An assessment of mercury (Hg) contamination and possible human exposure and environmental damage associated with artisanal gold mining in the catchment of the Agusan River in eastern Mindanao, the Philippines, was undertaken by the British Geological Survey (BGS) in collaboration with the Philippines Mines and Geosciences Bureau (MGB). Funding was provided by the UK Overseas Development Administration (ODA) under Technology Development and Research (TDR) programme R6226: Mitigation of Mining-Related Mercury Pollution Hazards.
The study aimed to ascertain the mercury contents of stream water and stream sediment in the Agusan catchment especially in those areas which have been subject to very rapid and largely unregulated growth in small-scale artisanal gold mining over the last ten years following the discovery of rich gold deposits in the area. The typical low-tech, mining and extraction methods, using amalgamation with liquid mercury as a means of concentrating fine gold, are hazardous both to the health of the miners and the general environment. Several cases of severe mercury poisoning and a number of fatalities have been recorded, mostly from the inhalation of Hg vapour during torching', i.e. evaporation of Hg amalgam to recover the gold. The wider environmental and long-term consequences of the use of Hg in the area are not so well characterised.
In this study, samples of stream water and sediment were taken from a large number of sites in the Agusan catchment, both from known mining sites such as Diwalwal and areas well away from these to establish peak and background levels, and downstream dispersion patterns. High levels of Hg were found near some of the mining sites, and the dispersion patterns suggest that although dissolved Hg is fairly rapidly lost from solution, a reservoir' of Hg may build up in the stream sediments causing a potential long-term pollution problem. However, much of the severe contamination is localised and the main Agusan river is relatively little affected. The growth of more highly-organised mining using cyanidation rather than amalgamation and the relative decline of the larger ad-hoc mining operations may indicate that the overall level of Hg contamination is on the decline, though it is recommended that Hg contents in some streams should be regularly monitored.
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