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Abstract:

A previous study made by our group indicated that the changes in the rare earth element (REE) distribution pattern, within the upper 30cm of sediment column from Sepetiba bay, are related to Paraiba do Sul River (PSR) diversion. Here we show that REE fractionations in the water column are directly related with the construction of PSR and Guandu river watershed diversion system. Surface water samples were collected during the dry and wet season in specific sites along the transposition system. A tangential ultrafiltration system was used to obtain specific fractions: particulate (SPM load < 63 μm to > 0.1 μm) colloidal (colloidal load >10 kDa) and dissolved (dissolved load >1 kDa) fractions. The nutrients (H₄SiO₄, PO₄^3-, F^-, Cl^-, SO₄^2-, Na⁺, K⁺, Ca²⁺, and Mg²⁺) and REE were analyzed in each of fractions. The relatively large total REE concentrations found in the PSR diminished substantially when reaching the downstream section of the tributary, showing lower concentrations when compared to a nearby river unaffected by the diversion. Some of the factors that may control the self-purification of REE, include the trapping of the particulate fractions associated with suspended matter in dams along the basin, uptake of the dissolved fraction by aquatic plants in nutrient rich waters of the tributaries, and co-precipitation with oxides/hydroxides as the redox potential increased downstream from the water diversion.