Integrated assessment of groundwater salinity sources and inter-aquifer mixing dynamics in Siwa Oasis, Egypt: A multi-method approach using self-organizing maps, isotopic tracers, and numerical modeling

Groundwater salinization threatens water security for 30,000 inhabitants of Siwa Oasis, Egypt, who depend on two aquifer systems: The shallow Tertiary Carbonate Aquifer (TCA) and deeper Nubian Sandstone Aquifer (NSSA). This study elucidates salinity sources and quantifies inter-aquifer mixing dynamics using an integrated approach combining self-organizing map (SOM) analysis, NETPATH inverse modeling, multiple isotopic tracers (δ 1 ⁸O, δ 2 H, ⁸⁷Sr/⁸⁶Sr, 13 C, 14 C), and 3D numerical modeling with reverse particle tracking. Analysis of 153 water samples revealed three distinct hydrochemical groups: High salinity TCA waters influenced by evaporation and recharge from salt lakes, intermediate salinity mixing zones, and low-salinity NSSA paleowaters aged 4,300–14,200 years. Salt lake contributions to TCA groundwater salinity range from 0.6–18%, while fault-controlled NSSA discharge contributes 52–90% to TCA recharge. Predictive modeling indicates the non-renewable groundwaters in the NSSA will experience 30–40 m of drawdown by 2100 under current groundwater abstraction rates, exacerbating TCA salinization. These findings provide a framework for sustainable groundwater management in complex arid aquifer systems and highlight the urgent need for protection of fault-controlled recharge zones.