Environmental and human health risks of mercury released from artisanal gold mining in Tanzania

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Date

2008

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Ghent University

Abstract

Artisanal gold mining is one of the major sources of mercury (Hg) contamination in Tanzania. Whilst the gold extraction process (known as amalgamation) is a simple technology, it is potentially very harmful to the environment and can contaminate air, soil, rivers and lakes with mercury. High levels of environmental mercury can cause harm to organisms living in the various environmental compartments. Also the health of the miners and other people living within the contaminated area may be adversely affected through inhalation of mercury vapour, direct contact with mercury, through eating Hg contaminated fish and other food items, or through ingesting Hg contaminated water. The Mugusu mine, which is privately owned by Chipaka family, is one of the active artisanal gold mines in Geita district, Tanzania. The current status of mercury use, environmental contamination and the potential risks of Hg to humans and the environment at the Mugusu mine arc not well understood. This doctoral study aimed at using the Mugusu mine as a model to asses the risks of Hg use in artisanal gold mining in Tanzania. In this thesis, the current status of mercury use and environmental contamination at Mugusu mine was established. The knowledge and perception of miners about the possible effects of mercury to the environment and human health were explored using structured questionnaire and focused group discussion. Based on chemical analysis and ecotoxicity testing the possible ecological effects to the aquatic ecosystem of wastes released from artisanal mine was studied. The potential health risk to humans was assessed by estimating mercury exposure of women residing in a fishing village near Mugusu mine through fish consumption. Concentrations of mercury in muscle and liver tissues of free grazing cattle and scavenging domestic fowl in gold mining centres were determined and used to assess the potential contribution of this exposure route to the Hg intake. Additionally, a total diet study approach was used to estimate the dietary mercury intake of people living and working in the Mugusu village. Finally, through head hair monitoring we established the current exposure of mercury to artisanal gold miners at the Mugusu mine. This study identified that intensive use of mercury in gold extraction and the associated environmental pollution is occurring at Mugusu mine. The current mining practice involves excavation of gold rich ore by using hand hoes, chisels and sledge hammers. After removing the mineralized bearing material, the ore is grinded in the ball mills and soil concentrate is recovered in sluice boxes which are lined with sisal sacks or any other fibre materials. The soil concentrate obtained is subsequently amalgamated with mercury to trap fine gold from the ore pulp. A piece of cloth is used to squeeze off excess mercury’ by twisting it and leaving behind the amalgam. Once the amalgam is obtained it is burned in open air to release Hg. Retorts arc not used, neither is any other form of protection against mercury contamination and/or exposure. Most miners lack knowledge on the possible environmental and human health problems associated with mercury release and exposure. Amalgamation mainly takes place adjacent to the river Mabubi that in turn drains into Lake Victoria, thus potentially contaminating Lake Victoria resources. Results from the present study show that use of mercury in gold mining contaminates nearby water resources and soils. To perform an exploratory’ environmental risk assessment, selected environmental compartments were studied. Analysis of water and sediment samples collected from the river Mabubi demonstrated that Hg concentrations in sediment and water decreased towards the river mouth. The highest concentrations of Hg in sediment (2.3 pg g’1 dw) were measured three kilometres downstream of the mine. As in previous studies in the Lake Victoria goldfields, we showed that the distribution of Hg in river sediments away from the mine is relatively restricted. Probably Fe-rich laterites and seasonal swamps act as natural barriers restricting the movement of Hg downstream into the Lake Victoria. The soil samples collected near the mine contained up to 1.2 pg g’1 dw. In the ecotoxicity assessment, sediments collected up to six kilometres downstream of Mugusu mine significantly reduced survival and growth of the African catfish (Clarias gariepinus). The calculated 5 day-LCso value (larval survival) was 1.75 pg Hg g'1 dw (95% CL of 0.72 - 2.53), the 5 day-NOEC for hatching was >2.3 pg Hg g‘‘ dw and that for larval survival and growth was 0.23 pg Hg g1 dw. These results were in agreement with results obtained from laboratory toxicity evaluations using mercury’ spiked artificial sediments and the benthic invertebrate Chirotiomus riparius. Indeed, the emergence of C. riparius midges was significantly delayed at Hg concentrations of 0.93 pg Hg g 1 dw. Mercury affected some life characteristics of C. riparius at concentrations lower than the levels of mercury which have been measured in different artisanal gold mining impacted watersheds in Tanzania. For example, in our assays Hg delayed emergence at concentrations that was 2.5 times lower than those measured three kilometres downstream of the mine in the Mabubi river. Overall, the chemical analysis and the ecotoxicity results from this study indicate that sediments downstream of the mine in the Mabubi river and probably in other artisanal gold mining impacted watersheds in Tanzania pose a risk to aquatic fauna. To assess the risk associated with Hg to the local human population the Hg intake through consumption of Hg contaminated fish, meat and other food types was estimated. Additionally, head hair analysis was done to establish the concentrations of Hg in the body of people living and working in Mugusu gold mining village. Results from the fish consumption survey indicate that, the main fish caught and consumed by the people residing in the Nungwe village that borders the Mugusu mine included nile perch (Lates niloticus), tilapia (Oreochromis niloticus, catfish (Clarias gariepinus), sardine (Rastrineobola argentea), and lung fish (Protopterus aethiopicus). Catfish contained marginally (0.51 ± 0.17 pg g’1 ww) higher concentration of mercury than the WHO safety limit of 0.5 pg Hg g'1 ww; the four other species had Hg levels below this safety limit. Because of high fish intake (144 g/day), the calculated weekly McHg intake in this village (4.0 pg/kg bwt/day) was above the Provisional Tolerable Weekly Intake (PTWI) recommended by WHO/FAO for protection of the unborn child and young children (1.6 pg/kg bwt/weck). On the other hand, the total diet study showed that daily Hg dietary intake for people living and working within the Mugusu mining village exceeded the provisional tolerable daily intake recommended by WHO/FAO for protecting unborn children (15.5 pg/day) by a factor of two. Thus, children born to mothers living in this village arc at risk of intrauterine mercury poisoning. The contribution to overall Hg exposure of meat obtained from cattle and domestic fowl reared in the mining villages was low. Indeed through analysis of muscles obtained from these animals we established that Hg levels in these products were rather low and ranged from 22 to 81 pg/kg ww. These Hg concentrations are far lower than the international safety limits of 500 pg/kg ww. In general, chemical analysis of Hg in head hair collected from people working and living in the Mugusu mining village indicated low exposure. The mean concentration (1.55 ± 2.2 pg g’1 dw) was lower than the warning limit (10 pg Hg g’dw) established by WHO for pregnant women. However, hair obtained from individuals involved in amalgam roasting contained significantly more Hg than those involved in other mining activities. Compared with studies performed ten years ago, the mercury concentrations in the hair of miners at Mugusu mine have slightly increased. In conclusion, this doctoral study has demonstrated that, artisanal gold mining at the Mugusu mine causes contamination of all the environmental compartments which lead to increased risks to the environment and human health. The current level of Hg concentrations in the rivers and streams draining the Mugusu mine into Lake Victoria pose risk to aquatic biota. Based on the survey conducted in this study it is clear that miners lack clear knowledge and understanding of the possible effects of Hg on the environment and human health. Furthermore, based on the current local food consumption pattern and Hg concentrations in different food items it is concluded that there is no risk to the general public but there is a health risk to unborn children. It is therefore recommended that more efforts are made (1) to reduce the use of Hg in gold mining and (2) to educate the miners and surrounding communities on the effects of Hg on the environment and human health.

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Keywords

Gold mining, Tanzania, Ghent University, Amalgamation

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