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
Abstract
The studied area, Akaki River Catchment, is Iocaled at tile headwaters of Ihe 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.
Basic concepts and over views/definitions 01 groundwater aquifer vutnerabiUly mapping was given in chapter 5 as it is necessary to make de<l(
how we carry OUI such vulnerability mapping or assessment Afier we have seen conceptS 01 vulnerability mapping, we need 10 decide and
define which method of vulnerability anatysis wiD be used (chapter 6). Hence, DRASTIC model was selected as toollar vulnerability analysis
and mapping. DRASTIC factors use a numerical ranking system 10 assess groundYIater pollution potential in hydrogedogic settings. The
system consists 01 three pans, which are designated as: ranges, rSlings and weights. Review of experiences with the model, its inherent
assumptions and ambiguities, and why it was particularly selected and its potential uses especially, with GIS was outlioed. Data base was
designed and constructed for attribute inputs for spatially distributed geographic data to make use 01 GIS as it is described in chapter 7. Afier
data base was designed and constructed, attributes were extracted and modelled to get the different DRASTIC parameters or layers to work
within the GIS latter on (chapter 8) .. Furthermore Chapter 8 gives the description/definition of each DRASTIC parameter and how each of them
contributes to groundv<ater aquifers contamination or protection. Rating and weighing was made according to the predefined ranges or values
given for the seven factors (Aller et aI., 1987). The land use/cover of Addis AbabafFinfinne city was rated to irldicate the potential risks 110m the
land use in the city.
The ultimate goal of this vulnerability mapping is the subdivision 01 an area into several units ShOwif'IQ the differential potential for pollution or for
a specified purpose and use based on an index map. The lesults of vulnerability assessment are portrayed on a map showing various
homogeneous areas, sometimes called cells, which have different levels of vulnerability. Accordingly, using a cell size 01 30m subjectively
defined in this thesis, and is believed to provide suffICient resolution; the catchment was divided into 1708 rows and 1807 columns. After the
seven DRASTIC layers were made read the following results were obtained by employing an overlay opera~on/anatysis: the minimum
normal/general DRASTIC index is 73 and the maximum normal/general DRASTIC index is 249; whereas, the minimum pesticide DRASTIC
index is 78, and the maximum pesticide DRASTIC index is 260. Both types 01 the above DRASTIC index maps show that the nonhem part of
the study area is relatively more vulnerable to pollution than olher areas except some kx::a\ized areas showing higher index values as well
(Chapter 9).
Furthermore, the environmental impact assessment (EIA), and an environmental police, legislation, etc. of Ethiopia was pin pointed (Annex 8).
This research work was ambitious in the sense that aquifer vulnerability assessment was planned & conducted under serious constraints of
resources. Even though this might affect the quality of the research result, all possible endeavors and necessary scarifications have been made
to organize data found in archives of different organizations and field surveys for the model input to tackle these problems. Finally, as this work
is the first application of the groundwater aquifer vulnerability assessment method; I recommended further detail research work (chapter 10) to
fill the knowledge gap and provide detailed data for future aquifer vulnerability mapping and groundYIater protection using various methods.
Description
Keywords
Akaki Ri ver, allu vial, aquifers, attributes, contamination/pollution, DRASTIC, fault, GIS, groundwater, hyd rogeology, intrinsic/specific/integrated vulnerabil ity, spatial, volcanic v