Água da chuva na Amazônia Ocidental: química e composição isotópica

Abstract

Rainfall water were monthly collected throughout 2006, at Boa Vista, Apuí, Tabatinga, Itapiranga and Parintins, in open áreas. They were also collected at Manaus both in open área and below the forest cover. Wet deposition was the adopted collecting method and, pH, conductivity, alkalinity, Na+, Ca2+, Mg2+, K+, H+, Cl-, SO4 2-, NO3 -, PO4 3-, F-, SiO2, Al, Zn, Fe, Sr, Mn, Cu, Ba, Ni, Pb, Rb, V, Mo and oxygen and hydrogen isotopes were determined. The pH was usually acid, especially in the dry period, varying between 2.8 and 5.9. Conductivity and chemical concentration of the rain waters were higher for Manaus forest station and Apuí station especially during the dry period. Boa Vista and Parintins presented a similar sequence both for cátions anions with predominance on Ca2+ and Cl-, respectively (Ca2+>Na+>K+> Mg2+>H+ e Cl->NO3 ->SO4 2->SiO2>PO4 3-~F-). At Manaus and Apuí stations the Mg2+ is lower than H+; Tabatinga is the only place where Na+>Ca2+ and Itapiranga is the station differentiating the most from the others (H+>Na2+>Ca2+>K+>Mg2+). Manaus and Manaus floresta stations the same sequence of anions (SO4 2-> NO3 ->Cl->SiO2>PO4 3->F-) and Apuí, Tabatinga and Itapiranga are different among each other. In general, the rain waters are more diversified in anion than cation ratios. In the distribution of trace elements there are just similarities on Al, Zn, Fe and Sr, with equal sequences for Boa Vista and Manaus (Al>Zn>Fe>Sr) and for Parintins and Manaus Florest (Al>Fe>Zn>Sr). Like for anions, trace elements at Apuí, Tabatinga, and Itapiranga stations differentiate from others and among one another. δ18O and δD values present wide variations and showing they do not follow a single fractionating pattern on the rainwater s isotopic composition. These behaviours are the results fron to local influences, temperature and, rainfall intensity anda ir mass movement. The similarity between Manaus and Manaus Forest rainfalls which are isotopically homogeneous, demonstrate that fractionation are not to be no directly influenced by vegetation. Rainwater chemical composition is directly linked to aerosols vegetation, biomass burning, rock weathering processes (Fe, Mg, Mn, Ba, Rb e V), marine aerosols (Na, Mg e Cl), and gases produced in the burning of fóssil fuels (Zn, Cu, Pb and Mo) as well as from biological sources (SO4 2-, NO3 - and PO4 3-).