Environmental Stable and Radioactive Isotopes in the Assessment of Thermomineral Waters in Lisbon Region (Portugal): Contributions for a Conceptual Model

Abstract

The existence of thermomineral waters in the Lisbon region (western Portugal), has been known for centuries. Several authors have investigated the characteristics of these thermomineral waters, but their conceptual model (recharge zone, processes controlling the origin and chemical evolution, depth of circulation, etc.) remains unknown. This work highlights the advantages of assessing the conceptual model in urban coastal areas, where different saline sources coexist, combining chemical analyses with environmental isotopes and geochemical modeling. The final physicochemical composition of thermomineral waters seems to be mainly controlled by water–rock interaction processes and different mixing degrees between HCO3-Ca regional groundwaters, cold dilute shallow groundwaters and highly salinized fluid (brackish or brine or non-recent seawater) probably trapped at depth. Based on these results, neither the contribution of polluted water nor the leaching of evaporite rocks seems to be the main salinization factor of the warmer and saline waters. A positive correlation exists between the lowest radiocarbon activities (average apparent ages ~6 ka BP) and highest water temperature and salinity (up to 39 °C and 6.3 g/L), suggesting progressive salinization and deeper circulation (up to 1200 m depth) into the hosting formations, where mixing with trapped salinized fluids probably took place, as confirmed by geochemical modeling. Finally, very favorable geological conditions in the Estoril region allow for fast rising and the occurrence of these thermomineral waters at relatively shallow depths.