Assessment of recharge and flowpaths in a limestone thermomineral aquifer system using environmental isotope tracers (Central Portugal)

Abstract

We conducted chemical and isotopic analyses to develop and test a hydrogeological model of thermomineral water circulation in a limestone aquifer system at Caldas da Rainha (Central Portugal), contributing to future borehole-drilling and development strategies, with the aim of extracting waters with the best possible flow and/or temperature. The thermomineral waters (T ≈ 33 °C) discharge from springs and boreholes located close to a locally N–S-oriented oblique fault (60°E) that places loamy and detritic Upper Jurassic rocks and Hettangian-Rhaetian marls (and evaporitic deposits) in contact. 14C determinations indicate a pmC content between 29.33±0.14 and 44.39±0.20 pmC. The presence of HCO, Ca2+ (and Mg2+) are ascribed to water–limestone interactions, while Na+, Cl− and SO concentrations are mainly associated with the dissolution of halite and gypsum lenses found along the regional syncline structure. The δ18O values of Caldas da Rainha thermomineral water were slightly lower than those of shallow groundwater from the Upper Jurassic rocks, suggesting the existence of two distinct aquifer systems. The different isotopic composition of water also indicates that the main recharge of the thermomineral waters likely occurs in the Middle and Lower Jurassic limestone formations of the Candeeiros Mountains. The presence of 3H (from 1.1 to 2.8 TU) in some thermomineral borehole waters (showing rather similar geochemical signatures) suggests mixing of small amounts of shallow groundwater with thermomineral waters, as a result of leaking borehole casing construction or a recharge when the 3H content in the atmosphere was higher than that at present. Caldas da Rainha thermomineral waters having δ34Ssulphate and δ18Osulphate values between+14.9 and+19.1 ‰ and+11.1 and+16.2 ‰, respectively, indicate that the sulphate is the result of water–rock interaction with evaporitic formations. The obtained chemical and isotopic data have informed the further development of a hydrogeological model that will be used by decision-makers, in order to contribute to the socio-economic development of the spa region.