The hydrogeological and tectonic features as the crucial factors for the hydro-geochemistry of surface and groundwater in active geothermal systems: The case study of Methana Peninsula in Greece

Abstract

Active geothermal systems developed in coastal volcanic environments are characterized by complex interactions among magmatic fluids, groundwater, seawater, and structurally controlled permeability. The Methana Peninsula (western Hellenic Volcanic Arc, Greece) represents a low-temperature geothermal system where such interactions remain insufficiently constrained. This study integrates new hydrochemical data (80 physicochemical parameters from 11 cold and thermal water samples), structural analysis, and satellite-based thermal remote sensing to evaluate the origin, evolution, and discharge pathways of fluids within the Methana geothermal system. Thermal waters (29.9–35.7 °C) are characterized by Na–K–Cl–SO₄ compositions, elevated electrical conductivity (up to 62,354 µS/cm), and significant enrichment in Li, B, Sr, Rb, and Cs. Binary and ternary geochemical diagrams indicate extensive mixing between hydrothermal fluids and seawater, with subordinate interaction with carbonate-hosted groundwater. Cold groundwater exhibits enrichment in Li, B, Sr, and Rb, reflecting water–rock interaction with overlying calc-alkaline volcanic formations rather than direct mixing with thermal waters. Na–K–Mg relations classify the waters as immature due to Mg enrichment from mixing, whereas Cl–SO₄–HCO₃ projections suggest deeper fluid contributions masked by seawater dilution. Winter Landsat-9 thermal imagery reveals submarine discharge zones spatially aligned with mapped and previously unmapped fault systems, confirming tectonic control on fluid upflow and coastal discharge. Integration of hydrochemical and structural evidence supports a conceptual model in which deep hydrothermal fluids ascend along active fault zones, undergo significant seawater mixing, and discharge both onshore and offshore. The results refine the hydrogeological conceptual model of Methana and highlight the dominant role of tectonics and seawater mixing in shaping the chemistry of coastal geothermal systems within the Hellenic Volcanic Arc.