Contribution of Isotopes of the Water Molecule to Determine Recharge Altitude of the Main Springs Welling Up in the Middle Atlas Limestone (Morocco)

The present study contributes to characterize the Middle Atlas limestone karst aquifers by implementing water stable isotope techniques, to determine the recharge altitudes of major springs issuing from the Liassic dolomitic limestone, and helping to delineate protection zones of the springs. Chemical analyses of spring water show a composition that reflects both reservoir lithology and chemical evolution over the flow transit through the reservoir. Analysis of the evolution of deuterium as a function of oxygen-18 suggests a local water line similar to that derived for rainwater at the Fez weather station, indicating that the concerned aquifers are recharged through rapid infiltration, without appreciable evaporation of rainwater. Application of the established −0.27 ‰ per 100 m altimetry gradient for oxygen-18 to the isotopic signatures of other analyzed springs enables the backtracking of recharge altitudes of these springs. Comparison of these results with local and regional hydrogeological data shows a notable agreement among springs for which the drainage basin is known, and allows for more precise localization of recharge area of springs for which the drainage basin is poorly known, with recharge altitudes up to 1,090 m higher than their emergence zones. To a certain extent, these results help to determine recharge zones for Middle Atlas springs and thus help in strategizing protection for this resource.