STRUCTURAL CONTROL OF THE DEEP NUBIAN SANDSTONE AQUIFER SYSTEM CONSTRAINED BY HYDROCHEMICAL, STABLE ISOTOPE AND NOBLE GAS DATA

The Nubian Sandstone Aquifer System (NSAS) of northeastern Africa is one of the largest confined fossil water aquifer systems in the world, with several nations relying on it (Egypt, Libya, Sudan and Chad). This groundwater is the biggest and in some cases the only future source of water to meet the development goals of each NSAS country. Groundwater extraction in the past 40 years has led to a continuous drop of water level and to the disappearance of most of the naturally flowing wells and springs. This study focuses on the connection between water quality, water management and tectonics. Results of this study will help in the development and sustainable management of the continental-scale Nubian Sandstone Aquifer System.

Groundwater samples were collected from the deep NSAS in the western and eastern Egyptian desert and from the shallow Nile valley alluvial aquifer in the eastern Egyptian desert. Helium isotope (³He/⁴He) data from NSAS (western Egyptian desert) provide evidence for inputs of mantle-derived helium and other deeply sourced fluids. The mantle-derived fluids are leaking into the NSAS along faults from below the aquifer. The highly fault-controlled confined deep wells in the Dakhla basin of the western desert of Egypt have the highest ³He/⁴He values up to 0.363 R_A, with P_CO2 values ranging between 10 -170 times atmospheric value of 10^-3.5. The isotopic composition of groundwater samples from NSAS in the western desert ranges from -72.3‰ to -82.7‰ for δD and -9.4‰ to -11.1‰ for δ¹⁸O; the isotopic composition of groundwater samples from NSAS in the eastern desert ranges from -49.5‰ to -58.2‰ for δD and -5.6‰ to -7.9‰ for δ¹⁸O; and the isotopic composition of groundwater samples from the Nile valley aquifer ranges from -4.1‰ to -29.6‰ for δD and 1.1‰ to -3.4‰ for δ¹⁸O. The isotopic composition of the deep NSAS and the upper alluvial aquifer suggest an upward leakage of the NSAS water into the alluvial aquifer through deep-seated faults.

Faults may also cause hydrologic partitions and connections between aquifer sub-basins that need to be considered in flow models for the NSAS. Tectonic influences on the NSAS also have implications for variability of the water quality that may become more pronounced as the aquifer is increasingly utilized.