## 5 Discussion
### The Geological Model of the Cascalheira Stream Alluvial Aquifer  
MASW and ERT geophysical techniques were integrated to define the geological model of aquifer H. The VS models were intended to this purpose exclusively, whereas the ER models were used to disambiguate geological structures having similar VS and different ER, and to complete geological information in areas not covered by MASW. As described in Section 2.3, local faulting narrowly reproduces the regional structural setting determined by the conjugate NW-SE and NE-SW strike-slip fault systems (Figure 8a) [44; 45]. These fault systems affect the Upper Miocene--Quaternary sedimentary record, thus determining the accommodation space for Holocene sedimentation in valleys and plains in the southwest coast of Portugal [43; 47; 48]. As shown in VS (Figure 5) and ER (Figure 6) models, the NW-SE strike-slip fault system generates small horsts and grabens perpendicular to the coast (Figure 8b). From north to south, the VS and ER models identified three small NW-SE faults called F6, F7, and F8; F8 was also inferred from regional geological mapping [43; 47; 48]. Out the area covered by MASW and ERT surveys, another small NW-SE fault called F9 was inferred after regional geological mapping and direct field observation. F6 and F8 are SW-vergent, whereas F7 and F9 are NE-vergent. The NE-SW strike-slip fault system is NW-vergent, is younger than the NW-SE system, and determines the progressive deepening of the geological formations toward the coast (Figure 8b). From East to West, two small NE-SW faults called F1 and F2 were inferred from regional geological mapping [43; 47; 48], two others called F3 and F5 were deduced after direct field observation, and a fifth called F4 was inferred after analyzing time-lapse ERT at site 2. The conjugation of NW-SE and NE-SW fault systems compartmentalizes the area into small NW-vergent blocks. The Casalheira Stream valley is the result of these conjugate strike-slip fault systems; first, the NW-SE system generates a small graben, and later, the NE-SW system deepens and widens the aquifer H toward the coast. This structural scheme is described in Figure 8b. In detail, the NW-SE faults F6 and F7 at site 1 (MASW1) were identified (Figure 8c). At site 3, the traces of NW-SE faults F6, F7, and F8 (MASW3) were also identified (Figure 8c). At sites 1 and 3, no NE-SW fault was identified because the MASW and ERT surveys were almost parallel to these faults (Figure 8c). However, at site 2, the ER model enabled us to deduce the NE-SW fault F4 as a preferential path for low-salinity water transference from Pleistocene formations to aquifer H (Figure 6b). At site 3, the ER model enabled us to infer geology after VS model gap in MASW3 at distances 130-220 m and to disambiguate the geological structure of this profile sector, which is infilled by Holocene sediments and Pleistocene sand dunes having similar VS (MASW3, Figure 5b) but different ER (ERT3, Figure 6c). After the ER models, some sedimentological features of hydrological interest have been deduced. At site 1, the current stream channel appears migrated towards the southern valley boundary, probably as the central valley was progressively infilled since early Holocene. This feature is also observed at site 3, determining that freshwater discharge from aquifer H over the saltwater lens in Pleistocene formations flow through what seems to be two incisive paleochannels (Figure 6c). Figure 8c presents the geological model of aquifer H. As shown, the Casalheira Stream valley is encased into a small graben structure contoured by small NW-SE and NE-SW strike-slip fault systems. In detail, aquifer H occupies one small graben bounded by faults F6 and F7 upstream and two small grabens (bounded by faults F6-F7 and F8-F9) separated by a small hornst (bounded by faults F7-F8) downstream.