## 2 Geomorphology, Geology, and Hydrogeology  
The study area comprises three main geomorphologic units (Figure 2): mountains, plateaus, and depressions [5; 23; 24]. The Red Sea mountainous terrains are located in the east and are composed mainly of igneous and metamorphic rocks. The plateaus consist of El Maaza limestone and El Ababda sandstone highlands. The limestone plateau occupies the western portion of the studied basin, and the sandstone plateau breaks up into low ridges and isolated hills [25]. The characteristics of these elevated areas have notable impacts on the hydrogeologic setting [23; 24]. The morphotectonic depression represents one of the most noticeable topographical features in the basin, a wide lowland that extends from the northern portion of Wadi Qena to the north of the Qena-Safaga road, linking the Qena district with the Red Sea coast. The Qena basin stratigraphy is composed of Precambrian (igneous and metamorphic) rocks overlain by Cretaceous to Quaternary sedimentary success (Figure 2). The stratigraphic sequence consists of rock units [1; 26; 27; 28; 29; 30; 31] arranged from base to top as indicated in Figure 2: (1) Cretaceous, represented by Wadi Qena, Galala, Umm Omeiyied, Abu Aggag, Hawashiya, Quseir variegated shale, Duwi, Rakhiyat, and Sudr formations; (2) Paleocene, represented by Dakhla, Tarawan, and Esna formations; (3) Early Eocene, represented by the Thebes formation, and (4) undifferentiated Pliocene deposits beside the alluvial Quaternary sediments. The Wadi Qena basin was affected by shear faults, folds, and fractures, which are attributed to the Pan African orogen and a series of tectonic reactivations, mostly during the Cretaceous and Oligocene eras [32; 33; 34; 35]. Figure 1: Location of groundwater samples. Samples 1 to 48 were collected in November 2018; samples 49 to 104 were collected later [21; 22]. The groundwater is exploited from the Quaternary aquifer, the PNA, and the NSA ([PERSON] 1997 [5]). The Quaternary aquifer at Wadi Qena is the main groundwater source for irrigation; it is composed of gravel, coarse to fine sand, and conglomerates; its thickness increases downstream, attaining approximately 100 m (Figure 3; [5; 37; 38]. The aquifer is characterized by unconfined conditions; the water depth ranges from 1.9 to 20 m (Table 1), the maximum penetration depth from the ground surface elevation is 28 m, and the groundwater generally flows from northeast to southwest [25]. The Post-Nubian aquifer (PNA) contains water bearing limestones and sandstones of the Pliocene and/or Eocene ages. The groundwater is characterized by free water table conditions, the depth to groundwater level (DTW) ranges from 28 to 76 m, and the maximum groundwater well total depth (TD) is 200 m (Table 1). Figure 3 shows that the PNA aquifer is directly overlain by the Quaternary aquifer, where it is not separated by a confining layer of shale or clay sheets. Hence, mixing may occur naturally or be induced by pumping, affecting the groundwater chemistry. The NSA is formed mainly of sandstone intercalated with shale interbeds of the lower Cretaceous age, and is considered one of the primary groundwater sources. The groundwater in the NSA is confined by impervious Quseir variegated shale overlying the aquifer [5]. The groundwater flows from northeast to southwest [25]. Figure 2: Wadi Qena geological map [36]. \begin{table}
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