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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Keywords
Adigrat and Surrounding Areas