## 2 Study Area
### Geological and Hydrogeological Setting  
The study area (Figure 1b) belongs to the Alpujarride tectonic complex from the Internal Domain of the Alpine Betic Cordillera [38; 39]. The area is tectonically active as a consequence of the convergence between the African and Eurasian Plates, which ended with the collision of the Internal and External Betic domains during the early Miocene [39; 40]. The combination of active tectonics and sea-level changes controls the accommodation space for Upper Miocene to present sedimentation [39; 41].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Acronym** & **Description** \\ \hline g1 & Pleistocene colluvial 1 \\ g2 & Pleistocene colluvial 2 \\ g3 & Pleistocene colluvial 3 \\ GDE & Groundwater-dependent ecosystem \\ GPR & Ground Penetrating Radar \\ MASW & Multichannel Analysis of Surface Waves \\ NSUS & Northern steep urban sector \\ PGS & Potential groundwater storage \\ SFUS & Southern flat urban sector \\ t1 & Pleistocene littoral facies 1 \\ t2 & Pleistocene littoral facies 2 \\ VE & Velocity of electromagnetic waves \\ VS & Shear-wave velocity \\ WB & Paleozoic weathered, fissured crystalline bedrock \\ \hline \hline \end{tabular}
\end{table}
Table 1: Cont.  
[PERSON] et al. (2002) [19] classified the geological record of Adra town into twelve geological formations attending to age, origin, and geometry. The two pre-orogenic formations included Paleozoic low- and medium-grade mica-schists and quartz-schists (crystalline bedrock). The ten post-orogenic formations included Pliocene to Quaternary sedimentary formations unconformably deposited over the bedrock (Figure 1b) as (1) a Pliocene deltaic formation; (2) five Pleistocene formations including two generations of littoral facies (t1 and t2) and colluvial (g1 and g2), and a cemented colluvial (g3); and (3) four Holocene formations including colluvial, the Adra River Delta (ARD) alluvial, present littoral facies, and anthropogenic filling. Direct field observations proved that the official 1:50,000-scale geological mapping [38] is detailed enough to explain the hydrogeological functioning in the peri-urban area (Figure 1b).  
From a regional hydrogeological point of view, the eastern and southern sectors of Adra town are emplaced on the ARDG, which is located at the ARB outlet (Figure 1a). The ARDB has a surface of 49.2 km\({}^{2}\) and an average saturated thickness of 100 m. The average groundwater recharge and discharge are around 25 Mm\({}^{3}\) year\({}^{-1}\)[14, 42, 43, 44].  
In the NSUS, Paleozoic weathered, fissured crystalline bedrock (WB), Pleistocene t1 and t2, and Holocene alluvial form a marginal aquifer not officially catalogued as a  
Figure 1: (**a**) Location of the study area in southern Spain, showing geographical sites cited in the text. (**b**) After the Geological Survey of Spain [38], the simplified 1:50,000-scale geological map of the urban and peri-urban areas; legend as (1) Paleozoic metamorphic formations, (2) Pliocene formations, (3) Pleistocene formations, (4) Holocene formations, (5) Undifferentiated geological contact, (6) Urban area, (7) Roads, (8) Adra River, (9) Urban basins, and (10) ARDB boundary. (**c**) After the Andalusian Environmental Information Network [[[http://www.juntadeandalucia.es/mediaombiente/site/rediam](http://www.juntadeandalucia.es/mediaombiente/site/rediam)]([http://www.juntadeandalucia.es/mediaombiente/site/rediam](http://www.juntadeandalucia.es/mediaombiente/site/rediam))], the 1:25,000 scale land-use map of the urban basins, accessed on 15 January 2021.  
groundwater body despite its historical relevance for water supply at homes and sparse traditional irrigated crops [15]. These formations are the subject of this paper.