Tailings Pore Water Research Articles

Phytostabilisation of severely contaminated mine tailings using halophytes and field addition of organic and inorganic amendments

Phytostabilisation strategies have proven to be an efficient remediation option for mine tailings, but the adequate plant species and amendments have to be carefully selected. A remediation experiment was carried out at the semi-field level in tailings (pH 3.2, ≈1100, 4700 and 5000 mg kg−1 of As, Pb and Zn, respectively) from the mining district of La Unión-Cartagena (SE Spain). A red mud derivative (Fe/Al oxides), its combination with compost, and hydrated lime (Ca hydroxide) were applied in field plots of 0.25 m2. After four months of field stabilisation, tailings were transferred unaltered to a plant growth facility, and Atriplex halimus and Zygophyllum fabago (halophytes) were sown. Three months later, trace element (TE) solubility, plant accumulation and chemical speciation in the tailings pore water were studied. In unamended tailings, soluble TEs concentrations were very high (e.g., 40 mg Zn l−1), the dominant species being free ions and SO42-- complexes (>70%). The addition of amendments increased tailings pH (6.7–7), reduced TEs solubility and extractability (>80–99%) and changed the dominant species of soluble Al, Cu, Pb and Zn to hydroxides and/or organo-metallic complexes, but increased slightly the extractable As and soluble Tl concentrations. Plants were able to grow only in amended tailings, and both species presented low levels of Al, As, Cd and Zn. Therefore, the use of combined red mud derivative and compost and halophytes was shown to be a good phytostabilisation strategy, although the dose applied must be carefully chosen in order to avoid possible solubilisation of As and Tl.

Electrokinetic thickening of mature fine oil sands tailings

Mature fine oil sands tailings (MFT) are the fine parts of oil sands tailings that remain suspended in tailings disposal pond for decades because of the low sedimentation/consolidation rate. This study applies electrokinetics to thicken, that is, to increase the solid content, of the MFT suspension. The geotechnical properties of MFT solids and chemical analysis of tailings pore water are measured, followed by electrokinetic (EK) tests. The results of this study indicate that EK thickening is very effective for the settling of MFT suspensions under an applied voltage of 219 V/m, the final solid content of the sediment reached 18·75% from an initial 5% after 7 h. The EK treatment is also very effective in clarifying the supernatant. The final turbidity of the supernatant is 30·6 NTU under 219 V/m applied voltage gradient. Regression models are developed using statistical software MINITAB 15 in coded and uncoded units to relate the final solid content of the sediment with the initial solid content and applied voltage gradient. The models and the independent variables are statistically significant at 95% confidence level based on F test and t test results, respectively.

Subsurface variations in arsenic mineralogy and geochemistry following long-term weathering of gold mine tailings

Variations in arsenic (As) mineralogy and geochemical controls on its mobility were evaluated in subsurface tailings at the historical Montague and Goldenville mine sites in Nova Scotia, Canada. Tailings at these sites contain some of the highest As concentrations in Nova Scotia and are located in close proximity to local communities. Pore water in the subsurface tailings is characterized by circumneutral to alkaline pH (6.2 to 8.7) and mildly reducing to oxidizing redox conditions (+130 mV to +347 mV). Bulk chemistry, scanning electron microscopy, and synchrotron micro-X-ray diffraction analyses showed As mineral hosts differ with depth. The deepest tailings (max. 2 m) are in direct contact with partially decomposed vegetation, which supports reducing conditions and the precipitation of authigenic As and Fe sulfides. Under reducing conditions, dissolved As concentrations are also controlled by desorption of As from dissolution of Fe and Mn oxides and the sorption or co-precipitation of As with carbonates. These geochemical controls differ from those influencing dissolved As concentrations under oxidizing conditions. In the near surface, As mobility is controlled by oxidative dissolution of primary arsenopyrite, precipitation of secondary Fe arsenates, Fe oxyhydroxides and Mn oxides, secondary Ca-Fe arsenates, and sorption onto Fe oxyhydroxides and gangue minerals. Some of these mineral species are stable under different conditions yet occur in close association, indicating the importance of microenvironments. The results of this study show that the weathering characteristics of these tailings vary with depth, leading to the formation of new As hosts that are distinct from those observed in the near surface. Identification of these As hosts provides an understanding of current controls on As mobility and has implications for future reprocessing and/or remediation efforts.

Screening of bacterial strains isolated from uranium mill tailings porewaters for bioremediation purposes

The present work characterizes at different levels a number of bacterial strains isolated from porewaters sampled in the vicinity of two French uranium tailing repositories. The 16S rRNA gene from 33 bacterial isolates, corresponding to the different morphotypes recovered, was almost fully sequenced. The resulting sequences belonged to 13 bacterial genera comprised in the phyla Firmicutes, Actinobacteria and Proteobacteria. Further characterization at physiological level and metals/metalloid tolerance provided evidences for an appropriate selection of bacterial strains potentially useful for immobilization of uranium and other common contaminants. By using High Resolution Transmission Electron Microscope (HRTEM), this potential ability to immobilize uranium as U phosphate mineral phases was confirmed for the bacterial strains Br3 and Br5 corresponding to Arthrobacter sp. and Microbacterium oxydans, respectively. Scanning Transmission Electron Microscope- High-Angle Annular Dark-Field (STEM-HAADF) analysis showed U accumulates on the surface and within bacterial cytoplasm, in addition to the extracellular space. Energy Dispersive X-ray (EDX) element-distribution maps demonstrated the presence of U and P within these accumulates. These results indicate the potential of certain bacterial strains isolated from porewaters of U mill tailings for immobilizing uranium, likely as uranium phosphates. Some of these bacterial isolates might be considered as promising candidates in the design of uranium bioremediation strategies.

Coupled effects of cement type and water quality on the properties of cemented paste backfill

The significant coagulation delay and poor strength performance of cemented paste backfill (CPB) are two of the biggest challenges faced by the backfilling system in Chihong mine. These factors significantly affect the mining and backfill cycle. Hence, CPB property tests (e.g., slump, coagulation, and strength) were conducted to investigate the influence of different cement types and mixing water qualities (tailings pore water and distilled water). Results of the tests for slump, initial coagulation time (ICT), and unconfined compressive strength (UCS) of the CPB samples are as follows: (i) slump variation of different CPB samples is insignificant. (ii) The ICT changing trends of the CPB samples are in line with the ICT of their mixing cement slurry. In addition, zinc ions in the tailings pore water are detrimental to CPB coagulation. (iii) Cement has an obvious influence on the UCS of CPB, and water quality barely affects the UCS of CPB. These results indicate that cement choice and water chemical components are important factors in CPB design and mine operations.

The Occurrence and Implications of Efflorescent Sulfate Minerals At the Former Sherritt–Gordon Zn–Cu Mine, Sherridon, Manitoba, Canada

The former Sherritt–Gordon sulfide deposit in Sherridon, Manitoba, Canada, was mined for zinc, copper, silver, and gold between 1930 and 1951. Since mine closure, two high-sulfide tailings impoundments underwent extensive oxidation, resulting in the release of very high concentrations of dissolved sulfate and metals to the tailings porewater. During precipitation events and Spring freshet, surface seeps develop along the flanks of the impoundments, discharging groundwater with a pH as low as 0.39 and dissolved concentrations of sulfate and iron up to 203 g L−1 and 68 g L−1, respectively, along with other metals at elevated concentrations. Several efflorescent minerals were observed within groundwater-seepage areas. Using a combination of powder X-ray diffraction and energy-dispersive spectroscopy, secondary efflorescent sulfate minerals at seepage zones were identified, including melanterite, rozenite, halotrichite, chalcanthite, alpersite, copiapite, hexahydrite, jurbanite, pickeringite, jarosite, and gypsum. According to geochemical modeling of seepage waters using a modified WATEQ4F database for ferrous and ferric sulfate minerals, the occurrence of most of the observed sulfate salts was favored. In the geochemical modeling, we used data available for seepage waters that were representative of discharging water after mineral precipitation had already occurred. Discrepancies between some of the observed and modeled results likely reflect the use of water compositions representative of the later stages in the reaction sequence rather than the composition during actual formation of the minerals. The formation of these secondary efflorescent minerals resulted in a temporary removal of sulfate and metals from solution. These minerals rapidly dissolved during laboratory dissolution tests; this would result in increased loading to receiving surface waters during precipitation events and Spring freshet.

Extremely High Phosphate Sorption Capacity in Cu-Pb-Zn Mine Tailings.

Elevated inorganic phosphate (Pi) concentrations in pore water of amended tailings under direct revegetation may cause toxicity in some native woody species but not native forbs or herb species, all of which are key constituents in target native plant communities for phytostabilizing base metal mine tailings. As a result, Pi sorption capacity has been quantified by a conventional batch procedure in three types of base metal mine tailings sampled from two copper (Cu)-lead (Pb)-zinc (Zn) mines, as the basis for Pi-fertiliser addition. It was found that the Pi-sorption capacity in the tailings and local soil was extremely high, far higher than highly weathered agricultural soils in literature, but similar to those of volcanic ash soils. The Langmuir P-sorption maximum was up to 7.72, 4.12, 4.02 and 3.62 mg P g-1 tailings, in the fresh tailings of mixed Cu-Pb-Zn streams (MIMTD7), the weathered tailings of mixed Cu-Pb-Zn streams (MIMTD5), EHM-TD (fresh Cu-stream, high magnetite content) and local soil (weathered shale and schist), respectively. Physicochemical factors highly correlated with the high Pi-sorption in the tailings were fine particle distribution, oxalate and dithionite-citrate-bicarbonate extractable Fe (FeO and Fed), oxalate-extractable Al and Mn, and the levels of soluble Cd and Zn, and total S and Fe. Large amounts of amorphous Fe oxides and oxyhydroxides may have been formed from the oxidation of pyritic materials and redox cycles of Fe-minerals (such as pyrite (FeS2), ankerite (Ca(Fe Mg)(CO3)2 and siderite (FeCO3), as indicated by the extractable FeO values. The likely formation of sparingly soluble Zn-phosphate in the Pb-Zn tailings containing high levels of Zn (from sphalerite ((Zn,Fe)S, ZnS, (Zn,Cd)S)) may substantially lower soluble Zn levels in the tailings through high rates of Pi-fertiliser addition. As a result, the possibility of P-toxicity in native plant species caused by the addition of soluble phosphate fertilizers would be minimal.

Copper and zinc adsorption by softwood and hardwood biochars under elevated sulphate-induced salinity and acidic pH conditions

Biochar adsorption may lower concentrations of soluble metals in pore water of sulphidic Cu/Pb–Zn mine tailings. Unlike soil, high levels of salinity and soluble cations are present in tailing pore water, which may affect biochar adsorption of metals from solution. In the present study, removal of soluble copper (Cu) and zinc (Zn) ions by soft- (pine) and hard-wood (jarrah) biochars pyrolysed at high temperature (about 700°C) was evaluated under typical ranges of pH and salinity conditions resembling those in pore water of sulphidic tailings, prior to their direct application into the tailings. Surface alkalinity, cation exchange capacity, and negative surface charge of biochars affected Cu and Zn adsorption capacities. Quantitative comparisons were provided by fitting the adsorption equilibrium data with either the homogeneous or heterogeneous surface adsorption models (i.e. Langmuir and Freundlich, respectively). Accordingly, the jarrah biochar showed higher Cu and Zn adsorption capacity (Qmax=4.39 and 2.31mg/g, respectively) than the softwood pine biochar (Qmax=1.47 and 1.00mg/g). Copper and Zn adsorption by the biochars was favoured by high pH conditions under which they carried more negative charges and Cu and Zn ions were predicted undergoing hydrolysis and polymerization. Within the tested range, salinity had relatively weak effects on the adsorption, which perhaps influenced the surface charge and induced competition for negative charged sites between Na+ and exchangeable Ca2+ and/or heavy metal ions. Large amounts of waste wood/timber at many mine sites present a cost-effective opportunity to produce biochars for remediation of sulphidic tailings and seepage water.

Sediment Diagenesis Module for CE-QUAL-W2 Part 2: Numerical Formulation

Pit lakes are used worldwide as part of closure schemes for mining operations. Pit lakes have also been proposed in the Athabasca Oil Sands Region of Canada where bitumen is close enough to the surface to be recovered using conventional surface mining and chemical extraction procedures. Many of the pit lakes are designed to cap tailings with a substantial water layer. The pore water in tailings will contain various amounts of labile and refractory organic compounds, nutrients, trace metals, and ion concentrations that, ultimately, will be released to the water cap as the tailings consolidate. In addition to advective release of constituents from the tailings pore water, chemical reactions, more specifically, diagenetic processes, within the tailings will result in changes to the redox state, speciation, and phase of several chemicals. A module that quantifies diagenetic processes has been developed for the 2-dimensional hydrodynamic and water quality model, CE-QUAL-W2. The module includes processes such as biogenic gas production (build up and release of methane, hydrogen sulfide, and ammonia); physical release of bubbles through the water column; unconsolidated sediment re-suspension during bubble ebullition; and dynamic oxygen consumption at the sediment–water interface and in the water column from bubbles. This paper presents the numerical framework used to develop the diagenesis module and the chemical basis for the algorithms.

Identification of individual thiophene-, indane-, tetralin-, cyclohexane-, and adamantane-type carboxylic acids in composite tailings pore water from Alberta oil sands.

Naphthenic acids (NAs) accumulate in oil sands process-affected water (OSPW) as a result of the water-based extraction processes, and represent one of the toxic fractions in OSPW. They exist as a complex mixture and so the development of an analytical method to characterize and quantify individual acids has been an on-going challenge. The multidimensional separation technique of two-dimensional gas chromatography (GC × GC) has the potential to provide a fingerprint of the sources of NAs and can potentially resolve individual analytes for target analysis. However, the identity and toxicity of a large proportion of the acids present in tailing waters are still unknown. Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC/TOFMS) was used to characterize NAs in a pore water sample from a Syncrude composite tailings (CT) deposit in Fort McMurray, Alberta, Canada. The extractable organic acid fraction was derivatized with diazomethane and the structures of selected resolved esters were elucidated through interpretation of their electron ionization (EI) mass spectra and, if available, confirmed by comparison with the spectra of reference standards. The high resolving power of the GC × GC/TOFMS technique allowed for the structural elucidation of numerous as yet unidentified acids in the CT pore water sample such as carboxylic acids containing a thiophene, indane, tetralin or cyclohexane moiety. Seventeen members of the previously reported class of adamantane-type carboxylic acids in oil sands process water could also be identified in the sample. This study underlines the complexity of naphthenic acid isomer distributions in composite tailings and provides a useful inventory of individual acids.

Uranium Association with Iron-Bearing Phases in Mill Tailings from Gunnar, Canada

The speciation of uranium was studied in the mill tailings of the Gunnar uranium mine (Saskatchewan, Canada), which operated in the 1950s and 1960s. The nature, quantification, and spatial distribution of uranium-bearing phases were investigated by chemical and mineralogical analyses, fission track mapping, electron microscopy, and X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies at the U LIII-edge and Fe K-edge. In addition to uranium-containing phases from the ore, uranium is mostly associated with iron-bearing minerals in all tailing sites. XANES and EXAFS data and transmission electron microscopy analyses of the samples with the highest uranium concentrations (∼400-700 mg kg(-1) of U) demonstrate that uranium primarily occurs as monomeric uranyl ions (UO2(2+)), forming inner-sphere surface complexes bound to ferrihydrite (50-70% of the total U) and to a lesser extent to chlorite (30-40% of the total U). Thus, the stability and mobility of uranium at the Gunnar site are mainly influenced by sorption/desorption processes. In this context, acidic pH or alkaline pH with the presence of UO2(2+)- and/or Fe(3+)-complexing agents (e.g., carbonate) could potentially solubilize U in the tailings pore waters.

Microscale mineralogical characterization of As, Fe, and Ni in uranium mine tailings

Uranium (U) ores can contain high concentrations of elements of concern (EOCs), such as arsenic (As) and nickel (Ni) present in sulfide and arsenide minerals. The U in these ores is often solubilized by adding H2SO4 to attain a pH ∼1 under oxic conditions. This process releases some EOCs from the primary minerals into solution. The barren raffinate (solution remaining after U extraction) is subsequently neutralized with Ca(OH)2 to a terminal pH of ∼10.5, resulting in a reduction in the aqueous concentrations of the EOCs. These neutralized raffinates are mixed with the non-reacted primary minerals and discharged as tailing into tailings management facilities (TMFs). To aid in the accurate characterization and quantification of the mineralogical controls on the concentrations of EOCs in the tailings porewater, their spatial distribution and speciation were studied at the micron scale in tailings samples collected from the Deilmann U Tailings Management Facility (DTMF), northern Saskatchewan, Canada. Backscattered electron images of the tailings samples generated using an electron microprobe show the presence of nodules (10–200μm size) surrounded by bright rims. Wavelength dispersive spectrometric (WDS) and synchrotron-based micro-X-ray fluorescence (μ-XRF) elemental mapping show that the nodules are dominated by Ca and S (as gypsum) and the bright rims are dominated by Fe, As, and Ni. Micro-X-ray absorption near-edge structure (μ-XANES) spectra collected within and near the rims indicate that the Fe and Ni are present mainly in the +3 and +2 oxidation states, respectively; for As, the +5 oxidation state dominates but significant amounts of the +3 oxidation state are present in some areas. Linear combination fit analyses of the K-edges for the Fe, As, and Ni μ-XANES spectra to reference compounds suggest the Fe in the rims is present as ferrihydrite with As and Ni are adsorbed to it. Energy dispersive spectrometric (EDS) data indicate that isolated, highly reflective particles distributed throughout the tailings matrix are primary As-, Cu-, Fe-, and Ni-bearing minerals. Geochemical modeling of the neutralization process shows that the nodules (gypsum) formed at pH ∼1 and acted as a substrate for the precipitation of ferrihydrite at pH ∼3.4. The As and Ni subsequently adsorb to the ferrihydrite. Overall, the microscale data suggest that the As and Ni adsorbed onto the ferrihydrite should remain stable for many years and continue to the control the Fe, As, and Ni concentrations in the tailings porewater.

Arsenic attenuation in tailings at a former Cu–W–As mine, SW Finland

Nearly half a century after mine closure, release of As from the Ylojarvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As2O5) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kankite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylojarvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondarymore » Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit« less

Occurrence and mobility of As in the Ylöjärvi Cu–W–As mine tailings

Fifteen tailings samples were investigated for mineralogy by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), 12 for geochemical distribution by sequential extractions successively attacking water-soluble, adsorbed-exchangeable-carbonate, Fe (oxy)hydroxide, Fe oxide and sulfide fractions, as well as 13 tailings pore water and two groundwater samples for geochemical composition and physicochemical conditions at the former Ylöjärvi Cu–W–As mine in SW Finland. Mineralogical observations and sequential extractions demonstrated the depletion of arsenopyrite and other sulfide minerals from the superficial and deeper coarse-grained tailings resulting in high concentrations of As in pore water and groundwater beneath the tailings heap (5140 and 14 500μgL−1, respectively), and lowering the pore pH (from ca. 7.8 to 5.3). The solubility of As was controlled by pH (and Eh) as well as precipitating Fe(III) (oxy)hydroxides and Fe(III) arsenates. Although these phases are an important sink for As, it accumulates in the percolating water. Further acidification of the tailings after the consumption of scarce neutralizing calcite is probable and will affect the stability of the remaining arsenopyrite and As-bearing secondary phases.

The behavior of arsenic and antimony at Pezinok mining site, southwestern part of the Slovak Republic

Arsenic and antimony contamination is found at the Pezinok mining site in the southwest of the Slovak Republic. Investigation of this site included sampling and analysis of water, mineralogical analyses, sequential extraction, in addition to flow and geochemical modeling. The highest dissolved arsenic concentrations correspond to mine tailings (up to 90,000 μg/L) and the arsenic is present predominately as As(V). The primary source of the arsenic is the dissolution of arsenopyrite. Concentration of antimony reaches 7,500 μg/L and its primary source is the dissolution of stibnite. Pore water in mine tailings is well-buffered by the dissolution of carbonates (pH values between 6.6 and 7.0) and arsenopyrite grains are surrounded by reaction rims composed of ferric iron minerals. Based on sequential extraction results, most solid phase arsenic is in the reducible fraction (i.e. ferric oxyhydroxides), sulfidic fraction, and residual fraction. Distribution of antimony in the solid phase is similar, but contents are lower. The principal attenuation mechanism for As(V) is adsorption to ferric oxide and hydroxides, but the adsorption seems to be limited by the competition with Sb(V) produced by the oxidation of stibnite for adsorption sites. Water in mine tailings is at equilibrium with gypsum and calcite, but far from equilibrium with any arsenic and antimony minerals. The concentrations of arsenic and antimony in the surrounding aquifer are much lower, with maximum values of 215 and 426 μg/L, respectively. Arsenic and antimony are transported by ground water flow towards the Blatina Creek, but their loading from ground water to the creek is much lower compared with the input from the mine adits. In the Blatina Creek, arsenic and antimony are attenuated by dilution and by adsorption on ferric iron minerals in stream sediments with resulting respective concentrations of 93 and 45 μg/L at the site boundary south of mine tailing ponds.

Organic carbon amendments for passive in situ treatment of mine drainage: Field experiments

Abstract A field-scale experiment was conducted to evaluate various organic C sources as amendments for passive treatment of tailings pore water. Varied mixtures of peat, spent-brewing grain (SBG) and municipal biosolids (MB) were assessed for the potential to promote dissimilatory sulfate reduction (DSR) and metal-sulfide precipitation. Five amended cells and one control were constructed in the vadose zone of a sulfide- and carbonate-rich tailings deposit, and the geochemistry, microbiology and mineralogy were monitored for 4 a. Increases in pore-water concentrations of dissolved organic C (DOC) and decreases in aqueous SO 4 concentrations of >2500 mg L −1 were observed in cells amended with peat + SBG and peat + SBG + MB. Removal of SO 4 was accompanied by shifts in δ 34 S-SO 4 values of >+30‰, undersaturation of pore water with respect to gypsum [CaSO 4 ·2H 2 O], and increased populations of SO 4 -reducing bacteria (SRB). Decreases in aqueous concentrations of Zn, Mn, Ni, Sb and Tl were observed for these cells relative to the control. Organic C introduction also supported growth of Fe-reducing bacteria (IRB) and increases in Fe and As concentrations. Enhanced Fe and As mobility occurred in all cells; however, maximum concentrations were observed in cells amended with MB. Subsequent decreases in Fe and As concentrations were attributed to DSR and metal-sulfide precipitation. The common presence of secondary Zn-S and Fe-S phases was observed by field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDS) spectroscopy. Selective extractions indicated that large decreases in water-soluble SO 4 occurred in cells that supported DSR. Furthermore, amendments that supported DSR generally were characterized by slight decreases in solid-phase concentrations of extractable metal(loid)s. Amendment of tailings with organic C amendments that supported ongoing DOC production and DSR was essential for sustained treatment.

Geochemistry and stable isotope composition of fresh alkaline porphyry copper tailings: Implications on sources and mobility of elements during transport and early stages of deposition

Prevention of acid mine drainage (AMD) in sulfide-containing tailings requires the identification of the geochemical processes and element pathways in the early stages of tailing deposition. However, analyses of recently deposited tailings in active tailings impoundments are scarce because mineralogical changes occur near the detection limits of many assays. This study shows that a detailed geochemical study which includes stable isotopes of water ( δ 2H, δ 18O), dissolved sulfates ( δ 34S, δ 18O) and hydrochemical parameter (pH, Eh, DOC, major and trace elements) from tailings samples taken at different depths in rainy and dry seasons allows the understanding of weathering (oxidation, dissolution, sorption, and desorption), water and element pathways, and mixing processes in active tailings impoundments. Fresh alkaline tailings (pH 9.2–10.2) from the Cu–Mo porphyry deposit in El Teniente, Chile had low carbonate (0.8–1.1 wt.% CaCO 3 equivalent) and sulfide concentrations (0.8–1.3 wt.%, mainly as pyrite). In the alkaline tailings water, Mo and Cu (up to 3.9 mg/L Mo and 0.016 mg/L Cu) were mobile as MoO 4 2− and Cu(OH) 2 0. During the flotation, tailings water reached equilibrium with gypsum (up to 738 mg/L Ca and 1765 mg/L SO 4). The δ 34S vs. δ 18O covariations of dissolved sulfate (2.3 to 4.5‰ δ 34S and 4.1 to 6.0 ‰ δ 18O) revealed the sulfate sources: the dissolution of primary sulfates (12.0 to 13.2‰ δ 34S, 7.4 to 10.9‰ δ 18O) and oxidation of primary sulfides (− 6.7 to 1.7‰ δ 34S). Sedimented tailings in the tailings impoundment can be divided into three layers with different water sources, element pathways, and geochemical processes. The deeper sediments (> 1 m depth) were infiltrated by catchment water, which partly replaced the original tailings water, especially during the winter season. This may have resulted in the change from alkaline to near-neutral pH and towards lower concentrations of most dissolved elements. The neutral pH and high DOC (up to 99.4 mg/L C) of the catchment water mobilized Cu (up to 0.25 mg/L) due to formation of organic Cu complexes; and Zn (up to 130 mg/L) due to dissolution of Zn oxides and desorption). At 1 m depth, tailings pore water obtained during the winter season was chemically and isotopically similar to fresh tailings water (pH 9.8–10.6, 26.7–35.5 mg/L Cl, 2.3–6.0 mg/L Mo). During the summer, a vadose zone evolved locally and temporarily up to 1.2 m depth, resulting in a higher concentration of dissolved solids in the pore water due to evaporation. During periodical new deposition of fresh tailings, the geochemistry of the surface layer was geochemically similar to fresh tailings. In periods without deposition, sulfide oxidation was suggested by decreasing pH (7.7–9.5), enrichment of MoO 4 2− and SO 4 2−, and changes in the isotopic composition of dissolved sulfates. Further enrichment for Na, K, Cl, SO 4, Mg, Cu, and Mo (up to 23.8 mg/L Mo) resulted from capillary transport towards the surface followed by evaporation and the precipitation of highly soluble efflorescent salts (e.g., mirabilite, syngenite) at the tailing surface during summer.

Technical Report: In-situ Monitoring and Validation of a Uranium Mill Tailings Management Facility Design Using X-ray Absorption Near Edge Structure (XANES) Spectroscopy

Uranium ore containing elevated concentrations of arsenic (up to 2 wt %) are processed at the McClean Lake Operation in the Athabasca Basin of northern Saskatchewan (Figure 1). This region of northern Saskatchewan is responsible for 33% of the world production of uranium [1]. A major concern of AREVA Resources Canada Inc. (AREVA) as well as provincial and federal regulators is the potential long-term risk that significant amounts of arsenic may pose to surrounding lakes and waters after their disposal in the JEB Tailings Management Facility (TMF). The regulatory limit is set at 5 mg/L for tailings pore water with an action level set by AREVA of 2 mg/L As.

Stable Hydrogen and Oxygen Isotope Composition of Waters from Mine Tailings in Different Climatic Environments

The stable isotope composition of waters (delta2H, delta18O) can be used as a natural tracer of hydrologic processes in systems affected by acid mine drainage. We investigated the delta2H and delta18O values of pore waters from four oxidizing sulfidic mine tailings impoundments in different climatic regions of Chile (Piuquenes at La Andina with Alpine climate, Cauquenes and Carén at El Teniente with Mediterranean climate, and Talabre at the Chuquicamata deposit with hyperarid climate). No clear relationship was found between altitude and isotopic composition. The observed displacement of the tailings pore waters from the local meteoric water line toward higher delta18O values (by approximately +2 per thousand delta18O relative to delta2H) is partly due to water-rock interaction processes, including hydration and O-isotope exchange with sulfates and Fe(III) oxyhydroxides produced by pyrite oxidation. In most tailings, from the saturated zone toward the surface, isotopically different zones can be distinguished. Zone I is characterized by an upward depletion of 2H and 18O in the pore waters from the saturated zone and the lowermost vadose zone, due to ascending diffused isotopically light water triggered by the constant loss of water vapor by evaporation at the surface. In zone II, the capillary flow of a mix of vapor and liquid water causes an evaporative isotopic enrichment in 2H and 18O. At the top of the tailings in dry climate a zone III between the capillary zone and the surface contains isotopically light diffused and atmospheric water vapor. In temperate climates, the upper part of the profile is affected by recent rainfall and zone III may not differ isotopically from zone II.

Mercury mobility in unsaturated gold mine tailings, Murray Brook mine, New Brunswick, Canada

Elevated concentrations of Hg are present (averaging 36 μg/g), mainly as cinnabar, in the Murray Brook Au deposit, located in northern New Brunswick, Canada. After the mined ore was subjected to CN leaching, the tailings were deposited in an unsaturated pile, and 10 a after mine closure an estimated 4.7 × 10 3 kg of CN and 1.1 × 10 4 kg of Hg remain in the pile. Elevated Hg concentrations have been measured in the groundwater (up to 11,500 μg/L) and surface water (up to 32 μg/L) down-gradient of the tailings. To investigate the controls on Hg mobility and leaching persistence, laboratory experiments were conducted using unsaturated columns filled with tailings. Within the first 0.2 pore volumes (PV) eluted, the concentrations of Hg and CN increased to peak concentrations of 12,900 μg Hg/L and 16 mg CN/L, respectively. In the subsequent 0.9 PV, concentrations decreased to approximately 1300 μg Hg/L and 2.8 mg CN/L. Thermodynamic calculations demonstrate that >99.8% of the mobilized Hg in the tailings pore water is in the form of Hg–CN complexes, indicating that Hg mobility to the surrounding aquatic environment is directly dependent on the rate of CN leaching. One-dimensional transport simulations suggest that leached CN can be partitioned into conservative (24%) and non-conservative (76%) fractions. Extrapolation of simulation results to the field scale suggests that CN, and by extension Hg, will continue to elute from the tailings for at least an additional 130 a.