Groundwater recharge sources and mechanisms in the Ethiopian central Afar rift: insights from isotopic and hydrogeochemical tracers

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Abstract The study of deep groundwater recharge and its flow pattern in a continental escarpment-rift interface zones is one of the most challenging subject in the field of hydrogeology. The central Afar rift and the associated western marginal grabens and Northwestern Plateau (NWP), in northeastern Ethiopia are an excellent examples, where the groundwater recharge and flow patterns are insufficiently addressed. Existing conceptual models argued on the existence of deep groundwater recharge to the rift, considering the marginal area as an overall barrier. But the role of interacting linking zones between the marginal grabens are commonly unaccounted for. Hydrogeochemical, stable isotopes of water and knowledge of geological structures are applied to determine groundwater recharge sources and mechanisms. There is a notable difference in isotopic compositions between the deep and shallow groundwater systems resulting from difference in recharge conditions. The major ion chemistry and EC of groundwaters in some corridors of western parts of the Afar rift show a clear geochemical evolution along the flow paths. The results of stable isotope and groundwater geochemistry revealed the existence of preferential deep groundwater recharge from the NWP, across the highly fractured linking zones, to the deeper volcanic aquifers of the central Afar rift. Overall, the groundwater flow from the NWP to the Afar rift is chiefly controlled by the orientations of the faulted marginal grabens and the linking zones between them. The suggested model of this study make a significant contribution to better understand groundwater recharge mechanisms in other similar continental rift zones.

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