The sedimentary coastal basin of Togo: example of a multilayered aquifer still influenced by a palaeo-seawater intrusion |
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Authors: | Ari Akouvi Martial Dray Sophie Violette Ghislain de Marsily Gian Maria Zuppi |
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Institution: | (1) Université Pierre et Marie Curie-Paris 6, UMR 7619-Sisyphe, Case 105, 4, place Jussieu, 75252 Paris Cedex 05, France;(2) CNRS, UMR 7619 Sisyphe, Case 105, 4, place Jussieu, 75005 Paris Cedex 05, France;(3) Present address: Burgéap, 27, rue de Vanves, 92772 Boulogne-Billancourt Cedex, France;(4) Université Pierre et Marie Curie-Paris 6, Centre de Recherches Géodynamiques, BP 510, 47, avenue de Corzent, 74203 Thonon cedex, France;(5) Dipartimento di Scienze della Terra, Università di Torino, Laboratorio di Idrogeologia e Geochimica Applicata, Via Valperga Caluso 35, 10125 Torino, Italia;(6) Present address: Dipartimento di Scienze Ambientali, Università Ca’ Foscari di Venezia, Dorsoduro 2137, 30123 Venice, Italia |
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Abstract: | In Togo, the hydrogeology of the sedimentary coastal aquifers along the Gulf of Guinea has been studied for the last three decades to define the recharge processes and the origin and evolution of the salinity. Isotope hydrology and fluid geochemistry suggest that the current recharge of all aquifers, both confined and unconfined, occurs through the crystalline basement and the Plio-Quaternary deposits. Two main groundwater mineralization processes are observed: the first one, in recharge areas, is due to farming, village and city life and concerns unconfined aquifers (crystalline basement, Continental Terminal and Quaternary sediments); the second one is a mixing process between recent freshwater and fossil saline water still present in the deep confined aquifers inland, several kilometers away from the coast. Brackish water was trapped in low-permeability lenses of confined aquifers (Eo-Palaeocene and Maastrichtian) during the Quaternary, in periods of low recharge, notably during the last glacial maximum (LGM), and has not yet been flushed out. Hydrodynamic simulations indicate that, at that time, the aquifers experienced a maximum seawater intrusion as far as 20–22 km inland, depending on the palaeo-recharge value at the outcrops. |
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Keywords: | Togo Groundwater age Saltwater/freshwater relations Paleohydrology Numerical modeling |
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