A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil

Current Hg0 emissions from goldmining activities in the Amazon basin reach 100 tons·year−1, but very little is known about Hg methylation rates in local aquatic systems. Simplified versions of the Furutani and Ruddi (1980) radiochemical method were tested and applied in the determination, by in-situ incubations, of specific net Hg methylation rates in water and surface sediments of the Madeira river goldmining area. Efficiency for Me203 Hg extraction from water and sediment was in the 75.3–96.77% range, depending on sample type and extraction procedure. Methylation was not detectable in water but rates in surface sediments ranged from 10−5 to 10−1% · g−1. h−1, the higher rates being found in sites in a reservoir area. Methylation was absent or strongly depressed in sterilized sediments. The effect of delays between sampling, incubation and extraction on methylation rates and Me203Hg recovery was documented. Despite their limited spatial and temporal range, these data are further evidence that concerns about the potential toxicological impacts of Hg emissions in the Amazon basin are justified.