HYDROCHEMICAL CHARACTERISTICS AND POTENTIAL TECTONIC INFLUENCES ON GROUNDWATER QUALITY OF THE CONTINENTAL-SCALE NUBIAN SANDSTONE AQUIFER SYSTEM (NSAS)

The continental-scale Nubian Sandstone Aquifer System (NSAS) is one of the largest groundwater aquifers in the world. It lies in northeastern Africa and is shared among Egypt, Libya, Sudan and Chad. Groundwater in the NSAS is the only source of the water for consumption, agriculture, and industrial activities in the Western Desert Oases of Egypt. Pumping of groundwater at non-sustainable rates is taking place to support relocation of populations to the oases (Egypt), creation of artificial rivers (Libya and Egypt), and other development (Sudan), highlighting the continued need for studies of the aquifer system to inform management decisions.

Groundwater samples from the NSAS and potential recharge units form the basis of this study. Samples were analyzed for major and minor element composition, stable water isotopes and helium isotopes. The western suite of water samples were collected from deep wells distributed along the Egyptian western desert from north (Al-Bahariya Oasis) to south (Al-Kharga Oasis). Initial results show variable temperatures (20-45^oC) reflecting depth of source; dissolved inorganic carbon ranges to more than 1000 mg/l as bicarbonate; TDS and pH values are low, and high iron content impairs water quality. Highest alkalinities are observed in the fault-controlled Kharga oasis. Previous workers have suggested deeply sourced fluids and a potential magmatic input from the mantle into the aquifer system suggested by the high CO₂ and by ³He/⁴He values of up to 0.1 RA. Our working hypothesis is that mantle-derived fluids are leaking into the NSAS along the fault system from below the aquifer. Faults may also cause hydrologic partitions and connections between aquifer sub-basins and, consequently, indicate more complex flow models for the NSAS.

Four samples from the Nubian aquifer and twenty six samples from the Quaternary aquifer were collected from the eastern desert around Qena area to determine the hydraulic connection between the Nubian aquifer (deep aquifer) and the Quaternary aquifer (shallow aquifer) in the Nile valley. Preliminary results of isotopic analysis suggest mixing between the Nubian aquifer water and the Quaternary aquifer water. Temperature from the Nubian aquifer wells ranging between 34.6 to 47.6 ^oC and between 25.3 to 34.3 ^oC for the Quaternary aquifer wells.