Preliminary hydrogeological investigation of the Upper Tekeze catchement.
Mr. Engida Zemedageghu
Seifu Kebede (PhD)
Ebenezer Kefeni
Water Works Design and Supervision Enterprise,
Abstract
The upper Tekeze basin has complex hydrogeological characteristics that are mainly attributed to
geomorphologic and topographical settings. The catchment is highly dissected, degraded and affected
by regional and local structures (running in the N-S and E-W direction). The exceeding percentage of
the area is characterized by steep slopes. Topographical elevation ranges in between 4500m and
1000m; mountain range of Ras Dashen (the highest point of the country) is also located in this basin.
Volcanic products, the oldest deeply weathered Ashangi basalt marks large area coverage with
localized Aiba and Tarmaber formation mainly basalt. The Mesozoic formation (lower sand stone) is
confined to north and slightly north east, which out crop is associated with tectonic episodes.
Northern end of the study area below is characterized by Paleozoic crystalline rocks that comprises
schist . Patches of alluvial deposit located in the east parallel to Tirare River course has implication of
palochannels effects on groundwater occurrence.
Groundwater potential of UpperTekeze is not yet quantified in detail even though previous studies
indicate presence of low yielding shallow groundwater. Few boreholes result for town water supply
suggests possibility of deep groundwater occurrence in limited hydrogeological regimes. Semen
mountain massifs may serve as recharge zone. Available perennial rivers and periodical streams are
restricted in profoundly cut narrow gorges. Shallow groundwater, springs and small streams are used
for water supply and small scale irrigation. Water supply deep boreholes (>200m) with good yield
(10l/s) are tapped in Lalibela and Sekota area, their result suggest presence of promising confined
aquifers. Aquifer geometry and setting of the basin is considerably influenced by geology, topography
and structural condition.
Field and Laboratory water quality analysis of Upper Tekeze basin demonstrates its suitability for
both irrigation and water supply. Water samples collected from the highland are fresher than those
obtained from the low lands. The maximum chemical concentration record is noted in water samples
taken from the sedimentary formation in Sekota area. Due to ambient rock water interaction,
solubility of the rock and residence time samples from the sedimentary formation tends to have
higher TDS. Generally, the water is Ca-HCO3- type having 470µS/cm average electrical conductivity
seldom showing considerable concentration of chloride and sulphate.
Geological, Hydrogeological and Geophysical investigations were so far used to delinate potentially
high groundwater potential zones. The zones are localized to the Belesa plain in the left bank of the
Tekeze River, the Lalibela plain in the right bank and the Sekota area. Possibilities of using the
groundwater for irrigation at large scale and house hold scales will be investigated under the project.
The principal objective of this work is to map the groundwater resources and quantify the
groundwater potential of the basin. Recharge, aquifer geometry, groundwater occurrence, water
quality will be investigated. It is envisaged to produce hydro-geological map at scale of 1:250,000.
Hydrogeological maps at scale of 1:50000 will be prepared for selected sites.