Hydrogeochemistry and isotope compositions of multi-layered aquifer systems in the Recife Metropolitan Region, Pernambuco (NE Brazil): An integrated approach using multivariate statistical analyses

Abstract

The Paraíba and Pernambuco basins are part of the Northeast Brazilian rift system, which contains a series of Cretaceous terrestrial to marine sediments covered by Neogene-Quaternary deposits. Their main aquifers (i.e. Barreiras, Beberibe, Cabo, Quaternary and, Fractured) constitute multi-layered aquifer systems and are the main strategical source of drinking water for the Recife Metropolitan Region. In this research, 323 physical-chemical samples along with 25 isotopic groundwater analyses were used to characterize the main hydrochemical processes controlling groundwater's chemistry and to identify recharge sources. Graphical, ionic ratios and multivariate statistical methods were used to achieve these goals. The correlation matrix was used to evaluate significant associations between 11 geochemical parameters. Principal Component Analysis was employed and a two-factor model is suggested explaining 72.54% of the total variation within the dataset. PC1 is the major contributor to groundwater mineralization and is assigned to ‘natural’ processes (e.g. rock weathering, ion exchange, etc.), and PC2 is related to ‘anthropogenic’ processes (e.g. nitrate contamination). Hierarchical cluster analysis was employed and partitioned the water samples into 28 distinct hydrochemical water clusters (B1–B4, Barreiras; BE1–BE6, Beberibe; C1–C6, Cabo; Q1–Q7, Quaternary; and F1–F5, Fractured). The majority of the cluster samples have Na+–Cl− to Na+–HCO3− water type, including the low salinity groundwater, which in a first approximation could be spatially tied to the recharge zones, and intermediate ones probably related to transition zones. Na+–Cl−to mixed–Cl− water type is typical for high salinity groundwater that more probably results from mixing with infiltrating highly mineralized water. The groundwaters' major ion compositions are primarily controlled by the following interactions: (1) mixing of Pleistocene marine transgression with meteoric Holocene infiltrating water, besides atmospheric deposition; (2) water–rock interactions from silicate rock-forming minerals; (3) ion-exchange reactions; and (4) input from animal/human wastewater. The analyzed groundwater samples showed isotopic signature ranging from −1.98 to +0.36‰ and −2.0 to +7.0‰ for δ18O and δ2H respectively, which is similar to precipitation. These isotopic depleted signatures suggest that most of the deep groundwater was likely recharged from colder epochs and/or from a selection of winter infiltrating precipitation through time.