Hydrogeochemistry and isotope hydrology were used to investigate groundwaterrnrecharge and flow processes in Borena lowlands. The study area is characterized byrnsemi-arid and arid climate. Surface water flows are intermittent. The available waterrnresource, therefore, restricted to groundwater. Precambrian basement, Tertiary andrnQuaternary basalt, and recent alluvial and elluvial sediments are major geologic units.rnFractured and weathered basalt is the principal aquifer.rnThe hydrochemical analysis shows that Borena groundwater varies in type over a widernrange even within few kilometer intervals and broadly distributed rather than formingrndistinct clusters. However, three major water types can be distinguished depending onrntheir position on a Piper diagram. These are Ca-Mg-HCO3, Na-HCO3 and Ca-Mg-SO4-rnCl/Na-Cl/SO4 water types. In most cases Ca-Mg-HCO3 type waters occur in areas ofrnrecharge and Na-Cl/Na-SO4 type waters in rift/lower lying section. The intermediate waterrntypes are mixed and lay between these two end members. Ten subgroups werernexamined from the hierarchical cluster analysis in which distinct hydrgeochemicalrnprocesses were clearly observed. The dissolution of minerals like olivine, pyroxenes,rnplagioclase, anhydrite, halite and CO2 gases are deriving the observed naturalrngroundwater chemistry, which are in most cases, balanced by the precipitation of calciternminerals and weathering/formation of clay minerals like Ca-montmorilonite, chalcedony,rnillite, and K-micas. Cation exchange reactions, particularly in Sarite lower plains, werernalso controlling chemical composition of the groundwater.rnThe isotopic composition of _18O and _2H in groundwater varies from -5.080/00 to 0.080/00rnand from -290/00 to -2.40/00 respectively. In this work the altitude effect on _18Orncomposition shows a depletion of -0.10/00 per 100m elevation rise. Three rechargernmechanisms; direct precipitation, regional groundwater and/or fast selective and flashrnfloods recharge mechanisms were known from the groundwater isotope interpretation.rnThe hydrogeochemical and environmental isotope analysis result clearly indicatesrnregional groundwater flows from elevated sections for instance; Elwaya, Yabello, Utalo,rnGololcha via the middle Gelchet plains to main Ririba fault. A flow from Mega andrnMegado escarpments to megado/Biloko lowlands is also identified.rnFor the sub basin recharge rate was estimated using chloride mass balance method andrnis 28mm/yr. The recharge rate shows variations mainly with topography, groundwaterrnchloride content and annual rainfall amounts. The northwestern (Mermero), northeasternrn(Yabello) and eastern (Mega) parts receives highest recharge where as the southwesternrnand southern sectors get the lowest groundwater recharge rates.rnKeywords: hydrogeochemistry, environmental isotopes, groundwater recharge, semi-arid, Borena lowlands