Résumé
The Trarza aquifer extends over about 40 000 km(2) in southwestern Mauritania, between the Senegal River in the south, the Atlantic Ocean in the west and the Mauritanides metamorphic chain in the north and east. The Trarza aquifer lies in Continental Terminal and Quaternary sediments. The field investigations conducted between 2010 and 2012 significantly added to the few older measurements. The hydrodynamic and geochemical approaches showed that in this semi-arid area, the groundwater dynamics is influenced by many environmental changes, past and present (from Quaternary transgressions to the various dams regulating the Senegal River). Groundwater is mainly recharged by lateral infiltration of surface water of the Senegal River, and to a lesser extent, by rainfall infiltration. Groundwater mineralization is controlled by water-rock interactions with silicates and alumino-silicate minerals and is locally influenced by marine water resulting from past Quaternary transgressions. Evaporation strongly affects surface waters before and during their infiltration. Very preliminary calculations suggest an annual recharge of less than or equal to 5% of precipitation, i.e. about 5 to 10 mm.