Using ion and isotope characterization to delimitate a hydrogeological macrosystem. Sierra de Gádor (SE, Spain)

•Carbonate aquifer does not appear to behave homogeneously.•The combination of geochemical and isotope tools reveals the aquifer's compartmentalisation.•Dedolomitization becomes predominant when the water flows through interbedded gypsum.

Sierra de Gádor is a karstic macrosystem characterized by highly complex geometry. The two main economic activities in this area – the highly profitable irrigated agriculture and tourism – are supported by water resources from Sierra de Gádor, hence the interest in understanding this aquifer system. The highly variable precipitation in the area is characteristic of the arid conditions. The aim of the study presented in this paper was to define the principal hydrogeochemical and isotopic characteristics of the area and to characterize the different types of freshwater, some of which are affected by the process of mixing with saltwater – particularly those at the east and west extremes of the Sierra de Gádor. The hydrogeochemical analysis enabled the principal geochemical processes occurring in the aquifer to be established. The waters can be grouped into four categories, each defined by a distinctive solute. Dissolution of dolomite determines the chemical composition of less mineralized water. Dedolomitization (dolomite dissolution together with calcite precipitation caused by dissolution of gypsum) becomes predominant when the water flows through interbedded gypsum. Marine intrusion results in highly-mineralized waters and leads to base exchange reactions. The variation of δ18O content with altitude (− 0.19‰/100 m) allows the main recharge areas to be determined: 2000–1500 m a.s.l., 1500–900 m a.s.l., and below 900 m a.s.l., according to the different hydrodynamic characteristics of each area. In some sectors, it is possible to detect the current recharge from rainwater infiltration or seawater intrusion, based on its tritium content.