Groundwater of the eastern Egyptian desert: Age and salinity patterns

The increasing development of groundwater dependent irrigation projects and desalination activities along the Red Sea coast raises questions about its recharge mechanisms and the geochemical processes driving salinity variations. To address these questions, several wells and springs, along the Red Sea Hills have been sampled and analysed for radiocarbon, tritium, δ 13 C, δ 18 O, δ 2 H and their chemistry. For the same period between 2019 and 2021, rain and floodwater samples have been collected from sporadic rain events in the region. The sampling locations are geographically clustered and classified into six distinct groundwater types based on geochemical composition and isotopic signatures. The distinguished types exhibits a diverse range of geochemical and isotopic characteristics, reflecting variable flow paths, recharge periods, and lithological interactions. Freshwater springs in Wadi Araba are characterized by minimal salinisation and light stable isotopes, suggesting Pleistocene recharge. These springs, emerge from fractures and are influenced by the weathering of Na-bearing minerals in the underlying Nubian Sandstone. In contrast, groundwater from the coastal plain shows significant contributions from seawater evaporation brines, with isotopic signatures following an evaporation trend. These waters vary widely in age, from recent to Pleistocene, indicating similar lithological interactions despite differing flow paths. Another group of groundwater is influenced primarily by the weathering of halite, gypsum, and dolostone, showing no significant brine contribution. Highly saline groundwater are rare and stands out with extreme brine contributions likely sourced from deep crustal formations. Groundwater with elevated Na/Cl ratios, found in regions of Wadi Araba and the Red Sea Hills, reflects the weathering of Na-bearing minerals such as feldspars in magmatic rocks or clastic sandstones. These waters contrast sharply with those influenced by seawater or evaporation brines, highlighting the complex interplay of lithology, recharge processes, and tectonic activity in shaping the region's groundwater systems. • Large scale area along the Red Sea in the Eastern Desert of Egypt has been considered in this study. • During 3 constitute years, flood and rainwater samples belonging to different events have been sampled in different parts of the investigated area and were analysed on chemical and stable isotope composition. • Groundwater provide a wide range of salinities (2–85 ms/cm). The deep groundwater in the central area display a residence time of 18,000 years. The molar ratios show intense admix of seawater evaporation brines, indicated by very low Na/Cl, high TDS and high Br/Cl ratios. • Within the Red Sea Hill area, groundwaters show the highest salinity, indicating the origin of the brine not necessarily in the coastal plain but within deep crustal formations and due to the dissolution of different distinguished minerals.