| Summary: | The geological and hydrogeological approach of the structure of the basin OuladBouSbaâ led to the definition of the geometry of the main aquifers. In general, the profiles show the complexity of the geological configuration. The filling of the depression of OuladBouSbaâ is from the Eo-Cretacian. At this level, the aquifer is recharged by direct water infiltration. The quaternary, Eocene, and Cenomanian-Turonian formations constitute the main aquifers. Horizontal as well as vertical heterogeneity lead to a higher diversification of aquifer characteristics. To define the origins and understand the groundwater flows in this complex zone, we used a multi-tracer approach with the analysis of major elements and the isotopes of δ<sup>2</sup>H and δ<sup>18</sup>O. The chemical composition is mainly governed by the interaction with the rock with low electrical conductivity except in areas around domestic landfills. Geochemical results analyzing groundwater in the Piper diagram show two distinct chemical facies: the sulfated calcium and magnesium, and the hyper-chloride calcium. The levels of δ<sup>18</sup>O range from −7.60 to −4.25 while those of δ<sup>2</sup>H vary between −53.07 and −27.03. Analyses of signature isotopes differentiate two groups. The first contains high levels of heavy isotopes (highest levels of δ<sup>2</sup>H and δ<sup>18</sup>O) having therefore been submitted to evaporation. The second with lower levels of δ<sup>2</sup>H and δ<sup>18</sup>O did not undergo evaporation. The first one belongs to the unconfined free aquifer while the second corresponds to the captive aquifer.
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