Auswahl der wissenschaftlichen Literatur zum Thema „U mine tailings“

Environmental context Uranium and arsenic, two elements of human health concern, are commonly found at sites of uranium mining, but little is known about processes influencing their environmental behaviour. Here we focus on understanding the chemical and physical processes controlling uranium and arsenic transport at an abandoned uranium mine. We find that the use of sedimentation ponds limits the mobility of uranium; however, pond conditions at our site resulted in arsenic mobilisation. Our findings will help optimise restoration strategies for mine tailings. Abstract Although As can occur in U ore at concentrations up to 10wt-%, the fate and transport of both U and As at U mine tailings have not been previously investigated at a watershed scale. The major objective of this study was to determine primary chemical and physical processes contributing to transport of both U and As to a down gradient watershed at an abandoned U mine site in South Dakota. Uranium is primarily transported by erosion at the site, based on decreasing concentrations in sediment with distance from the tailings. Sequential extractions and U X-ray absorption near-edge fine structure (XANES) fitting indicate that U is immobilised in a near-source sedimentation pond both by prevention of sediment transport and by reduction of UVI to UIV. In contrast to U, subsequent release of As to the watershed takes place from the pond partially due to reductive dissolution of Fe oxy(hydr)oxides. However, As is immobilised by adsorption to clays and Fe oxy(hydr)oxides in oxic zones and by formation of As–sulfide mineral phases in anoxic zones down gradient, indicated by sequential extractions and As XANES fitting. This study indicates that As should be considered during restoration of uranium mine sites in order to prevent transport.

Tailings from inactive uranium mine sites represent a potential secondary source of rare earth elements (REEs). For this study, two mine tailings (DT and RAT) from restored uranium sites in Ontario, Canada, were used. Bioleaching experiments were conducted with a mix of native sulfur- and iron-oxidizing bacteria to test the solubilization of REEs, U and Th at different temperatures (20, 30 and 40 °C). The selective recovery of REEs from bioleaching solution was evaluated using different ion exchange resins. The mineralogical characterization revealed that DT tailings were mainly composed of quartz, pyrite, gypsum and silicates, whereas RAT tailings were mainly composed of quartz. The maximum solubilization of heavy and light REEs (HREEs and LREEs, respectively), Th and U reached 54%, 6%, 60% and 51% for RAT after 35 days at pH 2, T = 30 °C and pulp density = 10% (w/v). Higher extraction yields were obtained for DT, with 58% of HREEs, 14% of LREEs, 85% of Th and 89% of U solubilized under the same conditions. The use of Lewatit TP272 resin for the recovery of Sc (94%) and U (99%) followed by the Lewatit SP112 resin for the recovery of Th (57%) and REEs (81% LREEs and 65% HREEs) seemed a promising method for the co-extraction of the key elements from the bioleaching solution.

Abstract Technogenic soils (Technosols) developed from mine tailings containing iron sulfides occurring in the area of the abandoned .Siersza. hard coal mine in Trzebinia and the abandoned .Staszic. pyrite mine in Rudki were investigated in order to assess their properties. The study revealed that the most adverse properties of the technogenic soils investigated are: strong acidity (pH below 3), the presence of large amounts of rock fragments containing unweathered sulfides, as well as the occurrence of heavy metals (e.g. Pb, As, and Tl) and radioactive elements (U and Th). All these properties should be taken into account during management of the studied mine tailings.

Barite is ubiquitous and known to incorporate 226Ra through the formation of a solid-solution. In U mining mill tailings, barite is one of the dominant sulfate-binding minerals. In such environments, sequential extractions are generally used to identify the U- and 226Ra-binding phases and their associated reactivity. To better decipher the main processes governing the behavior of 226Ra during such sequential extractions, a geochemical model was developed with PHREEQC mimicking the sequential extraction of U and 226Ra from Bois-Noirs Limouzat U mine tailings, France. The model results were compared with a dataset produced by an experimental sequential extraction from the same mine tailings and including data on the solids and selective extraction results with the major elements, U and 226Ra. The simulations reproduced the results of the experimental chemical extractions accurately, with iron oxyhydroxides being the major U binding phase. However, the modeling indicated rather that barite would be the main 226Ra binding phase, instead of the iron oxyhydroxides identified by the experimental extractions. This is consistent with the 226Ra concentration measured in pore water, but in disagreement with the direct interpretation of the sequential extractions. The direct interpretation disregarded the role of barite in the geochemical behavior of 226Ra because barite was not specifically targeted by any of the extraction steps. However, the modeling showed that the dissolution of 226Ra-binding barite by reactants would lead to a 226Ra redistribution among the clay minerals, resulting in a skew in the experimental results. Similar results were achieved by referring simply to the bulk mineralogy of the tailings. This study highlights the importance of considering the mineralogy, mineral reactivity and retention capacity for more realistic interpretation of sequential extractions. Moreover, this paper provides new perspectives on the long-term consequences of these mill tailings in which barite controls the geochemical behavior of the 226Ra.

This study was aimed at determining whether heavy metals in tailings from Ranger Uranium Mine (N.T.) change in chemical form in such a way that they will become more mobile, or bioavailable, after they are mixed with extremely acidic soils from downstream of the mine. Four soils were studied: two samples were acid sulfate (jarositic or pyritic) materials and two were acidic materials overlying acid sulfate horizons. Copper, iron, manganese, lead, uranium and zinc fractions were determined in soils to which uranium mill tailings had been added. Total and exchangeable 226Ra were also determined in selected samples. The tailings-soil mixtures were incubated for up to 4 months and included a comparison of reactions under continuously moist conditions and when subjected to a saturation and drying cycle. The tailings had considerably greater concentrations of total Mn, Pb, U and 226Ra than the soils. The heavy metals in the tailings occurred as relatively immobile forms. In the non-pyritic soils, the distribution of the metals between the fractions did not change much during 4 months of reaction. In the pyritic soil, which underwent oxidation and acidification during incubation, there were 2- to 3-fold increases in the exchangeable fractions of Fe, Mn, Cu and U. The metals in the tailings and soil behaved similarly. There appeared to be more likelihood of increased mobility of metals from oxidation of pyritic materials than from addition of tailings. The fraction of total 226Ra that was exchangeable decreased from 11% in the original tailings to 2-7% after reaction with three of the soils but increased to 44% in one soil. At estimated long-term erosion rates, the tailings are not likely to be a source of heavy metal pollution, but addition of 226Rato soils presents a possible radiological hazard.

The environment of Northern Kosovo and Metohija has been remarkably influenced for a long time by the huge metallurgical complex "Trepča". The technological process of ore exploitation and processing in the Mining and Metallurgical Chemical Company "Trepča" has resulted in a large amount of discharged material, deposited on metal tailings that are mostly located in inappropriate areas, partially covered with vegetation, mostly consisted of several invasive plants. The activity of the enzyme catalase was investigated in plant species collected from abandoned metal tailings "Žitkovac" and compared with the same plant species from the environment from the vicinity of Niš city. Measurements of enzyme activity were carried out in the underground and above-ground parts of plant species Artemisia vulgaris, Cichorium intybus, Erigeron canadensis, Robinia pseudacacia, Medicago sativa, Teucrium chamaedrys, Plantago lanceolata, Rumex acetosella, Tanacetum vulgare and Euphorbia cyparissias using gasometric method. The results have indicated that an increase of catalase activity in tested plants from the metal tailings is possibly a consequence of stress caused by specific environmental conditions.

AbstractThe Athabasca oil sands deposit in Alberta contains ~5 x 109 m3 of bitumen accessible by surface mining. During bitumen separation from the mined ore, ultra-fine (<300 nm) aluminosilicate clays only a few layers thick (U/F) are mobilized and become dispersed in the process water. In this water containing dissolved salts from natural deposits, U/F are capable of forming thixotropic gels. The consequence of this is the production of large volumes of mature fine tailings (MFT) with a high water holding capacity. For mine planning purposes, the objective of predicting and possibly mitigating MFT formation requires an understanding of the colloidal behaviour of U/F particles in salt solutions. In this work, photon correlation spectroscopy and the deuterium NMR method are used to provide an insight into the U/F floc formation process. These results are correlated with conventional analysis of settling data.

In this study, the activity concentrations and radiological risks associated with exposure to NORMS in soils around the gold mine tailings in Ilahun – Ijesa, Obokun Local Government, Osun State, Nigeria was evaluated. A Sodium Iodide (NaI) gamma spectroscopy was used to determine the activity concentrations of the natural radio nuclides in soil samples that were collected from the gold mine tailings. The mean activity concentration for 238 U, 232 Th and 40K were 14.50±2.40, 10.45±3.20 and 332.70±6.10 Bqkg-1 respectively. The average absorbed dose was calculated to be 26.4 nGy/h and the corresponding average annual effective dose was found to be 0.0031 mSv, which is less than the total annual effective dose of 1 mSv recommended by ICRP for public exposure control. Radiological hazard parameters were evaluated from activity concentrations and the average values were found to be lower than recommended safety limits.

The potential toxic elements (Be, U, As, Cd, Pb, Sb, Bi) were detected for mine landscape of Sherlovaya Gora tin-polymetallic deposit. Thermodynamic calculation of equilibrium for tailings dump pond water was carried out by means of “Selektor” program complex based on Gibbs free energy minimization algorithm at 25°C and 1 bar total pressure. It turned out that the mine water is supersaturated with respect to many sulphates of Ca, Mg, Sr, Zn, K, Cu, Ni, Cd, Be, Al, Ce and Y, fluorides of (Ln and Y, Sc), and Y phosphate.

Le 226Ra, descendant radioactif de l’238U et isotope majoritaire naturellement présent sur Terre, suscite de nombreuses problématiques environnementales en raison de sa demi-vie de 1600 ans dans des industries variées : hydrothermalisme, désalinisation de l’eau de mer et production de zircon par exemple ; mais surtout dans les industries extractives : pétrole et gaz de schiste, charbon, phosphate et uranium. Le 226Ra est retenu au sein des résidus de traitement des mines d’U et sa mobilité est contrôlée par les mécanismes de sorption à la surface de minéraux (oxy-hydroxydes de fer, phyllosilicates, zéolithes), ou de la matière organique ou par la formation de solutions solides (minéraux sulfatés comme la barytine et minéraux carbonatés). La concentration moyenne dans les roches lithosphériques étant de 32 Bq/kg, soit 1 ppt, l’identification des mécanismes de rétention de ce radionucléide à l’échelle du matériau échantillonné sur le terrain est rendue difficile par son caractère ultra-trace. Les extractions séquentielles, technique classique pour le suivi des éléments traces, peuvent être sujettes à de nombreux artefacts exacerbés pour les éléments ultra-traces. Dans le cadre de ce travail, une modélisation géochimique d’expériences de lixiviations séquentielles a en effet montré que cette technique conduit à des interprétations biaisées en particulier dans le cas du 226Ra, soumis à de nombreux mécanismes de remobilisation aux différentes étapes de lixiviation. Afin de mieux comprendre les processus de rétention du 226Ra et la distribution de celui-ci dans les matériaux hétérogènes et finement divisés, dont font partie les résidus de traitement de l’extraction minière, une nouvelle approche a été développée couplant autoradiographie alpha, cartographies chimiques élémentaires et caractérisations minéralogiques obtenues entre autres par MEB/EDS sur lames minces pétrographiques. Une analyse globale directe de l’ensemble de l’activité de l’échantillon à l’échelle de la lame mince pétrographique est ainsi possible. Cette méthode a été qualifiée tout d’abord sur des échantillons de référence contenant un seul minéral synthétique ou naturel jouant un rôle important dans la rétention du 226Ra en milieu naturel, puis à un assemblage de trois des principaux minéraux responsables de la rétention, à savoir : barytine, minéraux argileux et oxy-hydroxydes de fer. Enfin, elle a été directement appliquée à des résidus de traitement. Un premier corpus est issu des sites français de stockage post-miniers de Bellezane, où les résidus de traitement sont stockés sous couverture solide, et de Bois Noirs Limouzat, qui utilise une couverture liquide. Sont également considérés des résidus du site en activité de traitement de minerai de McClean Lake, au Canada, qui utilise un processus de neutralisation des résidus par précipitation de barytine
226Ra, a radioactive decay product of 238U and the most prevalent naturally occurring isotope of radium leads to many environmental issues in various industries due to its half-life of 1600 years: hydrothermal energy, seawater desalination and zircon production among others. The most impacted industries are the extractive ones: shale oil and gas production, coal, phosphate and uranium extraction. 226Ra remains in tailings from U mines and its mobility is controlled by retention mechanisms: sorption on mineral surfaces (iron oxy-hydroxydes, phyllosilicates, zeolites) and organic matter, or by the formation of solid solutions (sulfate minerals such as barite and carbonate minerals). The average concentration in lithospheric rocks being 32Bq / kg, or 1ppt, the identification of the retention mechanisms of this radionuclide at the scale of the material sampled in the field is made difficult because it is an ultra-trace element. Sequential extractions are commonly used to assess the retention of trace elements, but this technique is subject to experimental and analytical artefacts which are exacerbated in the case of an ultra-trace element. In this work, geochemical modeling of sequential extractions experiments has indeed shown that this technique leads to biased interpretations, particularly in the case of 226Ra which is remobilized during the different extraction steps. In order to have a better understanding of the retention of 226Ra and its distribution in heterogeneous and fine-grained materials, including mine tailings, a new approach has been developed. This approach combines alpha autoradiography, chemical elemental cartographies and mineralogical characterizations obtained on petrographic thin sections. A direct global analysis of the activity of the sample at the petrographic thin section scale is thus possible. This method was first qualified on model samples containing a single synthetic or natural mineral playing an important role in the retention of 226Ra in the natural environment. It was then tested on an assemblage of three of the main minerals responsible for the retention, namely: barite, clay minerals and iron oxy-hydroxydes. Finally, it was applied to U-mine tailings. A first set of samples comes from the French post-mining storage sites of Bellezane, where the tailings are stored under a solid cover, and from Bois Noirs Limouzat, which uses a liquid cover. A second set of tailings sample comes from the on-going ore processing facility of McClean Lake, Canada, which uses a tailings neutralization process by barite precipitation. The results show that barite is the main trap of 226Ra via the formation of a solid solution (Ba, Ra)SO4 in all these tailings from different sites. Over a few years, with or without neutralization by barite precipitation, it appears that this solid solution tends towards a recrystallization equilibrium which controls the concentration of 226Ra in solution. These results will subsequently be integrated into reactive transport type modeling to predict the long-term behavior of these tailings

Uranium ore mining and milling have been terminated in the Mecsek Mountains (southwest Hungary) in 1997. Mine tailings ponds are located between two important water bases, which are resources of the drinking water of the city of Pe´cs and the neighbouring villages. The average U concentration of the tailings material is 71.73 μg/g, but it is inhomogeneous. Some microscopic particles contain orders of magnitude more U than the rest of the tailings material. Other potentially toxic elements are As and Pb of which chemical state is important to estimate mobility, because in mobile form they can risk the water basis and the public health. Individual U-rich particles were selected with solid state nuclear track detector (SSNTD) and after localisation the particles were investigated by synchrotron radiation based microanalytical techniques. The distribution of elements over the particles was studied by micro beam X-ray fluorescence (μ-XRF) and the oxidation state of uranium and arsenic was determined by micro X-ray absorption near edge structure (μ-XANES) spectroscopy. Some of the measured U-rich particles were chosen for studying the heterogeneity with μ-XRF tomography. Arsenic was present mainly in As(V) and uranium in U(VI) form in the original uranium ore particles, but in the mine tailings samples uranium was present mainly in the less mobile U(IV) form. Correlation was found between the oxidation state of As and U in the same analyzed particles. These results suggest that dissolution of uranium is not expected in short term period.

Laboratory lysimeter studies were undertaken to evaluate the leaching characteristics and mobility of Ra-226 and other heavy metals (U, Th and Pb) from pyritic uranium mine tailings under sub-aqueous disposal conditions for assessing the long-term radiological stability of such waste repositories. The experiments were conducted using three types of un-oxidized tailings: fine, coarse and gypsum depleted mill total tailings. The results showed that Ra-226 was leached from surface of the submerged tailings and released to both surface water and shallow zone pore water during initial low sulphate ion concentration of the surface water cover in all three cases. The release of Ra-226 was further enhanced with the onset of weak acidic conditions in the surface water covers of both coarse and gypsum depleted mill total tailings. With additional acid generation and increasing sulphate and iron concentrations, the dissolved Ra-226 concentrations in the water covers of these tailings gradually decreased back to low levels. Pb was also leached and mobilized with the development of moderate acidic conditions at the surface of the submerged coarse and gypsum deplete tailings. No leaching of U and Th was observed.

The aim of this study was to determine the concentration of some metals (Ca, Mg, Mn, Fe, Zn, Ni, Cr, Cd) in the soil and species of Tussilago farfara L. and Clematis vitalba L. on the tailings and flotation of the mine DOO "Rudnik". Concentrations in the soil were: Pb and Cu - higher than the maximum allowed, limit and remediation values; Cd, Cr and Ni - higher than the maximum allowed and limit values, and Zn higher than the limit values (according to the regulations of the Republic of Serbia). Better bioaccumulation of all tested metals was shown by T. farfara species, especially leaves for Zn. The results indicate the possible use of aboveground parts of both plant species in the revitalization of tailings by bioaccumulation of tested metals.

Abstract Problems arising from uranium dispersion from mines and mine tailings, and the remediation of uranium contaminated areas, are discussed in this paper. In an experimental remediation study, a mixture of 70 vol.% of uraniferous mining wastes and 30 vol.% of a natural ceramic were used. The preliminary observations are discussed, and a model is proposed for the long term stabilization of mining tailings. Observations and monitoring of contaminated sites carried out during the last 25 years have revealed local impacts of uranium on the environment in Lower Silesia, Poland. Uranium pollution is limited to waste dumps, mine tailings, and their close vicinities at Kowary Podgórze, Radoniów, Kopaniec and Kletno. Uranium dispersion takes place mechanically due to transport by river waters, chemically by rain and ground waters, and anthropogenically when the wastes are utilized in construction. Floods are an additional mechanism responsible for the mechanical dispersion of uranium. As a result of these uranium transport mechanisms, in order to minimize the impacts of uranium on the environment, the covering of dumps with non-radioactive material is suitable only for sites located away from populated areas. Redox reactions have been observed at the Kowary tailings. During these reactions, iron hydroxide (goethite), hematite, and gypsum, are precipitated as solids. These observations provide a good prognosis for the long-term stabilization of radionuclides which can be incorporated into proposals for the construction of tailings sites. Using Eh-pH diagrams (system U-C-O-H, 25°C, 1 bar), UO2 is stable over the whole range of naturally occurring pHs, and is affected by Eh only in the range −0.4 to +0.1 volts in acidic environments, and below −0.4v in basal environments. BaSO4 and RaSO4 are stable under almost the same conditions as UO2. An environmentally significant redox boundary (FeS2 versus Fe2O3) occurs in the middle of the UO2 stability field. The geochemical and environmental behaviour of the elements discussed above suggest a mechanism for stabilizing radionuclides within stored wastes. The solidification of wastes should occur concurrently with naturally occurring redox reactions. During oxidation, an active iron-hydroxide gel is produced. This gel is then dehydrated and converted into limonite (a mixed compound), a monohydrate (goethite), hydro-hematite (Fe2O3·1/2H2O) and hematite (Fe2O3). This reaction occurs in neutral or weakly acidic environments. A key problem in the proposed remediation project, therefore, is pH stabilization in order to maintain the required environment for oxidation and cementation reactions. In order to achieve such an environment and to stabilize the reactions, a construction method is proposed for new waste storage systems, based on mixed layers of waste and barrier components composed of natural materials. The presence of CaO or Ca(OH)2 and anhydrite in the proposed internal membrane will reduce the vertical migration of sulphates. Redox reactions will be responsible for the secondary precipitation (reduction) of uranyl. These same reactions occur naturally during the precipitation of uranium ores. Iron oxidation is the other process in the redox pair required to reduce [UO2]+2 to UO2. The resultant pitchblende is insoluble in normal oxidizing environments. To minimize the dissolution of UO2 by sulphuric acid generated during the iron oxidation reaction, the construction of pH active membranes containing calcium oxide or hydroxide are recommended. These compounds will react with the free acid to precipitate gypsum. Although several elements can be mobilized as a result of oxidation, radium remains in insoluble solid phases such as the common Ca, Ba and Sr sulphates.

The Uranium mining and milling activities in Eastern Germany before reunification produced more than 232 000 t of U. Following reunification, £ 6.6 billion were committed to remediation of the left behind liabilities. The inventory of the liabilities comprises operations areas (37 km2), waste rock dumps (311 M m3), tailings (160 M m3), an open pit (84 M m3) and five large underground mines (1.53 M m3). The specific activities are 0.5 to 1 Bq/g for the waste rock, 10 Bq/g for the tailings, up to 500 Bq/g for the water treatment residues and 0.2 to 1 Bq/g for scrap metal. The remediation of the risk associated with this inventory is carried out by WISMUT GmbH. The legal framework of the remediation is set by the Federal Mining Act, the Atomic Act, the Radiation Protection Ordinance and the Water Resources Management Act. The large number and variety of objects that release contaminants at very different rates require, remedial measures to be planned and optimized in an integral way for each site. The integration is done on the basis of Conceptual Site Models (CSM). The CSM helps to balance among the objects the remedial effort, the allocation of resources and allows to flexibly adapt remedial measures to the site/object-specific conditions while maintaining conceptual consistency and focus on the overall remediation goals without compromising essential details. The remediation necessity of individual objects or areas is investigated, justified and the type of remedial measures selected on the basis of Remedial Investigation/Feasibility Studies (RI/FS). In the RI/FS the calculated individual effective dose to the public caused by the object/area in the nonremediated and remediated state is compared with the reference level of 1 mSv per year. Based on RI/FS contaminated areas are remediated either for unrestricted or for restricted use. Waste rock piles are remediated by covering in situ, by relocation and/or by backfilling into an open pit. Currently, approximately 40,000 tons of waste rock are backfilled into a pit per day. Backfilling follows a geochemically optimized placement procedure. In cases where the remediation object was judged vulnerable, remediation was supported by risk assessment. A probabilistic risk assessment was used to justify the dry remediation of the tailings ponds. Technically, the most challenging part of dry tailings remediation is the stabilization of the soft, under-consolidated slimes having a high excess pore water pressure and very low shear strengths. Because total cleanup and relocation of contaminants are not always feasible, the remediation is commonly done by covering of the contaminated object or area, i.e. by confinement. The covers used are either barrier covers that limit infiltration by having a low permeability layer incorporated or an evaporative cover which maximizes infiltration storage till it is removed by evapotranspiration. The largest sources of contaminant release are the discharges from flooded mines and from dewatering of the tailings ponds. Discharge rates vary from 30 m3/h to 1000 m3/h. Because the contaminants load in the discharging mine water decreases with time causing the conventional water treatment to become uneconomic, various alternative water treatment technologies are tested at WISMUT to identify suitable and cost efficient replacement options. Considerable amounts of contaminated debris and scrap metals arise from decommissioning and demolition of the structures. The aim is to categorize and recycle the uncontaminated portion of the scrap metal. The categorization of the scrap metal into contaminated and uncontaminated is by measuring the beta-count rate in the field. To improve the selectivity of the field monitors, specially prepared standards reflecting the operational history of the metal at the particular site are used to calibrate the instruments. Approximately £ 3.9 billion were invested into the remediation by end of 2002. A rough calculation of the specific costs of WISMUT remediation when using re-assessed total costs turned out to be approximately £ 22.6 per kg of U3O8 produced. Considering that this sum includes the indirect costs, the specific remediation costs appear in an international comparison very reasonable.

The area of the town of Mailuu Suu, Kyrgyzstan, is polluted by radionuclides and heavy metals in tailing dumps and heaps resulting from the historic exploitation of U-mines in the Mailuu Suu area in Kyrgyzstan. Radioactive substances are stored in 23 tailings and 13 mining debris heaps situated along the Mailuu Suu River. The objective of the EC-TACIS funded project is to evaluate and prepare measures to be taken by the authorities to reduce the radiological exposure of the population and to prevent environmental pollution by radionuclides and heavy metals in case of loss of tightness of dams and damage to dumps and heaps from mining and milling by land and mudslides and to propose sustainable remedial options, accepted by the public. The actual radiological situation is of no immediate concern for most of the population of Mailuu Suu. From the gamma radiation monitoring campaign (monitoring network in Mailuu Suu, Kara Agach and vicinity), it showed that the background radiation is 100–120 nSv/h. On the tailings radiation is on average twice background. The average outside and in-house radon concentration is 175 Bq/m3. In 3 of the houses monitored at Kara Agach the radon level is between the exemption limit for new (200 Bq/m3) and old (400 Bq/m3) houses. The concentration of uranium in the Mailuu Suu river water is far below the exemption limit for drinking water set in Kyrgyzstan. Additional dose from irrigation with Mailuu Suu river water is small in actual conditions (< 0.1 mSv/year). However, there is an important possibility that triggered by an eathquake or a landslide, (part of the) tailing(s) content may be directed to the river Mailuu Suu. Doses to the affected population may increase to multiple ten mSv per annum. Given the actual limited stability of Tailing N°3, the potental of such a disaster to occur isnon-neglegible. To impede the consequences of a potential disaster, under the TACIS project different remedial options are evaluated for Tailing 3 including in situ stabilisation and tailing translocation. Also more global remedial options for the Mailuu Suu River valley are studied (translocation of other tailings, tunnel to deviate river, partial protection of river from landslide blockage). It was proposed to acquire a phased approach in time performing urgent limited stabilisation options for Tailing 3, improve the stability of the Tailing 3 by COLMIX-columns in the medium term, while investigating and evaluating further two long-term remedial options: the translocation of the tailings and the long-right-bank river diversion tunnel.

Environmental impacts associated with the mining of carbonatite deposits are an emerging concern due to the demand for critical metals. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Its objectives are to characterize the mineralogy of the tailings and their pore water and effluent chemistries. Geochemical mass balance modeling, constrained by aqueous speciation modeling and mineralogy, is then used to identify reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. The tailings are composed mainly of REE-enriched calcite (82 wt. %), biotite (12 wt. %) and fluorapatite (4 wt. %). Minor minerals include chlorite, pyrite, sphalerite, molybdenite and unrecovered pyrochlore. Secondary minerals include gypsum, barite and strontianite. Within the unsaturated zone, pore water chemistry is controlled by sulfide oxidation and calcite dissolution with acid neutralization. With increasing depth below the water table, pore water composition reflects gypsum dissolution followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Concomitantly, incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F, forming kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. At a pH of 8.3, Mo is highly mobile and reaches an average concentration of 83 µg/L in tailings effluent. Although U also forms mobile complexes, concentrations do not exceed 16 µg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 µg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 µg/L in pore water and effluent, respectively. Mine tailings from carbonatite deposits are enriched in a variety of incompatible elements with mineral hosts of varying reactivity. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.