Water Quality Andgroundwateriakaki River Interaction in the Sekelo Basin (Lower Akaki River Sub-Basin)
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Date
1999-05
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Addis Ababa Universty
Abstract
The thesis presents the results of geological, hydrogeological and hydrogeochemical studies
in the Akaki subbasin (Sekelo). This study was to provide an overview of the water quality
and the mixing of the groundwater and the surface water. In the study the geology and the
hydrogeology of the area was investigated to provide basic information concerning the
different lithologies and the local and sub-regional groundwater flow patterns. The chemistry
and the stable isotopic composition of both the groundwater and the river water were
considered in order to understand the nature and origin of the waters and to further explain
groundwater and river water mixing patterns.
Analysis of the relationship between rainfall and Akaki river shows that the highest flow and
level in the river correspondences to the rainy season (June-September). Therefore, influent
condition is anticipated during this rainy season. Preliminary water balance estimates using
soil moisture budgeting method in the study area suggest 137 mm groundwater recharge
annually.
Geological and hydrogeological mapping of the Sekelo basin was conducted in the field and
mainly from aerial photograph interpretation. Detailed studies of the geology and the
hydrogeology was carried out in the Akaki Well Field D. The rocks of the area are generally
volcanic rocks with only minor fluvial sediments., The volcanic rocks are distinguished into ,,-._-- .--
lower basalt flows and young basaltic scoria and lava.
Hydrogeological data were collected. These are information from 34 boreholes in the Akaki
Well Field D abstracted from records and supplemented by data collected in the field. The types of data collected included lithological information, aquifer parameter data and borehole
construction details.
Analysis of these data has shown that on regional scale the groundwater flows from the water
divide to the discharge area on the river valleys. The permeabilities of the volcanic rocks in
this area are generally high. The important aquifer in the area is the lower basalt in which the
aquifers are found in the fractured volcanics and are usually semi-confined. The young
basaltic scoria is the main recharge area.
Water samples for hydrochemical and stable isotopic analysis were taken from 9 sites both
from the groundwater and river water. Chemical analysis data of 27 boreholes in the Akaki
Well Field D was collected from Addis Ababa Water Supply and Sewage Authority
(AAWSA) 1996-97. In addition, stable isotopic data of rainfall for Addis Ababa were
collected from International Atomic Energy Agency (IAEA) Vienna. Correction was made on
these data for altitude difference to get the local meteoric water line.
Analyses and evaluation of these data enabled to obtain information on the nature of the
water, in particular the mixing between the groundwater and the river water. The chemical
analysis of the groundwater and the river water shows similar Ca-Mg bicarbonate water type.
Based on U.S. Environmental Protection Agency's (USEPA) drinking water standard the
groundwater proved to have good quality. However the river water which is remarkably
influenced by human activities is highly polluted. The evaluation of the chemical composition
and stable isotope techniques led to the conclusion that the groundwater can be explained in
terms of mixing series between the rain water and the river water
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Keywords
Sekelo Basin