Variação sazonal e espacial da concentração de elementos-traço nos rios Urucu, Solimões e Negro - Amazônia Ocidental, Brasil

Abstract

The hydrological cycle of the rivers is the main source of the renewal of the fertility on floodplains and has a great influence on the limnological characteristics of Amazonian waters. The extraction of petroleum in the headwaters of the Urucu River and the construction of the Urucu-Coari-Manaus gas pipeline are sources of possible anthropic impacts in the region. The seasonal and spatial variation of temperature, pH, electrical conductivity and dissolved oxygen were studied in the water column of the Urucu, Solimões and Negro rivers. Also, major cations (Na, K, Ca and Mg) and the trace elements (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb e Zn) were studied in the sub-surface waters of the same rivers during the rising water period, peak high water period, falling water period and peak low water period during the hydrological cycle of the year 2006. The variables in the water column were measured with digital probes. For all other parameters water samples were collected just below the surface with a Ruttner sampler, filtered and preserved. The alkali metals were analyzed by Flame Photometry and the alkaline-earth metals and trace elements by Flame Atomic Absorption Spectroscopy. The data were evaluated with ANOVA to establish the existence of the variation of the chemical elements between the different periods of the hydrological cycle and between rivers. In order to recognize patterns in the behavior of the variables a Principal Components (PCA) and a Hierarchical Cluster (HCA) analysis were applied. In general there are hidrochemical similarities between the Urucu and Negro rivers, in the Solimões River the concentration of the elements is greater, especially calcium and there is variability depending on the phase of the hydrological cycle. In the Urucu River the limnological variables increased from the rising water period to the peak low water period. In the Solimões River pH and dissolved oxygen increased from the rising water period to the falling water period while temperature and electrical conductivity decreased from the rising water period to the peak high water period and increased thereafter up to the peak low water period. In the Negro River pH and oxygen increased during the peak high water period and peak low water period. Temperature decreased from the rising water period to peak high water period and increased from the high water period to peak low water period, conductivity was constant during the hydrological cycle with a small increase at the peak high water period and lower values during the peak low water period. In all three rivers the greatest temperatures were registered during the low water period. Considering the entire hydrological cycle the dominance of the major elements was K+>Ca2+>Na+>Mg2+ for the Urucu and Negro Rivers and Ca2+>Mg2+>Na+>K+ for the Solimões River. Throughout the hydrological cycle the mean sum of all the major elements in the Solimões River (14,003mg/L) was about 5.3 times greater than that of the Urucu River and 14 times greater than the Negro river, with emphasis on calcium which was 10 times greater than the greatest element in the other two rivers. According to the ANOVA there was no significant variation between the different periods of the hydrological cycle in the Urucu River for the concentrations of Cd, Cr e Ni; in the Solimões River for the concentrations of Na, Cd, Co e Cu; and in the Negro River of K, Ca, Mg, Cu, Ni e Pb. Between the rivers, for the major part of the hydrological cycle, principally during the rising water period, there was no significant variation between the Urucu and Negro rivers. The PCA and the HCA lend support to the similarities between the Urucu and Negro rivers, the differences between these rivers and the Solimões River and the significant correlations between all the trace elements and the major elements with the exception of Fe which had no correlation with any element. Therefore this study shows that there are seasonal and spatial variations in the concentrations of major and trace elements between the three rivers. The data should be looked upon as those of a practically unaltered equatorial ecosystem and in this manner become part of a basic data base for posterior comparisons and subsidize research with respect to nutrient fluxes in the drainage basins of the three rivers for the evaluation of an eventual anthropogenic impact.