LAUSR

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 (δ 18 O/δ 2 H), 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 3 H, 3 He, 4 He, 85 Kr, and 39 Ar 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.