Chemical composition of black-watered rivers in the western Amazon Region (Brazil)

Most investigations addressing Amazonian water chemistry are focused on the Solimões, Amazonas and Negro rivers. Knowledge of the chemical composition of their smaller tributaries is restricted to some few, punctual data. The smaller rivers, that only present inputs from their catchments, are very important to understand the overall mechanisms controlling the chemistry of larger rivers of the region. With this objective the chemical composition of the principal Solimões river black-watered tributaries in the western Brazilian Amazon during the low water period were determined. The data reveal the black water chemical composition to be highly variable and strongly influenced by the local geological environment: the Badajós basin being chemically more diluted; the Coari basin presenting higher SiO2 contents, as well as smaller lakes having higher pH, conductivity, Ca2+, Mg2+ and Sr, yet not as much as those found in the Solimões river. The chemical composition of these waters is compatible with the low physical erosion and the region's highly leached tropical environment from which most soluble elements were quickly removed.