This work is the study of the hydrogeochemical conditions of the 1506km2 Mojo RiverrnBasin, within the Ethiopian Rift Valley, found about 70km2rnsouth east of Addis Ababa,rnbetween longitudes 380 56' E â€“ 390 17'E and 80 34' N â€“ 90 05' N latitude. The Mojo RiverrnBasin is in a state of progressive population growth and industrialization as well asrnagricultural activities, which are all potential sources of surface and groundwaterrnpollution in the Basin.rnThe main objectives of the present research are identifying the major pollutant in thernMojo River, study the groundwater quality in the basin and identify its pollutionrncondition of groundwater using hydrogeochemistry. The final use of such a study is tornindicate the possible measures to be applied to prevent or reduce local sources ofrnpollution of Mojo River in turn regional groundwater pollution.rnCollection of water samples from surface (stream) and groundwater (borehole, dug wellrnand spring) was done in two field operations at two different time periods. Physical andrnchemical tests have been carried out in the field and in the laboratory. In the fieldrnphysical parameters such as pH, temperature and conductivity were measured in situ.rnThe main rock types in the Mojo River Basin range in age from Upper Miocene tornHolocene and consist of consolidated volcanic(plateau and Rift Volcanic) and loosernLacustrine and alluvial deposit.rnOverall, the water in the study area is a calcium-Sodium -Magnesium-Bicarbonate typernwhich is characterized by a high concentration of Ca++, Na, Mg++ and HCO3rn-. Mojo Riverrnis used for variety of purposes by inhabitants along their course. However, the dischargernof domestic and industrial wastes from the town highly limits the applicability of surfacernwater and also the quality of the groundwater in the area is deteriorated by infiltration ofrnpolluted water.