Isotopic and hydrochemical data as indicators of recharge areas, flow paths and water–rock interaction in the Caldas da Rainha–Quinta das Janelas thermomineral carbonate rock aquifer (Central Portugal)

Abstract

Summary An updated conceptual circulation model for the Caldas da Rainha and Quinta das Janelas thermomineral waters was developed. These thermomineral waters (T ≈ 33 °C) are related to a huge syncline ascribed to the regional flow paths. Two diapiric structures were responsible for the uplift and subsequent folding of regional Jurassic carbonate rocks. Environmental isotopic (δ2H and δ18O) data indicates that the main recharge area of the thermomineral system is linked to the Jurassic limestones (Candeeiros Mountains, E border of the syncline). The thermomineral waters belong to the Cl–Na sulphurous-type, with a total mineralization of about 3000 mg/L. The thermomineral aquifer system seems to be “isolated” from anthropogenic contamination, which is typical for the local shallow groundwater systems, due to the existence of impermeable layers composed of a series of loamy and detritic rocks of the Upper Jurassic. The presence of 3H in some thermomineral borehole waters, not accompanied by an increase in SO 4 2 - and NO 3 - , could be ascribed to different underground flow paths and different mean residence time. The δ34S(SO4) and δ18O(SO4) values of dissolved sulphate of groundwaters of the Caldas da Rainha Spas indicate that the sulphate is the result of water–rock interaction with evaporitic rocks (e.g. gypsum and anhydrite) ascribed to the regional synclinal structure.