The Impact of Gold Mining on Mercury Pollution in the Witwatersrand Basin, South Africa E.M. Cukrowska1, J. Lusilao-Makiese1, E. Tessier2 , B. Yalala1, Hlanganani Tutu1 and Luke K Chimuka1 11School 2 of Chemistry, University of the Witwatersrand, P. Bag X3,WITS 2050, Johannesburg,South Africa Laboratory of Bioinorganic and Environmental Analytical Chemistry, CNRS, University of Pau, France E-mail: [email protected] In South Africa (SA), the largest anthropogenic point sources of mercury emissions are combustion sources followed by gold mining activities (reprocessing of old tailing dams and artisanal mining). In the year 2000, South Africa was ranked 2nd in the world for total Hg emissions and 4th for Hg emissions arising from stationary fossil fuel combustion (Pacyna et al., 2006), though recent studies critically revised previous emission inventories. Trend of global anthropogenic emissions by region based on Pirrone et al. (1996) (a), Pacyna et al. (2003) (b), Pacyna et al. (2006) (c), and Pirrone et al. (2010) (d). Data reported in Fig. d) are for most contributing countries. AF-Africa; AS-Asia; EU-Europe; NA-North America; OC-Oceania; SA-South America. The mercury speciation ABSORPTION BIOAVAILAVILITY METHYLATION TOXICITY of Hg depends on its…. chemical form Total elemental analysis is not enough to provide such information Research motivation and objectives Motivation: Most of the data on mercury in SA concerns total Hg determination only and mostly from coal fired power stations. There is a need for measured speciation data near sources of concern, in order to get a better quantitative understanding of Hg chemistry in the SA semiarid environment (atmosphere, soils, water bodies and biota). Objectives: Development, optimization and validation of reliable analytical methods for the determination of mercury species in different environmental matrices (air, water, sediments and biological materials). Assessment of the extent of Hg contamination from historic gold mining sites i.e. to characterization of specific “hot spots” and to determination of HgTOT, IHg and MeHg in water, soil, and biota. To use obtained data for understanding the biogeochemical speciation of Hg, its distribution, transport and fate in environmental compartments impacted by mineral processing and energy production in South Africa. Hg contamination from active gold mining sites Sampling locations Sediment, water and plant samples were collected near TSFs and in watersheds from goldfields within the Wits Basin. A pilot study was carried out on a few collected air samples from a closed ventilation shaft (Vaal River site) in order to assess the level of atmospheric Hg emission. Dust samples were collected from Greater Johannesburg area. Speciation of Hg in sediment cores: sample preparation procedure Derivatization 1μl of organic phase Sampling GC-ICPMS Spiking Freezing (~18C) and Dissection Data acquisition Freeze-drying and Grinding Microwave Assisted Extraction Case 1: Vaal River West Complex: a. sediments US EPA Threshold Effect Level (TEL) value for Hg in sediment: 174 ng g-1 950 – 220 ppb from dry to rainy season 120 – 1000 ppb from dry to rainy season 8345-1500 ppb From dry to rainy season 0.005-1.000 ppb in H2O From dry to rainy season 4200ppb Remobilization of Hg from the water dam to surrounding sediments. Methylmercury formation Methylation occurs at deeper layers in sediments. MeHg enrichment in the region of high IHg and low redox. There is a risk for MeHg to enter the water system! Gaseous Hg (TGM) measurements in air samples from the ventilation shaft High TGM concentrations at the outlet of the ventilation shaft Typical background TGM in pristine and open areas: 1 - 4 ng m-3 (Wangberg et al., 2008) TGM within urban areas: up to 15 - 25 ng m-3 (Wang et al., 2007) The obtained TGM concentrations are likely to trace an underground gaseous Hg source from the old mine shafts connected to the ventilation shaft. Case 2: Hg contamination from historic gold mining sites Krugersdorp Game Reserve Mining Area High Hg concentration in sediment and water! Hg concentration much higher within the mining site than in the Game reserve West Rand HgTOT 3000 [Hg] (ng g-1) 2500 2000 1500 1000 500 0 80A 86A 87 92 93 95 97 98 100 105 106 Hg speciation in Randfontein sediment West Rand sediments [Hg] (ng g-1) 2000 1500 IHg 1000 10 x MHg 500 0 86A 87 92 93 97 98 100 93 93 %MHg %MeHg 0 1 IHg (μg kg-1) [IHg] (ug/kg) 2 3 1200 0 4 8 12 8 12 16 16 20 20 93 93 %C %C %S %S 0,3 0,45 0,6 0,15 0,75 0 0 4 4 8 12 Depth (cm) Depth (cm) 3200 4 Depth (cm) Depth (cm) 2200 0 0,3 0,45 0,6 8 12 16 16 20 20 Correlation with Suphur and Carbon Hg in water samples: Typical case of AMD Hg speciation in borehole water Methylation occurs in deep water (reducive conditions) but, due to its mobility, MHg migrates to shallow levels. Case 3: Characterisation and modelling of mercury speciation in urban air affected by gold mining - assessment of bioavailability (pilot study) Investigations of atmospheric mercury have been mostly done on gaseous species. Although, to assess human expose to mercury, especially in urban areas, the inhalable dust should be included in a study. The aim of this study was to determine the magnitude of mercury pollution in this urban area and assess its bioavailability. Dust samples were collected on inhalation levels (1-2 m above a ground). They were later separated into different fractions by micro sieving. Bioavailability of mercury in inhalable dust (25 µm) was tested by leaching collected samples with artificial lung fluid (ALF, pH 4.5), Gray’s solution (pH 7.4) and water. The leaching conditions were selected to mimic lungs environment (incubator at 300C, time 24 hrs, rotation of samples 150 rpm). Total concentrations of mercury in dust fractions were also determined after microwave digestion. Sample Analysis HgTOT in dust was determined, after MW digestion, by Anodic Stripping Voltammetry with a gold rotating disc electrode (ASV-RDE) Chemical Phosphoric acid Sodium chloride Ammonium chloride Sodium dihydrogen phosphate K-acid-phthalate Calcium chloride Sodium acetate trihydrate Sodium bicarbonate Sodium citrate Sodium carbonate Sulphuric acid Glycine Citric acid Magnesium chloride Potassium chloride di-sodium hydrogen phosphate Sodium sulphate anhydrous Gray's solution (g L-1) pH 4.50 1.200 2.300 5.300 1.700 artificial lung fluid (ALF) (g L-1) pH 4.50 6.0193 - 0.200 0.290 0.580 2.300 0.590 0.630 0.510 0.450 0.420 - 0.2771 0.9526 2.6042 0.0970 0.2033 0.2982 0.1417 - 0.0710 Composition of lung leaching solutions Total mercury in dust +20 km Sandton 22 ± 1 µg kg-1 Northern Residential Area Cemetery 2011 ± 327 µg kg-1 CBD North 59 ± 6 µg kg-1 CBD 111 ± 9 µg kg-1 Tailings footprint 2081 ± 165 µg kg-1 Reprocessed tailings 3196 ± 133 µg kg-1 Mineral processing plant 1037 ± 36 µg kg-1 Industrial area 1102 ± 98 µg kg-1 Industrial area 1177 ± 91 µg kg-1 PPC cement 2265 ± 71 µg kg-1 Mining & Industrial Area Soccer City 954 ± 124 µg kg-1 Soweto 1575 ± 213 µg kg-1 Southern residential 466 ± 26 µg kg-1 > 2000 µg kg-1 2000-1000 µg kg-1 100 – 999 µg kg-1 <100 µg kg-1 Dust samples leaching results Bulk – Coarse dust HgTOT (µg kg-1) Gray’s Solution (µg kg-1) % ALF (µg kg-1) N3 fly over 37.86 ± 4.32 26.77 ± 1.14 71 23.43 ± 0.60 61.9 15.10 ± 1.35 RCS entrance 28.71 ± 1.05 20.70 ± 1.05 72 17.31 ± 0.67 60.3 11.07 ± 3.01 RCS exit 15.26 ± 3.57 11.48 ± 1.6 75 9.86 ± 3.15 64.6 7.79 ± 0.52 De Korte Str. 6.07 ± 1.15 4.87 ± 0.3 80 4.09 ± 1.73 67.3 3.23 ± 0.55 Smit Street 6.11 ± 1.80 4.92 ± 0.6 81 4.22 ± 1.16 69.1 3.25 ± 0.60 Sample ID % H2O (µg kg-1) (Mean ± % Standard deviation) Detection Limit: 6.24 ng L-1 < 25 µm particle size fraction HgTOT (µg kg-1) Gray’s Solution (µg kg-1) % ALF (µg kg-1) % H2O (µg kg-1) N3 fly over 22.77 ± 3.10 14.31 ± 7.03 63 10.86 ± 2.15 48 9.32 ± 1.31 RCS entrance 20.20 ± 3.29 16.84 ± 7.12 83 13.76 ± 5.86 68 7.89 ± 1.19 RCS exit 11.12 ± 4.57 8.24 ± 1.85 74 7.18 ± 2.12 65 5.76 ± 0.83 De Korte Str. 4.40 ± 1.56 3.46 ± 1.34 79 3.04 ± 4.15 69 2.18 ± 1.10 Smit Street 4.34 ± 2.14 3.02 ± 1.14 70 2.77 ± 1.52 64 2.18 ± 0.77 Sample ID (Mean ± % Standard deviation) Detection limit: 7.11 ng L-1 Summary The extremely high Hg concentrations found in water and sediment suggest that post-goldmining activities may be important contributors of Hg to watersheds through probable remobilization that might occur at specific hot spots on a seasonal basis. Localized point sources of Hg likely exist throughout the entire gold mining region. Methylation of Hg is occurring close to the sources, which can allow MeHg to enter the food web. These point sources offer the most treatable target areas for investigation of possible remediation projects. TGM determination has shown a probable underground pollution source. Although, further air sampling in both shafts and open sites is needed to confirm these preliminary results. Extremely high concentration levels of mercury was found in air and dust in industrial areas. Especially high levels were detected around presently reprocessed old gold tailings dumps. The levels dropped significantly in CBD area but still showing elevated concentrations. They depend strongly on prevailing wind directions, dry and wet seasons, and day time. Leaching experiments showed that the Gray’s solution extracts more mercury from the dust followed by ALF and much lower by water. This suggests that most of the mercury inhaled with dust will be extracted in the deep interstitial parts of the lungs. In the upper region of lungs extraction will occur to a lesser extent. Acknowledgements I. Weiersbye (APES, Wits University) NRF (South Africa) THRIP (South Africa) Anglo Gold Ashanti (South Africa) Thank You!