## 5 Discussion  
For the period 2020-2040, climate change projections foresee declining surface water sources in the Andean highlands, while the population in the MDQ could increase from 2.7 to about 4.2 million inhabitants [6,8,10]. The consequence is more groundwater exploitation to supplement the increasing urban water demand [37]. Water Authority of Ecuador and EPMAPS are aware of this problem and have already begun to promote applied groundwater research for sustainable use. Geophysical prospecting surveys can contribute to improve the hydrological conceptualization. However, the existing geophysical surveys explored different observations scales aimed to cover different research interests. So, the geophysical information must be examined and integrated before use in groundwater research. Most geophysical surveys have been performed in the most populated IAV sector (Figure 1b, Table 2), where groundwater exploitation is concentrated and signs of degradation have been reported [36,78]. To examine the usefulness of the subsurface geophysical information in groundwater research in the IAV, the area covered by each geophysical prospecting survey (defined by the corresponding prospecting length and depth) (Table 2) was superimposed onto a synthetic stratigraphic column in the southern border of the MDQ (Figure 1c). Of the compiled 23 geophysical prospecting surveys (Table 2), only the 20 ones located in the IAV (Figure 5) were selected and classified into three methodological groups (electrical, seismic, and electromagnetic) covering two observation scales and two aquifer typologies: shallow Holocene and late Pleistocene aquifers and thick regional middle-early Pleistocene and late Pliocene aquifers. The VES and ERT surveys were used to define the geometry of shallow Holocene and late Pleistocene aquifers, deduce the regional piezometric level, and qualify pore-water conductivity. The prospecting depth was 150-500 m for the VES and 25-165 m for the ERT surveys (Figure 5). Reinterpretation of the 2D ER models shows that the ER range was 17-203 \(\Omega\) m for VES and 3-150 \(\Omega\) m for ERT in those geological formations catalogued as aquifers. These figures agree with the expected EC variability in saturated media associated with variable contributions of natural (e.g., recharge, thermalism, mineral dissolution) and anthropogenic (e.g., domestic, agriculture, industry) salinity sources. Groundwater conductivity deduced in shallow aquifers was higher than in thick regional aquifers. In those geological formations catalogued as aquitards, the ER range was 215-850 \(\Omega\) m for VES and 210-320 \(\Omega\) m for ERT. These figures agree with the expected lower variability of conductivity induced by the homogeneous clay content and barely variable lower pore-water content. The regional piezometric level varied depending on the aquifer hydraulic functioning, explored aquifer zone (recharge, transit, and discharge), and topography (Table 2). REMI surveys were originally performed in geotechnical research for civil works. The 2D VS models were reinterpreted for the geometric definition of shallow Holocene and late Pleistocene aquifers, which is an innovative research application. The prospecting depth was 40 m (Figure 5). The VS values were less than 200 m s\({}^{-1}\) in recent anthropogenic fillings, soils, and lacustrine formations; 200-550 m s\({}^{-1}\) in Holocene formations; and more than 500 m s\({}^{-1}\) in late Pleistocene formations. VLF-EM and low-frequency MTS surveys provided the geometry and structure of Holocene to late Pliocene formations resulting from the action of first-order thrusts and strike-slip faults. The prospecting depths were 130 m for the VLF-EM survey and 1500, 1800, and 4000 m for the three low-frequency MTS surveys, of which only the first two were in the IAV sector (Figure 1b, Table 2).