Impacts environnementaux liés à l'utilisation du mercure lors de l'exploitation artisanale de l'or dans la région de Kédougou (Sénégal oriental)

L'objectif de l'étude est d'évaluer les impacts environnementaux liés à l'utilisation du mercure lors de l'exploitation artisanale de l'or. Les analyses ont permis de révéler que malgré son introduction très récente dans la région de Kedougou, les quantités de mercure mesurées dans les sédiments, les sols et l'eau dans les sites d'exploitation d'orpaillage dépassent de loin les normes internationales et commencent à induire des effets réels sur l'environnement, la faune et menace la santé des populations. Les mollusques présentent des teneurs en mercure au-dessus de la norme de l'OMS, alors que les poissons révèlent des teneurs en-dessous de cette norme. 30% de la population locale des sites d'orpaillage présentent des teneurs en mercure au niveau des cheveux dépassent la concentration de référence édictée par USEPA. Ce travail a permis aussi de mettre en évidence des bactéries ayant acquis des gènes de résistance au mercure dans les sédiments des sites d'orpaillage

Quantification and characterization of mercury resistant bacteria in sediments contaminated by artisanal small-scale gold mining activities, Kedougou region, Senegal

This study describes Hg-resistant bacterial present in the aquatic sediments artisanal small-scale gold mining ASGM activities along Gambia River Kedougou, Senegal. Mercury (Hg) is used for gold amalgamation in artisanal small-scale gold mining (ASGM) activities. The level of total Hg in sediment samples was determined by automatic mercury analyser. Bacterial (colony-forming units) susceptibility to Hg was evaluated by minimal inhibitory concentrations. The phylogenetic diversity analysis of the Hg-resistant bacteria was performed by PCR amplification of 16S rDNA on isolated bacterial strains, followed by restriction fragment length polymorphism, cloning and sequencing. The results documente high concentrations of Hg in ASGM activity areas, ranging between 2.4 to 6.2 mg kg-1. Population densities of heterotrophic bacteria in wet sediment ranging from 3.7x106 to 4.6x108 CFU g-1. The isolated bacterial strains from highly Hg-contaminated sites can grow to medium containing up to 17 mg L-1 of Hg2+. In this study, bacterial strains resistant to Hg are Stenotrophomonas maltophilia, Dyella ginsengisoli, Arthobacter defluvi, Arthobacter pascens, Bacillus firmus and Pseudomonas moraviensis. Our results demonstrate the occurrence the presence of diverse groups of bacterial strains resistant to metal (Hg) under tropical conditions. The isolated strains are particularly interesting for further studies to evaluate their role in bioremediation of Hg in contaminated aquatic ecosystems

Impact of recent artisanal small-scale gold mining in Senegal: Mercury and methylmercury contamination of terrestrial and aquatic ecosystems

In Senegal, the environmental impact of artisanal small-scale gold mining (ASGM) using mercury (Hg) is poorly documented despite its intensification over the past two decades. We report here a complete dataset including the distribution and speciation of Hg in soil, sediment, and water in pristine and ASGM impacted sites of the Gambia River ecosystem (Kedougou region - eastern Senegal). Selective extraction showed that soils surrounding ASGM activities were contaminated with elemental Hg [Hg(0)] at concentrations up to 3.9 mg kg−1. In the Gambia River, high total Hg (THg: 1.16 ± 0.80 mg kg−1) and methylmercury (MeHg: 3.2 ± 2.3 ng g−1) were also measured in sediment samples collected at ASGM sites. Along the stream, THg concentrations in sediment decrease with distance from the ASGM sites, while those of methylmercury increase downstream. The study of THg and MeHg partitioning between filtered surface water and suspended particles demonstrate that particulate transport is responsible for the downstream dissemination of the Hg contamination from ASGM sites. Sedimentation of fine particles enriched in Hg downstream ASGM sites likely favors MeHg production and accumulation in sediment. Although elemental Hg is weakly labile, surface soil erosion may also provide important and long-term Hg inputs to downstream aquatic ecosystems, where it can be oxidized and methylated. Finally, the dissemination of THg and MeHg downstream from the ASGM sites in the Gambia River may constitute a long-term source of contamination and can have a large scale impact on the aquatic ecosystem through biomagnification

Human exposure to mercury in artisanal small-scale gold mining areas of Kedougou region, Senegal, as a function of occupational activity and fish consumption

We investigated mercury (Hg) exposure of food web and humans in the region of Kedougou, Senegal, where Hg is used for gold amalgamation in artisanal small-scale gold mining (ASGM). For this purpose, total mercury (THg) concentration was determined in eight fish species and two shellfish species from Gambia River and in human hair from 111 volunteers of different age and sex, living in urban locations (Kedougou and Samekouta) or in ASGM areas (Tinkoto and Bantako). THg concentrations in fish samples range from 0.03 to 0.51mgkg−1 wet weight (ww) and 0.5 to 1.05mgkg−1 ww for shellfish. THg concentrations in fish are below the WHO guideline of 0.5mgkg−1 ww, whereas 100% of shellfish are above this safety guideline. In the entire set of fish and shellfish samples, we documented a decrease of THg concentrations with increasing selenium to mercury (Se:Hg) ratio suggesting a protection of Se against Hg. However, local population consuming fish from the Gambia River in the two ASGM areas have higher THg concentrations (median = 1.45 and 1.5mgkg−1 at Bantako and Tinkoto) in hair than those from others localities (median = 0.42 and 0.32mgkg−1 at Kedougou town and Samekouta) who have diverse diets. At ASGM sites, about 30% of the local population present Hg concentrations in hair exceeding 1mgkg−1, defined as the reference concentration of Hg in hair. We also evidence a higher exposure of women to Hg in the Tinkoto ASGM site due to the traditional distribution of daily tasks where women are more involved in the burning of amalgams. The discrepancy between the calculated moderate exposure through fish consumption and the high Hg concentrations measured in hair suggest that fish consumption is not the only source of Hg exposure and that further studies should focus on direct exposure to elemental Hg of population living at ASGM sites

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

Abstract 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 analysed 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 confirms 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

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

Concentration of toxic metals and potential risk assessment in edible fishes from Congo River in urbanized area of Kinshasa, DR Congo

The objective of this research is to determine the concentration and potential health risks of toxic metals in six most consumed freshwater fish species (Auchenoglanis occidentalis, Bagrus ubangensi, Citharinus gibbosus, Distichondus fasciolatus, Marcusenius moorii and Schilbe mystus). A total of 126 samples of these fish species were collected from seven sites of Congo River in the vicinity of Kinshasa City, Democratic Republic of the Congo. Toxic metals (Cr, Co, Ni, Zn, Se, Ag, Cd, Sn, Sb and Pb) levels in fish muscle tissues were determined using Inductively Coupled Plasma Mass Spectrometry whereas Hg analysis was carried out using atomic absorption spectrophotometry. The maximum metal concentration (mg kg-1 wet weight) of Zn (59.72), Sb (5.98) and Pb (4.96) was detected in muscle tissues of Marcusenius moorii, Citharinus gibbosus and Distichondus fasciolatus respectively. Pb, Zn, and Hg values in many fish samples exceeded the permissible levels set by Food and Agriculture Organization and World Health Organization for human consumption. Given the concentrations of Pb in all fish species and Zn in Marcusenius moorii, human health implications are likely to occur on continuous consumption. Addition, the evaluation of potential health risks for fish consumers indicates that there may or not be adverse effects

Trace metal pollution in aquatic sediments and some fish species from the Kwilu-Ngongo River, Democratic Republic of Congo (Bas-Congo)

In this study, the concentrations of some metals in sediments and consumable fish from the Kwilu Ngongo River, province of Bas-Congo, Democratic Republic of Congo, are investigated. The digested muscle tissue, whole fish, and river sediment samples were subjected to trace element analysis by inductively coupled plasma mass spectroscopy and by using an automatic mercury analyzer. The highest values of trace metals in fish muscle tissue were found for Cr, Cu, Pb and Hg, with maximum values of 8.6 (Cr), 15.7 (Cu), 6.0 (Pb) and 3.3 (Hg) mg kg _1. These values are higher than maximum tolerable limits recommended by the European Commission Regulation for Food. The highest metal concentrations in sediments were 81 (Cr), 86 (Cu), 21 (Sb), 41 (Pb), and 0.8 (Hg) mg kg _1. Sediment toxicity tests using benthic crustaceans (ostracods) revealed mortality rates of up to 30% after six days of exposure to the sediments near the sugarcane industry outlet pipe discharge. The contamination of the sediments and fishes by the elements and the bio-accumulation of mercury in fish most likely arise from the adjacent agricultural fields where intensive sugarcane cultivation since 1925 has probably contaminated the soils through the long-term use of fertilizers and pesticides

Trace metal distributions in the sediments from river-reservoir systems: case of the Congo River and Lake Ma Vallée, Kinshasa (Democratic Republic of Congo)

The contamination of drinking water resources by toxic metals is a major problem in many parts of the world, particularly in dense populated areas of developing countries that lack wastewater treatment facilities. The present study characterizes the recent evolution with time of some contaminants deposited in the Congo River and Lake Ma Vallée, both located in the vicinity of the large city of Kinshasa, capital of Democratic Republic of Congo (DRC). Physicochemical parameters including grain size distribution, organic matter and trace element concentrations were measured in sediment cores sampled from Congo River (n = 3) and Lake Ma Vallée (n = 2). The maximum concentration of trace elements in sediment profiles was found in the samples from the sites of Pool Malebo, with the values of 107.2, 111.7, 88.6, 39.3, 15.4, 6.1 and 4.7mgkg−1 for Cr, Ni, Zn, Cu, Pb, As and Hg, respectively. This site, which is characterized by intense human activities, is especially well known for the construction of numerous boats that are used for regular navigation on Congo River. Concerning Lake Ma Vallée, the concentration of all metals are generally low, with maximum values of 26.3, 53.6, 16.1, 15.3, 6.5 and 1.8mgkg−1 for Cr, Ni, Zn, Cu, Pb and As, respectively. However, the comparison of the metal profiles retrieved from the different sampled cores also reveals specific variations. The results of this study point out the sediment pollution by toxic metals in the Congo River Basin. This research presents useful tools for the evaluation of sediment contamination of river-reservoir systems

Microbiological and physicochemical characterization of water and sediment of an urban river: N'Djili River, Kinshasa, Democratic Republic of Congo

Microbial and toxic metals contamination of freshwater resources is still a major problem in many parts of the world. In this study, water and sediment samples (n=9) were subjected to the microbiological and some physicochemical analysis to assess the water quality of the N'Djili River (Kinshasa, Democratic Republic of Congo). Microbiological analysis was performed for faecal indicator bacteria (FIB) including Escherichia coli (E. coli) and Enterococcus (ENT). The FIB characterization was performed for general E. coli, Enterococcus faecalis (E. faecalis) and human-specific bacteroides by PCR, using specific primers. The physicochemical parameters including pH and electrical conductivity were measured in water samples, and grain size distribution, organic matter and total mercury (Hg) were measured in sediments samples. The results revealed high concentration of FIB, with the maximum values of 1.6x103 and 2.7x103 CFU 100 mL-1 for E. coli and ENT, respectively. The FIB in sediment samples present higher concentration than in water, with maximum values of 9.4x105 and 1.2x105 for E. coli and ENT, respectively. The PCR assays for human-specific bacteroides HF183/HF134 indicated that more than 90% of bacteria were from human origin. The Hg concentration in sediment samples reaches the values of 0.5 mg kg-1. Thus, our results indicate that the uncontrolled landfills and mixing of untreated urban and industrial effluents lead to the deterioration of the water quality of the rivers traversing the economically important cities. This study represents useful tools incorporated to evaluate water and sediment quality in river systems which can be applied to similar aquatic environments