Abstract
The disruption of lithologies by cross-cutting faults and the variability in volcanic structures make the hydrogeology of the rifted volcanic terrain in Ethiopia very complex. Along two transects, selected due to their hydrogeologic characteristics, groundwater flow, depth of circulation and geochemical evolution have been conceptualized. The groundwater flow continuity between the high rainfall plateau bounding the rift and the rift valley aquifers depends principally on the nature of the bounding faults. Up to 50% of recharge to the rift aquifers comes from the plateau as groundwater inflow where the rift is cross cut by transverse fault zones. Recharge from the mountains is found to be insignificant where the rift is bounded by marginal grabens; channel loss and local precipitation are the principal sources of recharge to the rift aquifers in such cases. At a regional scale, there is a clear zonation in the geochemical compositions of groundwaters, the result of aquifer matrix composition differences. The environmental isotope results show that the majority of the aquifers contain modern groundwaters. In a few localities, particularly in thermal groundwaters representing deeper circulation, palaeo-groundwaters have been identified. Deeper groundwaters in the rift floor have a uniform 14C age ranging between 2,300 and 3,000 years.
Résumé
La rupture des lithologies par des failles transversales les recoupant, et la variabilité au sein des structures volcaniques rend l’hydrogéologie des terrains volcaniques riftés en Éthiopie très complexe. Le long de deux transversales, sélectionnées pour leurs caractéristiques hydrologiques, l’écoulement de l’eau souterraine, la profondeur de la circulation et l’évolution géochimique ont été conceptualisées. La continuité de l’écoulement de l’eau souterraine entre le plateau bordant le rift et caractérisé par des précipitations élevées, et les aquifères de la vallée du rift, dépend principalement de la nature des failles de bordure. Jusqu’à 50 % de la recharge des aquifères du rift provient du plateau via la venue des eaux souterraines le long des failles découpant le rift. La recharge en provenance des montagnes apparaît insignifiante là ou le rift est bordé par des grabens marginaux. ; dans ces cas là les pertes au niveau des canaux et les précipitations locales sont les principales sources de recharge des aquifères du rift. A l’échelle régionale une zonation nette des compositions géochimiques des eaux souterraines apparaît, résultat des différentes compositions des aquifères. Les résultats apportés par les isotopes environnementaux montrent que la majorité des aquifères contiennent des eaux souterraines récentes. Dans quelques localités et particulièrement dans les eaux souterraines thermales représentant des circulations plus profondes, des paléos-eaux souterraines ont été identifiées. Les eaux souterraines plus profondes au mur du rift présentent des âges 14C uniformes, entre 2300 et 3000 ans.
Resumen
Los cambios litológicos por la existencia de fallas transversales y la variabilidad de las estructuras volcánicas hacen que la hidrogeología de los terrenos volcánicos en el Rift Etíope sea muy compleja. Se han conceptualizado dos transeptos, seleccionados por sus características hidrogeológicas, el flujo del agua, la profundidad de la circulación y su evolución geoquímica. La continuidad del flujo del agua subterránea entre la plataforma lluviosa elevada que bordea el rift y los acuíferos del Valle del rift depende principalmente de la naturaleza de las fallas de los bordes. Por encima del 50% de la recarga a los acuíferos del rift procede de la plataforma así como de la entrada de agua subterránea en las zonas donde el rift está cortado por zonas de fallas transversas. Se ha encontrado que la recarga procedente de las montañas es insignificante donde el rift está bordeado por graben marginales; en estos casos, la pérdida a partir de los cauces y la precipitación local constituyen las fuentes principales de recarga a estos acuíferos. En una escala regional, existe una zonación clara en la composición geoquímica de las aguas subterráneas, como resultado de diferencias en la composición de la matriz del acuífero. Los resultados de isótopos ambientales muestran que la mayoría de los acuíferos contienen aguas subterráneas modernas. En algunas localidades, particularmente las aguas termales representan circulaciones más profundas y se han identificado aguas subterráneas antiguas. Las aguas subterráneas más profundas en el suelo del rift tienen unas edades de 14C uniformes que varían entre 2300 y 3000 años.
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Acknowledgements
This paper addresses part of the objectives set by the Sustainable Management of Water Resource in the East-African Rift system (MAWARI) project. The data used in this paper is a compilation from previous works related to the International Atomic Energy Agency Technical Cooperation projects (ETH8003, ETH8005, ETH8006, ETH8007) conducted in collaboration with the Ethiopian Ministry of Water resources; the Ethiopian Geological Survey and the Addis Ababa University. SK acknowledges the financial support provided by International Center for Training and Exchanges in the Geosciences (CIFEG) (the coordinator of MAWARI project). SK and AA would like to thank the Swedish International Development Agency/The Department for Research Cooperation (SIDA/SAREC) for the seed grant meant for field investigations.
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Kebede, S., Travi, Y., Asrat, A. et al. Groundwater origin and flow along selected transects in Ethiopian rift volcanic aquifers. Hydrogeol J 16, 55–73 (2008). https://doi.org/10.1007/s10040-007-0210-0
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DOI: https://doi.org/10.1007/s10040-007-0210-0