Ikingura J R, Akagi H, Mujumba J, Messo C
University of Dar es Salaam, P.O. Box 35052 Dar es Salaam, Tanzania.
J Environ Manage. 2006 Oct;81(2):167-73. doi: 10.1016/j.jenvman.2005.09.026. Epub 2006 Jun 19.
Environmental dispersion and transformation of mercury discharged from gold mining operations has been investigated in field and laboratory studies in order to provide better understanding of the degree of mercury (Hg) pollution and bioavailability in the Lake Victoria goldfields (LVGF) ecosystems. This paper reviews results already published elsewhere and presents additional data on Hg dynamics in the LVGF. Studies conducted at the Mugusu and Rwamagaza artisanal mines indicated different degrees of Hg contamination and dispersion in environmental matrices. Mercury concentration in contaminated river sediments near the Mugusu mine varied from 6.0 to 0.5 mg/kg on a dry weight basis. The highest Hg contamination levels (165-232 mg/kg) were associated with mine tailings at the Rwamagaza mine. Mercury concentrations in fish representing different dietary habits on the southwestern shore of Lake Victoria at the Nungwe Bay were very low (2-35 microg/kg) and thought to represent background levels. These and other results suggested that the use of Hg in gold extraction in the LVGF has not caused high Hg levels in lake fish. The study of Hg in lichens showed Parmelia lichen to be an effective bioindicator for atmospheric Hg contamination due to Hg emissions from gold-amalgam firing and purification operations. The Hg levels in the lichens around the Mugusu mine ranged from 3.1 to 0.1 microg/g; the highest levels were recorded in the lichens sampled close to gold-amalgam processing sites. The regional background level in the Parmelia lichen was 0.05-0.10 microg/g, with a mean level of 0.07 microg/g. Studies of Hg transformation in the mine tailings revealed unexpectedly high methylmercury (MeHg) levels in the tailings (629-710 ng/g), which indicated that oxidation and methylation of metallic Hg in the tailings occurred at significant levels under tropical conditions. Re-equilibration of the tailings with freshwater (FW) indicated the MeHg was firmly bound in the tailings and therefore very little MeHg was released to the water column (0.2-1.5 ng/L). The methylation of Hg in tropical loamy clay soil contaminated with HgCl(2) (5 mg Hg/kg) yielded MeHg concentrations of 11 and 14 ng/g when inundated with seawater and FW, respectively, for 4 weeks. Little MeHg was transferred from the soil to the equilibrated water (< or = 0.4 ng/L). Atmospheric exposure of the soil pre-inundated with FW resulted in net degradation of MeHg during the 1st week of exposure, followed by net production and accumulation of MeHg in the soil (up to 15.5 ng/g) during atmospheric desiccation. Mercury uptake by fish from the Hg(0)-contaminated aquatic sediment-tailings system in the aquarium experiment was found to be low, suggesting the low availability of MeHg for bioaccumulation in the system. These and other results provide useful insights into Hg transformation, mobility and bioavailability in tropical aquatic systems affected by Hg pollution from gold mining operations.
为了更好地了解维多利亚湖金矿区（LVGF）生态系统中汞（Hg）污染程度和生物有效性，已在实地和实验室研究中对金矿开采作业排放汞的环境扩散和转化进行了调查。本文回顾了已在其他地方发表的结果，并展示了LVGF中汞动态的更多数据。在Mugusu和Rwamagaza手工矿进行的研究表明，环境基质中汞污染和扩散程度不同。Mugusu矿附近受污染河流沉积物中的汞浓度以干重计在6.0至0.5毫克/千克之间变化。Rwamagaza矿尾矿中的汞污染水平最高（165 - 232毫克/千克）。在Nungwe湾维多利亚湖西南岸代表不同饮食习惯的鱼类中的汞浓度非常低（2 - 35微克/千克），被认为代表背景水平。这些以及其他结果表明，LVGF金矿开采中使用汞尚未导致湖鱼中汞含量升高。地衣中汞的研究表明，由于金汞齐烧制和提纯作业排放汞，梅衣属地衣是大气汞污染的有效生物指示物。Mugusu矿周围地衣中的汞含量在3.1至0.1微克/克之间；在靠近金汞齐加工地点采集的地衣中记录到最高含量。梅衣属地衣的区域背景水平为0.05 - 0.10微克/克，平均水平为0.07微克/克。尾矿中汞转化的研究揭示尾矿中甲基汞（MeHg）含量出乎意料地高（629 - 710纳克/克），这表明热带条件下尾矿中金属汞的氧化和甲基化程度显著。尾矿与淡水（FW）重新平衡表明，甲基汞牢固地结合在尾矿中，因此很少有甲基汞释放到水柱中（0.2 - 1.5纳克/升）。用HgCl₂（5毫克汞/千克）污染的热带壤土粘土在分别用海水和淡水淹没4周后，汞的甲基化产生的甲基汞浓度分别为11和14纳克/克。很少有甲基汞从土壤转移到平衡水中（≤0.4纳克/升）。预先用淡水淹没的土壤在大气暴露的第1周导致甲基汞净降解，随后在大气干燥期间甲基汞在土壤中净产生和积累（高达15.5纳克/克）。在水族箱实验中发现鱼类从受Hg(0)污染 的水生沉积物 - 尾矿系统中吸收汞的量很低，这表明该系统中甲基汞生物累积的可用性较低。这些以及其他结果为受金矿开采作业汞污染影响的热带水生系统中汞的转化、迁移和生物有效性提供了有用的见解。
