Evaluation of risks of groundwater quality alteration in Recife urban area (Pernambuco, Brazil) using a multi-isotopic approach
Résumé
The Recife Metropolitan Region (RMR) is a heavily urbanized area located in a estuary zone and over a multilayered
sedimentary system on the Brazilian Atlantic coast. In a context of increasing land use pressures, involving
aquifer overexploitation and surface water contamination, and repeated droughts, the identification of groundwater
quality risks in RMR is a necessary management requirement. In this perspective, this work focused on the two
shallow aquifer systems, named Boa Viagem and Barreiras aquifers, located at the interface between the city (the
consumers) and the deeper semi-confined Cretaceous Cabo and Beberibe aquifers. The Holocenic Boa Viagem and
Tertiary Barreiras formations conform unconfined sedimentary aquifers, with no more than 80 m of thickness. Cabo
is the most important groundwater body for Recife private complementary water supply and it has experienced an
intense exploitation in the last three decades. In contrast, Boa Viagem and Barreiras aquifers are more restrictively
used, but it is important to understand their water quality degradation,because of hydraulic connections with deeper
aquifers, mainly in the littoral part of Recife, where hydraulic potentiometric head of the Cabo aquifer is 60 m below
sea water level in some places, with conditions for recharge from shallower aquifers. Through a multi-isotopic
characterization (87Sr/86Sr, 11B, 18O-SO4, 34S-SO4) of sampling of 19 wells and 3 surface waters, carried out
during two field campaigns with additional geochemical parameters (major ions, noble and major gases, CFC´s and
SF6), the spatio-temporal variability of groundwater quality was investigated. The detection of CFC´s, implying a
modern 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 to
the coast or estuary, with higher well and population densities, were attributed to water-rock interactions along the
natural 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]<375,4 mg/L). Nitrate and sulfate are also potentially produced or consumed within the system, featured by
varying sediment texture and organic contents and therefore apparent to a patchwork of biogeochemical reactors, as
demonstrated by 18O-SO4, 34S-SO4 and dissolved major gases. Further, on the cost line high well-density area,
intensive pumping, through intrusion andhydraulic lift, probably explains temporary salinization in some wells.