Mechanism of degradation of the quality of natural water in the lakes region of the ethiopian rift valley

The natural waters of the Lakes Region in the Main Ethiopian Rift (MER) show serious problems of chemical quality. The high content of fluoride reaches 300 mg l−1 and affects the health of the population who live in this area. Furthermore, the alkaline and sodic characteristics (82(Ca+Mg)], which results from the weathering and dissolution of the volcanic rocks. The predictive qualities of the model AQUA were tested in the context of this region. A simulation of the concentration of the waters by evaporation has shown chemical variations relatively similar to those obtained with field data which include the minor chemical elements such as the fluoride or the dissolved silica. For field data, enrichment is more variable and may represent localised conditions of mineralization. This model is then used to study a method of defluoridation by supplying these waters with calcium in the form of gypsum. This method also allows the reduction of the alkalinizing and sodifying characteristics of the waters without reaching the fluoride standard concentration accepted for drinking waters. For drinking purpose, waters of the Ethiopian rift should undergo another method of defluoridation.