Diagnostic de la contamination des eaux par les éléments traces métalliques dans la zone aurifère de Komabangou – Tillabéri, Niger

A Komabangou, dans la région de Tillabéri au Niger, l’or est exploité, durant des décennies, de manière artisanale avec l’utilisation des méthodes inadéquates et des substances peu respectueuses de l’environnement comme le mercure et le cyanure. La présente étude a pour objectif de déterminer les teneurs en éléments traces métalliques (ETM) des eaux de Komabangou afin de prévoir les risques environnementaux et sanitaires liés à leur utilisation. Ainsi, des prélèvements d’eau ont été effectués sur 8 sites d’extraction d’or, 4 forages et 3 mares, et leurs concentrations en métaux tels que As, Cd, Cu, Hg, Ni, Pb et Zn ont été analysées par spectrométrie de masse avec plasma à couplage inductif (ICP-MS). Les résultats obtenus révèlent une contamination importante de ces eaux et soulignent une variabilité significative des concentrations des ETM en fonction du métal analysé et du type d’eau. Dans la plupart des eaux, les valeurs limites réglementaires notamment celles de l’OMS ont été dépassées, cela pourrait poser des problèmes de toxicité pour les plantes, les animaux et la population locale. Par conséquent, la mise en place d’un programme de surveillance et de traitement des eaux contaminées de la zone de Komabangou serait nécessaire pour réduire les risques de contamination humaine.Mots clés : ETM, eaux, risque, site aurifère, Komabangou, Niger

Bioavailability in soils

The consumption of locally-produced vegetables by humans may be an important exposure pathway for soil contaminants in many urban settings and for agricultural land use. Hence, prediction of metal and metalloid uptake by vegetables from contaminated soils is an important part of the Human Health Risk Assessment procedure. The behaviour of metals (cadmium, chromium, cobalt, copper, mercury, molybdenum, nickel, lead and zinc) and metalloids (arsenic, boron and selenium) in contaminated soils depends to a large extent on the intrinsic charge, valence and speciation of the contaminant ion, and soil properties such as pH, redox status and contents of clay and/or organic matter. However, chemistry and behaviour of the contaminant in soil alone cannot predict soil-to-plant transfer. Root uptake, root selectivity, ion interactions, rhizosphere processes, leaf uptake from the atmosphere, and plant partitioning are important processes that ultimately govern the accumulation ofmetals and metalloids in edible vegetable tissues. Mechanistic models to accurately describe all these processes have not yet been developed, let alone validated under field conditions. Hence, to estimate risks by vegetable consumption, empirical models have been used to correlate concentrations of metals and metalloids in contaminated soils, soil physico-chemical characteristics, and concentrations of elements in vegetable tissues. These models should only be used within the bounds of their calibration, and often need to be re-calibrated or validated using local soil and environmental conditions on a regional or site-specific basis.Mike J. McLaughlin, Erik Smolders, Fien Degryse, and Rene Rietr

Spatially Explicit Analysis of Metal Transfer to Biota: Influence of Soil Contamination and Landscape

Concepts and developments for a new field in ecotoxicology, referred to as “landscape ecotoxicology,” were proposed in the 1990s; however, to date, few studies have been developed in this emergent field. In fact, there is a strong interest in developing this area, both for renewing the concepts and tools used in ecotoxicology as well as for responding to practical issues, such as risk assessment. The aim of this study was to investigate the spatial heterogeneity of metal bioaccumulation in animals in order to identify the role of spatially explicit factors, such as landscape as well as total and extractable metal concentrations in soils. Over a smelter-impacted area, we studied the accumulation of trace metals (TMs: Cd, Pb and Zn) in invertebrates (the grove snail Cepaea sp and the glass snail Oxychilus draparnaudi) and vertebrates (the bank vole Myodes glareolus and the greater white-toothed shrew Crocidura russula). Total and CaCl2-extractable concentrations of TMs were measured in soils from woody patches where the animals were captured. TM concentrations in animals exhibited a high spatial heterogeneity. They increased with soil pollution and were better explained by total rather than CaCl2-extractable TM concentrations, except in Cepaea sp. TM levels in animals and their variations along the pollution gradient were modulated by the landscape, and this influence was species and metal specific. Median soil metal concentrations (predicted by universal kriging) were calculated in buffers of increasing size and were related to bioaccumulation. The spatial scale at which TM concentrations in animals and soils showed the strongest correlations varied between metals, species and landscapes. The potential underlying mechanisms of landscape influence (community functioning, behaviour, etc.) are discussed. Present results highlight the need for the further development of landscape ecotoxicology and multi-scale approaches, which would enhance our understanding of pollutant transfer and effects in ecosystems

Contamination, risk, and source apportionment of potentially toxic microelements in river sediments and soil after extreme flooding in the Kolubara River catchment in Western Serbia

Climate change is contributing to an increase in extreme weather events. This results in a higher river flooding risk, causing a series of environmental disturbances, including potential contamination of agricultural soil. In Serbia, the catastrophic floods of 2014 affected six river basins, including the Kolubara River Basin, as one of the larger sub-catchments of the large regional Sava River Basin, which is characterized by large areas under agricultural cultures, various geological substrates, and different types of industrial pollution. The main aim of this study was to establish the sources of potentially toxic elements in soil and flood sediments and the effect of the flood on their concentrations. Field sampling was performed immediately after water had receded from the flooded area in May 2014. In total, 36 soil samples and 28 flood sediment samples were collected. After acid digestion (HNO3), concentrations of the most frequent potentially toxic elements (PTE) in agricultural production (As, Cd, Cr, Cu, Ni, Pb, Zn) and Co which are closely related to the geological characteristics of river catchments, were analyzed. The origin, source, and interrelations of microelements, as well as BACKGROUND: values of the PTE of the river catchment, the pollution index (Pi), enrichment factor (Ef), and geological index (Igeo), were determined, using statistical methods such as Pearson correlations, principal component analysis (PCA), and multiple linear regression (MLRA). The content of the hot acid-extractable forms of the elements, PCA, and MLRA revealed a heavy geological influence on microelement content, especially on Ni, Cr, and Co, while an anthropogenic influence was observed for Cu, Zn, and Cd content. This mixed impact was primarily related to mines and their impact on As and Pb content. The pseudo-total concentrations of all the analyzed elements did not prove to be a danger in the catchment area, except for Cu in some samples, indicating point-source pollution, and Ni, whose pseudo-total content could be a limiting factor in agricultural production. For the Ef, the Ni content in 59% soil and 68% flood sediment samples is classified into influence classes. The similar pseudo-total contents of the elements studied in soil samples and flood sediment and their origin indicate that the long-term soil formation process is subject to periodic flooding in the Kolubara River Basin without any significant changes taking place. This implies that floods are not an endangering factor in terms of the contamination of soil by potentially toxic elements in the explored area

Evaluation of metals that are potentially toxic to agricultural surface soils, using statistical analysis, in northwestern Saudi Arabia

© 2015, Springer-Verlag Berlin Heidelberg. Heavy metals in agricultural soils enter the food chain when taken up by plants. The main purpose of this work is to determine metal contamination in agricultural farms in northwestern Saudi Arabia. Fifty surface soil samples were collected from agricultural areas. The study focuses on the geochemical behavior of As, Cd, Co, Cr, Cu, Hg, Pb and Zn, and determines the enrichment factor and geoaccumulation index. Multivariate statistical analysis, including principle component analysis and cluster analysis, is also applied to the acquired data. The study shows considerable variation in the concentrations of the analyzed metals in the studied soil samples. This variation in concentration is attributed to the intensity of agricultural activities and, possibly, to nearby fossil fuel combustion activities, as well as to traffic flows from highways and local roads. Multivariate analysis suggests that As, Cd, Hg and Pb are associated with anthropogenic activities, whereas Co, Cr, Cu and Zn are mainly controlled by geogenic activities. Hg and Pb show the maximum concentration in the analyzed samples as compared to the background concentration

Rhizosphere pH dynamics in trace-metal-contaminated soils, monitored with planar ph optodes

The present study presents new insights into pH dynamics in the rhizosphere of alpine pennycress (Noccaea caerulescens (J. Presl & C. Presl) F.K. Mey), maize (Zea mays L.) and ryegrass (Lolium perenne L.), when growing on three soils contaminated by trace metals with initial pH values varying from 5.6 to 7.4. The pH dynamics were recorded, using a recently developed 2D imaging technique based on planar pH optodes. This showed that alpine pennycress and ryegrass alkalinized their rhizosphere by up to 1.7 and 1.5 pH units, respectively, whereas maize acidified its rhizosphere by up to -0.7 pH units. The alkalinization by the roots of alpine pennycress and ryegrass was permanent and not restricted to specific root zones, whereas the acidification along the maize roots was restricted to the elongation zone and thus only temporary. Calculations showed that such pH changes should have noticeable effects on the solubility of the trace metal in the rhizosphere, and therefore on their uptake by the plants. As a result, it is suggested that models for trace metal uptake should include precise knowledge of rhizospheric pH conditions

Intérêt de différents réactifs d'extraction chimique pour l'évaluation de la biodisponibilité des métaux en traces du sol

L’évaluation de la biodisponibilité des métaux en traces du sol intéresse deux grands domaines d’application : d’une part, le diagnostic de fertilité chimique basé sur l’établissement de seuils de carence, employé depuis plusieurs décennies dans différents pays ; d’autre part, l’estimation du risque de phytotoxicité ou de contamination de la chaîne alimentaire qu’entraîne la pollution du sol par les éléments en traces. Dans ce cas, très peu de pays sont allés jusqu’à l’élaboration de références de diagnostic. Afin de guider le choix d’une méthode d’extraction chimique pour permettre l’ébauche de telles références en France, une synthèse bibliographique a été entreprise. Elle reprend les principaux résultats obtenus depuis les vingt dernières années concernant l’évaluation de la biodisponibilité de Cd, Cu, Zn, Pb, Cr et Ni. De cette étude, il ressort que les solutions salines non tamponnées semblent les mieux adaptées à l’estimation rapide du transfert des éléments du sol aux végétaux et à la mise au point de valeurs guides permettant de statuer quant aux risques de toxicité susceptibles d’être engendrés par des sites pollués.The prediction of soil trace metal bioavailability using extractions has two direct applications: i) evaluation of soil chemical fertility and nutrient deficiency, as has been used widely in different countries for many years; and ii) risk assessment of phytotoxicity and contamination of the food chain induced by polluted soils. In this latter case, few countries have defined guide values. In order to choose one of the extraction methods proposed in the literature, and then define such references for France, a review of research concerning the chemical estimation of Cd, Cu, Zn, Ni, Cr and Pb plant uptake in the last twenty years was undertaken. In conclusion, the use of unbuffered salt solutions seems to be the most suitable way to i) estimate trace element transfert from polluted soil to plant and ii) define guide values for risk assessment

A comparison between three methods for the determination of cation exchange capacity and exchangeable cations in soils

The object of this study is to compare the results obtained with three standardized methods of determination of cationic exchange capacity (CEC) and exchangeable cations (Ca, Mg, K) in soils. The three methods are based on different exchange reagents: cobalt hexamine (Cohex) trichloride, barium chloride and ammonium acetate. Exchange procedures are different as well; they are, respectively, single extraction, successive extractions and percolation. Values measured with barium and Cohex as index cations are very comparable and reveal the uniformity of the physicochemical processes at play. Concerning these processes, the two methods can be considered as equivalent. Ammonium acetate, buffering the pH of the extracts, causes significant variations in the proportion of negatively charged sites and particularly those bonded to organic matter. This produces large divergences from CEC values obtained with the two other methods. Nevertheless, it is possible to reduce these differences for a given soil sample, by considering its pH and organic carbon content. Quantities of calcium and potassium extracted may also change due to the dissolution of carbonated species and ammonium ability for exchanging fixed potassium.Comparaison de trois méthodes de détermination de la capacité d'échange et des cations échangeables des sols. L'objet de cet article est de comparer les résultats obtenus au moyen de trois méthodes normalisées de détermination de la capacité d'échange cationique des sols (CEC) et des cations échangeables (Ca, Mg. K). Les méthodes examinées utilisent des réactifs d'échange différents : le chlorure de cobaltihexammine, le chlorure de baryum et l'acétate d'ammonium. Les procédures d'échange varient également : ce sont, la simple agitation, les extractions successives et la percolation. Les teneurs mesurées avec le chlorure de baryum et le chlorure de cobaltihexammine sont étroitement corrélées et montrent l'uniformité des processus physicochimiques mis en jeu. De ce point de vue, les deux méthodes peuvent être considérées comme équivalentes. L'acétate d'ammonium, en tamponnant le pH du milieu d'extraction, induit des variations importantes dans les proportions de sites électronégatifs et en particulier, ceux liés à la matière organique. La prise en compte du pH et de la teneur en carbone des échantillons nous a cependant permis de réduire les différences entre méthodes. Les quantités de calcium et de potassium extraites varient également à cause de la dissolution d'espèces carbonatées et l'aptitude particulière de l'ammonium à s'échanger avec le potassium

Determination of cation exchange capacity and exchangeable cations in soils by means of cobalt hexamine trichloride. Effects of experimental conditions

Different ways of using solutions of cobalt-hexamine (Cohex) trichloride, [Co(NH3)6]Cl3, to deterr the cation exchange capacity (CEC) and the quantity of exchangeable cations in soil samples are compared. F various techniques for measuring the residual concentration of Cohex ions left in solution after the exchange reac with soil are examined. Cobalt can be determined by flame atomic absorption spectrometry and ammonium nitro by spectrocolorimetry. However, a direct spectrocolorimetric determination of the coloured Cohex ions at 425 proves to be easier to carry out with a good repeatability, provided that non-specific absorptions have been prop corrected in advance. That method was retained to compare three procedures of ion exchange: single extract successive extractions and percolation. The latter study allows us to chose a particular procedure with a knowledg its limitations, advantages and disadvantages. In spite of a slightly lower reaction yield, results obtained with a si extraction remain quite appropriate to quantify the magnitude of the exchange reactions.Détermination de la capacité d'échange et des cations échangeable dans les sols avec le chlorure de cobaltihexammine. Choix des conditions expérimentales. On décrit et compare différentes manières d'utiliser des solutions de chlorure de cobaltihexammine [Co(NH3)6]CI3 pour mesurer la capacité d'échange cationique (CEC) et les quantités de cations échangeables d'un échantillon de sol. On décrit dans un premier temps différentes techniques de dosage de l'ion cobaltihexammine en solution. Le cobalt peut être dosé par absorption atomique de flamme et l'ion ammonium par spectrocolorimétrie. Néanmoins, la spectrocolorimétrie directe des solutions de cobaltihexammine présente les avantages d'une grande simplicité de mise en oeuvre et d'une bonne répétabilité, sous réserve que l'on s'affranchisse correctement des absorptions non spécifiques. Elle a été retenue pour comparer différentes procédures d'échange entre l'échantillon de sol et la solution de réactif : la simple agitation, les extractions successives et la percolation. Cette étude, en décrivant l'influence de certains paramètres expérimentaux importants, permet de justifier le choix d'une procédure particulière en précisant ses limites d'utilisation, ses avantages et ses inconvénients. Malgré un rendement légèrement plus faible, les résultats obtenus avec une simple agitation restent tout à fait appropriés pour mesurer l'amplitude des réactions d'échange