Geochemical distribution and threshold values determination of heavy metals in stream water in the sub-basins of Vermelho and Sororó rivers, Itacaiúnas River watershed, Eastern Amazon, Brazil

Abstract

RESUMOLevantamento geoquímico nas sub-bacias Vermelho (SBV) e Sororó (SBS) da bacia do Itacaiúnas no sudeste da região Amazônica revela que parâmetros físicoquímicos da água superficial não mostram variações acentuadas entre as estações chuvosa e seca. Com exceção de Fe e Mn, em geral os demais metais revelam baixas concentrações na água e não foram observadas evidências significativas de contaminação. Conteúdos comparativamente mais elevados foram obtidos principalmente durante a estação chuvosa. Fe e Mn se distribuem regularmente na água na área estudada e o aumento de seu conteúdo foi favorecido pelo desmatamento. A concentração de Mn aumenta durante a estação seca possivelmente devido a processos biogeoquímicos. Altas concentrações de Fe e Mn em águas superficiais são inerentes às condições locais da região Amazônica. Fatores geogênicos influenciam a distribuição na água de Co, Cr, Cu, Ni, Pb, Sn, V e Zn. Efeitos antrópicos são subordinados, exceto para Fe e Mn ou em áreas localizadas. A definição de valores de threshold para As, Co, Cr e Pb, apresentou limitações. Valores de threshold puderam ser definidos em pelo menos uma estação nas duas sub-bacias para Cu, Ni, Sn, V e Zn. No caso de Fe total, foram obtidos valores de threshold de 5 a 6 mg/L e de 2 a 3 mg/L nas estações chuvosa e seca, respectivamente. Os valores de threshold para Mn são de 0,30 a 0,45 mg/L na estação chuvosa e, na estação seca, decrescem para 0,20 a 0,30 mg/L na VSB e aumentam para 1,3 a 1,4 mg/L na SSB. Palavras-chave: Geoquímica de água fluviais; Valores de threshold; Metais pesados; Bacia do rio Itacaiúnas; Amazônia Oriental. ABSTRACTGeochemical survey in the Vermelho (VSB) and Sororó (SSB) sub-basins of the Itacaiúnas basin in southeastern Amazonian region has shown that the physicalchemical parameters of stream water do not display accentuated variations between the rainy and dry seasons. Except for Fe and Mn, in general most metals show low contents in water and evidences of significant contamination were not observed. Higher contents were mostly registered during the rainy season. Fe is regularly distributed in water in the studied area and the increase of its concentration was favored by deforestation. Mn contents increase during the dry season possibly due to biogeochemical processes. High Fe and Mn contents in water are inherent to the specific local conditions prevalent in the Amazonian region. Geogenic influence in metal distribution in water is significant for Co, Cr, Cu, Ni, Pb, Sn, V, and Zn. Anthropic effects are subordinate except for Fe and Mn or in local areas. The definition of threshold values for As, Co, Cr, and Pb, was limited, however, threshold values for Cu, Ni, Sn, V, and Zn were estimated for at least one season in each sub-basin. Threshold values of total iron were obtained (5 to 6 mg/L and 2 to 3 mg/L in the rainy and dry seasons, respectively). Estimated Mn threshold values are 0.30 to 0.45 mg/L in the rainy season and in the dry season they decrease to 0.20 to 0.30 mg/L in VSB and increase to 1.3 to 1.4 mg...

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