Geothermal activities in the Main Ethiopian Rift: Hydrogeochemical characterization of geothermal waters and geothermometry applications

Ethiopia holds an enormous capacity to generate geothermal energy in the volcano-tectonically active zones of the East African Rift System. In this study, we investigate the potential of three geothermal prospect areas in the Main Ethiopian Rift (Dofan-Fantale, Gergede-Sodere, Aluto-Langano). We examine existing and new data from a water sampling campaign, both of which are evaluated in terms of geochemistry and applicability for the estimation of geothermal subsurface temperatures. Several solute geothermometers, Cl–SO4–HCO3 and Na–K–Mg ternary diagrams as well as silica-enthalpy and chloride-enthalpy mixing models were applied to the prevailing alkaline and Na–HCO3 or Na–Cl–HCO3 dominated waters. Mixing was most pronounced in the individual subregions of Dofan-Fantale and Gergede-Sodere, yet these areas still indicate high heat-generating capabilities. The applied enthalpy-chloride mixing model suggests a subsurface temperature of 190 ± 20 °C for these hot springs. This temperature estimate is in good agreement with those obtained from the Na–K and Na–K–Ca geothermometers (185 ± 20 °C) for both geothermal areas. Additionally, for Gergede-Sodere it agreed well with the silica-enthalpy mixing model hot spring water results with the assumption that steam loss occurs before mixing (170 ± 20 °C). Furthermore, the enthalpy-chloride mixing model refers to reservoir temperatures between 300 °C and 370 °C for the Aluto-Langano geothermal fluids, which are in the same magnitude than the ones obtained by the silica-enthalpy mixing model for hot springs, if no steam loss occurs before mixing (270 ± 30 °C). In addition, they are comparable with few known data collected in drilled wells in the Aluto-Langano geothermal field (231–282 °C for LA-4 and LA-8 as well as 315–363 °C for LA-3 and LA-6).