Hydrodynamic functioning of a Mediterranean multi-aquifer system inferred from isotopic and hydrochemical tracers

Groundwater resources in Mediterranean semi-arid basins are increasingly challenged by climate variability and agricultural intensification, yet recharge processes in multi-aquifer systems remain insufficiently constrained. This contribution presents research from Central-Western Tunisia focusing on the Sidi Marzoug–Sbiba basin, a strategic agricultural area dependent on cascading Cretaceous, Miocene, and Mio-Plio-Quaternary aquifers.

A multi-tracer approach was implemented combining piezometric mapping, major-ion geochemistry, and stable and radioactive isotopes (δ¹⁸O, δ²H, ³H, ¹⁴C). The data reveal a coherent regional flow from SW highlands toward the NE Sbiba plain, controlled by major faults and fractured outcrops that enhance lateral hydraulic connectivity. The upstream sector stores isotopically young waters with homogeneous compositions, indicating active meteoric recharge originating from relatively high elevations during the wet season (November–April). Downstream, decreasing ³H and ¹⁴C values together with higher TDS and sulfate-rich facies reflect older, slowly circulating groundwater affected by dissolution of surrounding gypsum-bearing formations and mixing with evaporated surface water from dams. Localized nitrate levels exceeding the 50 mg/L WHO guideline highlight emerging contamination risks despite the overall dominance of geogenic salinization.

These findings demonstrate how complementary tools can be jointly used to reduce uncertainty in recharge assessment of complex Mediterranean semi-arid aquifers and to guide sustainable management and monitoring priorities.