Hydrogeochemistry of the thermal waters from the Pamukkale and Karahay?t Geothermal Fields (Denizli Basin, Southwestern Anatolia, Turkey)

Abstract

The chemical and isotopic properties of thermal waters (Pamukkale and Karahay?t) and cold springs from the Pamukkale and Karahay?t geothermal fields (PGF and KGF, Southwestern Anatolia, Turkey) are investigated in order to establish a conceptual hydrogeochemical-hydrogeological model. These thermal waters derive from metamorphics of Menderes Massif and carbonates of Lycian Nappes and emerge along northern normal faults in the Denizli Basin; they are commonly used for heating of greenhouses and bathing facilities. Discharge temperatures of thermal waters are mean of 33°C for Pamukkale and 53°C for Karahay?t, whereas cold groundwaters are mean of 13°C. Pamukkale and Karahay?t thermal waters are mostly of Ca-Mg-HCO3-SO4 type, whereas cold groundwaters are mainly Ca-HCO3 types. In the reservoir of the geothermal system, dissolution of host rock and ion-exchange reactions change thermal water types. High correlation in some ionic ratios (e.g., Na vs. Cl, Mg vs. Cl, K vs. Cl, HCO3 vs. Cl, SO4 vs. Cl) and high concentrations of some minor elements (e.g., Sr, B, Cl, F) in thermal waters likely originate from enhanced water-rock interaction. Pamukkale and Karahay?t thermal waters are oversaturated at discharge temperatures for carbonate and silica minerals allowing increase to a carbonate- and silica-rich scale and correspond to travertine/tufa precipitation in the discharge area. Water samples from PGF and KGF have not reached complete chemical re-equilibrium, possibly as a result mixing with groundwater during upward flow. Geothermal reservoir temperatures are calculated as 57-61°C (mean 59°C) for Pamukkale and 41-73°C (mean 57°C) for Karahay?t fields, based on the chalcedony geothermometry. Very negative ?18O and ?2H isotopic ratios (~-9 ‰ and ~-58 ‰) and high tritium values (3-10 TU) of the Pamukkale thermal waters are indicative of a shallow circulation and a meteoric origin, and radioactive decay with a mixture of pre-modern (old) water mixed with modern (new) water recharge or recent additions. The other hand, negative ?18O and ?2H isotopic ratios (~-8 ‰ and ~-51 ‰) and low tritium values (1 TU) of the Karahay?t thermal waters reflect a deep circulation pathway and a meteoric origin. These waters likely derived from the infiltration of rainwater through fractures and faults to the deep hot reservoir. Subsequent heating by conduction in the high geothermal gradient setting (resulting from regional crustal thinning) controls geothermal waters upwards along faults and fractures that act as hydrothermal pathways. The CO2 gases of Pamukkale field have negative ?13C values with ~-4 ‰, probably indicating mantle-derived magmatic origin. This study was sponsored by the Scientific and Technical Research Council of Turkey (TÜB?TAK research grants ÇAYDAG113Y551).