Recent seawater intrusion into deep aquifer determined by the radioactive noble-gas isotopes ⁸¹Kr and ³⁹Ar

Radioactive noble-gas isotopes tracers ⁸¹Kr and ³⁹Ar are used for the first time to measure the residence times of deep (∼1000 m) saline coastal groundwater, and to determine its connection mode with the sea. The average rate of seawater intrusion into the deep aquifer in Israel, located near the Mediterranean Sea, is estimated. ⁸¹Kr-ages of the saline water samples, found to be younger than 40 ka, contradict previously estimated ages of up to several million years based on hydrogeological considerations. The new results imply a stronger and more recent connection between the aquifer and the sea, and indicate that the intrusion occurred during the sea-level rise that began about 20 ka ago. These coastal aquifers need to be managed with caution because lowering of the adjacent fresh water level due to over pumping could accelerate seawater intrusion in a relatively short time. This study demonstrates the suitability of these two noble-gas tracers for the examination of hydrogeological systems in general and for the study of seawater intrusion in particular.