The objective of this research was to determine the recharge of the Cretaceous aquifers by the High Atlas, as well as the interaction and possible mixing phenomena between the waters of the different aquifers, by investigating the hydrochemical and isotopic evolution of groundwater flow paths from the limestone karst systems of the High Atlas to the Cretaceous basin of Errachidia. Geological techniques were used to investigate and confirm the chemical and isotopic characteristics of the waters. Although the Gibbs diagram shows that water–rock interaction is the dominant hydrochemical process, some water samples in the Cretaceous basin are influenced by both evaporation and water–rock interaction, indicating a mixture of rainfall and deep waters. A saturation index study indicated that limestone minerals were supersaturated in parts of the groundwater samples (calcite and dolomite). This result was confirmed by isotope data. Indeed, some Cretaceous basin samples show isotopic similarities to those from the Jurassic High Atlas. The geological cross-sections illustrate that the High Atlas Jurassic limestones are in direct contact with the Cretaceous basin’s permeable rocks, allowing groundwater to circulate from the High Atlas to Errachidia’s Cretaceous basin.