Avaliação de impactos ambientais nos tributários do lago Paranoá, Brasília - DF




The high level of urbanization in Distrito Federal in Brazil is contributing to modify the environment particularly the quality of surface water resources. The historical of environment aggression to the Paranoá Lake through urban expansion, land use and land occupation, is studied as environmental changes. In this work, physical-chemistry conditions and geochemistry of water and sediments of the Paranoá Lake contributories is studied, trying to understand the behavior of the chemical elements and their interactions, as well to verify the possible inputs of anthropogenic contamination. The study area is formed by the Paranoa Lake Basin, subdivided in six hydrographic units: Torto, Bananal, Riacho Fundo, Gama and Paranoá Lake. Concerning the geology of the studied area, the Ardósia Unit (shales) of the Paranoá Group is the main bedrock unit. The physical chemistry parameters in water was measured using spectrofotometer UV- visible. The Inducted Coupled Plasma Atomic Emission Spectrometer (ICP/AES) was utilized to determine Sr, La, Pb, Cd, Y, Ti, Ca, V, Mg, Fe, Si, Ni, Cu, Al, Cr, Ba, Zn, P and Mo (sediment 0,063 mm fraction), X-ray diffraction was used to identify the mineralogical composition of suspended and bedload sediments . The results of analysis of water samples show high concentrations during the rainy season, in Riacho Fundo (R2), for ammonia (0,73mg/L), and RG1 for alkalinity (50,4 mg/L and during dry season at R3 for phosphate (1,76 mg/L),and RG2 for alkalinity 48mg/l) . These high concentrations are associated to the domestic wastewater, garbage discharges, fertilizers and agricultural inputs, transported by the drainage waters or discarded in Guará and Vicente Pires streams, Riacho Fundo stream tributaries. Therefore, this last tributary is directly influenced by antropogenic actions with a high sediments discharge. The results of the chemical analysis of trace elements (Zn, Cr, V, Ba and Sr) in sediment, samples show that high concentrations are associated with inputs from building sites (cement and sand inputs) carried by these tributaries. Comparing with previous works, we show that the Gama Stream has higher concentrations for nitrate (from 0,12 to 1,85mg/L) and TDS (from 4,2 to 7,3 mg/L), resulting for the increase of suspended solids and organic sludges (wastewaters) inputs, transported to the lake. The Torto tributary demonstrated some anomalous values associated to the bridge construction and metallic wastes. The Bananal tributary shows higher concentrations for bedload sediments, comparing with other affluents, and this could be due to minerals transport, as gibbsite and kaolinite. Iron oxides (Fe O ) could be derive from 2 3 goethite and hematite minerals. The values obtained for trace elements were referenced to geologic materials with the influence of antropogenic material, whose clay minerals can adsorbed these elements (Sr, Cu, Y, Zn, V and Cu) by Al and Fe in amorphous form. The suspended solid material has an average composition: Al 55%, Fe 31%, Ca 9%, Mg and Ti 2% and trace elements 1%. The obtained data shows that all tributaries are influenced by antropogenic inputs, but the region with highest urban impact is the Riacho Fundo tributary, where we found discharge of wastewater, garbage and agricultural inputs. The less polluted is the Bananal tributary, because it belongs to a conservation area inside the Parque Nacional. As conclusion, we recommended a constant monitoring for every tributary, in order to avoid contaminations in Paranoa Lake due to transported material, using chemical O for indicators ass nitrate, phosphate and ammonia for water and Zn, CaO and K2O sediments . We recommend also the recovery of damaged areas.


urbanização lago paranoá geologia impacto ambiental

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