Investigation of the hydrogeochemical processes affecting the Nubian sandstone horizons: West El Mouhoub, Western Desert, Egypt

The Nubian Sandstone Aquifer System (NSAS) is the world's largest known fossil water aquifer system. It is located underground in the Eastern end of the Sahara Desert and spans the political boundaries of four countries in north-eastern Africa. NSAS covers a land area spanning just over two million km 2 , including north-western Sudan, north-eastern Chad, south-eastern Libya, and most of Egypt. Containing billions of cubic meters of groundwater, the significance of the NSAS as a potential water resource for future development programs in these countries is extraordinary. Therefore, understanding of the hydrogeochemical processes and continuous monitoring of its groundwater regionally and locally is so vital. The present study has been conducted to investigate these processes in a pilot area of Western Desert of Egypt (West El Mouhoub). To accomplish this objective, surface water and groundwater samples were collected and analyzed to estimate the chemical properties of these water samples. The ion ratios, water type, facies, hypothetical salts, geochemical diagrams were integrated with geostatistical analyses [factor analysis (FA) and correlation analysis (CA)] to characterize the hydrogeochemical processes that are controlling the Nubia sandstone aquifer system (NSAS). The obtained results revealed that, the Sabaya and Six Hills Formations represent the main aquifer horizons where the groundwater exists under confined conditions. The groundwater within these aquifer units is mainly fresh and the salinity increases from southeast to northwest direction in the Sabaya Formation groundwater. Both the hypothetical salts assemblages and the groundwater genesis confirm that there is a connection between the Sabaya and Six Hills Formations in the investigated area. The results show a chemical development from the groundwater that dominated by bicarbonate salts to that dominated by chloride salts. The main process controlling the groundwater is the rock-water interactions. total respectively). It shown that, the in the is rich in ions and this can be attributed to the