Hydrogeology of Adigrat and Surrounding Areas Eastern Tigray, Northern Ethiopia

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Date

2006-06

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Addis Ababa Universty

Abstract

The study area has a total surface area of 152km2 with a wide difference in altitude ranging from 2000 m.a.s.l to 3000 m.a.s.l (meters above sea level). The high altitude topography mainly occupies the western scarp of the study area. The central and eastern parts in turn have rugged topography with flat to gentle slopes. The paleozoic-mesozoic sedimentary rocks cover larger part of the study area. The dominant lithological units in the area are the alluvial sediments unit, the tertiary basalt unit, the Adigrat sandstone unit, the Edagaarbi glacial unit, the Enticho sandstone unit and the metamorphic rocks unit.The main structures of the area are sedimentary beds, cross bedding, fracture/joint sets and foliation. From long term mean monthly rainfall data, the area receives 694mm mean annual rainfall. The potential evapotranspiration of the area calculated using the Penman combined method is 1261mm.The actual evapotranspiration computed by the Thorthwaite-Mathes soil balance model is 583mm. The mean annual surface run off that leaves the catchment is 62mm of water. The amount of groundwater recharge is 46.4mm. Based on pumping test data analysis results and additional qualitative field descriptions such as degree of weathering, extent and frequency of fractures, the rocks in the study area are classified in to four hydrogeological units These are rocks with high permeability (K>1.45m/d), rocks with high to medium permeability (K=1.45m/d to 0.15m/d), rocks with medium to low permeability (K= 0.15m/d to 0.09m/d) and rocks with very low permeability (K<0.09m/d). The highly permeable lithology corresponds to the alluvial sediments and fractured Enticho sandstone. The Edagaarbi glacial and the Adigrat sandstone rock units that occupy high topography area with steep slope are under the very low permeability group. Hydrochemical study indicated the presence of two major water types in the study area. These are Ca-Mg-HCO3 and Ca-Na-HCO3. One borehole water sample shows Ca and NO3 as dominant cation and anions respectively. The high nitrate concentration in the borehole is attributed tocontamination by domestic wastes. Bacteriological analysis result conducted in the years 2000 and 2005 indicated the presence of E.coli and coliform bacteria, above permissible limit for drinking, in a borehole that supply water to the residences of Adigrat town. With exception of the above, the groundwater quality with respect to drinking, agricultural and industrial water quality standards is within the permissible limits set by different organizations. However, it has some limitations for textile and paper manufacturing plants.

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Adigrat and Surrounding Areas

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