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Broad hydrochemical survey has been carried out to study the spatial variation of the major ions composition of the surface and groundwater systems in the Ethiopian volcanic terrain and associated Plio-Quaternary sediments. The result revealed wide hydrochemical variations controlled by geological, geochemical, geomorphological and climatological factors. The total ionic concentration increases dramatically from the highlands towards the rift valley following the regional groundwater flow directions to low-lying regions characterized by low annual rainfall and high evapotranspiration. In the rift the total dissolved solids (TDS) variation is dramatic (in places more than 50 fold). The highland waters are predominantly fresh with low total ionic concentrations, TDS varying often from 50 to 1200 mg/l; with dominant ions of Ca, Mg and HCO 3 . The rift valley waters have very high TDS (varying in a wide range between 200 and 73,015 mg/l) characterized by high Na and HCO3, in places highly enriched with F. Waters of the southern and central Main Ethiopian Rift (MER) are dominated by HCO 3 tending to SO 4 and Cl type in northern and central Afar mainly due to changes in lithology related to evaporite deposits and due to the natural chemical evolution of waters as they move from highland recharge to low-lying rift discharge areas. Deep geothermal waters, alkaline terminal lakes of the MER and small highly evaporated saline and brackish lakes of the Afar have the highest ionic concentrations. Keywords : Ethiopian rift, hydrogeology, ionic concentration, TDS, volcanic terrain SINET: Ethiopian Journal of Science Vol. 28(2) 2005: 171-188

Major ions composition of the groundwater and surface water systems and their geological and geochemical controls in the Ethiopian volcanic terrain