Aplicação da composição isotópica de multielementos no monitoramento ambiental de área circunvizinha à barragem de rejeito: o caso da mina de cobre da Serra do Sossego, Canaã dos Carajás – PA

Abstract

Tailings dams from ore mining activities are relevant compartment for environmental monitoring because the dam can present undesirable forms of water outlets such as infiltration. The present thesis proposed multielement isotope study in order to evaluate the use of the isotopic composition of oxygen, hydrogen, strontium and lead as a tool for the environmental monitoring of groundwater vulnerable to the tailings dam. This research was conducted in the area of the Serra of Sossego copper mine and, in addition to the isotope study, the hydrochemical characterization of groundwater and surface water was caried out. Besides, the direct reading of the isotopic composition of lead (without chemical treatment of the water sample) was applied for the first time, using multicollector inductively coupled plasma mass spectrometry, in the laboratory Para-Iso. The hydrochemical characterization of groundwater in the Serra do Sossego mine area showed that it is moderately acid (pH ~ 6). Only the AP15 and PS32 wells, under the influence of the drainage of the mixed pile (ore waste + sterile), presented high electrical conductivity (599 μS / cm2 and 694 μS / cm2) and the higher sulfate content (1.82 to 6.57 Meq/L). In general, iron (29.7 mg/L), manganese (69.7 mg/L) and copper (24.9 mg/L) present the highest metal content in the groundwter. In terms of the isotope studies, the groundwater of PS17, PS04, AP15 and PS32 wells and the water of the Dam present the least radiogenic lead isotopic composition. The water sampled in the AP15 and PS32 wells would be representative of the lead isotope signature of groundwater in the mine area (206Pb/207Pb = 1.1481 to 1.1663) and, probably, is indicative of the lead isotope composition of the meteroric water. The lead isotope composition of the groundwater of the wells PS04 (206Pb/207Pb = 1.1784-1.1850) and PS17 (206Pb/207Pb = 1.2100 to 1.2160) may indicate a small contribution of the rocks of the region (206Pb/207Pb > 1.6807). In turn, the lead isotope signature of water of the Dam (206Pb/207Pb = 1.2279) suggests, as expected, a slightly more pronounced contribution of the ore and the rocks. On the other hand, the lead isotopes of the water sampled in the wells MNA23 and PS06 show a more radiogenic signature (206Pb/207Pb = 1.6741 to 1.9196), which is similar to those of the chalcopyrite and the diabase of the Sequeirinho open pit, revealing a more significant contribution of the ore and rocks. As the lead isotope composition of water from the dam is different from that of the groundwater, it can be used for the environmental monitoring of the influence of the water from the dam in groundwater in the Serra of Sossego mine area. In those wells with less radiogenic lead isotope signature, a possible contribution of the water from the dam would result in a more radiogenic lead isotope composition in the groundwater. Conversely, in those wells with more radiogenic lead isotope signature, a contribution of the water dam for the groundwater would decrease the values of the measured lead isotope ratios. On the other hand, the application of the sulfur isotopes for environmental monitoring presented some limitation in the study area, due to the lack of contrast of the sulfur isotope composition among the sulfides of the Sossego deposit (δ 34S ~ 2-7 ‰), the water from the dam (δ 34S ~ 2 ‰), and the drainage of the mixed pile (δ 34S ~ 2‰). This prevents the recognition of a possible contribution of the tailings dam to groundwater. Mixing models using the 87Sr/86Sr ratio as a function of the strontium concentration and δ 18O values, also showed some limitations, since there is no contrast between the values of the 87Sr/86Sr ratio of the water from the dam (0. 7458 to 0.7539) and the rocks of the mine area (granite ~ 0.7474). Such similarity, in principle, would not allow identifying the contribution of the water from the dam. However, since groundwater generally exhibits less radiogenic values of the 87Sr/86Sr ratio (0.7161 to 0.7283) and well below the dam values, the strontium isotopic composition can be used for environmental monitoring of groundwater. Hydrogen and oxygen isotope data revealed that, in the rainy season, the groundwater presented values of δ 18O and δ D varying from -2.74 to -7.17‰ and from -15.6 to -46.0‰, respectively. In turn, an enrichment of 18O and D, with values of δ 18O between -1.67 and -7.29 ‰ and of δ D between -0.23 and -46.1 ‰, is observed in the dry season. Hydrogen and oxygen isotope composition of the water from the dam (δ 18O from 1.24 to 2.79 ‰ and δ D from 1.40 to 7.0 ‰) is very different from those of the groundwater and of the drainage of the pile. A mixing model performed with oxygen and hydrogen isotopes showed that these elements are the ones that best respond as indicators of the contribution of the waters from the dam to groundwater. The application of this model revealed no influence of the water from the dam on the groundwater during the period of this study. The results obtained in this multielement isotopic study suggest that the environmental monitoring of the groundwater of the Serra do Sossego mine to investigate the possible contribution of the water from the dam, can be done more efficiently with the oxygen and hydrogen isotopes. The isotopic compositions of lead and strontium can also be used for monitoring the groundwater, but this has to be done more systematically due to the small contrast between the isotopic composition of these elements in the dam water and in the groundwater. Finally, the isotopic composition of sulfur is the one that would contribute least to the environmental monitoring of the groundwater of the study area.