Geochemical evidence of seawater intrusion into a coastal geothermal field of central Greece: example of the Thermopylae system

Thermal water of Thermopylae and from other geothermal fields located in the southern part of the Sperchios basin (central Greece) are characterized by high salinity (total dissolved salts, or TDS, range from 1.2 to 30.3 g L-1) associated with a degassing of CO2. To determine the mineralization processes, geochemical and isotopic investigations (major elements, O-18, H-2 and C-13) have been carried out upon 17 thermal waters from springs and boreholes. This study emphasizes that all the thermal waters result from the mixing of a seawater end-member, several fresh water components depending on the field location, and a mantle-derived CO2 rising upward through an E-W fault system. The seawater identified in the thermal mixture is likely to be evolved Aegean seawater (ASW). Once intruded into the basin sediments, the trapped seawater has its chemical content modified by both water-rock interactions and massive dissolution of the deep CO2 (pCO(2) of 100.5 atm). The modelling performed with PHREEQC indicates that the anomalous major ion ratios measured in the so-called evolved ASW are explained by the dissolution of calcite and dolomitization process associated to precipitation of gypsum within the thermal aquifer.