Abstract

Alasehir graben is located in the southern edge of the Gediz graben, which is an important graben for geothermal activity. Thermal waters are hosted by Menderes massif metamorphic rocks, which are made of gneisses, schists and marbles. Impermeable clayey units of the neogene sediments are cap rocks of the geothermal system. Presence of geothermal waters is closely related to normal fault systems and graben tectonic. Meteoric waters recharging the reservoir rocks are heated at depth with geothermal gradient. The AK-2 and KG-1 wells have the third and fourth highest temperatures of Turkey, respectively. Reservoir temperatures of the geothermal system are estimated to vary between 125 and 225°C by mineral equilibria geothermometer, vary between 160 and 240°C by Giggenbach triangular diagram and vary between 150 and 250°C by silica enthalpy-mixture model. Cold water contributions to thermal waters vary from 75 to 95%. Na-HCO₃ water type is dominant for thermal water. Major reaction of the thermal water to change facies is softening reaction. The temperatures obtained from silica enthalpy model, mineral equalibria geothermometers, Na/K, Na/Li geothermometers are more useful than others in the study area. Scaling tendencies of the thermal water are examined. The major environmental problem in the groundwater is high boron concentration which is harmful for agricultural irrigation.

Key words: Alasehir, geothermal systems, hydrogeochemistry, saturation index, geothermometer.