Investigação de mistura de águas entre o sistema Barreiras e Pirabas com base na assinatura isotópica de Estrôncio (Sr) e hidrogeoquímica em Ananindeua, Belém e backgrounds em Benevides e Capanema, Pará

Abstract

Until the 70's was believed that the northeastern region of Para, as well as other areas of Cenozoic age in Brazil was stable, without evidence of recent tectonics. However, field investigations and remote sensing analysis identified features and tectonic structures that characterize the neotectonic process this region. Concomitant with the above studies were performed in water in the metropolitan area of Belem and demonstrate the vulnerability of aquifers deeper by neotectonic processes, suggesting mixing between Barreiras and Pirabas aquifers. Results obtained from the seasonal variation reveal distinct hydrogeochemical characteristics, the parameters pH (4,27 to 6,19 in Barreiras; 7,00 to 8,02 in Pirabas), Electrical Conductivity (11,1 to 92,2 μS/cm in Barreiras; 222-406 μS/cm in Pirabas), Total Dissolved Solids (10-87 mg/l in Barreiras, 105-181 mg/l in Pirabas), Cl- (0,18 to 0,42 meq/l in Barreiras; 0,05 to 0,12 meq/l in Pirabas), Na+ (0,24 to 0,50 meq/l in Barreiras; 0,05 to 0,18 meq/l in Pirabas), Mg2+ (0,001 to 0,061 meq/l in Barreiras; from 0,68 to 0,128 meq/l in Pirabas), Ca2+ (0,01 to 0,33 meq/l in Barreiras; 1,78 to 2,53 meq/l in Pirabas) and Sr2+ (0,0002 to 0,0066 meq/l in Barreiras; 0,016 to 0.023 meq/l in Pirabas), all data with considerable variation between the aquifer systems. The analysis results from the correlation matrix between the aquifer systems in dry and wet periods showed correlation coefficients (r) above 0,8 on various parameters studied, suggesting a direct contribution from the aquifers. Regarding the hydrogeochemical facies are divided in two types: 1) Barreiras aquifer, dry period, the samples behave as water-type Cl--Na+, divided in two types Cl-Na-Ca-SO4 and Cl-Fe-Na; while it have water during the rainy season of the type Cl-Na-Ca-SO4-Cl and Cl-Na-Ca, pointed to lower stability of this aquifer and its facie typically input of meteoric water, changing the type of water as the water cycle and 2) Pirabas aquifer, both in the dry season and the rainy behaved like water typeHCO3-Ca, showing great stability of hydrogeochemistry facie. The diagram Schöller type composition for Barreiras and Pirabas aquifers, from 10% hydrogeochemistry modeling indicates mixture of these samples aquifers. The 87Sr/86Sr isotopic show in general the waters of Barreiras aquifer are more radiogenic (continental) that the aquifer Pirabas (Navy). It’s noteworthy that during the rainy season in general there is homogeneity in the isotopic ratios obtained (below 0,71), corroborated by the isotope ratio of rainwater (0,705316), suggesting mixing between aquifers studied. As the scatter plots of 87Sr/86Sr-STD is possible to see the type of process that controls the aquifer (in the Barreiras case shows the weathering of the silicate, while in Pirabas shows the dissolution of the calcite). The results of discriminant analysis using isotopic and hydrogeochemical parameters (87Sr/86Sr-Cl--SO4 2--K+-Mg2+) suggest an excellent breakdown of Barreiras and Pirabas samples during the rainy season, as well as analyzes using PCA (87Sr/86Sr) and (87Sr/86Sr-SO4 2--K+)indicate mixtures between the aquifer systems in the rainy season. The combined results of isotopic and hydrogeochemical parameters in dry and rainy season based on the Sr isotopic signature, multivariate statistics, Schöller diagram and correlation analyzes suggest mixing processes, especially in the rainy season between Barreiras and Pirabas aquifers, corroborating structural studies. They indicate the mobility of water in these aquifers mix, mainly by neotectonic features existing in the metropolitan region of Belém.