Considerations concerning the origin of the Estoril (Portugal) thermal water

Abstract

In the urban area of Estoril, a Portuguese tourist village 20 km westward from Lisbon, hot mineral waters (thermal waters) spout out from natural springs which have been known since the 19th century. The thermal waters are of the sodium chloride type, with total dissolved solids and temperatures higher than the ones of the regional waters. Isotopic data are consistent with a meteoric origin for both the regional and thermal waters of the Estoril area: δ180 (‰), -4.16 to -3.52 and δ2H (‰), -25.5 to -18.7. The thermal water composition can be derived from the regional water composition assuming the dissolution of evaporite minerals, cation exchange and precipitation of calcite. The thermal water flow system has probably the recharge area somewhere in between Estoril and the Sintra mountain. The elevation difference between the recharge area and the sea provides the driving force for groundwater movement to the Estoril area where the upward movement of the mineralised and warm water is controlled by an impermeable barrier of dykes and open fractures in the pre-existing rocks.