Hydrogeochemical and isotopic characteristics of thermal waters in Gazlıgöl (İhsaniye-Afyonkarahisar) region

This study was carried out to determine the hydrogeochemical properties, origin, recharge areas and isotopic composition of the hot waters in the Gazlıgöl geothermal area of İhsaniye District, which is approximately 36 km northwest of Afyonkarahisar Province. 12 cold water samples (deep well, shallow well and spring) and 8 thermal water samples (7 thermal water wells and 1 reinjection well) representing the study area were collected. The temperatures of cold waters vary between 14-24,5 °C, pH values between 7.08 -8.25 and electrical conductivity values between 136-1927 µS/cm. The temperatures of thermal waters vary between 60-74 °C, pH values 7.05 -7. 54 and electrical conductivity values vary between 3920-3975 µS/cm. Thermal waters are Na-HCO3 type and cold waters are Ca-Mg-HCO3, Ca-Mg-HCO3-SO4, Ca-Na-HCO3, Na-Ca-HCO3 and Na-Ca-HCO3-SO4 type waters. The reservoir of the thermal waters in the Gazlıgöl geothermal area is composed of quartzite, quartzschist and calcschists with abundant fractures, cracks and melting gaps belonging to Precambrian-Cambrian aged Afyon metamorphics. The clayey levels of the Neogene units, and the micaschists in the Afyon metamorphics constitute the cap rocks. Meteoric waters collected in the permeable units of Afyon metamorphics forming the main lithology of the recharge regions, infiltrate into the deep and heat up due to the high geothermic gradient. During ascent towards the surface, thermal waters cool by conduction and mixing with the colder groundwater at the shallower depths. The δ18O and δD contents of cold waters respectively vary from -11.03‰ to -6.09 ‰ and -80.17 ‰ and -48.23 ‰, and the δ18O and δD contents of the thermal waters respectively range from -11.08 ‰ to -10,67 ‰, and -84.14 ‰ to -83.38 ‰. The δ18O and δD values of thermal and cold waters indicate a meteoric origin. However, more negative δ18O and δD values of the thermal waters compared to cold waters indicate that they are recharged from higher elevations. The thermal waters have low tiritium (average 0.31 TU), high chloride and EC values indicater that the thermal waters have a deeper circulation and a longer residence time in the aquifer than cold waters. However, the high tritium (average 5.35 TU), low chloride and EC values of cold waters indicate that they have a much shallower circulation and shorter residence time than hot waters. The reservoir temperatures calculated using various geothermometers for the Gazlıgöl geothermal field are in the range of 100-149 °C.