Hydrogeochemical and isotopic characterization of the El-Tarf geothermal aquifer (Algerian−Tunisian border): Implications of the regional geodynamic structure and the water−rock interactions

The geothermal aquifer complex investigated in this study comprises a component of the eastern geothermal reservoir of Northern Algeria that is located close to the Algerian−Tunisian border. Seven thermal and two cold water springs were sampled in the framework of this study. The area hosts the region's second hottest spring discharging at a temperature of 67 °C. The thermal waters are carbogaseous type and are dominated by a Na−HCO₃ composition. Isotopic analyses of deuterium and oxygen-18 confirm their meteoric origin. Radiogenic strontium isotopes (⁸⁷Sr/⁸⁶Sr up to 0.710841) suggest interactions with a silicate basement, probably composed of granodiorites and micaschists. Interaction with the Triassic evaporites results in a total dissolved solids (TDS) concentration of up to 1773 mg L⁻¹. The thermal waters emerging through the faults have a temperature ranging between 36 and 67 °C. These waters mix with cooler waters during their ascent. The use of various chemical geothermometers yielded a maximum reservoir temperature of 78–162 °C for the Bouhadjar region. Using the average regional geothermal gradient (43.5 °C·km⁻¹) and the aforementioned temperature interval, an estimated depth range of 1793–3731 m was computed for the geothermal water reservoir. All the thermal waters are found oversaturated with respect to kaolinite and K−mica reflecting the dissolution of granodiorites and muscovite-rich micaschists. High P_CO2 levels (0.07–1.07 atm) suggest the existence of a thermo-decarbonation reaction taking place in the tectonic collision zone. This highlights the important role played by major geological structures occurring in such a geothermal system.