Aquifers developed on basement rocks of Ethiopia: their genesis, properties and classification
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Date
2015-02
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Addis Ababa Universty
Abstract
The basement rocks of Ethiopia were traditionally described as a system of regional aquiclude.
This attribution ignores regional differences in aquifer properties, structure and functioning. In
contrast, this work presents how the interactions of evolutionary and modern geological
processes determine aquifer development and hydrogeological characteristics of the basement
terrain of Ethiopia. It specifically addresses the role of geomorphic history (deep weathering
and stripping) in affecting aquifer genesis and their control on vertical and spatial
heterogeneity. In addition, the study provides field evidences for existing literature based
weathering-stripping model previously developed for the country. Detailed examination of
geological, hydrological and hydrogeological evidences enabled the understanding of the
causes of regional variation in aquifer properties. The aquifers exhibit noticeable regional
differences. Thus, conceptual hydrogeological model depicting three coherent categories of
aquifers are developed: (a) in western basement terrain, aquifer is relatively extensive in the
thick weathered mantle over fractured bedrocks of low to high-grade metamorphic rocks. High
groundwater storage but low hydraulic conductivity characterizes this aquifer. (b) in the
northern, groundwater occurs in fractures and discontinuities in bedrocks toward the surface
and tectonically induced relatively deeper fracture zones. These aquifers have high hydraulic
permeability but low storage capacity. (c) in the Borena lowlands of southern basement region,
groundwater occurs in wadi beds, fractures, and preferentially weathered mantles. The
orientations of wadi beds follow regional fractures, which control groundwater flow regime and
enhance preferential weathering of bedrocks. Aquifers are of intermediate type with regard to
hydraulic properties. The variations in low-flow indices and shapes of flow duration curves of
streams manifest contrast in bulk storage capacity and hydraulic conductivity of catchments in
the three basement regions. This contrast has been used as evidences for characterization of
aquifers.The implication of these regional differences on groundwater exploration and
exploitation has been the subject of this study.
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Keywords
Basement rocks