Evaluation of risks of groundwater quality alteration in Recife urban area (Pernambuco, Brazil) using a multi-isotopic approach

International audienceThe Recife Metropolitan Region (RMR) is a heavily urbanized area located in a estuary zone and over a multilayeredsedimentary system on the Brazilian Atlantic coast. In a context of increasing land use pressures, involvingaquifer overexploitation and surface water contamination, and repeated droughts, the identification of groundwaterquality risks in RMR is a necessary management requirement. In this perspective, this work focused on the twoshallow aquifer systems, named Boa Viagem and Barreiras aquifers, located at the interface between the city (theconsumers) and the deeper semi-confined Cretaceous Cabo and Beberibe aquifers. The Holocenic Boa Viagem andTertiary Barreiras formations conform unconfined sedimentary aquifers, with no more than 80 m of thickness. Cabois the most important groundwater body for Recife private complementary water supply and it has experienced anintense exploitation in the last three decades. In contrast, Boa Viagem and Barreiras aquifers are more restrictivelyused, but it is important to understand their water quality degradation,because of hydraulic connections with deeperaquifers, mainly in the littoral part of Recife, where hydraulic potentiometric head of the Cabo aquifer is 60 m belowsea water level in some places, with conditions for recharge from shallower aquifers. Through a multi-isotopiccharacterization (87Sr/86Sr, 11B, 18O-SO4, 34S-SO4) of sampling of 19 wells and 3 surface waters, carried outduring two field campaigns with additional geochemical parameters (major ions, noble and major gases, CFC´s andSF6), the spatio-temporal variability of groundwater quality was investigated. The detection of CFC´s, implying amodern recharge component, highlighted the vulnerability of Boa Viagem and Barreiras to surface contaminations.The increasing mineralization and decreasing 87Sr/86Sr from the inland sector wells to the wells located close tothe coast or estuary, with higher well and population densities, were attributed to water-rock interactions along thenatural or human-induced potentiometric gradients. Along with this trend appeared an environmental pressure gradient,highlighted by 11B related to sewage or surface channel networks. These sources are purveyors of chloride,nitrate and sulfate, which are found in varying amounts (13,6< [Cl]<2826,8 mg/L; 0<[NO3]<103,9 mg/L; 1,7<[SO4]