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Earthworms as a bioindicator of mercury pollution in an artisanal gold mining community, Cachoeira do Piriá, Brazil Hinton, Jennifer

Abstract

The health and welfare of millions of inhabitants of Latin America are directly and indirectly influenced by artisanal mining activities. The rudimentary methods that characterize artisanal gold mining, particularly in association with the misuse of mercury in gold amalgamation, often generate extensive environmental degradation, which persists long after mining activities cease. This is the case in the municipality of Cachoeira do Piria, located in Para, Brazil, where approximately 5,000 artisanal miners discharged more than four tonnes of mercury into soils, air and aquatic systems between 1980 and 2000. An area covering approximately 85 ha located adjacent to the town of Cachoeira is moderately to highly contaminated with Hg (>1000 ppb), although Hg contamination is dispersed across an area of approximately 2100 ha. Mercury discharged into the environment can undergo many changes, including the transformation to a readily bioavailable and highly toxic form of mercury, methylmercury. Due to its behaviour in the food chain, methylmercury poses a serious risk to humans reliant on fish as a protein source. A laboratory methodology using the earthworm Eisenia foetida was developed in this research to assess mercury bioavailability in mine tailings and aqueous solutions, and examine the influence of certain factors on mercury bioavailability, in particular organic acids. The quick, inexpensive and simple earthworm protocol described herein was developed to prioritize contaminated sites based on the potential for mercury incorporation into biota with specific consideration of the technical and financial limitations inherent in artisanal mining communities. Using samples collected in Cachoeira, this research indicates that the reaction of mercury with organic acids is an important pathway for the incorporation of mercury into the food chain. Earthworms (E. foetida) are capable of accumulating Hg from solutions and soils, and this potential increases markedly in the presence of organic acids. In addition, evidence suggests that earthworms may be converting mercury associated with organic acids to its highly toxic, readily bioavailable form, methylmercury, in the intestinal tract. The influence of organic acids on Hg bioaccumulation and intestinal methylation is extremely significant in terms of understanding biogeochemical cycling of Hg in darkwaters and assessing ecological and human health risks from Hg in these aquatic systems. Due to the demonstrated bioavailability of Hg in soils, sediments and tailings, and elevated Hg levels in fish sampled in Cachoeira, the earthworm methodology was also applied to assess the suitability of local materials, specifically clay, lateritic and organic soils, for capping of Hg hot spots in order to inhibit its mobility. It was found that clay and lateritic soil have some capacity to inhibit Hg uptake, particularly in comparison to the organic-rich soil. However, these tests were conducted using distilled water, and increased bioaccumulation would be anticipated in the presence of organic acids. Non-technical measures (e.g. educational campaigns) to reduce Hg risks were also explored.

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