Fresh and saline groundwater ages and flow dynamics in a perturbed coastal aquifer

Excessive freshwater pumping in coastal areas result in an inland intrusion of the saltwater wedge underlying the freshwater aquifer, leading to salinization of coastal aquifers. While multiple approaches and techniques in the form of analytical solutions, numerical simulations, mass balance calculations and laboratory experiments have been used to study the flow dynamics of the density-driven circulation of saltwater, little is known regarding the groundwater ages in coastal aquifers and the process timescales. The current study makes use of multiple age tracers such as ³H, ⁸⁵Kr and ³⁹Ar, allowing to calculate both modern and sub-modern groundwater ages. The ages derived based on the measured tracer values were compared to a numerical flow simulations. The results indicate that seawater intrusion in the Nitzanim coastal reserve (prompted by excessive freshwater pumping upstream) has a different effect on the ages of the shallow groundwaters in comparison with the deeper groundwater. Close to the shoreline (<~200 m) at shallow depths (<40 m below sea level), the intruding saline groundwater is younger than the freshwater system leading to young ages of <50 years. Further away from the shoreline (~700 m), at greater depths (~60 m below sea level), the deep intruding saline groundwater is dated to ~350 years, which is significantly older than fresh groundwater ages in this part of the coastal aquifer. The approach brought forward in the paper allows to better understand the timescales of density-driven circulation mechanism and the effect of seawater intrusion on groundwater ages.