## 2 Study Area
### Geology and Hydrogeology  
The study area belongs to the Pacific Ring of Fire, a highly active belt of volcanic and seismic activity originating from the subduction of the oceanic Nazca Plate beneath the South American Plate, which is the source of the compressive tectonics and arc magmatism of the Andes Cordillera [27; 28]. The MDQ is located in the westernmost part of the NNE-trending fault-bounded Andean compressive structure, which includes the IAV between the WAC and EAC (Figure 1b).  
The basement of the MDQ includes a variety of geological formations (Figure 1c). Upper Cretaceous oceanic, arc-island sequences, and volcano-sediments (codes 12-14 in Figure 1c) form the non-metamorphic basement of the WAC, which underlies Paleocene to Eocene marine turbidites and limestones (code 11 in Figure 1c), and is locally intruded by Miocene granodiorites (code 10 in Figure 1c) [29]. Subparallel belts of Paleozoic metapeilic rocks (code 15 in Figure 1c) and other volcanic-arc rocks accreted against the stable craton during the early Cretaceous form the western metamorphic basement of the EAC [30]. At present, the IAV basement depth and typology remain unknown, although Bouguer gravity anomaly data [31] would suggest an east-verging tectonic wedge of the Cretaceous WAC basement [29], which is covered by Pliocene andesitic lavas (code 9 in Figure 1c), and Pleistocene (codes 5-8 in Figure 1c) and Holocene (codes 1-4 in Figure 1c) sedimentary and volcano-sedimentary formations [32].  
The IAV and EAC sectors occupy a large portion of the Quito-Machachi Hydrogeological Unit and a small part of the Cayambe-Ruminahui Hydrogeological Unit (Figure 1b). The WAC sector is officially catalogued as a regional impervious area. In hydrogeological terms, the above geological formations can be classified into five groups attending to the permeability type and effective porosity reported in the consulted literature [25; 26; 33; 34]: (1) the Paleozoic metapeilic EAC basement is a low-permeability formation representing the impervious lower boundary of the eastern aquifers; (2) the Late Cretaceous sedimentary and volcanic WAC basement is assumed to include low- to moderate-permeability formations comprising the impervious lower boundary of the aquifers in the western IAV sector; (3) the Pliocene and Pleistocene andesitic lavas and pyroclastic flows form thick regional compartmentalized aquifers of moderate permeability, with yield dependent upon the degree of fissuring and fracturing; (4) the Pleistocene and Holocene ash, tuff, and lahar are low- to moderate-permeability formations, often confining the above Pliocene and Pleistocene aquifers; and (5) the Pleistocene and Holocene fluocene and high-permeability aquifers with intergranular porosity (Figure 1c). Table 1 summarizes the compiled information regarding the permeability and effective porosity of representative geological formations in the MDQ.  
Hydrogeological functioning in the MDQ depends on: (1) the low permeability of the EAC (metapelitic rocks) and WAC (sedimentary and volcanic rocks) basements; (2) the compartmentalization, thickness, and degree of fissuring and fracturing in Pliocene and Pleistocene andesitic lavas, which determine the storage capacity and permeability of these aquifers in the IAV; (3) the extent and thickness of low-permeability Pleistocene and Holocene volcano-sedimentary formations forming aquitards in the IAV; (4) the hydraulic connectivity between Pliocene and Pleistocene aquifers and between Pleistocene and Holocene aquitards, favoring the deep percolation of aquifer recharge and localized aquifer discharge; and (5) the extent and thickness of Pleistocene and Holocene coarse-grained sediments for draining runoff and aquifer discharge [9; 11; 12; 18; 26; 35; 36].