Earth and Environmental Sciences

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Effectiveness of Organic Carbon Cover Systems on Sulfide-rich Tailings
(University of Waterloo, 2021-09-27) McAlary, Mason; Ptacek, Carol
Acid mine drainage (AMD) occurs when sulfide minerals oxidize; generating low pH water and the release of dissolved metals. This phenomena is of primary concern in the mining industry, where the future loading of dissolved metals from inactive mine sites could continue for decades to centuries, with recent estimates of total liability related to AMD remediation exceeding $10 billion in Canada alone (Mining Watch Canada, 2017). Traditionally, remediation of AMD generally is focused on collection and treatment of water from surface-water bodies (i.e., ponds, ditches and streams) and treatment through pH neutralization using lime (CaO). While this approach improves water quality, it represents a recurring cost for the mining companies who need to continue operating these systems after mine closure. Due to this reality, numerous approaches have been developed over the past few decades to passively mitigate and remediate AMD by limiting the supply of O2 and water to tailings rich in sulfide minerals. These passive approaches can be done through the use of: 1) cover systems by limiting O2(g) diffusion and/or O2(g) consumption; and 2) waters covers by limiting O2(g) diffusion. Examples of option 1) include biosolids, wood-waste, peat, compost, geosynthetic clay liners (GCL), covers with capillary barrier effects (CCBEs), and monolayer covers with an elevated water table; while an example of option 2) is subaqueous tailings disposal. The performance of a cover system, consisting of a 0.5 m layer of biosolids fertilizer and municipal compost underlain by a ~ 2 m layer of thickened desulfurized tailings (DST), was studied. Six locations were investigated, including four locations with a 2-layer cover system, one location with a 1-layer cover of DST only, and one location without any cover system. The DST layer had been in place for up to 12 years and the organic cover components in place between 5 and 8 years. Results demonstrated that the cover system was able to consume all atmospheric O2(g) prior to the base of the cover system, resulting in improved water quality when compared to the location without a cover system. Comparison between the locations with a 2-layer cover system and the location with a 1-layer cover revealed that the current geochemical conditions were similar, except that the organic layer consumes a portion of incoming O2(g) and leaches alkalinity to the shallow porewater; thus improving the acid-neutralizing capacity. This research shows that cover systems which use an organic layer over top of a low sulfur (S) thickened tailings layer with an elevated water table can limit oxidation of the tailings and neutralize acidity. However, in order for the organic carbon portion of this cover system to operate as an effective O2 barrier in the long-term, occasional replenishment of organic material is required.
Geochemical and microbiological characterization of effluent and pore water from low-sulfide content waste rock
(University of Waterloo, 2013-04-22T17:45:10Z) Bailey, Brenda Lee
Laboratory and field studies were completed to characterize the geochemistry and microbiology of drainage emanating from low-S content waste-rock test piles at the Diavik Diamond Mine (Diavik) from 2007 through 2010. The potential use of small-scale laboratory humidity-cell experiments to predict the water quality from larger-scale field-based experiments also was examined. Waste rock at Diavik is segregated into three categories according to sulfide content: Type I (target concentration: < 0.04 wt. % S), Type II (target concentration: 0.04 to 0.08 wt. % S) and Type III (target concentration: > 0.08 wt. % S). Four high-density polyethylene tanks, 2 m in diameter by 2 m in height, were filled with and surrounded by waste rock (active zone lysimeters; AZLs) at the Diavik site to study the upper 2 m of the active zone within a waste-rock pile and to evaluate the quality of effluent released from waste rock with differing S contents (Type I AZLs: 0.014 wt. % S and Type III AZLs: 0.035 wt. % S). In addition, three waste-rock test piles also were constructed at Diavik, two uncovered test piles (Type I test pile: 0.035 wt. % S and Type III test pile: 0.053 wt. % S) and a third pile was constructed based on the mine-closure plan which consists of waste rock (Type III: 0.082 wt. % S) capped with a 1.5 m layer of till and a 3 m layer of Type I material (Covered test pile). Each test pile is underlain by a high-density polyethylene geomembrane that captures and directs water to outflow drains. Results show that the release and transport of blasting residuals could be used as a resident tracer, indicating the first flush of water through the AZLs and the test piles. Variations in concentrations of blasting residuals and the gradual rate of dissipation provide an indication of the heterogeneity of the distribution of blasting residuals and the relative contributions of water and solutes from different flow paths. As temperatures within the test piles increase in response to ambient air temperature increases, larger proportions of the test pile contributed to the outflow, and increased concentrations of blasting residuals were observed in waste-rock test pile effluent. Effluent from the Type I AZLs and test pile maintained near-neutral pH (ranged from 5.8 to 8) with concentrations of SO₄²⁻ < 500 mg L⁻¹. These results suggest that the near-neutral pH values were associated with the presence of carbonates in the waste rock and the lack of intense acid generation. As ambient air temperatures increased in spring and summer of each year, the measured pH in the Type III test-pile drainage decreased from near-neutral in May (pH 7.5) to acidic conditions by October (ranged from 5 to 4.5). As the pH in the Type III test pile decreased, concentrations of SO₄²⁻ and dissolved metals increased (e.g. SO₄²⁻ > 1500 mg L⁻¹) suggesting sulfide oxidation was occurring. Maximum concentrations of SO₄²⁻, Al, Zn, Ni, Co, and Cu were observed in 2009 during the first flush of water through the Type III test pile. A sequence of acid-neutralization reactions was inferred based on the water chemistry of the effluent derived from the Type III AZLs and waste-rock test pile. This acid-neutralization sequence is similar to those observed at other AMD impacted sites. A series of mineral dissolution-precipitation reactions controlled pH and metal mobility; carbonate-mineral dissolution consumed H⁺ generated from sulfide-mineral oxidation at near neutral pH and the dissolution of Al and Fe (oxy)hydroxides consumed H⁺ at pH < 5.0. The cover system on the Covered test pile dampened the effects of ambient air temperature on the internal temperatures within the core of the Covered test pile. As a result, the Covered test pile had a relatively steady change in flow rate, with decreased flow from June to August, which led to a slow but prolonged release of sulfide-mineral oxidation products, such as SO₄²⁻ and dissolved metals, including Ni, Co, Zn, Cd, and Cu, compared to the uncovered Type III test pile. The pH decreased in 2008 and remained low for the duration of the study, whereas the pH in the uncovered test pile was near-neutral at the beginning of each field season in May and decreased to < 4.2 by the end of the field season in November. The microbiological-community profiles observed in the AZLs and waste-rock test piles suggest typical AMD-related species were present in acidic effluent with elevated concentrations of metals, whereas typical soil microbes were present in effluent with a near-neutral pH and lower concentrations of SO₄²⁻ and dissolved metals. The Type III AZLs, Type III test pile, and Covered test pile maintained populations of acidophilic Fe-oxidizers, whereas, the Type I AZLs and Type I test pile maintained populations of neutrophilic S-oxidizers. Laboratory humidity-cell (1 kg) results were scaled up to estimate the water quality from the Type III AZLs (6 t) using measured physical and chemical parameters. The results suggested over-prediction of SO₄²⁻ and metal concentrations when low mean annual precipitation occurred, limiting flushing of predicted oxidation products. In subsequent years with higher mean annual precipitation oxidation products from previous years were liberated and resulted in the under prediction of SO₄²⁻ and metal concentrations. Additionally, Fe and Al were over-predicted because Fe and Al concentrations in the AZL effluent may be controlled by the solubility and formation of secondary minerals, such as Fe oxyhydroxides, jarosite, and goethite, which were not included in the scaling procedure.
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.
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.

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Browsing Earth and Environmental Sciences by Subject "acid mine drainage"