.99‰, with lighter Ni isotopes partitioned into the precipitates. Both the results of the solid-phase and isotope analyses suggest a combination of Ni removal mechanisms, including complexation, precipitation, co-precipitation and sorption, thus complicating the interpretation of the isotope data. Previous studies involving calcite interaction with Ni have shown the potential of calcium carbonate in the sequestration of Ni from impacted aquatic and groundwater environments. A flow-through cell experiment was conducted to assess Ni isotopic fractionation during Ni treatment by calcite. Synthetic Ni-contaminated groundwater was pumped through a custom-made flow-through cell containing crushed natural calcite. Measurements oh pH were conducted on the effluent and aqueous samples were collected for determination of the concentrations of anions and cations and Ni isotope analyses. X-ray absorption spectroscopy (XAS) was performed via a Kapton window in the cell to gather information on the speciation of Ni in the system. Confocal X-ray microfluorescence imaging (CMXFI) analysis was also conducted on Ni-bearing calcite particles from the cell to further characterize the mechanisms of Ni removal. Results indicate that there are multiple processes controlling Ni removal by calcite inside the flow-through cell, and that greater enrichment of 60Ni over 58Ni in the effluent compared to the input solution, is associated with higher rates of Ni removal. Nickel isotope analysis was conducted on pore water samples extracted from the Moose Lake tailings impoundment as a supportive tool in the characterization of the processes controlling Ni mobility in the tailings within the ML 25 storage facility which are subject to sulfide mineral oxidation. Samples of the tailings material were analyzed using multiple analytical techniques including transmitted and reflected microscopy, X-ray fluorescence, and synchrotron based X-ray absorption spectroscopy (XAS). The highest Ni concentration (578.9 mg L 1) in pore water was found to coincide with the depth of the front of active sulfide-mineral oxidation, whereas highest Ni concentrations in the tailings solid material (1473 and 5000 mg kg-1) corresponded to the highest peaks in sulfur content. Results show a good correlation between Ni isotopic signatures of the pore water and the weathering zones characteristic of different depths of the tailings impoundment, thus suggesting the potential of Ni stable isotopes as tracers in environmental geochemistry.
Oxidation characteristics, acid neutralization, secondary minerals, and trace elements associated with pyrrhotite oxidation in historical waste rock
(University of Waterloo, 2022-07-27) Smith, Lianna; Blowes, David
The Detour Lake Mine is an open pit, greenstone-hosted gold mine in Ontario, Canada. Mining produced waste-rock piles that were constructed from 1983 – 1999. Redevelopment and expansion of the open pit required the excavation and relocation of the waste-rock piles, providing an opportunity to collect samples of waste rock that had been weathering in situ for 30 years. Samples of this weathered waste rock and extracted porewater were analyzed for solid-phase and aqueous geochemistry, characteristics of sulfide-mineral oxidation, stable calcium isotopes (40/44Ca), and secondary minerals. Characterization of weathered waste rock suggest the waste rock was potentially acid generating or of uncertain acid-generating potential and had depleted neutralization potential compared to unweathered samples. Porewater was circumneutral with measurable alkalinity. Mineralogical observations identified pyrrhotite as the sulfide mineral with greatest abundance and oxidation characteristics. Pyrite was present in abundances of approximately half that of pyrrhotite; chalcopyrite was present in lesser amounts and trace amounts of sphalerite and pentlandite were identified. Pyrrhotite was typically strongly altered, with intensity ranging from thick rims surrounding intact-cores to complete replacement by iron-(oxyhydr)oxides. Pyrrhotite, pyrite, chalcopyrite and pentlandite contained trace elements; no sphalerite grains were encountered during the measurements. Of the trace elements measured, Ni occurred in the highest concentrations in pyrrhotite, pyrite, and alteration rims associated with these sulfide minerals. Nickel concentrations in the associated porewater were elevated and sorption sites were calculated to be saturated, suggesting the oxidation of sulfide minerals released Ni that accumulated in porewater, with some attenuation by sorption to iron-(oxyhydr)oxide phases. Synchrotron studies identified Ni in alteration rims associated with an oxidizing pyrrhotite grain, in addition to phases consisting of mixed oxidation states of Fe and S. An additional synchrotron study focused on iron speciation at 1 µm intervals across transects of three partially oxidized pyrrhotite grains to better understand how pyrrhotite oxidation proceeds. Grains were selected from an unweathered sample, a sample of intermediate weathering characteristics, and a sample with more advanced weathering characteristics. Analysis of the spectral features suggests that at the 1 µm scale oxidation products were mixtures of Fe-(oxyhydr)oxides, Fe-(hydroxy)sulfates, and Fe-depleted sulfides. Measurements on visually unoxidized pyrrhotite grains in the samples suggested variable bonding arrangements of Fe among the grains studied, either due to the mineral structure or early-stage oxidation that were not apparent in other features of the spectra. The pyrrhotite standard and some ferrous iron [Fe2+] standards had discordant spectral features; ratios of ferric iron [Fe3+] to [Fe2+ + Fe3+] calculated for the transect spots exhibited a clustering at a Fe3+ component of 0.2. Together, these results suggested oxidation from Fe2+ to Fe3+ may retain characteristics of the Fe2+ pre-edge spectral feature until a threshold component of Fe3+ is exceeded. Existing datasets of porewater and solids chemistry were evaluated from Detour Lake Mine and Diavik Diamond Mine to attempt to discriminate the contributions of carbonate minerals and non-carbonate minerals to acid neutralization based on Ca concentrations. Diavik waste rock had low concentrations of calcite and Ca-bearing minerals, and periods with no measurable alkalinity in the drainage water, in contrast to Ca-rich Detour waste rock and porewater samples with measurable alkalinity. The porewater from both Diavik and Detour had higher molar proportions of Ca to major cations than the solids, indicating preferential release during weathering and/or preferential retention in the aqueous phase. The formation of the secondary Ca-bearing phase gypsum, but not Mg- or K-bearing phases, affected the molar proportions of Ca in porewater. Calculations based on measured alkalinity and concentrations of calculated dissolved inorganic carbon provided a lower bound for concentrations of dissolved Ca derived from carbonate-mineral dissolution. Results were consistent with expectations based on Diavik and Detour lithology and measured porewater alkalinity trends. A small number of samples were analyzed for stable Ca isotopes. This is the first study believed to apply stable Ca-isotopes to mine-waste systems. No trends were discernable between the Ca-isotope values and alkalinity or ratios of major cations, but the dataset was small. Two-component mixing using assumed carbonate and non-carbonate endmembers appeared to over-ascribe Ca to carbonate minerals in the Diavik samples, indicating endmembers may not have been representative, and/or another mechanism was affecting porewater values. Mixing calculations could not be completed on the Detour samples because Ca-isotope values of the endmembers were lower than the porewater values, suggesting a confounding mechanism, likely gypsum precipitation/dissolution, affected the Ca-isotope ratios. Calculated fractionation factors were consistent with previously reported fractionation factors for gypsum. Secondary minerals associated with sulfide-mineral oxidation were characterized by automated quantitative mineralogy (mineral liberation analysis, MLA). Grain sizes, modal abundances and mineral habits of gypsum produced by MLA suggested geochemical conditions were spatially and temporally variable in the historical waste-rock pile. The secondary mineral jarosite was inferred to be present by MLA and by X-ray diffraction analysis, though porewater was calculated to be undersaturated with respect to this phase. Calculated mineral associations and grain habits produced by MLA suggested that jarosite formed in microenvironments associated with oxidizing sulfide minerals. MLA calculates elemental concentrations based on idealized mineral formulae in the database, and comparisons to results from four-acid digestions of solid samples revealed that the MLA results under-represented concentrations of some trace elements by an order of magnitude. Modal abundances provided by MLA and concentrations of trace elements measured by electron-probe microanalysis were used to calculate concentrations of trace elements in sulfide minerals and their alteration rims. Combining these techniques resulted in an increase of up to 170% (relative percent difference) of some trace elements compared to MLA, but concentrations remained lower than the four-acid digestion results. The discrepancy may be (i) an artefact of the MLA resolution, which may not capture mineral grains < 1.5 µm; (ii) the MLA sample size, which considers a much small number of particles compared to four-acid digestion; and/or (iii) the incorporation of trace elements as impurities in silicate minerals, which are digested by the four-acid method. This research provided new insights into the complex processes of pyrrhotite oxidation that affect solid phase and aqueous phase geochemistry of mine-waste systems. Results of this research illustrate that integrating standard and novel bulk and microanalytical techniques will contribute to more robust predictive models of mine-drainage chemistry and, therefore, reduced environmental risk from mine drainage.
Removal of Arsenic and Metals from Mine-impacted Groundwater Using Organic Carbon and Zero-valent Iron in Passive Remediation Systems
(University of Waterloo, 2020-09-21) Angai, Joanne; Blowes, David; Ptacek, Carol
Arsenic (As) is a wide-spread contaminant, often encountered in drainage associated with Au mining. The oxidation state controls the mobility and toxicity of As in water. Passive remediation is a potential management approach for removing As and other contaminants from mine waters. Permeable reactive barriers (PRBs) are a passive management technology that utilizes reactive material to target the contaminant of interest through chemical interactions, including precipitation, reduction, and adsorption. The Long Lake field site is an abandoned Au mine located near Sudbury, ON, characterized by acidic conditions and high concentrations of As, Fe, SO42-, and metals in the tailings porewater. This project aims to evaluate the potential for passive remediation to remove As and other contaminants from mine drainage at the Long Lake field site. A series of laboratory column experiments were conducted to determine the potential of a reactive mixture, containing organic carbon substrates (OC), granular zero-valent iron (ZVI), limestone, and silica sand, to remove As and increase the pH of the water. Groundwater was collected directly from the Long Lake site and used as the influent solution for the column experiments. Results indicated an increase in pH and removal of As within the first 3 cm of reactive material. Removal of As in the three treatment columns represented 99.9% of the total As in the water. A decrease in Eh, the production of H2S, a decline in SO42- concentrations, an enrichment in δ34S, and the presence of microbial communities, indicated the presence of bacterially-mediated SO42- reduction. The percentage of total reads that were sulfate-reducing bacteria (SRB) ranged from 2.4 – 10.0%. Sulfate reduction rates ranged from 0.18 to 0.20 mg L-1 d-1 g-1 dry wt. % OC for the three treatment columns. Synchrotron-radiation bulk S X-ray absorption near-edge structure (XANES) indicated that accumulation of reduced S phases including pyrite and elemental S occurred in the solid material during the experiments. Geochemical modelling results further indicate that precipitation of sulfides including mackinawite, greigite, pyrite, sphalerite, and chalcopyrite, was favoured. Removal of metals, including Cu, Ni, and Zn, is attributed to the precipitation of low-solubility metal sulfides following SO42- reduction. Synchrotron As µXANES indicated that As was present in secondary precipitates in both the reduced phase, as realgar, orpiment, and arsenopyrite, and in the oxidized phase, as As(V) sorbed onto ferrihydrite. The addition of OC contributed to the development of sulfate-reducing conditions and resulted in bacterially-mediated SO42- reduction. The presence of ZVI led to the formation of ferrous iron and ZVI corrosion products, providing additional surface sites for As adsorption. Two separate field-reaction cell (30 cm inner diameter by 99 cm length) trials, were conducted at the Long Lake mine site, one in the summer (mean air temperature of 19 ℃) and one in the autumn (mean air temperature of -1 ℃), to evaluate the effect of temperature on As removal. A reactive mixture containing ZVI, OC, limestone, and pea gravel (at similar proportions to the column experiments) was utilized. The results from the summer field cell were similar to those observed in the laboratory column experiments. A decrease in As, metals, SO42-, and acidity, were observed within the first 9 cm of reactive media. Reactions contributing to metal and As removal include precipitation of low-solubility metal sulfides and adsorption on ZVI corrosion products. The results from the autumn field cell indicated that the development of bacterially-mediated SO42- reduction was limited, with lower percentages of SRB observed in the autumn cell compared to the summer cell and laboratory column experiments. Removal of As, metals, and an increase in pH was observed in the autumn cell, however, aqueous chemistry results did not show a decline in SO42- concentrations or an enrichment in δ34S. Optical microscopy indicated the presence of pyrite and pyrrhotite in the autumn cell material, but abundance was lower in the autumn cell than in the other two experiments. The results from bulk S XANES indicated the accumulation of sulfides in the solid material was also limited. The difference in results between the summer cell and autumn cell may be attributed to colder outside temperatures during the field trial or the shorter duration of the experiment. The results from all three experiments indicate that the addition of OC to the reactive mixture was important for the development of sulfate-reducing conditions and the growth and activity of SRB. The addition of ZVI further enhanced the removal of As, metals, and Fe from the water through the formation of corrosion products and metal sulfide precipitation. Removal of As and metals and an increase in pH was observed in all three experiments despite varying flow rates and fluctuating temperatures. These results indicate that a mixture of OC and ZVI will likely be effective at removing As and metals from mine drainage waters under a range of flow rates and temperature conditions.
Source and Fate of Contaminants in a Pit Lake and Water Treatment at the Faro Mine Complex
(University of Waterloo, 2022-05-20) Larrea, Esteban; Blowes, David
The Faro Mine Complex (FMC) is an abandoned zinc-lead mine in Yukon, Canada that requires extensive remediation efforts to manage the environmental liabilities associated with mine wastes stockpiled at the site. Active care and maintenance are necessary to manage surface water and groundwater impacted by acid rock drainage (ARD) and metal leaching (ML). The Faro Pit lake (FPL) is the largest reservoir of water at the FMC, multiple mine impacted water sources are pumped to the pit lake for storage and mixing. The FPL is also the main influent source to the FMC treatment plant. Active pumping from other mine components and continuous degradation of ARD seepage quality flowing into the pit lake alter the concentration and distribution of contaminants at the FMC. Changes to the load balance of the site may therefore affect contaminant removal processes at the FMC treatment plant. This research project aims to combine a mass balance of contaminant loading in water at the FMC with the characterization of metal removal during the treatment of mine waters. The results would characterize the source, distribution, and fate of contaminants within the FPL. A sampling apparatus was installed into the FPL on two occasions, in September 2019 and September 2021. Profile samples and in situ measurements were obtained to a depth of 47 m in 2019 and 25 m in 2021. In both years, the pit lake was thermally stratified with a well-defined thermocline between 8 m and 10 m depth. The stratification is caused by the difference in specific density of water at 10 ⁰C and 4 ⁰C, in the epilimnion and hypolimnion respectively. Multiple parameters changed sharply at the well-defined thermocline because mixing between the epilimnion and hypolimnion is limited. In 2019, most concentrations of dissolved species were higher in the hypolimnion than in the epilimnion, whereas the concentrations were generally higher in the epilimnion in 2021. The pH was higher in the epilimnion in 2019, and in 2021 it was lower in the epilimnion at pH 3.8. Increasing contaminant loads from the tailings area Intermediate Pond and seepage from the mine waste surrounding the Faro Pit drive geochemical changes in the pit-lake water quality. Batch experiments were conducted to understand the processes resulting in removal of dissolved metals during treatment of mine-impacted waters. FPL water samples were placed in a stirred cell reactor (SCR) and dosed with calcium hydroxide to a pH of 10. Samples were collected for characterization of kinetic and thermodynamic controls during water treatment. Contaminant removal during lime treatment is controlled by processes that occur consistently over narrow pH ranges. Results of the batch experiments show that most Fe removal occurred at pH 6.5, Zn was removed at pH 9, Mn removal at pH 9.7, and Ni and Cd removal occurred above pH 10. Characterization of the precipitated solids showed that Fe and some Zn coprecipitated in the initial stage of the experiments. Later in the experiment, Zn precipitated separately, likely as amorphous Zn hydroxides, mixed with Mn (hydr)oxides. The solution remained undersaturated in respect to gypsum and minimal removal of SO4 from solution was observed. The results from all components of this study suggest that changing geochemical conditions and additional load sources have the potential to change the contaminant distribution and effectiveness of treatment process. This study increased the understanding of the necessary actions to efficiently remove contaminants from a mine with deteriorating water quality and changing operating conditions.
Stabilization of Mercury in River Water and Sediment Using Biochars
(University of Waterloo, 2016-11-29) Liu, Peng; Ptacek, Carol; Blowes, David
Mercury (Hg) is a common contaminant in air, oceans, lakes, rivers, soils, and sediments as elemental, inorganic and organic forms. Organic Hg (e.g., methylmercury (MeHg)), which is much more toxic than other forms and can cause central nervous system defects, and can be converted from inorganic or elemental forms by microbes. Efforts have been made to decrease the production of MeHg by dredging Hg-contaminated sediment, in situ capping, or by converting Hg to stable forms using reactive media to decrease its bioavailability. However, current remediation techniques are commonly burdened by high capital costs or by secondary contamination. The application of biochar, which is an alternative to activated carbon and can promote Hg stabilization, may be a cost-effective reactive material for managing Hg-contaminated sites. This thesis describes laboratory batch and anaerobic microcosm experiments for evaluating the addition of biochar for Hg stabilization in water and sediment. Laboratory batch experiments were conducted to evaluate the treatment of Hg in aqueous solution at environmental concentrations using 36 biochar samples. The biochars were prepared from various feedstocks (wood, agricultural residue, and manure) pyrolyzed at different temperatures (300, 600, and 700oC). The results indicate >90% removal of total Hg (THg) aqueous concentrations was achieved in systems containing biochars produced at 600oC and 700oC (high T) and 40-90% removal for biochars produced at 300oC (low T). Sulfur (S) X-ray absorption near edge structure (XANES) spectra obtained from biochars with adsorbed Hg were similar to those of washed biochars. Micro-X-ray fluorescence (μ-XRF) mapping results indicate that Hg was heterogeneously distributed across biochar particles. Extended X-ray absorption fine structure (EXAFS) modeling indicates Hg was bound to S in biochars with high S contents and bound to O and Cl in biochars with low S contents. These experiments provide information on the effectiveness and mechanisms of Hg removal in aqueous solutions. Components released from the biochars during these batch experiments include anions, cations, alkalinity, organic acids (OAs), dissolved organic carbon (DOC), and nutrients. These components may influence the speciation of Hg (e.g., complexation with Hg), facilitate the transport of Hg, promote the growth of organisms, and stimulate the methylation of Hg. The analyses show elevated concentrations of anions (e.g., for SO42- up to 1000 mg L-1 from manure-based biochars) and nutrients (NO3-, PO4-P, NH3-N, and K) were observed in the majority of aqueous solutions reacted with the biochars. The release of alkalinity OAs and DOC was highly variable and dependent on the feedstock and pyrolysis temperature. Alkalinity released from wood-based biochar was significantly lower than from others. Concentrations of OAs and DOC released from low-T biochars were higher than from high-T biochars. The carbon (C) in the OAs represented 1-60% of the DOC released, indicating the presence of other DOC forms. The C released as DOC represented up to 3% (majority <0.1%) of the total C in the biochar. The modeling analyses of Hg-dissolved organic matter (DOM) complexes suggest that the majority of Hg was likely complexed with thiol groups. Long-term microcosm experiments were carried out by co-blending Hg-contaminated sediment, biochar, and river water under anaerobic conditions, followed by monitoring for more than 500 days. Factors that may control the evolution of THg and MeHg in aqueous solution were evaluated. Furthermore, the Hg spatial distribution and speciation associated with biochar particles were investigated. The results indicate aqueous concentrations of 0.2-µm THg for co-blended systems were less than for sediment controls during the experimental period; 0.45-µm THg concentrations of co-blended systems decreased by 20-92% compared with the controls. Two peaks in MeHg concentrations were observed at early (~40 days) and late (~400 days) times. These peaks corresponded to the onset of iron and sulfate reducing conditions (early peak) and methanogenic conditions (late peak). The MeHg concentrations in the amended systems were less than those observed in the controls, except for late peaks in the microcosms containing a high-T oak wood biochar and switchgrass (600oC) biochars. Pyrosequencing analyses showed shifts in percentages of microbial sequences associated with fermenters, iron-reducing bacteria (FeRB), sulfate-reducing bacteria (SRB), and methanogens that were correlated to changes in C sources (DOC, OAs, and alkalinity) and electron acceptors (NO3-, Fe, and SO42-). Twelve mercury methylators grouped into SRB, FeRB, methanogens, and fermenters based on the pyrosequencing results were detected in the systems. The fate of the removed Hg was studied using X-ray absorption spectroscopy. Hg was observed to co-occur with S, Cu, Fe, Mn, and Zn on the surface and inside the biochar particles as indicated by µ-XRF mapping. EXAFS modeling showed that Hg was present in an oxide form on the surface of an iron (hydro)oxide particle from fresh sediment and in Hg-sulfide forms for the Hg-rich areas of biochar particles after co-blending with sediment. Sulfur XANES showed the presence of sulfide in these biochar particles. After the addition of biochars, a fraction of the Hg in unstable forms (e.g., dissolvable, HgO, colloidal, nano) in the sediment was likely converted to Hg-sulfide forms. These more stable forms bound on and within the biochar particles are anticipated to have decreased mobility and decreased bioavailability relative to the Hg in the unamended sediment. The detected fluorescence intensity in conventional XRF mapping represents the sum of information along the path of microbeam penetration into the sample, usually limited by the thickness of thin sections. To overcome this limitation, confocal X-ray micro-fluorescence imaging (CXMFI) was applied to delineate the three-dimensional spatial distribution of elements accumulated within switchgrass biochar particles. The maps indicate that Hg, Fe, Ti, Cr, Mn, Co, Ni, Cu, Zn, and As were distributed within the structural material of low-T switchgrass biochar, whereas these elements were preferentially observed on the surfaces of high-T switchgrass biochar. These observations suggest that the accumulations of Hg and other elements in biochars may be through an early-stage diagenetic process, rather than a simple surface adsorption reaction. Hard-wood biochar was sulfurized using CaSx (CPS-CL2) and a dimercapto-related (DMC-CL2) compound to further promote Hg removal. With an initial THg concentration of 17,800 ng L-1, final THg concentrations were 40 and 7.0 ng L-1 using CPS-CL2 and DMC-CL2, and 370 ng L-1 using unmodified hard-wood biochar; for an initial THg concentration of 245,000 ng L-1, final concentrations were 74 and 110 ng L-1 using CPS-CL2 and DMC-CL2, and 5,700 ng L-1 using the unmodified form; for an initial concentration of 4,960 µg L-1, final concentrations were 10 and 29 µg L-1 using CPS-CL2 and DMC-CL2, and 170 µg L-1 using the unmodified biochar. The THg removal percentages were >99.5% using the modified biochars for these three initial concentrations. A suite of synchrotron-based techniques was applied to characterize biochars loaded with Hg. S XANES analyses indicates polysulfur- and thiol-like structures were present in the modified biochars. The accumulated Hg was distributed primarily on the edges of the modified biochar particles as indicated by µ-XRF mapping and CXMFI techniques. Hg EXAFS analysis shows Hg was bound to S in sulfurized biochars. This study demonstrates that biochars may be effective reactive media for Hg removal from aqueous solution and for Hg stabilization in amended sediment. The extent of Hg removal was further enhanced after sulfurization of the biochar. The results also suggest that the Hg stabilization process may be effective for years or longer through strong binding with biochar particles.

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