The typical vertical profile comprises from top to bottom: i) The laterite layer with a few meters thickness, ii) The alterite (or regolith) layer, a few tens of meters thick, derived from the weathering of the basement rocks; iii) The fractured layer which consists of the massive rocks.
The Am Zoer site is located in the south-west, and that of Guereda-Iriba in the northeast (Figure 1).This region is regularly faced with recurring problems of water supply.This situation is linked to the climatic context of this region with a rather low rainfall, but also to the lack of knowledge about the basement aquifer.However, the basement aquifer of Wadi-Fira constitutes the main source of water supply for the populations of this region.The aquifer system has two main types of reservoirs, the alterites reservoir and the fractured reservoir.The formations of the Precambrian basement identified in Chad, of which the Ouaddaï massif forms part, covers between 15% and 20% of the territory of the country  .
The formations encountered are granitoids, metamorphic rocks (schists, quartzites, micaschists, gneiss, green rocks and migmatites) and volcano-sedimentary formations.
Transmissivities estimated by pumping tests on deep wells range between 1.9 × 10 were obtained at Diker (Am Zoer site) and on two wells at Guereda.
Such high discharges are linked respectively to a thick sedimentary layer (20 to 40 m) and a well-developed (10 and 26 m) weathered and/or fractured horizon. This map does not differentiate the laterite reservoir from the fractured rocks reservoir.
The stable isotopes were interpreted using conventional IAEA methods.
The results permitted to differentiate the laterite reservoir from the deep fractured reservoir.
The main process controlling groundwater mineralization is water-rocks interaction and natural minerals dissolution.