Sources of mercury in surface waters of the Upper Madeira erosive basins, Bolivia

The human activities impact on the mercury (Hg) distribution in the Amazon waters is still discussed but very few information regarding the Hg sources from the highly turbid Andean headwaters have been published. The annual load of suspended sediments eroded from the Bolivian Andes, averages 300 10(6) tons in the Beni river, of which till 97% is exported during the rainy season. At the edge of the Bolivian Andean piedmont, the total Hg concentration vary from 8 ng l(-1) during the dry season, to 1600 ng l(-1) during the high water period [1]. During this stage, it appears that the maximum concentrations of Hg associated with the fine particles and the highest Hg specific fluxes are observed in the rivers exploited for their alluvial gold during the last century. The black-shales series associated to the gold deposits are characterised by high natural Hg contents, which can be released in the hydrographic system by both chemical and physical weathering. But if we compare two clear water rivers characterised by the same suspended sediment concentrations, the maximum specific Hg flux varies between 126 kg km(-2)j(-1)' in the pristine river, to 590 kg km(-2)j(-1) in the river characterised by recent human activities.