Limited Groundwater Recharge and Long Residence Times in Salinized Endorheic Arid Aquifers: Evidence from Stable Isotopes, Noble Gases, and Radiocarbon Dating

Arid endorheic basins are increasingly dependent on deeper groundwater resources, making it essential to assess their renewability and long-term response to climate and pumping pressures. In the Bahira Basin (central Morocco), deep low-salinity groundwater is being exploited as the shallow aquifer has become extremely saline (up to 60,000 µS/cm). To explore the recharge rate and resilience of this deeper resource, a multi-tracer approach was conducted. Major ion chemistry, stable isotopes (δ¹⁸O, δ²H, δ¹³C), noble gases isotopes (He and Ne), and radiocarbon (¹⁴C) were analyzed in 19 groundwater samples.  The deeper aquifer shows moderate salinity (500-3,000 µS/cm) and isotopically depleted δ¹⁸O-δ²H values, suggesting recharge from higher elevations and cooler climatic periods. Substantial radiogenic ⁴He enrichment (> 4x10^-6 ccSTP/g) indicates very low recharge rates and long residence times. Radiocarbon analyses further support this interpretation, as most samples have ¹⁴C values below the detection limit (<1.12 pMC), corresponding to apparent ages >35 ka. These results suggest that modern recharge is extremely limited across most of the basin. However, a few wells near identified recharge areas show atmospheric He and Ne signatures and measurable ¹⁴C (~35 pMC), indicating the presence of a recent recharge mixed with older groundwater. These localized recharge zones illustrate the spatial heterogeneity of groundwater replenishment. Overall, our findings reveal the low renewability on human timescales and limited resilience under increasing abstraction pressures. The integration of geochemical, noble gas, and radiocarbon tracers proves essential for assessing aquifer vulnerability and supporting more informed groundwater governance in arid, data-scarce regions.