## 4 Results  
### Near-Surface Electrical Surveys  
Near-surface electrical techniques take voltage measurements between two potential electrodes installed on the land surface once direct current is injected into two current electrodes. Such techniques allow the calculation of subsurface electrical resistivity (ER) [\(\Omega\) m], reciprocal of subsurface electrical conductivity (EC). Penetration depth and resolution depend on subsurface EC, which is a function of transient pore-water EC and steady ground EC, the input voltage used, and the electrode spacing adopted [53; 54; 55].  
The ER data were acquired using the SuperSting R8/IP eight-channels and the SuperSting R1/IP single-channel Memory Earth Resistivity and IP Meter by Advanced Geosciences Inc., Austin, TX. Fifty-six electrodes were placed along each ERT profile using a variable 3-6-m spacing and applying an input voltage of 200 V. A Schlumberger electrode array was used. See [PERSON] and [PERSON] [49] for further methodological details.  
Figure 2b shows the location of selected ERT profiles, here called ERT1 and ERT2. The profile features were prospecting lengths of 330 (ERT1) and 165 m (ERT2), prospecting depths of 59 (ERT1) and 28 m (ERT2), ER in the ranges of 20-175 (ERT1) and 25-330 m (ERT2), and average root-mean-square errors (RMSE) of 1.80 (ERT1) and 2.94 (ERT2). Both ERT1 and ERT2 exposed quite similar horizontal and vertical ER distributions (Figure 2c). The ER values were typical of sedimentary and volcano-sedimentary rocks with a variable degree of saturation [56].  
Local hydrogeological information [26; 33; 34] was used to reinterpret the ER models. From top to bottom, the vertical ER distribution was as follows: (i) 1-5 (ERT1) and 1-8 m (ERT2) of discontinuous porous soils and anthropogenic fillings with ER in the 70-300 \(\Omega\) m range; (ii) 3-5 (ERT1) and 6-15 m (ERT2) of tuff and ash formation catalogued as aquit  
Figure 2: (**a**) General location of the selected ERT survey labelled 19 in Figure 1b and Table 2, updated and reinterpreted from Yautibug and Herrera [49]. (**b**) Detailed location of two selected ERT profiles, here called ERT1 and ERT2. (**c**) ERT1 and ERT2 profiles. The first groundwater observation (in this survey equivalent to the regional piezometric level) is singled out. Hydrogeological reinterpretation of ER models after [49] and local hydrogeological information [26; 33; 34]. Profiles are topographically corrected, and the vertical-to-horizontal scale ratio is 1:1.  
with ER in the 20-50 \(\Omega\) m range; (iii) 5-10 (ERT1) and 10-20 m (ERT2) of silty alluvial formation catalogued as aquitard with ER in the 40-80 \(\Omega\) m range; (iv) 5-50 (ERT1) and 10-50 m (ERT2) of coarse-grained alluvial formation catalogued as aquifer with ER in the 80-120 \(\Omega\) m range; and (v) 5-10 m (ERT1) of silty alluvial formation catalogued as aquitard with ER in the 40-80 \(\Omega\) m range. The interbedded coarse-grained alluvial formation between the above silty alluvial formation (which includes items iii and v) is part of a shallow aquifer that provided the first groundwater observation corresponding to the regional piezometric level. Changes in the thickness and spatial continuity in the vertical ER distribution are due to sedimentary processes (e.g., lateral facies changes and erosive channels) and the action of minor-order normal faults described in the area [26; 33; 34].