The studied area is located in the northen part of the central s!(5ctor of the MainrnEthiopian Rift. It is bounded by 7Â° 35' N & 812' N latitudes and 38 25' E & 39Â°15' Ernlongitudes. It is covered by Pliocene to Recent volcanic rocks and lacustrine sediments.rnVolcanic rocks are dominantly acid ignimbrites, pumices and lavas with a few basalticrnlava flows and strombolian scoriae. The maximum altitude in the area is 4080 metersrnabove sea level at the tip of Chillalo mountain and the lowest point is 1636 metersrnabove sea level at the western shore of Ziway lake.rnThe effective annual depth of precipitation in the area is 936.62 mm, the actual andrnpotential evapotranspirations are 735.87 mm and 893.64 mm respectively in thernlowlands and 639.06 mm and 695.83 mm respectively in the highlfnqs. From the totalrnmean annual volume of water entering thlla~e about 123.3 x 10 m of water leavesrnthe lake as surface runoff, about 88.4 x 10m of water leaves the lake as groundwaterrnoutflow and about 643.91 x 106 m3 of water leaves the lake in the form of evaporation.rnThe main aquifers in the area are coarse grained lacustrine sediments, scoria cones,rnhyaloclastites and highly weathered and fractured basalts and ignimbrites. Depth ofrnground water level ranges from less than two metres up to 130 meters and the generalrngroundwater flow direction is towards the lake from east, west and north of the area andrnout of the area from the southern part of the lake.rnThe water in the area is mainly of sodium bicarbonate type and a few are of calciumsodiumrnbicarbonate type. The chemical analyses of ground waters of the lowlandsrnshow high total dissolved solids and high fluoride concentrations.. High fluoridernconcentrations may result from input from fumarolic activity, water-rock interaction andrnlow Ca concentrations. Generally ground waters from the lowlands have poor qualityrnwhile surface waters and ground waters in the highlands have good chemical quality forrndifferent purposes.