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In this paper, we present a method for groundwater dating, using radium isotope ratios. We use this method to constrain ages of saline groundwater in two aquifers of different depth and age (shallow Pleistocene and deep Cretaceous) along the Israeli coast, with implications to seawater intrusion. In the Pleistocene aquifer, long to short-lived isotope ratios (e.g. ²²⁶Ra/²²³Ra and ²²⁶Ra/²²⁸Ra) in groundwater from both shallow and deep sub-units, up to >700 m from the sea, were far from equilibrium with radioactive parent ratios (²³⁸U/²³⁵U and ²³⁰Th/²³²Th, respectively). This suggests young ages, on the order of decades, which further implies high flow rates of ≥10 m yr⁻¹, probably related to recent over-pumping and seawater intrusion. On the other hand, most Cretaceous aquifer groundwater showed close to or higher than equilibrium ²²⁶Ra/²²³Ra, which implies ages of >1000 years, with the exact …