Caracterização química/geoquímica de sedimentos do baixo curso do Rio São Francisco visando avaliar riscos ecológicos

Abstract

The low course of the São Francisco River covers an area that is divided between the states of Sergipe and Alagoas, in the stretch after the Xingó Dam, downstream from the city of Piranhas, through the city of Pão de Açúcar, both in the state of Alagoas and following its course to Propriá and Ilha das Flores, in the state of Sergipe. The objective of this work was to characterize, through chemical and geochemical parameters, the sediments of the low course of the. River, aiming to evaluate possible ecological risks. Surface sediment samples were collected (0-10 cm) during three campaigns, two in the dry season (February 2014 and December 2015) and one in the rainy season (July 2016), on the two margins of four transects, totaling 8 sample points. Samples of sediment cores were collected in July 2016 at 7 locations (Sugar Loaf-AL, Niterói-SE, Traipu-AL, Gararu-SE, Xinaré-AL, Pindoba-SE, Flores Island-SE) 2 to 44 cm, to verify the influence of dams on sedimentation behavior. The Environmental Protection Agency (EPA-3051A) analytical procedure was applied to sediment samples to determine trace elements (As, Cd, Cr, Cu, Ni, Pb, P and Zn), which were analyzed by inductively coupled plasma optical emission (ICP OES). The origin and sources of organic matter were evaluated by elemental carbon analysis coupled with mass spectrometry. The results obtained after partial decomposition in microwave oven and ICP OES determination were evaluated considering the guiding values established by the sediment quality guides. The concentrations of As in Pão de Açúcar-AL (2014) and Ilha das Flores-SE (2015, 2016) exceeded the TEL and PEL criteria, respectively Threshold Effect Level and Probable Effect Level. The concentrations obtained for Cd in Pao de Açúcar-AL, Gararu-SE, Flores Island-SE (2014 to 2016), Niterói-SE, Xinaré-AL, Pindoba-SE (2014 and 2016), Traipu-AL (2016) and Penedo-AL (2015 and 2016) also exceeded the minimum value set by Canadian and US law, TEL and LEL (Lowest Effect Level). In Pão de Açúcar-AL (2014) and Niterói-SE (2016) the values were above what is established by Canadian law (PEL). Already in Ilha das Flores-SE (2016) this occurred in relation to the limits established by the American legislation, SEL (Severe Effect Level). The As and Cd in the various locations must be derived from agrochemicals used in several monocultures in the region. The Cr concentration found in the IF-SE sediments (2016) was above the lower limits established by US law (LEL). This contamination may come from the textile industry operating in the study area. The calculated potential ecological risk index reflects a warning for the localities Niterói-SE and Ilha das Flores-SE, ranging from moderate to high in 2016. Regarding the ecological risk factor calculated for the sediment of the different locations in the At different times, it can be seen that the elements As, Cr, Cu, Ni and Pb offer higher risk. With respect to phosphorus, it was observed that the decrease in flow reflected in Niterói-SE, Traipu-AL, Gararu-SE, Xinaré-AL, Penedo-AL and Flores Island-SE, causing an increase in its concentration in the sediment. , justified by the amount and type of clay minerals present, which allow the retention of P originated from anthropic activities. The results on the origin and source of organic matter (OM) revealed different constitutions composed of bacteria, seaweed and C3 plants, meaning respectively an influence of marine and anthropic conditions. The stable isotope ratios δ13C and δ15N evidenced the presence of sewage, fertilizers, cyanobacteria and typical estuary organic matter. Thus, it is inferred that the work carried out warns of the need for constant monitoring of sediments in the low course of the São Francisco River.