Fluid geochemistry of the Chios geothermal area, Chios Island, Greece

Two separate aquifers have been identified in Chios Island. The first one, Nenita, is found in the southern part of the island and the other one, Aghiasmata and Aghia Markela, in the northern part, which is characterized by high salinity waters. Chemical and isotopic contents were used for the investigation of the origin and evolution of thermal water in sedimentary and volcanic rocks, for the estimation of the mixing process between meteoric and seawater involved in the deep geothermal systems and for the evaluation of the deep aquifer temperature. t borehole and spring waters discharging in Chios Island, Greece, change in composition from earth–alkaline–bicarbonate-type to alkaline-type chloride. The chemical and physical characteristics of bicarbonate well waters show interaction between meteoric waters and Neocene rocks. In general, for these waters as their mineralization increases the Mg2+ contents increase. The deuterium and oxygen contents of these water samples indicate a meteoric origin. + and Cl− ions dominate the chemistry of the thermal waters of Aghia Eleni spring. This thermal water appears to be a mixture of seawater and ground water. The marine contribution for this sample is 80–89%. The chemical and isotopic data of the thermal Cl-rich water springs of the northern part of the island, Aghiasmata and Aghia Markela, suggest that they are fed by thermal water mixed with local groundwater and seawater respectively. The parent geothermal liquid is either a mixture mad up of local groundwater (∼40%) and arc-type magmatic water (∼60%), that did not exchange oxygen isotopes, or a mixture constituted by local groundwater (∼70%) and seawater (∼30%), which experienced a significant oxygen isotope exchange. Assessments from chemical and isotopic geothermometer applied on the thermal waters springs suggest the probable existence of a deep geothermal reservoir of middle–high enthalpy (220 °C) in the northern (Aghiasmata and Aghia Markela) part of the island and low enthalpy (80 °C) in the southern part (Nenita–Patrika–Thimiana) of the island.