Hydrogeochemistry of the thermal waters from the Yenice Geothermal Field (Denizli Basin, Southwestern Anatolia, Turkey)

Abstract

The chemical and isotopic properties of thermal waters (Kamara and Çizmeli) and cold springs from the Yenice Geothermal Field (YGF), in southwestern Anatolia, Turkey are investigated in order to establish a conceptual hydrogeochemical-hydrogeological model. These thermal waters derive from Menderes metamorphic rocks and emerge along normal faults; they are commonly used for heating of greenhouses and bathing facilities. Discharge temperatures of thermal waters are 32°C to 57°C (mean 51°C) for Kamara and 35°C to 68°C (mean 47°C) for Çizmeli, whereas deep groundwaters are 15°C to 20.1°C (mean 17°C) and shallow groundwaters are 12 to 16°C (mean 15°C). Kamara and Çizmeli thermal waters are mostly of Na–Ca–HCO3–SO4 type, whereas deep groundwaters are Ca–Mg–HCO3 and Mg–Ca–HCO3 types and shallow groundwaters are mainly Mg–Ca–SO4–HCO3 and Ca–Mg–HCO3 types.In the reservoir of the geothermal system, dissolution of host rock and ion-exchange reactions changes thermal water types. High correlation in some ionic ratios (e.g. Na vs. Cl, K vs. Cl, HCO3 vs. Cl) and high concentrations of some minor elements (e.g., As, Sr, B, Cl, F) in thermal waters likely derive from enhanced water–rock interaction. Water samples from YGF have not reached complete chemical re-equilibrium, possibly as a result mixing with groundwater during upward flow. Geothermal reservoir temperatures are calculated as 89–102°C for Kamara and 87–102°C for Çizmeli fields, based on the retrograde and prograde solubilities of anhydrite and chalcedony.Based on the isotope and chemical data, a conceptual hydrogeochemical-hydrogeological model of the YGF has been constructed. Very negative δ18O and δ2H isotopic ratios (Kamara: mean of −8.43‰ and −56.9‰, respectively and Çizmeli: mean of −7.96‰ and −53.7‰, respectively) and low tritium values (<1 TU) reflect a deep circulation pathway and a meteoric origin. Subsequent heating by conduction in the high geothermal gradient setting (resulting from regional crustal thinning) drives geothermal waters upwards along faults and fractures that act as hydrothermal pathways. Positive δ13C ratios (+9.45‰ for Kamara and +7.28‰ for Çizmeli) indicate a metamorphic origin of thermal waters. Negative carbon isotope ratios (−8.40‰) found in the cold groundwaters are linked to exchange in freshwater carbonates of the Sazak Formation.