Hydrogeochemical pattern and environmental isotope hydrology of coastal Bademli geothermal area (BGA) in western Turkey (Dikili-İzmir): A new geothermal prospect

Abstract

Bademli geothermal area (BGA) is located on the coastline of Dikili-İzmir province and consists of Bademli spring and Hayıtlı areas with 36.8-51°C discharge temperatures, respectively. The waters of Hayıtlı borehole have remarkable seawater mixing ratios like Bademli spring and have Na-Cl water type. Dikili group pyroclastic volcanic units constitute the reservoir rock in the entire geothermal area. The heat source is relatively elevated geothermal gradient caused by extensional tectonics forming E-W trending grabens. Bademli thermal water is plotted in the "immature waters" area in the Na-K-Mg triangular diagram with a calculated seawater contribution of 18%. Therefore, some cation geothermometers are considered unreliable. On the other hand, the silica-enthalpy diagram showed an anticipated reservoir temperature of approximately 240 °C. Hayıtlı borehole water sample is plotted on the "partially equilibrated waters" area in the same triangular diagram and shows a reservoir temperature of 208 °C. The seawater contribution in the Hayıtlı area (16%) is less than the Bademli spring. On the other hand, based on the K/Mg geothermometry, the reservoir temperatures for Bademli spring and Hayıtlı borehole waters are 129 °C and 138 °C, respectively. B, Fe, Mn, and Sb concentrations exceed the tolerance limits of the EPA and Turkish drinking water standards in the area. In addition, due to silica-rich volcanic rocks, Ge solubility increased with temperature in thermal waters and reached 34-45 μg/L. Enrichment of δ1818O and δ2D values can be observed in Bademli spring and Hayıtlı with -3.70‰ and -4.63‰, and -34.5‰ and -37.9‰, respectively. From the chemical and isotopic results, it can be clearly said that this coastal geothermal area is high enthalpy, as evident from the equilibrium temperatures. However, when the thermal water rises to the surface, it mixes with both cold groundwater and modern seawater resulting in a decrease of the discharge temperatures of the springs. Moreover, according to the isotopic and hydrogeochemical data, the area has a high potential for thermal heating of the settlements in the area.