Hydrogeology and Hydrochemistry of Lake Ziway Area and the Surrounding

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Addis Ababa University


The studied area is located in the northerrt part of the central s~cto r of the Main Ethiopian Rift. It is bounded by 7° 35' N & 8 12' N latitudes and 38 25' E & 39°15' E longitudes. It is covered by Pliocene to Recent volcanic rocks and lacustrine sediments. Volcanic rocks are dominantly acid ignimbrites, pumices and lavas with a few basaltic lava flows and strombolian scoriae. The maximum altitude in the area is 4080 meters above sea level at the tip of Chillalo mountain and the lowest point is 1636 meters above sea level at the western shore of Ziway lake. The effective annual depth of precipitation in the area is 936.62 mm, the actual and potential evapotranspirations are 735.87 mm and 893.64 mm respectively in the lowlands and 639.06 mm and 695.83 mm respectively in the highI,Pndp. From the total mean annual volume of water entering thlla~e about 123. 3 x 10 m of water leaves the lake as surface runoff, about 88.4 x 10m of water leaves the lake as groundwater outflow and about 643.91 x 106 m3 of water leaves the lake in the form of evaporation. ~be main aquifers in the_area are coarse_grainedJ acustrine sediments, scoria cones~ _ hyaloclastites and highly weathered and fractu red basalts and ignimbrites. Depth of ground water level ranges from less than two metres up to 130 meters and the general groundwater flow direction is towards the lake from east, west and north of the area and out of the area from the southern part of the lake . The water in the area is mainly of sodium bicarbonate type and a few are of calciumsodium bicarbonate type. The chemical analyses of ground waters of the lowlands show high total dissolved solids and high fluoride concentrations. High fluoride concentrations may result from input from fumarolic activity, water-rock interaction and low Ca concentrations. Generally ground waters from the lowlands have poor quality while surface waters and ground waters in the highlands have good chemical quality for different purposes.



Hydrogeology and Hydrochemistry, Lake Ziway Area