Increase in mercury contamination recorded in lacustrine sediments following deforestation in the central Amazon

The geochemical study of surface sediments (vertical profiles of 30-80 cm) from lentic ecosystems of the Tapajos River permit the observation of envir onmental changes responsible for the mercury contamination of aquatic syste ms exploited by the human riverine population. The Arapiuns and Amazon rive rs are compared. Measurements of mercury, textural indicators (water conten t and dry density), mineralogic indicators (iron and aluminum associated wi th oxyhydroxides and aluminosilicates), and organic indicators (carbon, nit rogen, atomic C/N ratio) were performed over the full length of the cores. The results demonstrate that soil erosion is responsible for an increase in surficial sediment mercury concentrations in the different aquatic systems of the Tapajos and Arapiuns rivers. This increase is the result of the rel ative enrichment of the sedimentary deposits in fine particulates rich in a luminosilicates, oxyhydroxides and mercury, transported in suspension in th e water column. The oxyhydroxides of iron and aluminum associated with fine , clayey particles seem to control the accumulation of heavy metals in the sediments of the Tapajos, Arapiuns and Amazon rivers. Overall, the mercury levels in the sediments studied have the same relationship with the alumino silicates and the texture of the sediments. The quantity of aluminosilicate s permits the evaluation of diagenetic effects, the influence of the clay c ontent and the matrix effect on the levels of mercury in sediments. The act ivity of lead-210 measured in two cores suggests that the superficial sedim ents originate from eroded soils. A preliminary dating using the constant i nitial concentration model indicates that the environmental changes recorde d in the Tapajos River sediments would have been initiated some time betwee n the 1950s and 1970s. They would then coincide with the important coloniza tion of the Brazilian Amazon during this period. The results presented have important implications for the geochemical interpretation of anthropogenic disturbances in the Amazon. They demonstrate that the recent colonization of the drainage basins and the growing exploitation of new parcels of land in the central Amazon disturb the mineral and organic matter cycles, as wel l as that of mercury. The ensuing result is increased exportation of fine p articulate mercury from the surface of soils to drainage waters that transp ort them to fluvio-lacustrine systems where they finally settle out. (C) 20 00 Elsevier Science B.V. All rights reserved.