## 6 Conclusions  
Integrated MASW and time-lapse ERT geophysical techniques enabled building a predictive geological model and deducing some transient groundwater features of the CSB Holocene alluvial  
Figure 8: **(a)** Regional tectonic setting of CSB area, after [43; 44; 45], showing rose diagrams of faults orientation clustered by age of geological domains as in Figure 1b. **(b)** Local tectonic setting of aquifer H after [40; 43; 47], direct field observations, aerial photographs, and MASW and ERT surveys, showing theoretical structural and deformation schemes, and the conjugate NW-SE and NE-SW strike-slip fault systems. **(c)** Geological model of aquifer H, showing geological cross-sections 01 (site 1), 02 (site 2), and 03 (site 3) from integrated MASW (Figure 5) and time-lapse ERT (Figure 6) surveys at sites 1 to 3 and direct field observations, and a new geological cross-section called 00 and the southern end of the geological cross-section 03 inferred from geological mapping and direct field observation only; the vertical-to-horizontal scale ratio being 1:2.  
aquifer (aquifer H) in the southwest coast of Portugal. Findings from 2D VS and ER models were completed with official regional geological information, geological and geophysical data compiled from the scientific literature, and direct field observations. The conjugate NW-SE and NE-SW strike-slip fault systems determine compartmentalization of underlying geological structures and the subsequent accommodation space for Holocene sedimentation. The NW-SE system deepens the geological structures toward the coast, whereas the NE-SW system generates small horsts and grabens, the aquifer H being encased into one of these small grabens. From upstream to downstream, aquifer H thickness and width increase from 10 m to 12 m and from 140 m to 240 m, respectively. The two 2D VS models in sites 1 (upstream) and 3 (downstream) were addressed at defining the near-surface geological structure of aquifer H and underlying Pleistocene and Pliocene formations. Since VS is a site-specific steady variable which does not depend on ambient changes, the VS models can be used to interpret geological structures without the groundwater component influence. In interpreting VS models, some reference VS values compiled from scientific literature for equivalent lithologies were used. The ER models were aimed at inferring some shallowest geological structures after some VS gaps and disambiguating geological structures having different age, similar VS, and different ER. Unlike VS, ER is a site-specific variable which is subjected to ambient changes. In coastal porous media, ER is primarily governed by GER, which in turn depends on the recharge water salinity resulting from the variable combination of atmospheric bulk deposition salinity and actual evapotranspiration rates, which are predictable variables over space and time. These predictive variations allowed us to disambiguate inland recharged Holocene alluvial from locally recharged coastal Pleistocene sand dunes at the aquifer H outlet. For proper disambiguating, the time-lapse ERT from September 2014, in which GEC mass flow and aquifer storage variations are minimal, was selected. Groundwater predictions from numerical models can be quite inaccurate when inherent uncertainty of data used for aquifer conceptualization is neither appraised nor considered. In contrast to the sole use of qualitative raw geological data and mapping to prepare a geological model, normalized errors of the VS and ER models mean a quantitative quota of uncertainty from which groundwater models will add other types of uncertainty. The performance analysis of the VS and ER models shows satisfactory to very satisfactory matches between measured and predicted data. This means that the Pliocene to Holocene geological structures were well-conceptualized and they are homogeneous enough to predict the magnitude of VR and ER in non-surveyed areas from the results obtained in the surveyed ones.