Conceptual hydrogeological model of the Hüdai Geothermal Field (Afyonkarahisar, Turkey) based on hydrochemical and environmental isotopic data

Abstract A comprehensive conceptual hydrogeological model of the Hüdai Geothermal Field (HGF), an important balneotherapy destination in western Turkey, has been developed from water chemistry and isotope data collected from springs and wells across different seasons. Temperature and specific electrical conductivity of the Hüdai geothermal waters (HGWs) ranged from 65.4–82.4 °C and 1727–2156 µS/cm at 25 °C, respectively. The cool waters are mainly Ca–HCO 3 /SO 4 type and the hot waters are mainly Na–HCO 3 /SO 4 type. The δ 18 O and δ 2 H data suggest a meteoric origin for all groundwater. Recharge elevations and temperatures of the groundwaters are inferred to be 2105–2385 m above sea level and 5.5–6.9 ºC, respectively. The δ 13 C data suggest that the source of dissolved inorganic carbon (DIC) in cool waters is plant root respiration, whereas a crustal source is suggested for the HGWs. The radiocarbon ( 14 C) apparent ages of the DIC in the HGWs range from 28 to 48 ka, indicating groundwater recharge occurred throughout the late Pleistocene and early Holocene. The δ 34 S values suggest that coal, magmatic rocks, shale and limestone are the most likely sources of sulfur in the HGWs, whereas sulfur in the cool waters is derived from terrestrial evaporite minerals. Silica geothermometers estimate reservoir temperatures varying between 73 and 116 ºC. The maximum groundwater circulation depth is estimated to range from 0.93 to 2.09 km. The data suggest the HGF has been heated by crustal radioactive decay and heat flow linked with magmatism and metamorphism in western Anatolia during the late Miocene.