Hydrogeology of Nazret
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Date
1994-06
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Addis Ababa Universty
Abstract
The study area is found in the southern part of the
northern sector of the Main Ethiopian Rift some 97 km southeast
of the capital. It covers a surface area of about 370 sq.km.
Elevation varies from below 1600 m.a.s.l. to more than
1970 m.a.s.l .. More than 70% of the area represents low lying
flat land with slop~ of.1 to 10%.
The area is covered by various igneous rocks which are
products of volcanism_that spanned from Pliocene to Recent, and
also by Quaternary sediments. The volcanics include;
ignimbrites, basaltic flows, rhyolitic flows and domes,
unwelded tuffs, pumice and ash deposits. Lacustrine sediments,
alluvium and colluvium represent the non-volcanic deposits in
the area.
Mean annual rainfall of 822.53 mm and mean annual
temperature of 21'C characterize the study area. According to
water balance study over the small sub-catchment at the centre
of the area, generally about 49% of the mean annual rainfall is
lost through evapotranspiration. While about 20% is available
for runoff and the remaining about 30% is accounted for
infiltration to the ground.
The main aquifers in the area are coarse grained
lacustrine and alluvial sediments, pyroclastic rocks (cinders,
volcanic sand and pumice), and weathered or fractured volcanic
rocks mainly basalts and also somtimes ignimbiites
From the pumping test data, 'analyes permeabities and
transmissivities of rocks have been foundto vary: from O. 50 to
78.50 mid and 14.73 to 1355.20 m2/d respectively specific capacity also ranges from 0.26 to 9.13 mJ/hr/m. High
permeability values are observed for the aquifers in the
southern part of the area. The pumping test analyses also
shol1ed that there is a recharge from AI1ash river to the
aquifers at Melka Hida well field.
Depth to groundwater level ranges from fel" meters in the
southern part of the area to more than 170 meter in its
northern part. From the groundwater contour, it can be observed
that the general groundwater flow is to the south in the deeper
aquifers. In the shallow aquifer zone (that is , southern part
of the study area) groundwater flows towards Awash river in
locally different directions from both sides of the river.
The chemical analyses result showed that the ~Taters of the
area are mainly bicarbonate type varying from sodiumbicarbonate
to calcium-bicarbonate ones. The analyses also
showed that there is high fluoride concentration above the
acceptable limit for drinking in the groundwaters.
Potential places for groundwater in the area are
restr icted to its southern part. The remaining part is of
limi ted potential not only due to 1011 permeabili ties of
aquifers but also due to large depth to groundwater level.
Finally, it seems that conjunctive use of groundwater and
surface water from Awash river is the only solution to meet
future water demand of the town and of its surrounding.
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Keywords
Nazret