Comparative study of factors controlling the groundwater occurrence in Bir Kiseiba and Bir El Shab areas, south western desert, Egypt using hydrogeological and geophysical techniques

• Providing new concepts on understanding the hydrogeology of the Nubian Sandstone.
• Clarifying the role of basement configuration on the groundwater occurrence.
• Implementing groundwater stable isotopes as archives of paleoclimatic and recharge.
• Addressing the impact of hydrothermal solution on the Nubian Sandstone groundwater.

This research presents a clear example of the significant role of basement relief on the formation of aquifers and the impact of geologic structures on groundwater occurrence. A basement relief map was constructed using the depth to basement data acquired from 20 vertical electrical soundings (VESes), 3 land magnetic profiles, and 27 drilled wells tapping the basement rocks in addition to the elevations of the basement outcrops in the area of study. The map shows three basins underlying the area. The geoelectric survey shows that these basins were formed as a result of series of step faults. The largest basin underlies El-Shab area. The medium basin underlies the area of Bir Kiseiba whereas the smallest one underlies Bir Abu El-Hussein area. The Nubian Sandstone aquifer occurs only in El-Shab basin whereas the other basins are filled completely with the confining layer of Kiseiba Formation. The depth to basement in El-Shab basin ranges from 11 m. (ves-20) to 197 m. (ves-1) m.b.g.s. The depth to basement in Kiseiba basin ranges from 20 m. (Bir Kurayim magnetic profile) to 122 m. (ves-13) m.b.g.s. The depth to basement in Abu El-Husein basin ranges from 0 (basement outcrops) to 64 m. (Abu El-Husein magnetic profile) m.b.g.s. The aquifer thickness ranges from 0 m (where the aquitard rests directly on the basement) to 153 m. (El Shab well No. 79). The aquifer is uncoformably overlain by Kiseiba Formation which represents the aquitard layer at Bir El-Shab. The thickness of the aquitard ranges from 0 (in areas covered by the Nubian Sandstone) to 120 m (ves-13). Each of the aquifer and aquitard consist of three layers. Two of the aquitard layers are water-bearing. However, the estimated transmissivity of the aquitard is very low (11.9 m2/d). The groundwater moves vertically into the overlying aquitard at Bir El-Shab and subsequently flows in concentric pattern into the surrounding areas. Faulting controls groundwater occurrence and quality. Some springs lie on the basement high associated with step faulting at the edges of El-Shab basin. An ENE low-salinity zone is associated with the basement high which separates El-Shab basin from Kiseiba basin. Focused groundwater recharge through the faults and fractures from paleo playas could be the mechanism of the formation of this anomaly. The isotope data shows local recharge of the groundwater most likely during the Pleistocene time. Two-dimension (2D) Electrical Resistivity Tomography (ERT) profiles reveal that the evaporation process has the main role in increasing the salinity of some water points. It is highly recommended to delineate the southern boundary of El-Shab basin which is expected to extend into Sudan.