Characterization of the functioning of the Motril-Salobrena coastal aquifer (SE Spain) through the use of environmental tracers

Environmental tracers were used to characterize the origin and determine the age of the groundwater in the Motril–Salobreña aquifer (south-eastern Spain). The stable isotope concentrations (δ18O/δ2H), compared to the results obtained in previous studies, indicate that most of the recharge during the sampling period was from irrigation return flow and the carbonate Escalate aquifer. The combined dating of 3H, 3He, 4He, 85Kr, and 39Ar allowed establishing the presence of modern water throughout the aquifer, although with different mixing percentages. Thus, there is a large zone characterized by a fluvial domain with 100% young waters (< 5 years) due to the circulation of water through an area of high permeability sediments. In the discharge zone of the aquifer, older water is located (age > 170 years), and the percentage of young water is reduced (22.5%). This is explained by the greater distance that groundwater travels (aquifer thicknesses is over 250 m) and the lower permeability of the aquifer in the deeper sectors.