Groundwater Age Dating in Motril-Salobreña Coastal Aquifer with Environmental Tracers (δ¹⁸O/δ²H, ³H/³He, ⁴He, ⁸⁵Kr, and ³⁹Ar)

The Motril-Salobreña coastal aquifer (SE Spain) is under high anthropogenic pressure due to the changes in river infiltration, irrigation and pumping. There are several uncertainties regarding how groundwater flows from the recharge areas to the discharge areas, and how long groundwater circulates in the aquifer due to the presence of layers with different hydraulic conductivity. This study constrains the age distribution in the aquifer by dating water samples taken at three different locations (recharge sector, intermediate sector and discharge sector) and at variable depths. The environmental tracers used were δ¹⁸O/δ²H, ⁴He, ³H/³He, ⁸⁵Kr, and ³⁹Ar. The groundwater dating allows to define the flow distribution and preferential flowpaths, to determine the mean groundwater residence times in the system and to establish the connection between age distribution and saltwater intrusion processes. Mean groundwater residence times range from recent to ~170 years, with the presence of young water in almost all the areas of the aquifer at a shallow depth and an age gradient at deeper locations in the discharge zone. The presence of the salt wedge increases the age mixing processes. The groundwater flow in the shallow part of the aquifer (0–50 m depth) turns out to be fast, with a residence time lower than 5 years, while the flow in the deeper parts (50–150 m depth) is slower and groundwater has at least 170 years.