Groundwate Potential Evaluation & Pydrochemistry of Sululta Cacthment
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Date
2005-06
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Addis Ababa University
Abstract
Sululta catchment is located in the north part of Addis Ababa with east north-south stretch of 9°05'
OI"N to 9° 18' 45"N latitude and east west stretch of 38° 33' 15"E to 38° 50' 45" E longitude. It is a
. sub-catchment of Abay drainage basin with a total area of about 480.80 km2
The main rock outcrops in the catchment are alluvial deposit of Quaternary age, basaltic, trachytic,
rhyolitic lava flows & pyroclastic deposits of Tertiary age.
From long term mean monthly rainfall data annual precipitation of the catchment is calculated as
1201.31 mm per year. The potential and actual evapotranspiration for the catchment are 959.81 mm
and 684.82 mm respectively. The area has an estimated overland flow of 178mm/year. Hence, the
area's annual recharge value is 342 nun/year. When the value of annual base flow is subtracted
I 32mm/year depth of water is being added to the groundwater reservoir. This value is very impOliant to
categorize the groundwater development of the catchment exactly knowing the entire groundwater
withdra,wal per year.
The vertical electrical survey (VES) interpreted results, borehole logs, pumping test and qualitative
interpretation have indicated that the. upper 3-20 ,meters in most part of the central plain area is
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potential for hand dug well and shallow groundwater. This same area is potential for borehole of
maximum depth 120 following the main fracture zones. The main aquifer in catcl1l11ent is fractured
and/or weathered basaltic lava. Hand dug wells are potential sources of water in the area where
quaternmy alluvial deposits followed by thick weathered basaltic lava. Even though, the yield is low to
moderate both rhyolitic and trachyatic flow are aquifers near the Intoto ridge,
. Based on the qualitative and quantitative analysis of aquifer characteristics the catc!unent is categorized
into tln'~e permeability zones, The high permeable zone has transmissivity greater than 22 m2/c'ay and
the yield of wells is 2 to 4 Usec with partial penetration. The low permeable zone have trasmissivity
less than 5,5 nl Iday and yield less thanillsec. the formation mapped as m~derate permeable zone has - ., ~
values of transmissivity and yield between these zones.
The water in the catchment is mainly of Ca- Mg-HC03-C03 and Ca- Na-HC03-C03 water type. In
most samples percentage of Ca2+ +Mg2+ is greater than Na+ +K+. The TDS obtained for the catchment is
generally below 256mg/1 and maximum EC is 540 microsiemens (~lS). Redox Potential (Eh) is
generally positive nearby the recharge area and gradually decreases downstream part of the catchment.
Concerning water quality, except for turbidity, color and bacteria the rest parameters are within the
limit of acceptable value of WHO (World Health Organization) water quality standards fo), water
supply, 'irrigation and industries excluding trace elements. Hand dug well shows the bacterial pollution because of its shallow water table.
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Keywords
Hydrology