Geochemistry of metals in the bottom sediments of tropical dam reservoirs in San Francisco River (Três Marias, MG) and Tocantins River (Tucuruí, PA), Brazil.

Abstract

In tropical climates, the high rainfall and temperature, throughout the annual cycle, allow high leaching rates of metallic elements from the basin upstream, which accumulate in the reservoirs. However, the concentration of these elements in natural waters is usually lower than expected, due to the easiness of adsorption and co-precipitation in solid phases. We have studied two tropical dam reservoirs in Brazil, Três Marias (Minas Gerais) and Tucuruí (Pará), with the aim at understanding the correlation between physical-chemical parameters of the water column, chemical and mineralogical characteristics of the accumulated material and the solubility, mobilization and precipitation of metals in reservoirs. Metals speciation performed in selected samples assess that metallic micronutrients are preferentially adsorbed or retained through precipitation/co-precipitation onto fine-size charged crystalline/amorphous Fe-oxides. Under the prevailing reducing and low pH conditions of the bottom reservoirs some adsorbed metals (particularly Fe and Mn) are easily released from their solid hosts and mobilized to the aqueous phase of sediments, which show high levels of soluble forms of these elements. However, the solubilisation process and the release to the water column are not very extensive, as abundances of metals such as Fe and Mn in water are low, although increasing with depth.