Using radium isotopes to constrain the age of saline groundwater, implications to seawater intrusion in aquifers

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. Ra-226/Ra-223 and Ra-226/Ra-228) in groundwater from both shallow and deep subunits, up to >700 m from the sea, were far from equilibrium with radioactive parent ratios (U-238/U-235 and Th-230/Th-232, 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 Ra-226/Ra-223, which implies ages of >1000 years, with the exact minimum age dependent on radium adsorption in saline water. The higher than equilibrium ratios are attributed to a borehole stagnation effect, which results in the preferred decay of the short-lived Ra-223. Three samples showed significantly lower than equilibrium Ra-226/Ra-223 (Ra-226/Ra-223 = 4.5-7.6), which suggests ages of several hundreds to slightly over 1000 years. These results, together with recent relatively young (Holocene) Kr-81 and Ar-39 apparent ages of Yechieli et al. (2019), challenge the accepted paradigm of a Neogene age for the Cretaceous saline water and suggests that this water recently had hydraulic connection with the sea. The use made in this study of radium isotopes to constrain groundwater ages, in particular its relevance to the time window of 100's to 1000's years, is an important addition to the family of groundwater dating tools, which should be further elaborated on. It is especially important to dating of water from the fresh-saline water interface, since it is less sensitive than other methods to the mixing with fresh meteoric water.

Automated summary

This study presents a method for groundwater dating using radium isotope ratios along the Israeli coast, with implications for seawater intrusion. The results suggest that the age of Pleistocene aquifer groundwater is young, possibly due to recent human activities, while Cretaceous aquifer groundwater is older and may have had a hydraulic connection with the sea.