Proposição de valores de referência para concentração de metais e metalóides em sedimentos limnicos e fluviais da bacia hidrográfica do Rio Tietê, SP

Abstract

This project represents the first efforts in Brazil in the proposition and establishment of reference concentration values for metals and metalloids in limnic and fluvial sediments. The selected study site is the Tietê River basin (São Paulo State, southeast region), a watershed which is under significative environmental impact and degradation since the onset of the Brazilian revolution at early-midd 50 s. In order to consolidate the analytical values 1,100 analyses were performed encompassing 95 sampling points in 29 municipalities, from the upper stream parts of the river down to its mouth at the Paraná River, covering an extension of approximate 800 km. Sediment samples were collected in small streams and rivers as closest as possile to their upper reaches, as well as in rivers and reservoirs in locations minimally affected by human contamination. Samples were initially sieved for separation of the particles <63 µm, dryed at 50°C, and extracted with a mixture of strong acid solution (HNO3, HF, HClO4 and HCl). Chemical analyses were performed by induced coupled plasma atomic emission spectrometry for Cr, Ti, Zn and V, and via atomic absorption spectrometry using conventional flame for Co, Cu, Ni and Pb, graphite furnace for Cd and Ag, hydride generation for As and Se, and cold vapor for Hg. U and Th were analysed by molecular absorption spectrophotometry using the chromogenic reagent Arsenazo III. The reference concentrations determined by this study showed significant differences with respect to the assumed global geological reference for some elements; differences were also detected among the concentrations of the same element from different basin regions and are attributed to differences in regional geochemical characteristics. As a consequence of these lithological variations, four different sets of reference concentration values are proposed valid to the different regions of the studied basin. This study also presents application of different sediment quality classification models and potential ecological risk index for Bariri, Billings, Rasgão, Barra Bonita and Promissão reservoirs.