Hydrogeochemical characteristics and evaluation of the geothermal fluids in the Gazlıgöl geothermal field (Afyonkarahisar), Western Anatolia, Turkey

Abstract

The Gazlıgöl geothermal field located in 20 km northern of Afyonkarahisar in Western Anatolia region of Turkey is one of the most important geothermal areas in the Afyon-Akşehir Graben on the basis of potential. The present study has been performed to investigate the hydrogeochemical and isotopic characteristics and evaluate the origin, reservoir temperature and processes controlling the chemical composition of thermal waters of the Gazlıgöl geothermal field. Discharge temperature, electrical conductivity and pH of the thermal waters vary from 58.5 to 74 °C, 3910 µS/cm to 4050 µS/cm and, 7.05 to 7.54, respectively. The Gazlıgöl thermal waters are chemically of Na-HCO3 type, with high salinity, while the cold waters have mainly Ca-Mg-HCO3, Ca-Na-HCO3 and Ca-Mg-HCO3-SO4 types, with low salinity. Geochemical processes controlling chemical composition of the thermal waters are mainly water-rock interactions including dissolution and/or weathering of mostly silicates and partially carbonates and ion-exchange reactions. Besides, the bacterial sulfate reduction is another main process leading to the depletion of SO4 in the thermal water. Higher contents of some minor elements in the thermal waters, such as F, B, Li, As and Sr, are probably derived from enhanced water-rock interaction, and the minor elements can be regarded as indicator of residence times and flow paths.Geothermometer applications including quartz geothermometers and SiO2-K2/Mg, mineral saturation state and silica enthalpy mixing models provide the most reliable estimations of reservoir temperature in the varying of 100-150 °C for the Gazlıgöl thermal waters. Accordingly, the Gazlıgöl geothermal system can be classified as a low-moderate geothermal resource. The isotope data (δ18O, δ2H and 3H) of the thermal waters point to their deep circulating meteoric origin, and let estimating of recharge elevation varying from 1300 to 1400 m.a.s.l. These elevations suggest the mountainous region in the north-northeastern of the study area as the recharge area of the geothermal system. Long-term circulation of meteoric waters within the aquifer is confirmed by low tritium ratios (< 1 TU) of the thermal waters although the fluids do not achieve thermodynamic equilibrium. Based on the all gathering data, a conceptual model of the functioning of geothermal system has been constructed. Accordingly, geothermal fluid is heating at deep zones due to the high geothermal gradient caused by active fault systems and heated waters rise toward the shallow levels through faults and fractures. During moving to the surface, they undergo conductive cooling by mixing with the cold groundwater at shallow depths and during contacting with the colder rocks.