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

Abstract

The geochemical study of surface sediments (vertical profiles of 30–80 cm) from lentic ecosystems of the Tapajós River permit the observation of environmental changes responsible for the mercury contamination of aquatic systems exploited by the human riverine population. The Arapiuns and Amazon rivers are compared. Measurements of mercury, textural indicators (water content and dry density), mineralogic indicators (iron and aluminum associated with oxyhydroxides and aluminosilicates), and organic indicators (carbon, nitrogen, 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 Tapajós and Arapiuns rivers. This increase is the result of the relative enrichment of the sedimentary deposits in fine particulates rich in aluminosilicates, oxyhydroxides and mercury, transported in suspension in the 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 Tapajós, Arapiuns and Amazon rivers. Overall, the mercury levels in the sediments studied have the same relationship with the aluminosilicates and the texture of the sediments. The quantity of aluminosilicates permits the evaluation of diagenetic effects, the influence of the clay content and the matrix effect on the levels of mercury in sediments. The activity of lead-210 measured in two cores suggests that the superficial sediments originate from eroded soils. A preliminary dating using the constant initial concentration model indicates that the environmental changes recorded in the Tapajós River sediments would have been initiated some time between the 1950s and 1970s. They would then coincide with the important colonization 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 well as that of mercury. The ensuing result is increased exportation of fine particulate mercury from the surface of soils to drainage waters that transport them to fluvio-lacustrine systems where they finally settle out.