Quantification of inter-aquifer flow in a Multi-Aquifer System Using Regional Groundwater Modeling: Northwestern Desert, Egypt

Under severe water stress, intensified by the lack of rainfall and upstream regulation of freshwater, Egypt has little choice but to turn to alternative water resources, such as groundwater. However, the largest groundwater source—the Nubian Sandstone Aquifer—is non-renewable, and its connections to other aquifers are complicated and remain insufficiently studied. Modeling such an aquifer—one of the largest in the world, spanning approximately 2 million square kilometers across Egypt, Sudan, Chad, and Libya—is a complex task, with existing studies largely limited to local scales. This study aims to quantify inter-aquifer flow in the multi-layered hydrogeological system of the Northwestern Desert, Egypt, on a regional scale covering part of the Nubian Sandstone Aquifer. The system comprises five aquifers and two aquitards, namely the Marmarika Limestone and Moghra Aquifers, separated from the Eocene Limestone Aquifer by the Al-Dabaa Shale Aquitard. The Eocene Limestone Aquifer is horizontally connected to the Shallow Nubian Aquifer and underlain by the Abu-Rawash Shale Aquitard, which separates it from the Deep Nubian Aquifer. A three-dimensional regional groundwater model was developed using a comprehensive dataset of lithology, water level, and extraction data. Calibration with over 1,000 historical measurements spanning five aquifers yielded aquifer properties and recharge estimates. The model reveals the presence of vertical connectivity between the Eocene Limestone and Nubian Aquifer and interaction between Siwa lakes and the Limestone Aquifer. Results showed that discharge to Siwa lakes in 1960 was approximately twice the aquifer’s recharge from the Nubian Aquifer but has since declined by 30%, while recharge from the Nubian Aquifer increased by 10% by 2023. Flow from the Nubian Aquifer accounts for about 30% of current extraction from the Limestone Aquifer, indicating over-extraction in the region. Horizontal connectivity between the Limestone and Shallow Nubian Aquifers is minimal, contributing less than 2% of extraction in West Minya. This explains stable water levels in the Shallow Nubian Aquifer at Al-Bahariya despite significant extraction from the Limestone Aquifer at West Minya. The results offer valuable insights into the connections between the aquifers, supporting the development of future local models and clearly highlighting the risks of water salinization and aquifer depletion in the event of overextraction.