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Large quantities of mercury are being released into the waters of the Amazon and its tributaries by gold-mining activities. Large releases are also taking place into the air. In this paper, the fate of mercury released to the environment by gold mining in the Brazilian Amazon is reviewed. Mercury contamination is presently widespread in the Amazon region. A major source of mercury in the local environment is the burning of the gold–mercury amalgam, which releases from 30 to 150 t of mercury yearly into the Amazon atmosphere. Air samples collected close to mining sites showed extremely high mercury concentrations (up to 7.5 μg∙m−3). Inside gold-dealers' shops or in amalgam-burning stations, ambient air concentrations may reach 100 μg∙m−3. A secondary source is the discharge of metallic mercury into rivers during the amalgamation process. Sediment concentrations frequently range from 0.3 to 3.0 μg∙g−1 in contaminated sites. However, values as high as 19.8 μg∙g−1 have been reported in some sites. Waters of many rivers are also contaminated, although reliable data on dissolved mercury concentrations are still lacking. Local carnivorous fishes typically show mercury concentrations higher than 1.0 μg∙g wet wt.−1 in contaminated sites and methyl mercury represents over 90% of the total content. Human groups with fish-based diets frequently show evidence of mercury contamination, with high mercury concentrations in hair (up to 70 μg∙g−1). However, a reliable epidemiological study on the affected population still has to be carried out.Key words: mercury, gold mining, Amazon, sediments, aquatic biota, humans.

Abstract.Sediment cores from lakes located in the Pantanal Swamp, Central Brazil were analysed for the distribution of mercury released by the local gold mining. Atmospheric transport is the only pathway of mercury contamination of these remote lakes. Mercury concentrations were higher at the surface of sediments (62 to 80 ug.kg-') decreasing to values of 20 to 30 ug.kg-' in deeper layers. Mercury deposition rate was estimated as 90 to 120 ug Hg.m-*.yr-'. Although mercury concentrations were much lower than in industrialized areas, mercury deposition rate for these Pantanal lakes is of the same order of magnitude of deposition rates measured in lakes in industrialized areas.

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In studies of heavy metals in sediments, there is a need for standardization of the procedures for sample collection and preservation, chemical analyses and presentation of results. The method and depth of sampling depend on the aim of the investigation and on local sediment conditions, such as consistency of the sediment, rate of sedimentation, diagnetic processes and bioturbation. Therefore no general recommendations can be given in this respect. During collection and preservation, contamination and loss of constituents must be avoided. In sediments, the best means for estimating total contents of metals is digestion with HF, in combination with strong acids. Other methods include X-ray fluorescence and neutron activation analyses. The use of HF is considered objectionable by some laboratories. A reasonable alternative is aqua regia. Because variations in granular composition affect metal contents, it is advisable to use the fraction < 63 /Um for the analysis.Chemical partition of sediments provides an insight into the source of metallic constituents, and their pathways to deposition areas. A three-step extraction procedure, in the sequence 0. M hydroxylamine-HC1, H 2 0 2 30% and HF, is proposed.Finally, attention is paid to the anthropogenic enrichment of metals in sediments. The establishment of base-line levels is discussed.