Impact de l'exploitation minière en Guyane française sur les flux de mercure vers les écosystèmes aquatiques.

Thèse co-encadrée par Michel Grimaldi (IRD, Bélèm), Laurent Charlet (CNRS/LGIT, Grenoble) et Daniel Cossa (IFREMER, La Seyne sur mer)Pedological cover of French Guiana is a storage and transformation compartment for natural and anthropogenic mercury (Hg). Geochemical background of guianese soils is naturally high (0.11-0.50 µg g-1). In the hydromorphic soils of the Combat creek flat, Hg concentrations were high (0.82 ± 0.35 µg g-1) and from former gold mining origin. The study of Hg hydro-geochemical fluxes in the Combat creek watershed has shown that soils act as sinks for inorganic Hg and source of methylmercury (MMHg). Our results demonstrate that gold mining contribute to favor geochemical conditions for Hg methylation. Only the modification of mining practices and post exploitation sites rehabilitation can reduce its impact on hydrosystems and especially on the bioaccumulation of MMHg along the trophic chains.La couverture pédologique de Guyane Française est un compartiment majeur de stockage et de transformation du mercure (Hg) naturel et anthropique. Le fonds géochimique en Hg des sols guyanais est naturellement élevé (0.11-0.50 µg g-1). Dans les sols hydromorphes du bas fond de la crique Combat, les concentrations très élevées en Hg (0.82 ± 0.35 µg g-1) sont héritées de l'orpaillage ancien. L'étude des flux de mercure dans le bassin versant de la crique Combat a montré que les sols jouent un rôle de « puits » pour le Hg inorganique et de source de mono-méthylmercure (MMHg). Nos résultats démontrent que l'orpaillage est à l'origine des conditions géochimiques qui favorisent la méthylation du Hg. Seules la modification des pratiques minières et la réhabilitation des sites après exploitation peuvent limiter la bioaccumulation du MMHg le long des chaînes alimentaires

Geochimie haute résolution sur archives naturelles continentales (sédiments, spéléothèmes, cernes d'arbres): mesures de routine et développements analytiques

PosterUn scanner de carottes XRF de type Avaatech, a été installé en mai 2012 au laboratoire EDYTEM. Les avantages du scanner XRF sont nombreux. Il permet la mesure en continu et à haute résolution (de 2cm à 100µm) de nombreux éléments majeurs et traces (0-50keV). Cette méthode ne nécessite aucune préparation préalable puisque les analyses sont effectuées directement sur les prélèvements. Elle est donc utilisée en routine comme outil de logging et comme indicateur de changements environnementaux, en utilisant notamment les éléments majeurs. Ces applications de routine sont proposées à l'ensemble de la communauté scientifique qui assure 60% du temps d'utilisation de la machine. S'il est un outil de caractérisation chimique particulièrement adapté à l'étude des séries sédimentaires lacustres et marines, le scanner de carottes peut être adapté à un grand nombre d'archives naturelles et sa plage d'utilisation en termes d'éléments chimiques peut être optimisée. C'est dans ce contexte que s'inscrit la démarche du laboratoire EDYTEM en développant les mesures par scanner XRF comme un outil d'analyse automatisée multi-archives. Les principaux travaux de développement entrepris au laboratoire s'articulent autour de plusieurs axes de recherche qui combinent à la fois un travail de calibration des paramètres de l'appareil et l'utilisation de logiciels spécifiques pour le traitement des données (PyMca, MATLAB). Les trois axes, développés à ce jour, portent sur : (i) L'augmentation de la résolution spatiale des analyses (<100µm) pour tracer l'évolution chimique annuelle voir intra-annuelle des environnements à faible taux de sédimentation. (ii) L'optimisation des mesures pour tracer les changements environnementaux enregistrés dans les échantillons à forte composante matricielle (spéléothèmes, cernes de bois) (iii) L'amélioration des seuils de détection des contaminants métalliques en traces (Pb, Zn, Hg, Ni...) à partir de différentes archives (bois, sédiment, stalagmite) afin de fournir une chronique des pollutions à l'échelle de l'écosystème

Lead and copper isotopic tracing of human exposure and ore sources during Roman mining activities in the ancient city of Vienne (Isère, France).

International audienc

Synergistic effects of mining and urban effluents on the level and distribution of methylmercury in a shallow aquatic ecosystem of the Bolivian Altiplano

International audienceLake Uru Uru (3686 m a.s.l.) located in the Bolivian Altiplano region receives both mining effluents and urban wastewater discharges originating from the surrounding local cities which are under rapid development. We followed the spatiotemporal distribution of different mercury (Hg) compounds and other metal(oid)s (e.g.{,} Fe{,} Mn{,} Sb{,} Ti and W) in both water and sediments during the wet and dry seasons along a north-south transect of this shallow lake system. Along the transect{,} the highest Hg and metal(oid) concentrations in both water and sediments were found downstream of the confluences with mining effluents. Although a dilution effect was found for major elements during the wet season{,} mean Hg and metal(oid) concentrations did not significantly differ from the dry season due to the increase in acid mine drainage (AMD) inputs into the lake from upstream mining areas. In particular{,} high filtered (<0.45 [small mu ]m) mono-methylmercury (MMHg) concentrations (0.69 +/- 0.47 ng L-1) were measured in surface water representing 49 +/- 11% of the total filtered Hg concentrations (THgF) for both seasons. Enhanced MMHg lability in relation with the water alkalinity{,} coupled with abundant organic ligands and colloids (especially for downstream mining effluents){,} are likely factors favoring Hg methylation and MMHg preservation while inhibiting MMHg photodegradation. Lake sediments were identified as the major source of MMHg for the shallow water column. During the dry season{,} diffusive fluxes were estimated to be 227 ng m-2 d-1 for MMHg. This contribution was found to be negligible during the wet season due to a probable shift of the redox front downwards in the sediments. During the wet season{,} the results obtained suggest that various sources such as mining effluents and benthic or macrophytic biofilms significantly contribute to MMHg inputs in the water column. This work demonstrates the seasonally dependent synergistic effect of AMD and urban effluents on the shallow{,} productive and evaporative high altitude lake ecosystems which promotes the formation of natural organometallic toxins such as MMHg in the water column

Algal Bloom Exacerbates Hydrogen Sulfide and Methylmercury Contamination in the Emblematic High-Altitude Lake Titicaca

International audienceAlgal blooms occurrence is increasing around the globe. However, algal blooms are uncommon in dominantly oligotrophic high-altitude lakes. Lake Titicaca, the largest freshwater lake in South America, located at 3809 m above the sea level, experienced its first recorded algal bloom covering a large fraction of its southern shallow basin in March-April 2015. The dominant algae involved in the bloom was Carteria sp. Water geochemistry changed during the bloom with a simultaneous alkalinization in heterotrophic parts of the lake and acidification in eutrophic shallow areas. A decrease in oxygen saturation (from 105 to 51%), and a dramatic increase in hydrogen sulfide (H 2 S) concentrations (from <0.02 to up to 155 µg·L −1) resulted in the massive death of pelagic organisms. Such changes were brought by the exacerbated activity of sulfate-reducing bacteria (SRB) in this sulfate-rich lake. Although levels in total mercury remained stable during the event, MMHg % rose, highlighting higher conservation of produced MMHg in the water. Such an increase on MMHg % has the potential to produce exponential changes on MMHg concentrations at the end food web due to the biomagnification process. Our physicochemical and climatological data suggest that unusually intense rain events released large amounts of nutrients from the watershed and triggered the bloom. The observed bloom offers a hint for possible scenarios for the lake if pollution and climate change continue to follow the same trend. Such a scenario may have significant impacts on the most valuable fish source in the Andean region and the largest freshwater Lake in South America. Furthermore, the event illustrates a possible fate of high altitude environments subjected to eutrophication

Effect of recent artisanal small-scale gold mining on the contamination of surface river sediment: Case of Gambia River, Kedougou region, southeastern Senegal

We report here the first inventory of mercury (Hg) contamination in the region of Kedougou, Senegal, where mercury (Hg) is used for gold amalgamation in artisanal small-scale gold mining (ASGM) activities. Sediment cores were sampled during the dry and wet seasons, and at different locations along the Gambia River in the Kedougou region to evaluate the spatio-temporal trends of Hg distribution related to ASGM activities. The sediment samples were analyzed for total mercury and trace element concentrations (Cr, Cu, Zn, Pb, As, and Sc). The results highlight high concentrations of total Hg (reaching up to 9.9 mg.kg-1) in sediment cores sampled in the vicinity of ASGM operations, with the highest values reported for the dry season. The calculation of the Hg Enrichment Factor (EF) using Sc as refractory element, confirm high enrichments around active sites of gold mining being proportional to the density of miners and the duration of mining activities. The Hg spatial distribution reveals a very local contamination and a limited downstream dispersion of the contaminant. The high Hg concentrations obtained at ASGM sites largely exceed the Sediment Quality Guidelines (SQGs) and the Probable effect concentration (PEC) for the Protection of Aquatic Life recommendation. Our results demonstrate that the ASGM activities discharged large amounts of Hg into the Gambia River ecosystem and that the accumulation of Hg in sediments may represent a significant human health risk and a potential source of toxicity for aquatic organisms

Biogeoquímica del mercurio en el lago tropical de altura Uru Uru (Altiplano boliviano)

International audienceEn la región del Altiplano boliviano, de condiciones ambientales extremas (3700 m.s.n.m. de altitud, elevada radiación UV, menor disponibilidad de O2), se encuentra el lago Uru Uru. De aguas poca profundas (0-1 m), elevada gradiente en la salinidad de sus aguas y alta productividad primaria, es impactado por descargas de afluentes mineros y urbanos. Las elevadas concentraciones de monometilmercurio (MMHg), un neurotóxico muy potente, medidas en peces y aves (Molina et al., 2012), ha cuestionando la fuente y transformaciones del Hg para la producción de MMHg en los ecosistemas de altura. La producción de MMHg es un proceso clave que dirige la bioacumulación del Hg en la cadena alimentaria y los procesos de metilación y demetilación, son los mayores patrones de transformación que regulan la producción neta de MMHg. Para dar respuesta a estas preguntas concentraciones de metales, metaloides (Fe, Mn, Sb, Ti y W), elementos mayoritarios, especies de mercurio (mercurio inorgánico (IHg), MMHg, mercurio vapor (Hg°) y dimetilmercurio (DMHg)) fueron medidas en muestras de aguas; MMHg y mercurio total (THg) en sedimentos y aguas intersticiales de sedimentos del lago Uru Uru. Al mismo tiempo se realizó un seguimiento de la distribución espacio temporal de Norte a Sur del lago, para todos estos parámetros, durante las épocas seca y húmeda (octubre, 2010 y mayo 2011). Además se determinó los potenciales de metilación (M) y demetilación (D) en situ en muestras de sedimentos, aguas, periphyton y agregados bio-orgánicos flotantes en los sitios norte (NS) y sur (SS) del lago Uru Uru, utilizando trazadores isotópicos estables de 199Hg y MM201Hg. Entre los resultados más importantes se encuentra la elevada concentración de metales pesados y de MMHg en forma disuelta, la cual llega hasta el 49 ± 11 % con respecto al mercurio total disuelto (THgd). Se identifica a los sedimentos como la fuente principal de MMHg, cuyo flujo difusivo alcanzan hasta 227 ng m-2 day-1 de MMHg durante la época seca. Esta aseveración es sustentada con la medición de los potenciales de M/D en las diferentes matrices. Los potenciales Netos de M, medidos en el sitio Norte, indican que los efluentes mineros y urbanos promueven la producción de MMHg en las aguas y en los sedimentos (3,4±1,2 ng g−1 day−1) durante la época seca. Si bien se identifica a los agregados bio-orgánicos como los mayores productores de MMHg (5,8 ng MMHg g−1 day−1, época seca), son los sedimentos los mayores representantes de producción de esta sustancia tóxica, tomando en cuenta la diferencia de masa total de cada matriz en el lago. Se determina también que el rol del periphyton de las totoras es el de la descomposición de MMHg (-2,1 ng MMHg g−1 day−1), pero también el de almacenamiento de MMHg. En suma se demuestra que existe un efecto sinérgico de los drenajes ácidos mineros (DAM) según la temporada y los efluentes urbanos en los ecosistemas productivos, al cual la evaporación superficial promueven las elevadas emisiones de contaminantes organometálicos como MMHg en la columna de agua, cuya producción y la estabilidad se nutre de la abundante materia orgánica y de los ligandos presentes en el medio

Iron and sulfur biogeochemical processes involved in mercury mobility and speciation in an artificial aquifer

Although many data are available on the Hg cycle in terrestrial surface aquatic environments, little is known on its behaviour in subsurface and deep aquifers where environmental conditions (anoxia, water saturation) may be favourable to Hg methylation. Bioavailability of Hg, a prerequisite for its methylation by sulphate or iron reducing bacteria, is mainly controlled by physico-chemical conditions and the strong affinity of Hg for organic matter or iron (oxy)hydroxides. This work presents an original experimental setup combining geochemical and microbiological approaches in order to imitate reactions observed in aquifers (i.e., Hg biosorption, solubilisation versus sequestration, speciation, dual effect of iron and sulphate). Two columns were filled in the lower half with sterile sand and in the upper half with a sterile mixture of sand and iron oxides, initially enriched with Hg(II). The water flow was ascendant. Five septa set regularly along the columns enabled water sampling from the different layers of the column without perturbing water flow or in-situ experimental conditions. After an abiotic rinsing period, the system was inoculated with a bacterial consortium and physico-chemical and microbial parameters were monitored in time and space. The inflowing groundwater was supplemented with sulphate (370 g.L-1 MgSO42-) and lactate (830 mg.L-1 sodium lactate) to encourage sulphate-reducing bacteria in the first column (A) and with molybdate (0.40 mmol.L-1) to inhibit sulphate reduction and glucose (10 g.L-1) to favour iron-reducing bacteria in the second column (B). At the end of the experiment (130 days), microbial methylation potentials were evaluated using stable isotope-spiked incubations, diversity using Denaturing Gel Gradient Electrophoresis (DGGE) followed by band sequencing and Fluorescent in situ hybridisation (FISH). Raman spectrometry was carried out to identify neo-mineral formation and selective extractions provided information on Hg distribution between the solid carrier phases. In column A microbial activity assessed by substrate consumption began after 15 days and induced a progressive decrease in the sulphate concentrations at the column outlet and the visual observation of black FeS precipitates (identified as partially oxidized makinawite). When sulphate reduction peaked after 120 days, iron was leached from the column as well as monomethylmercury (MMHg) and Hg. Bacterial diversity did not vary significantly either in time or space, sulphate reducing bacteria were observed with FISH and a sequenced DNA band bore resemblance to Desulfotomaculum reducens. In column B bacterial activity directly induced iron reduction (measured in the outlet as Fe(II)) as well as Hg leaching and MMHg formation. Shewanella sp. was observed with FISH. In both columns methylation potentials averaged 1% and selective extractions showed a shift of Hg from the ferrihydrite towards the organic matrix and newly formed FeS. These results contribute to understanding the interactions between bacteria and geochemical mechanisms controlling Hg fate and behaviour in saturated aquifers

Linking Microbial Activities and Low-Molecular-Weight Thiols to Hg Methylation in Biofilms and Periphyton from High-Altitude Tropical Lakes in the Bolivian Altiplano

International audienceThe sources and factors controlling concentrations of monomethylmercury (MMHg) in aquatic ecosystems need to be better understood. Here, we investigated Hg transformations in sediments, periphyton associated with green algae's or aquatic plants, and benthic biofilms from the Lake Titicaca hydrosystem and compared them to the occurrence of active methylating microorganisms and extracellular Hg ligands. Intense Hg methylation was found in benthic biofilms and green algae's periphyton, while it remained low in sediments and aquatic plants' periphyton. Demethylation varied between compartments but remained overall in the same range. Hg methylation was mainly carried out by sulfate reducers, although methanogens also played a role. Its variability between compartments was first explained by the presence or absence of the hgcAB genes. Next, both benthic biofilm and green algae's periphyton exhibited a great diversity of extracellular low-molecular-weight (LMW) thiols (13 or 14 compounds) present at a range of a few nmol L−1 or μmol L−1 but clearly dominated by cysteine and 3-mercaptopropionic acid. Hg methylation was overall positively correlated to the total thiol concentrations, albeit to different extents according to the compartment and conditions. This work is the first examining the interplay between active methylating bacterial communities and extracellular ligands in heterotrophic biofilms and supports the involvement of LMW thiols in Hg methylation in real aquatic systems