The thesis presents the results of geological, hydrogeological and hydrogeochemical studiesrnin the Akaki subbasin (Sekelo). This study was to provide an overview of the water qualityrnand the mixing of the groundwater and the surface water. In the study the geology and thernhydrogeology of the area was investigated to provide basic information concerning therndifferent lithologies and the local and sub-regional groundwater flow patterns. The chemistryrnand the stable isotopic composition of both the groundwater and the river water werernconsidered in order to understand the nature and origin of the waters and to further explainrngroundwater and river water mixing patterns.rnAnalysis of the relationship between rainfall and Akaki river shows that the highest flow andrnlevel in the river correspondences to the rainy season (June-September). Therefore, influentrncondition is anticipated during this rainy season. Preliminary water balance estimates usingrnsoil moisture budgeting method in the study area suggest 137 mm groundwater rechargernannually.rnGeological and hydrogeological mapping of the Sekelo basin was conducted in the field andrnmainly from aerial photograph interpretation. Detailed studies of the geology and thernhydrogeology was carried out in the Akaki Well Field D. The rocks of the area are generallyrnvolcanic rocks with only minor fluvial sediments., The volcanic rocks are distinguished into ,,-._-- .--rnlower basalt flows and young basaltic scoria and lava.rnHydrogeological data were collected. These are information from 34 boreholes in the AkakirnWell Field D abstracted from records and supplemented by data collected in the field. The types of data collected included lithological information, aquifer parameter data and boreholernconstruction details.rnAnalysis of these data has shown that on regional scale the groundwater flows from the waterrndivide to the discharge area on the river valleys. The permeabilities of the volcanic rocks inrnthis area are generally high. The important aquifer in the area is the lower basalt in which thernaquifers are found in the fractured volcanics and are usually semi-confined. The youngrnbasaltic scoria is the main recharge area.rnWater samples for hydrochemical and stable isotopic analysis were taken from 9 sites bothrnfrom the groundwater and river water. Chemical analysis data of 27 boreholes in the AkakirnWell Field D was collected from Addis Ababa Water Supply and Sewage Authorityrn(AAWSA) 1996-97. In addition, stable isotopic data of rainfall for Addis Ababa wererncollected from International Atomic Energy Agency (IAEA) Vienna. Correction was made onrnthese data for altitude difference to get the local meteoric water line.rnAnalyses and evaluation of these data enabled to obtain information on the nature of thernwater, in particular the mixing between the groundwater and the river water. The chemicalrnanalysis of the groundwater and the river water shows similar Ca-Mg bicarbonate water type.rnBased on U.S. Environmental Protection Agency's (USEPA) drinking water standard therngroundwater proved to have good quality. However the river water which is remarkablyrninfluenced by human activities is highly polluted. The evaluation of the chemical compositionrnand stable isotope techniques led to the conclusion that the groundwater can be explained inrnterms of mixing series between the rain water and the river water