Using Earthworms to Assess Hg Distribution and Bioavailability in Gold Mining Soils

Abstract

Between 1980 and 2000, the municipality of Cachoeira do Piriá, located in Pará State, Brazil, experienced an intense gold rush with approximately 5,000 artisanal miners discharging more than four tonnes of mercury into soils, air and aquatic systems. Mercury is dispersed across an area of approximately 2,100 ha and concentrations in soils and sediments frequently exceed 1,000 μg.kg⁻¹. The metallic mercury discharged by miners into the environment has the potential to be transformed into a highly toxic form of mercury, methylmercury. A 28-day bioassay with the earthworm Eisenia fetida was used to assess mercury bioavailability in mine tailings, soils, and sediments. Experiments indicated that the highest Hg concentration in earthworms was associated with low-Hg-organic-rich soils collected from densely vegetated areas despite higher mercury concentrations in organic-poor tailings. This indicates that reaction with organic acids is an important pathway for mercury incorporation into food chains. The quick, inexpensive, and simple bioassay also provided a means to evaluate remedial measures (i.e. by capping “hotspots” with local soils). Earthworm experiments indicate that covering “environmental hotspots” (sites with high Hg bioavailability) with local clay-rich sediments is very effective in terms of preventing uptake of mercury from tailings, while organic-rich sediments are relatively ineffective.

Keywords:

• Amazon
• artisanal gold mining
• bioassay
• contaminated sites
• earthworms
• mercury
• remediation

Acknowledgements

The authors gratefully acknowledge the financial support of Natural Science and Engineering Research Council of Canada (NSERC) operating grant #217089–04, CETEM, and the Dept. of Mining Engineering. The review of this work by Dr. Ken Hall, Dr. Kevin Telmer, and Dr. Malcolm Scoble is also appreciated, as is assistance from Claudia Sandoval, many people at CETEM, in particular Zuleica Castilhos and Ricardo Melamed, and Terezinha Cid de Souza at the DNPM Mercury Laboratory in Belém. The authors would like to express their gratitude for the meticulous review of this paper by Dr. Kees van Gestel from the Department of Animal Ecology of the University of Vrije, The Netherlands. Finally, researchers would like to thank the people of Cachoeira do Piriá, Brazil, for their hospitality and kindness.