Chapter 2 Effect of recent artisanal small-scale gold mining on the contamination of
2.5 Conclusions
The aim of this study was to investigate the Hg contamination in sediments of the Gambia River impacted by ASGM activities in the region of Kedougou, eastern Senegal. Our results reveal that Hg contamination is very high in the vicinity of ASGM sites. The calculated enrichment factors in sediments confirm the severe Hg contamination in ASGM sites of Bantako 2, Dalacoye, Sabodala and Tinkoto. Additionally, our results demonstrate a seasonal variability with lower Hg concentrations in sediments during the rainy season likely
Chapter 2
related to remobilisation of surface sediments and/or dilution by fresh sediments. The variation in Hg concentrations between dry and wet seasons confirms anthropogenic introduction of this element into the ecosystem, which is the highest during the dry season when artisanal gold mining is at its maximum.
Comparison of THg concentrations in sediments collected at different sites with respect to the values recommended by MacDonald et al. (2000) for sediment quality guideline shows that the main active gold mining sites have concentrations above the PEC standards for THg, therefore they may cause toxic effects to the fish community and suggest potential health risk for local population consuming local fish.
After only about 10 years of Hg use in artisanal small-scale gold mining, environmental impacts of its abundant use are already noticeable. It is crucial to change the gold mining and ore dressing processes by promoting techniques recovering Hg, such as using retort.
Acknowledgements
The authors thank the Lombard Foundation (Geneva), Schmidheiny Foundation and Sida-UNESCO project 503RAF2000 for financing the study, and the Geological Survey of Senegal (DMG), University Cheikh Anta Diop (Dakar) and RandGold Company Senegal, for their logistic help. This is a contribution to IGCP projects 594 and 606.
2.6 References
Acevedo-Figueroa, D., Jiménez, B. D., Rodríguez-Sierra, C. J., 2006. Trace metals in sediments of two estuarine lagoons from Puerto Rico. Environmental Pollution 141, 336-342.
ANDS, 2010. Situation économique sociale et régionale. Agence National de la Statistique et de la Démographie, Service Régionale de la Statistique et de la Démographie de Kédougou.
Bassot, J. P., 1997. Albitisations dans le Paléoprotérozoïque de l'Est sénégal: relations avec les minéralisations ferriféres de la rive gauche de la Falémé. Journal of African Earth Sciences 25, 353-367.
Biney, C., Amuzu, A. T., Calamari, D., Kaba, N., Mbome, I. L., Naeve, H., Ochumba, P. B.
O., Osibanjo, O., Radegonde, V. Saad, M. A. H., 1994. Review of Heavy Metals in the African Aquatic Environment. Ecotoxicology and Environmental Safety 28, 134-159.
Boening, D. W., 2000. Ecological effects, transport, and fate of mercury : a general review.
Chemosphere 40, 1335-1351.
Burger, J., Gochfeld, M., 2006. Mercury in fish available in supermarkets in Illinois: Are there regional differences. Science of The Total Environment 367, 1010-1016.
Castilhos, Z. C., Rodrigues-Filho, S., Rodrigues, A. P. C., Villas-Bôas, R. C., Siegel, S., Veiga, M. M. Beinhoff, C., 2006. Mercury contamination in fish from gold mining areas in Indonesia and human health risk assessment. Science of The Total Environment 368, 320-325.
Cheng, J., Gao, L., Zhao, W., Liu, X., Sakamoto, M., Wang, W., 2009. Mercury levels in fisherman and their household members in Zhoushan, China: Impact of public health.
Science of The Total Environment 407, 2625-2630.
Dean, W., 1974. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. J Sediment Petrol 44, 242–248.
Donkor, A. K., Bonzongo, J. C., Nartey, V. K. Adotey, D. K., 2006. Mercury in different environmental compartments of the Pra River Basin, Ghana. Science of The Total Environment 368, 164-176.
Förstner, U. Wittmann , G. T. W., 1981. Metal Pollution in the Aquatic Environment. Berlin, Springer.
Guedron, S., Grangeon, S., Lanson, B. Grimaldi, M., 2009. Mercury speciation in a tropical soil association; Consequence of gold mining on Hg distribution in French Guiana.
Geoderma 153, 331-346.
Han, S., Obraztsova, A., Pretto, P., Choe, K.-Y., Gieskes, J., Deheyn, D. D. Tebo, B. M., 2007. Biogeochemical factors affecting mercury methylation in sediments of the venice lagoon, Italy. Environmental Toxicology and Chemistry 26.
Harada, M., Nakachi, S., Cheu, T., Hamada, H., Ono, Y., Tsuda, T., Yanagida, K., Kizaki, T.
Ohno, H.,1999. Monitoring of mercury pollution in Tanzania: relation between head hair mercury and health. Science of The Total Environment 227, 249-256.
Hissler, C., Probst, J.-L., 2006. Impact of mercury atmospheric deposition on soils and streams in a mountainous catchment (Vosges, France) polluted by chlor-alkali industrial activity: The important trapping role of the organic matter. Science of The Total Environment 361, 163-178.
Holmes, P., James, K. A. F., Levy, L. S., 2009. Is low-level environmental mercury exposure of concern to human health? Science of The Total Environment 408, 171-182.
Hylander, L. D., Meili M., Oliveira L. J., de Castro e Silva E., Guimarães J. R. D., Araujo D.
M., Neves R. P., Stachiw R., Barros A. J. P. and Silva G. D., 2000. Relationship of mercury with aluminum, iron and manganese oxy-hydroxides in sediments from the Alto Pantanal, Brazil. Science of The Total Environment 260, 97-107
Ikingura, J. R., Akagi, H., 1996. Monitoring of fish and human exposure to mercury due to gold mining in the Lake Victoria goldfields, Tanzania. Science of The Total Environment 191, 59-68.
Jewett, S. C., Duffy, L. K., 2007. Mercury in fishes of Alaska, with emphasis on subsistence species. Science of The Total Environment 387, 3-27.
Kainz, M., Lucotte, M., 2002. Can flooded organic matter from sediments predict mercury concentrations in zooplankton of a perturbed lake? Science of The Total Environment 293, 151-161.
Chapter 2
Kaisin, J., Dabo, B., Fall, M., Ndiaye, F., Barry, T.M.M., Diagne, E., 2010. Inventaire et prospection géochimique du Sénégal oriental. Ministère des Mines, de l’Industrie, de l’Agro-Industrie et des PME, Direction des Mines et de la Géologie, Dakar
Katner, A., Sun, M.-H., Suffet, M., 2010. An evaluation of mercury levels in Louisiana fish:
Trends and public health issues. Science of The Total Environment 408, 5707-5714.
Kim, N.-S., Lee, B.-K., 2010. Blood total mercury and fish consumption in the Korean general population in KNHANES III, 2005. Science of The Total Environment 408, 4841-4847.
Kojadinovic, J., Potier, M., Le Corre, M., Cosson, R. P. Bustamante, P., 2006. Mercury content in commercial pelagic fish and its risk assessment in the Western Indian Ocean. Science of The Total Environment 366, 688-700.
Li, P., Feng, X., Qiu, G., Shang,L., Wang S., 2012. Mercury pollution in Wuchuan mercury mining area, Guizhou, Southwestern China: The impacts from large scale and artisanal mercury mining. Environment International 42: 59-66.
Loizeau, J. L., Arbouillle, D., Santiago, S., Vernet, J. P., 1994. Evaluation of a wide range laser diffraction grain size analyser for use with sediments. Sedimentology 41, 353–
361.
Lusilao-Makiese, J. G., Cukrowska, E. M., Tessier, E., Amouroux, D, Weiersbye, I., 2013.
The impact of post gold mining on mercury pollution in the West Rand region, Gauteng, South Africa. Journal of Geochemical Exploration 134: 111-119.
Lynch, J.,1990. Provisional elemental values for eight new geochemical lake sediment stream sediment reference materials LKSD-1, LKSD-2, LKSD-3, LKSD-4, STSD-1, STSD-2, STSD-3, STSD-4, Geostandards Newsletter-the Journal of Geostandards and Geoanalysis 14, 153-167.
MacDonald, D. D., Ingersoll, C. G., Berger, T. A., 2000. Development and Evaluation of Consensus-Based Sediment Quality Guidelines for Freshwater Ecosystems. Archives of Environmental Contamination and Toxicology 39, 20-31.
Malm, O., Castro, M. B., Bastos, W. R., Branches, F. J. P., Guimarães, J. R. D., Zuffo, C. E.
Pfeiffer, W. C., 1995. An assessment of Hg pollution in different goldmining areas, Amazon Brazil. Science of The Total Environment 175, 127-140.
Maurice-Bourgoin, L., Quiroga, I., Chincheros, J. Courau, P., 2000. Mercury distribution in waters and fishes of the upper Madeira rivers and mercury exposure in riparian Amazonian populations. Science of The Total Environment 260, 73-86.
Ndiaye, P. M., Dia, A., Vialette, Y., Diallo, D. P., Ngom, P. M., Sylla, M., Wade, S. Dioh, E., 1997. Données pétrographiques, geochimiques et géochronologiques nouvelles sur les granitoïdes du Paléoprotérozoïque du Supergroupe de Dialé-Daléma (Sénégal Oriental): Implications pétrogénétiques et géodynamiques. Journal of African Earth Sciences 25, 193-208.
Ngom, P. M., 1985. Contribution à l'étude de la série birimienne de Mako dans le secteur aurifère de Sabodala (Sénégal oriental), Thèse Doc. Univ. Nancy, Fr., 134p.
Ntow, W., Khawaja, M., 1989. Mercury pollution in Ghana(West-Africa) coastal commercial fish. Environmental Technology Letters 10, 109-116.
Olivero-Verbel, J., Caballero-Gallardo, K., Negrete-Marrugo, J., 2011. Relationship Between Localization of Gold Mining Areas and Hair Mercury Levels in People from Bolivar, North of Colombia. Biological Trace Element Research 144, 118-132.
Osol,1998. Ordonnance du 1er juillet 1998 sur les atteintes portées aux sols (Osol). Office federal de la protection de l'environnement. 814.12.
Pardos, M., Benninghoff, C., Alencastro, L.P., Wildi, W., 2004. The impact of a sewage treatment plant’s effluent on sediment quality in a small bay in Lake Geneva (Switzerland–France). Part 1: Spatial distribution of contaminants and the potential for biological impacts. Lakes Reservoirs: Research and Management 9, 41-52.
Programme d'Appui au Secteur Minier PASMI , 2009. Cartographie géologique du Sénégal au 1/500000. Rapport final. Projet 9 ACP SE 009.
Pawlig, S., Gueye, M., Klischies, R., Schwarz, S., Wemmer, K. Siegesmund, S., 2006.
Geochemical and Sr-Nd isotopic data on the Birimian of the Kedougou-Kenieba Inlier (Eastern Senegal): Implications on the Palaeoproterozoic evolution of the West African Craton. South African Journal of Geology 109, 411-427.
Rasmussen, P. E., Villard, D. J., Gardner, H. D., Fortescue, J. A. C., Schiff, S. L. Shilts, W.
W., 1998. Mercury in lake sediments of the Precambrian Shield near Huntsville, Ontario, Canada. Environmental Geology 33, 170-182.
Reimann, C., de Caritat, P., 2005. Distinguishing between natural and anthropogenic sources for elements in the environment: regional geochemical surveys versus enrichment factors. Science of The Total Environment 337, 91-107.
Sanei, H., Goodarzi, F., 2006. Relationship between organic matter and mercury in recent lake sediment: The physical–geochemical aspects. Applied Geochemistry 21, 1900-1912.
Schumm, S.A., 1977. In The Fluvial System (Eds. J. Wiley & Sons), New York, pp. 338.
Schuster, E., 1991. The behavior of mercury in the soil with special emphasis on complexation and adsorption processes-a review of the literature. Water, Air, and Soil Pollution 56, 667-680.
Soto-Jimenez, MF, Paez-Osuna, F., 2001. Distribution and normalization of heavy metal concentrations in mangrove and lagoonal sediments from Mazatlan Harbor (SE Gulf of California). Estuar Coast Shelf Sciences 53, 259-274.
Sunderland, E. M., Krabbenhoft, D. P., Moreau, J. W., Strode, S. A. Landing, W. M.,2009.
Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models. Global Biogeochemical Cycles 23, GB2010.
Sylla, M., Ngom, P. M., 1997. Le gisement d'or de Sabodala (Sénégal Oriental): Une minéralisation filonienne d'origine hydrothermale remobilisée par une tectonique cisaillante. Journal of African Earth Sciences 25, 183-192.
Tomiyasu, T., Kono Y., Kodamatani H., Hidayati N. and Rahajoe J. S., 2013. The distribution of mercury around the small-scale gold mining area along the Cikaniki river, Bogor, Indonesia. Environmental Research 125: 12-19
van Straaten, P., 2000. Mercury contamination associated with small-scale gold mining in Tanzania and Zimbabwe. Science of The Total Environment 259, 105-113.
Chapter 3
Chapter 3
Mercury distribution and speciation in soil profiles and various forms of Hg in water around artisanal small-scale gold mining areas in Kedougou region, Senegal
Birane Niane 1, Stéphane Guédron 2, Robert Moritz 1, Papa Malick Ngom 3, Hans- Rudolf Pfeifer 4, John Poté 1
1Earth and Environmental Sciences, University of Geneva, rue des Maraîchers 13, CH-1205 Geneva, Switzerland
2 Institut des Sciences de la Terre Université Joseph Fourier, F-38041, Grenoble, France
3 Département de Géologie, Université Cheikh Anta DIOP, Dakar, Sénégal
4 Institut des Dynamiques de la Surface Terrestre, Université de Lausanne, Geopolis, CH-1015 Lausanne, Switzerland
Intended to be submitted to Geoderma
We report the first study focused on the distribution and speciation of mercury (Hg) in soil and water in the ecosystem of Gambia River in Kedougou region of eastern Senegal. Total Hg concentrations (THg) and Hg speciation were determined in soil and water at sites affected by artisanal small-scale gold mining (ASGM) and were compared to sites devoid of mining and ore dressing activities, not subject to anthropological influence in the Kedougou region. Total Hg concentrations in soils in sites without mining range between 7 µg kg-1 and 60 µg kg-1, with a mean value of 15 µg kg-1, which is slightly lower than THg contents in soil on a worldwide comparison basis. In contrast, soil in mining sites reveals THg concentrations ranging between 130 µg kg-1 and 3900 µg kg-1, which also yields high concentrations of elemental Hg (Hg0), but a very low soluble + exchangeable Hg fraction between 1 and 6 µg kg-1, suggesting low Hg soil mobility and bioavailability. This low water soluble + exchangeable fraction is very important from an environmental risk point of view due to its easy availability in environmental weathering conditions. The presence of higher amounts of Hg0 than the leached fraction from the soil suggests that Hg is almost immobile in the soil of our studied system. THg contents of 5.8 to 973.8 ng l-1, dissolved THg of 5.6 to 33.8 ng l-1, and methylmercury (MeHg) concentrations of 0.1 to 4.6 ng l-1 were measured in water samples, confirming the Hg-contamination and active methylation of the ecosystem. With the high THg concentrations found in water and the low leachable fraction in soil, the main environmental risk for the aquatic ecosystems during rain events is the transfer of Hg-rich particles from the downslope soil and waste of ASGM activities.
Keywords: Soil, Kedougou region, artisan small-scale gold mining, methylmercury
Chapter 3
3.1 Introduction
Mercury (Hg) is a ubiquitous toxic element present at trace concentrations in the environment, with a global soil average of 0.03 mg kg−1 and a range between 0.01 and 0.2 mg kg−1 (Adriano, 2001). Mercury exists in various chemical forms: elemental (Hg0), organic (MeHg) and inorganic (Hg2+). Methylmercury (MeHg) is the most dangerous species because of its bioavailability, its toxicity and its biomagnification in the aquatic food chain (Ullrich et al., 2001).
In addition to natural sources such as volcanic activity, Hg has been increasingly released in the environment by human activity, particularly by artisanal small-scale gold mining (ASGM) using Hg-amalgamation for gold recovery. According to Wade (2013), ASGM has become the world's leading source of mercury pollution, poisoning air, rivers, and people. During gold recovery processes, a large quantity of Hg is lost directly to the surrounding environment or emitted through the atmosphere (Guédron et al., 2009). Once Hg0 has been introduced into aquatic environments it can be converted to MeHg by bacterial action in the anoxic spaces surrounding aquatic plant roots (Mauro et al., 2001).Mercury can undergo changes in speciation that are either physicochemical or biologically induced, which results in changes in solubility, toxicity, and bioavailability (Biester et al., 2002). Mercury is bound to soil and sediment in various forms and to different constituents, representing different fractions of Hg. Speciation of Hg is fundamental, because it controls Hg solubility, reactivity, bioavailability, and therefore toxicity (Stein et al., 1996). Thus, understanding Hg speciation is essential to estimate the potential for Hg transformation processes such as methylation and to evaluate environmental risks. Numerous studies showed that the transport of contaminants through soil depends on physical–chemical characteristics both of the contaminants and of the site, such as soil type, soil heterogeneity, geochemical environment, and moisture content (Charbeneau et al., 2003; Guédron et al., 2009; Reis et al. 2014). Several studies have reported the dramatic Hg pollution of ecosystem compartments in the immediate neighbourhood of several mining districts over the world (Grimaldi et al., 2008; Ntow and Khawaja, 1989; Roulet et al., 1998;
van Straaten, 2000). However in Senegal, despite the intensive activities of ASGM recorded since 1995 in Kedougou region, only a few studies so far aimed at determining the effects of gold mining on ecosystems along the Gambia River basin (Niane et al., 2014). The present work is the first study addressing the pollution by Hg of soils and water affected by ASGM in the Gambia River basin at Kedougou, eastern Senegal. The aim of this study was to investigate mercury distribution and speciation through soil profiles and various forms of Hg in water around ASGM sites in the Kedougou region.
Kedougou region is situated on the eastern side of the Paleoproterozoic Kedougou-Kéniéba inlier, which covers eastern Senegal and western Mali (Fig. 3.1). The regional geology is dominated by so called Birimian volcano-sedimentary rock formations (also extending over to Ivory Coast, Burkina Faso and Ghana). According to Bassot (1987), the Birimian domain of the Kedougou-Kenieba inlier is subdivided into a western Mako supergroup (granite-greenstone belt) and an eastern Dialé-Daléma supergroup (sedimentary basin), separated by a regional-scale shear zone known as the Main Transcurrent zone. These rock formations are well known for their locally, high grade, shear zone-hosted auriferous mineralization in West Africa such as Ghana, Ivory Coast, Burkina Faso, and Mali (Lawrence et al 2013).
Figure 3.1: Geological map and location of the two sampling sites for soil modified after Niane et al. (2014).
According to Ngom and Sylla 1997, there are two types of gold mineralization in this region: primary gold lode deposits occur in basement rocks and rivers contain abundant alluvial
Chapter 3
gold. Mining activities take place along the Gambia River and inland around temporary water ponds.
3.3 Sampling and analysis 3.3.1 Soil and water sampling
Soil was sampled in two different locations along Gambia River, one in Samekouta, which is considered as the control site, where there is no mining, and a seasonal water local pond at Tinkoto, which is a producing ASGM site (Fig. 3.1). Three soil profiles were sampled at each location along a slope: Tinkoto 1, 2 and 3, and Samekouta 1, 2 and 3. The numbers 1, 2 and 3 refer to top soil at the upper location of the slope, the middle part of the slope and the lowermost location of the slope, respectively. In each profile location, soil samples were taken every 10 cm from the surface down to a maximum depth of 1.2 m, using an auger, and collected in sterile polyethylene bags. Soil profiles of less than 50 cm can be explained by the position of the bedrock in these profiles.
Water sampling was performed in the two locations described above, as well as in Mako, Bantako and Sabodala (Fig. 1), using ultra-clean techniques following the description by Cossa and Gobeil (2000). All materials in contact with samples were acid-washed during 5 days in 50%
Normapur HNO3 v/v, then for 3 days in Normapur HCl 10% v/v, and rinsed with ultrapure Milli-Q
® water. Unfiltered surface water samples for total mercury and total methylmercury analyses were collected in 250 ml Teflon bottles and acidified with HCl (0.5% v/v, Millipore Seastar; Parker and Bloom, 2005), while dissolved mercury (THg)D and dissolved methylmercury ((MeHg)D) samples were filtered with Sterivex®-HV 0.45 μm sterile filters. The difference between THg and dissolved Hg is termed particulate Hg.
3.3.2 Granulometry and chemical analysis
The particle size distribution was measured with a laser granulometer (model LS-100, Fullerton) following dispersion during 5 min by sonication in deionized water (Loizeau et al., 1994). The organic matter content (OM) was estimated by loss on ignition (LOI) by heating the sediment samples at 550 °C for 1 h (Dean, 1974).
Fe was determined by inductive coupled plasma-mass spectroscopy (ICP-MS - Agilent 7700x series). Total organic carbon (TOC) and sulfur (S) concentrations in soil were determined from the dry combustion of soil sample aliquots using a Fisons® (model 1500 CHNS) infra-red analyser. Water pH and Eh were determined in situ during sampling. Dissolved organic carbon
Geneva, Switzerland. The bulk chemical analysis of the soil was conducted using a dispersive-X-ray fluorescence spectrometer (WD-XRF) at the University of Lausanne, Switzerland.
3.3.3 Mercury and methylmercury analysis
The concentration of total mercury (THg) in soil samples was determined by atomic absorption spectrophotometry with catalytic decomposition and gold amalgamation (Guédron et al., 2009) with an automatic mercury analyser (Model AMA 254 Altec). A relative precision of ±10%
was determined from triplicate analyses. The accuracy of the measurements was obtained by repeated analyses of the certified reference material MESS-3 (0.091 ± 0.008 μg g−1). The measurement error was usually about 5% and always below 10%. The detection limit, defined as three times the standard deviation of the blank was 0.005 μg g-1.
According to Santos-Francés et al. (2011), three chemical extractions were chosen to selectively extract the main Hg carrier phases reported in the literature for tropical soils: elemental (Hg0), soluble + exchangeable, bound to organic matter. Hg0 in soil samples was evaluated by means of a pyrolysis technique, following the method of Biester and Scholz (1997). The difference in the Hg concentration between the raw and heated samples is defined as elemental Hg. The exchangeable Hg fraction in soil samples was determined following the DIN V 19730 (1995) procedure. In this method, a soil/extractant ratio of 1 g/2.5 mL is stirred for 2 h using 1 M NH4NO3
as extractant agent. Organic-bound Hg was extracted following the method of Wang et al. (2003), in which a 1% KOH solution is added to the residual of the previous step and shaken for 30 min and centrifuged. Hg concentrations in the corresponding extracts, THg and MeHg in water samples were determined after ethylation by GC-separation and thermo-decomposition by fluorescence spectrometer (GC-CVAFS, Merx BrooksRand). Quality (QA/QC) was checked using certified reference materials (i.e. Mess-3; National Water Research Institute, Canada).
Statistical treatment of data (Pearson product moment correlation) was made using SigmaStat 11.0 (Systat Software, Inc., USA).
3.4 Results and discussion 3.4.1 Soil characteristics
Table 3.2, table 3.2 of supplementary informations (SI) contain the bulk compositional data.
Figure 3.2 reveals noticeable negative correlations between Al2O3 (%) and SiO2 (%), and between
Chapter 3
Fe2O3 and SiO2. These results are consistent with the dominantly sandy silty composition of soils along the profiles in each location (Fig. 3.3).
Figure 3.2: Some correlation among major elements of soils, left: Tinkoto, right: Samekouta site.
The main properties of the studied soils (Table 3.1 and SI.3.1) can be summarised as follows: 1) the dominant textures are silty-sandy; 2) the mean OM content ranges between 5.78 to