GIS Based Groundwater Vulnerability Assessment in Akaki River Catchment, Addis Ababa (Central Ethiopia)
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Date
2003-11
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Addis Ababa,University
Abstract
The studied area, Akaki River Catchment, is Iocaled at tile headwaters of the Awash River Basin. The Int010 Mountain Ranges formed by
Ambo-Kassam regional fault form Ihe hydrographic divide between Awash and Blue Nile Rivers. The Akaki River Catchment has an
3pp1'oximale surface area 01 aboull464 km2 and perimeter 01216 km, and approximately bounded between SO 46' 5T' and go 13' 00" N and
JaG 35' 00" and 39005' OO"E. It lias an elevation range of less than 2040 to 3,200 m above mean sea level (amsl).
The studied area is made up of different volcanic rocks ranging in age from Ouaternary to Miocene, induding recent alluviaillesidual deposits.
There are also regional and local faults. Differenllype 01 day soil and lac\Jslrine deposits formed from Ihe weathering 01 volcanic rocks are the
dominant soil type that characterizes Ihe area. AI some localities these clay soil are either Ihin or totally absent.
The mean yearly rainfall depth of the Akaki River Catchment was 1150 mm (from 1964• 1998). The arithmetic mean monthly rainfall ranges
from 6 mm (in December) to 276mm (in August) for the three stations (see chapter 2) in too catchment for 35 years. The highest and icMtest
meilfl monthly maximum temperature occurs in months 01 March (25 Oc) and the lowest is in the month of August (20 Oc), while the minimum
mean monthly values were 8 Oc (in December) and 12 Dc ~n March) for Addis Ababa Observatory from 1951- 1998 respectively. The calculated
mean annual temperature was 16 Dc.
Studies show both surface and groundwater pollution iD the Akaki River Catchment; and associated it to the following major sources: industrial
activities, agricultural practices, munidpall domesticl wastes, fuel stations, garages, and health centers and also to graveyard (cemeteries),
quarry mining, and market areas (see chapter 4). In this chapter an accotJnt was made on possible potential pollution sources. The over view
include general potential pdlutionfcontamination sources and existing potentiaigrourKtwater polluting sources identified by earlier wor1c:s in the
studied area. To see spatial distribution oIgfoundwater contamination situation in the catchment the discussion was supported by groundwater
quality analysis data at some selected sites in the catchment extracted from recent study. For these purpose three impol1ant groundwater
quality indicators (CI, NOl, and TOS) was seleded.
The population density of Addis Ababa and its environs vary from more than 400 persons per hectare to less than 25 persons per hedare. The
existing land use pattems in the catchment were broadly divided into four groups as: agriOJlture/open area (69%), forest land (1 5%), urban
area &its associated lISes (15%) and water body and wetland covers (0.98%) 01 the total land use/cover.
The general objective of the study is to identify and map the aquifer vulnerability to pollution in the Maki River Catchment by using an empirical
modeIIsystem known as DRASTIC (Aller et. ai, 1987; Evans and Meyers, 1990) 10 assess relative groundwater pollution susceplibility using
hydrogeologic factors (intrinsic ralher Ihan SpecifIC or integrated approach) was adopted, with GIS there by to prepare vulnerability index maps.
One of the Specific objectives of this thesis was the application or use for the first lime in Ethiopia 01 an index method of aquifer vulnerability
assessment with Geograp/lic Information System (GIS).
Practically and academically, the research is of interest since there was no earlier comprehensive study concerned with groundwater
conlaminatiol1 proIfJdiol1. Thus, to supplement the policy makers on grovndwater rasource management and protectioo in this catchment, there
must be map-based informatiol1 that indicates spatial distribution of relatively vulnerable areas that is useful in major aspects of planning
(chapter 1). YVhere as, previous researchers dealt with general hydrogeology, hydrOChemistry, pollution condition, etc. in this catchment.
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Keywords
Akaki Ri ver, allu vial, aquifers, attributes, contamination/pollution, DRASTIC, fault, GIS, groundwater, hyd rogeology, intrinsic/specific/integrated vulnerabil ity, spatial, volcanic v