Conceptualization of Groundwater Flow System and Aquifer Characterization in Awassa Lake Catchment

No Thumbnail Available



Journal Title

Journal ISSN

Volume Title


Addis Ababa Universty


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.



Aquifer characterization, Awassa Lake catchment, Conceptual profile model, Ground cracks, Hydraulic conductivity