Conceptualization of Groundwater Flow System and Aquifer Characterization in Awassa Lake Catchment
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Date
2005-07
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Addis Ababa Universty
Abstract
In this work, hydrogeological study has been carried out in order to characterize the
aquifers and conceptualize the groundwater flow in Awassa Lake catchment, located
275km south of Addis Ababa in the central sector of the Main Ethiopian Rift. Hydrometeorological,
hydrological, geological, and structural, hydrochemical and isotopic
characteristics of the area are assessed through uses of various approaches and
methodologies. Fieldwork, observation and measurements of relevant variables along with
existing data accompanied by systematic analysis and interpretation helped so as to
understand the hydrogeological system.
The Awassa Lake catchment is a closed caldera Lake on its one segment overlaps with the
Main Ethiopian Rift. The main geological units in the area are lacustrine sediments
characterizing the floor; acidic volcanics covering the caldera rims and escarpment and
some basalts, hayaloclastites and scoria cones outcrop on the floor. As it is found on the
axial zone of the Ethiopian Rift, it is highly affected by rift structures, recent tectonic
features are also observed in the area.
The aquifers in the area generally occur in unconfined condition where their hydraulic
characteristics are spatially highly variable, which is the result of the complex nature of the
lacustrine sediments and the degree of fracturing of the volcanic rocks. The most permeable
unit is the lacustrine sediment with hydraulic conductivity ranges between 5m/day-
200m/day, where as the least are those acidic volcanics of the rhyolite, obsidian and lapilli
tuff having less than 0.1m/day. The others volcanic rocks lie in between depending on the
degree of weathering, fracturing or faulting.
The hydrochemical and isotopic analysis of 2H and 18O result signifies that groundwater
of the floor, caldera rims, and escarpment and highland is found in different flow regime.
This is also supported by the large hydraulic gradient between the floor and the highland.
Groundwater of the caldera rims, escarpment and highland is characterized by active,
shallow flow with low EC less then 300μs/cm and little isotopic fractionation. Intermediate
flow characterizes the remaining floor and western caldera rims with relatively higher ionic
concentrations (EC ranges between 1000-2400μs/cm) and enriched in isotopic composition.
These flows are controlled by the rift faults and structures, which depends on the
characteristics of these structures such as depth of penetration. The area gets higher
amounts of direct recharge from the eastern and south eastern caldera rims, escarpment and
highland where as the other parts are characterized by intermediate and localized recharges.
Similar to the groundwater flow, discharge areas are also restricted to rift structures except
the swampy areas found on the floor. Not only these structures but also recent tectonics
(ground cracks) developed on the area control the groundwater flow. These features
facilitate the groundwater flow toward the Lake Awassa by increasing the hydraulic
connections of the different rock units, which might be associated with the rise of the Lake
level, the main problem of the area.
Key words: Aquifer characterization, Awassa Lake catchment,conceptual profile model, ground
cracks, hydraulic conductivity.
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Keywords
Aquifer characterization, Awassa Lake catchment, Conceptual profile model, Ground cracks, Hydraulic conductivity