Isotopic composition and origin of mineral and geothermal waters from Tuşnad Băi Spa, Harghita Mountains, Romania

Different types of free water have been analysed for their deuterium and chemical composition in order to evaluate their source, mixing phenomena, and the underground dynamics. All types of ground water (mineral, geothermal and wellhead water) display δD values (− 82.6‰ to − 72.6‰) similar to surface waters from the studied area (− 77.1‰ to − 73.6‰). The global salt content varies from 1102 to 8707 mg/l for the groundwater, and from 46 to 392 mg/l for the surface water. From the co-variation between the δD values and the chemical composition of the waters, as well as from the seasonal variation of these two parameters, it is evident that the free water from Tuşnad Băi are meteoric in origin. The mineralization of the mineral water took place by means of an intense underground circulation, probably in the fault system developed within the Neogene magmatites.