Integrated Groundwater Potential Assessment of the Volcanic Plains of the
Borena Lowlands
(Case studies in Borena Zone – Part of South Eastern Oromia Development Corridor)
EAH1, 2nd Annual Congress (August 23-24, 2008 – MoWR2, Addis Ababa, Ethiopia)
By:
Abebe Ketema, OWWDSE 3
Key words:
Arid and semi-arid environment, volcanic aquifer, integrated approach,
Bulal basalt, water balance, test wells, hydrochemistry, isotope
Abstract
The Borena zone located in the arid and semi-arid environment of Southern Oromia is well known for its
high potential of livestock (the famous Borena cattle bread). However, due to recurrent drought in the area,
the people and their livestock are suffering, from year to year, from shortage of water supply and extreme
competition for grazing land. The area is generally characterized by scarcity of surface water. The importance
of groundwater in the region, can be manifested by the traditional wells in particular area used to be a focal
point for social organization and means of survival. The project entitled “Groundwater Potential
Assessment of the Volcanic Plains of South and South west of Yabello Town” has been launched over
an area of 16,495 km2 to study the resource in an integrated and comprehensive hydrogeological approach
formulated in three phases. The survey and investigation methodologies applied as well as the preliminary
results obtained during the present stage; phase I (detailed reconnaissance) are found promising.
The scope of this work includes: Geological mapping at 1:250,000 scale, hydrological studies,
hydrogeological survey, water point’s inventory and sampling, geophysical survey, test wells drilling,
hydrochemistry and Isotope studies.
The surface geology of the project area is made of three major rock groups: Precambrian crystalline
basement (13.9%), Tertiary volcanic products and Quaternary volcanic (42.3%) and sediments (alluvial
and elluvial) covering about 43.8 %. About 22 sub divisions of geological units have been mapped of
which 12 are basement series. Among the four Teritiary basalt units mapped in the area, the unit
designated as “Bulal basalt” covering about 28.4 % of the project area has been considered as a target
formation in the area from groundwater perspective view.
The major faults in the area trend in north- west south–east direction corresponding to the Mega Rift
system and the nearly north south trending faults (Ririba Rift system).
The general water balance analysis done on the seven sub-catchments indicated an estimated groundwater
recharge of the project area to be about 54.05mm/yr, which will be verified by other recharge estimation
methods as more data is obtained. A total of about 100 water points (66 boreholes, 13 springs and 21 hand
dug wells) have been inventoried out of which 85 groundwater samples have been collected for laboratory
analysis.
The geophysical investigation work done by the help of 121 VES for construction of regional geoelectric
sections along different transect lines, as well as for test wells sitting show that the area is underlain by four
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Ethiopian Association of Hydrogeologists
Minstry of Water Resources
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Oromia Water Works Design and Supervision Enterprise
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main resistivity layers. These layers correspond, from bottom to top: the Precambrian basement rocks, highly
weathered and/or fractured basalts (aquifer), slightly fractured to massive basalt and the surficial loose and
unconsolidated material. The thickness of unconsolidated materials is estimated to range from 50m to 90m.
The potentially water bearing part of the Bulal basalt is estimated to have thickness varying from about 40m
to 160m, while the volcanic rock resting over the basement is in general estimated to be about 240 – 290m
thick.
Out of the 12 test wells drilled only one is turned out to be dry. All the rest have succeeded to intercept
groundwater of variable water qualities. Drilling depths range from 108m to 294m. About 50 % of the wells
have succeeded to intercept the basement while the rest could not reach it due to drilling problems. Similarly,
only low yielding wells (3 l/s to 9 l/s) were able to be pump tested. The rest wells (above 12 l/s) are currently
waiting for high capacity pumps.
Depth to groundwater strike ranges from about 75m at Megado/Alabor locality to 157m (b.g.l) in the Gelchet
area, while the depth to water table/piezometric level ranges from about 18m at Mermero to about 156m
(b.g.l) at the area of Dillo. The groundwater system in the area is under confined/semi-confined condition.
The hydrochemical investigation revealed that, very high salinity groundwater exist at close proximity to
good quality groundwater. In general, larger portion of the groundwater system of the area has moderately
hard and potable water, while in the localities of craters and middle Sarite plain areas; the groundwater is with
concentrations of high sulphate, chloride, sodium and fluoride and therefore, has poor water quality.
In all localities for which environmental isotope data were available the dating returns modern age. Some
water with low tritium content are in the order of a few decades. Therefore, the aquifer system of the region
has connection with modern day recharge.
Integrated hydrogeological, geophysical, test drilling and water quality results helped to identify about nine
prospective groundwater zones (target well field areas) for further investigations. Hence, additional
hydrogeological, geophysics, drilling and testing, water quality surveys and groundwater regime observations
will be made during the subsequent stages for detailed analysis and ultimate development of groundwater
management models for the volcanic aquifer systems of the area; in order to aid the proper and sustainable
utilization of the resource.
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