Geochemical, Natural Radiogenic and Isotopic Implications for Nubian Sandstone Groundwater Assessment in Kharga Oasis, Western Desert, Egypt

Abstract

roundwater in the Kharga Oasis, located in the Western Desert of Egypt, is an essential resource for the local population. The Oasis is fed by the Nubian Sandstone Aquifer (NSA) of the Cretaceous age. This aquifer is one of the world's largest reservoirs, which provides fresh water for drinking, irrigation, domestic use, and industrial purposes. The NSA possesses four bearing zones; Zone A, Zone B, Zone C, and Zone D, which are separated by clay confining layers. The total penetration depth ranges from 12.5 to 800 meters, and the aquifer is underlain by granitic and basement rocks. The groundwater temperature ranges between 18 to 36.8°C, where high temperatures have been detected in the deeper zones (C and D), indicating geothermal activity. Generally, the quality of NSA water is fresh to brackish water types; however, the deeper aquifer zones (C and D) mainly contain freshwater. The concentrations of the trace elements and the natural radioactive nuclei (Rb, Pb, 232 Th, 238 U, and Sr) in the NSA waters are insignificant and far below the permissible drinking limits, and they are mainly coming from the geogenic source due to leaching of aquifer matrix. The stable isotopes (δ 18 O and δ 2 H) show great insights for recharge and salinization sources. The δ 18 O ranges from -11.96 to -6.11‰, while δ 2 H ranges between -83.5 to -67.7‰, indicating that the groundwater in the NSA is fossil water (paleometeoric origin) as the aquifer was recharged during the paleo heavy pluvial periods. Accordingly, the groundwater in Kharga Oasis is a paleo nonrenewable resource whose quality has been threatened mainly by geogenic natural pollution, agricultural activity, and overexploitation.