J. Phys. IV France 107 (2003) 327
DOI: 10.1051/jp4:20030308

Methylmercury formation in the anoxic waters of the Petit-Saut reservoir (French Guiana) and its spreading in the adjacent Sinnamary river

M. Coquer¹, D. Cossa², S. Azemard¹, T. Peretyazhko³ and L. Charlet<sup>3

1</sup>  International Atomic Energy Agency, Marine Environmental Laboratory, BP. 800, 98000 Monaco, Principauté de Monaco
²  Ifremer, Centre de Nantes, BP. 21105, 44311 Nantes cedex 03, France
³  Laboratoire de Géophysique Interne et Tectonophysique, Université de Grenoble l, BP. 53, 38041 Grenoble, France

Abstract
The present work was carried out on the hydroelectric reservoir Petit-Saut on the Sinnamary river in French Guiana. Measurements were performed during the wet and dry seasons along a longitudinal gradient, from upstream of the reservoir in two inflow rivers, to the Sinnamary estuary downstream of the dam. Gold extraction has led to a marked increase in suspended matter and total mercury (HgT) in one of the rivers. Dissolved monomethylmercury (MMHg) measured in surface waters were similar for both rivers: 0.03-0.05 ng l ^-1 (1.0-1.7% of the dissolved HgT). These results indicate similar methylation efficiency and/or transfer of MMHg into the dissolved fraction of the water column, independently of the amounts of inorganic mercury transported. Dissolved MMHg concentrations in surface waters of the reservoir were similar to those in the rivers, but were more than 10 times higher in deep anoxie waters, up to 0.6 ng 1-1 (20% of dissolved HgT). The MMHg concentration profiles in the water column suggest that methylation occurs mainly in anoxie waters and sediments in relation with the activity of sulfate reducing bacteria. Dissolved MMHg concentrations measured in the Sinnamary at the base of the dam were still high (0.5-0.6 ng l ^-1; 20 to 35% of the dissolved HgT).

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