## 4 Results
### Hydrogeological Conceptualization  
This section completes the two first goals of this paper, i.e., geometry and hydraulics definition of the geological formations in the NSUS. For geometry definition, geophysical findings, and previous [19] and new geological data were integrated on GIS to prepare the 1:5000 scale hydrogeological map of Adra town (Figure 6) and three representative hydrogeological cross-sections (Figure 7). For hydraulics definition, a permeability and effective porosity database was prepared from (1) compiled data from the literature de  
Figure 5: Selected sectors from GPR sections G2, G3, and G4 of especial hydrogeological interest; location is in Figure 2. The interpretation of radargrams was based on previous geological [19] and hydrogeological [15] data, and MASW sections (Figure 3). The projected piezometric level after the flash campaign carried out in June 2015 is included; location of open well and piezometer W1, W2, W3, W7, and W10 is in Figure 2. Profiles are topographically corrected and its vertical-to-horizontal scale ratio is 1:2.  
scribed in Section 3.1.1, which included official publications devoted to the Holocene ARD alluvial Formation in the SFUS, technical reports surveying the NSUS and SFUS, and scientific publications covering the NSUS and SFUS; and (2) data from specific field surveys in the NSUS, which included five double-ring infiltration tests in low-permeability formations, three pumping tests in open wells in high-permeability formations, and nine granulometric curves in different formations. On the basis of this information, the twelve geological formations have been classified into ten hydrogeological formations attending to the permeability type and storability (Table 2). The potential groundwater storage (PGS) of each hydrogeological formation is defined as the product of surface (direct and underlying outcrops), thickness (saturated and unsaturated), and effective porosity (Table 2). PGS must be considered a tentative magnitude of the maximum storability, not the actual storability. PGS has been calculated for all formations in the total urban area and for the existing ones in the NSUS. The Paleozoic crystalline (mica-schists and quartz-schists) bedrock is a low-permeability formation constituting the impervious base of aquifers in the urban area (Table 2). This is catalogued as aquiclude. The MASW sections (Figure 3) and maps (Figure 4) delineate the bedrock geometry through VS > 1200 m s\({}^{-1}\). The WB Formation is a porous media forming a moderate- to high-permeability aquifer. In the NSUS, the surface is 0.68 km\({}^{2}\), thickness is in the 1-18 m range, and its average PGS is around 0.27 Mm\({}^{3}\) (Table 2). The geometry of WB can be deduced through VS in the 800-1200 m s\({}^{-1}\) range from MASW sections (Figure 3) and maps (Figure 4). Recharge comes from direct rainfall and runoff infiltration, and urban and irrigation returns. The Pliocene deltalic facies formation is unconformably deposited over the bedrock in the SFUS. This formation is catalogued as a moderate- to high-permeability aquifer (Figure 6) of 0.09 km\({}^{2}\), thickness in the 4-31 m range, and average PGS around 0.03 Mm\({}^{3}\) (Table 2). Recharge comes from transference from the Holocene ARD alluvial formation. Figure 6: Hydrogeological map of Adra town at scale 1:5000, showing operative and historical groundwater observation points, sites where groundwater discharge to the sea is observable, and hydrogeological cross-sections I–I’, II–II’, and III–III’ as in Figure 7.  
The Pleistocene record includes two littoral-continental sedimentary sequences in the NSUS and a continental sequence in the SFUS (Figure 6). A brief description is below. In the NSUS, the two littoral-continental sequences are unconformably deposited over the bedrock (Figure 7). Each sequence includes littoral facies (t1 and t2) underlying clay-rich colluvial (g1 and g2) (Figures 6 and 7).