Hydrochemistry of the Çeşme geothermal area in western Turkey

Hydrochemical characterization of thermal waters discharged from springs and wells in the Çeşme geothermal area show that there are two groups. One is of thermal waters from a lower aquifer composed of Triassic karst limestones, which are the main potential reservoir of the area. They are of Na–Cl type and between 37 and 62°C and have total dissolved solids (TDS) with around 35 000 mg/l. The other group are thermal waters derived from an upper aquifer formed by Neogene terrestrial sediments. These have lower discharge temperatures (37–40°C) and lower TDS values due to their having mixed with local groundwater before emerging. The isotopic and chemical data shows that the initial aqueous solution is a mixture of modern seawater and meteoric water in various proportions. Enrichment in δ18O and δD values and tritium contents (8±1 TU) of thermal waters reflect a rapid circulation and the contribution of modern seawater. The thermal waters are undersaturated with respect to gypsum but oversaturated, or around equilibrium, with respect to dolomite and calcite. Several chemical geothermometry techniques applied to Çeşme geothermal waters gave estimated reservoir temperatures of around 85–100°C.