Hydrogeological Characterization of Tegona River Catchment, Bale Zone of Oromia Region
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Date
2014-05-05
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Addis Ababa University
Abstract
This research was focused on characterizing the hydrogeology of Tegona river catchment,
which is located in Bale administrative zone under the Oromia Regional State and situated at
about 430 km from Addis in the southeast direction. The hydrogeology and hydrochemistry
of the area was characterized by conducting detailed field mapping and sampling with
subsequent laboratory analysis. Water balance and chloride mass balance methods were
followed to estimate recharge. The main geological successions from youngest to oldest are
Quaternary sediments, Quaternary plateau basalts, Tertiary alkali trachyte flows and Tertiary
alkali trachyte and basalt flows. Different types of aquifers and aquicludes which range from
very high permeability aquifers to impermeable rock units (T>500 m2/d, Q ≥ 7 l/sec to T<5
m2/d, Q = 0-1.5 l/s) were identified in the study area. The very high permeability aquifers of
the area are Porous and weathered and/or fractured volcanic rocks mainly basalts and
possessing the transmissivity values greater than 500 m2/d and discharge of Q ≥ 7 l/s. The
high permeability aquifers of the area are made up of weathered and/or fractured volcanic
rocks mainly basalts and possessing the transmissivity values of 50-500 m2/d and discharge
of Q =4.5-7 l/s. The moderate permeability aquifers of the area are also characterized by
weathered and/or fractured volcanic rocks mainly basalts, with transmissivity values of 5-50
m2/d and discharge of Q =2-4.5 l/s. similarly, the low permeability aquifers of the area are
characterized by weathered and/or fractured basaltic rocks, with transmissivity values of T<5
m2/d and discharge of Q <1.5 l/s. Aquicludes and minor aquifers with dry and/or limited
ground water resources are considered as impermeable rock unit. The recharge as estimated
from water balance method was found to be 327.8 mm/year. Analysis of the area’s water
chemistry indicates that Ca2+ and Mg2+ bicarbonate water is the dominant water type of the
area. As far as water quality is concerned most of the water samples are found within the
range of WHO standard. However, anomalies of Fe, TDS, Ca and, Na concentrations beyond
the standard have been encountered in few water samples. Anomalous concentration of some
elements which in some cases lead for public outcry has been recommended to be further
investigated.
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Keywords
Hydrogeological Characterization, Tegona River Catchment, Bale Zone, Oromia Region