Water from ten thermal springs, five drinking-water boreholes and one hand-dug well from Hanlé-Gaggadé grabens has been analyzed for major-minor hydrogeochemical parameters and multi-isotope composition: δ2H(H2O), δ18O(H2O), δ18O(SO42−), δ34S(SO42−), δ13C(DIC), 14C(DIC), and δ11B. The interaction of the thermal water with both silicic (basaltic) and sedimentary rocks and its low enthalpy (T ≤ 150 °C) regime make this area an optimal site for testing chemical and isotope geothermometers and optimizing them for similar difficult cases. In this study, the use of radiogenic carbon (14C) reveals that the age of the deep aquifer is between 12,500 and 8700 years ago (late Pleistocene/early-to-mid Holocene), which places it within the East African Humid Period. The oxygen and hydrogen stable isotope ratios of the water molecules, δ2H(H2O), δ18O(H2O), show common characteristics between the groundwater from this area and that of groundwater from nearby local lakes (Asal-Abhé) and palaeolakes (Sakalol-Hanlé-Gaggadé-Dobi), which were interconnected during that period to form the terminal Awash River Basin. The comparison of these aquifers within the transboundary aquifer located between northeast Ethiopia and Djibouti helps to reconstruct both the palaeoenvironmental history and the hydrogeochemical characteristics of this regional aquifer. At present, these data are poorly defined, but it is of fundamental importance for the constructive use of water resources in arid areas. Finally, a conceptual model was proposed for the Hanlé-Gaggadé low-enthalpy geothermal system on the basis of the geochemical and isotope data on the thermal and non-thermal groundwaters combined with geological and tectonic information and the results of recent geophysical studies.
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