## 5 Discussion
### Use of the Groundwater Resource  
The sources and mechanisms for groundwater recharge and discharge in urban areas are more numerous and complex than in natural environments, as documented many urban groundwater research [70; 71; 72; 73; 74; 75]. Buildings and civil works combine with human-made drainage networks, sanitation systems, and paving to introduce new recharge and discharge components or modify the existing ones. In Adra town, this problematic increase because the study area must be extended to include a peri-urban area devoted to natural uses and irrigation agriculture that influences the urban hydrology downstream. In the NSUS, irrigation (0.17 Mm\({}^{3}\) year\({}^{-1}\)) and urban (0.10 Mm\({}^{3}\) year\({}^{-1}\)) returns are clearly higher than the recharge from precipitation (0.03 Mm\({}^{3}\) year\({}^{-1}\)). The consequence is an uncatalogued groundwater resource contributing to the SFUS. The ARDGB, where the SFUS is emplaced, provides most of the usable water, sustains some GDEs catalogued in the Ramsar Convention list [14], and is officially protected to avoid new exploitations [43; 44; 46]. However, the NSUS and the southern irrigation agriculture peri-urban sector form a not officially catalogued marginal hydrogeological system that contributes to the SFUS and therefore to the ARDGB. This groundwater resource discharges to the sea under Adra town. This paper conceptualizes the functioning of this marginal hydrogeological system and provides a preliminary evaluation of the groundwater resource contributed to the SFUS. This resource could sustainably be exploited downstream.  
Average groundwater discharge is around 0.28 Mm\({}^{3}\) year\({}^{-1}\), and may vary in the 0.03-1.02 Mm\({}^{3}\) year\({}^{-1}\) range when the minimum and maximum values of saturated thickness and permeability are considered (Table 3). Average aquifer recharge is around 0.31 Mm\({}^{3}\) year\({}^{-1}\). As expected in unexploited aquifers, the magnitude of average groundwater discharge and recharge in the NSUS is similar and lower than the calculated AGS of around 0.47 Mm\({}^{3}\) (Table 3). Three conclusions regarding the conceptualization of the NSUS aquifer functioning are gained: (1) The recharge-to-discharge absolute difference is 0.03 Mm\({}^{3}\) year\({}^{-1}\), so other sources and mechanisms for groundwater discharge probably occur and must be characterized, for instance occasional groundwater pumping to lower the piezometric level during building construction, groundwater up-take by phreatophytes and deep-rooted vegetation, and direct groundwater evaporation in sites having a quite shallow piezometry; (2) average saturated thickness is about 0.5-0.9-fold the total thickness of the hydrogeological formations, but there are desaturated sectors that must be characterized; and (3) renewability of the groundwater resource is high enough to sustain a durable small exploitation downstream, as deduced from an average groundwater turnover time less than one year, here tentatively expressed by means of the groundwater recharge (0.31 Mm\({}^{3}\) year\({}^{-1}\); Table 4) to AGS (0.47 Mm\({}^{3}\); Table 3) ratio [76; 77]. This exploitation may guarantee the permanent water supply to watering public gardens and urban cleaning, currently around 0.11 Mm\({}^{3}\) per year and tending to increase in coming years as new urbanized areas are being planned. Other ecological uses could be also considered.  
Other hydrogeological gaps to resolve in future research are (1) definition of the hydraulic behavior of fault as water-bearing or water-tight, (2) characterization of the hydraulic effect of civil works on groundwater flow, and (3) how climate change and subsequent land-use adaptations can affect this groundwater resource.