## 2 Study Area
### Geological and Hydrogeological Setting  
The study area (Figure 1b) belongs to the western Mesozoic extensional margin of the Iberian Plate, which has had a long distensive tectonic and magmatic history from the Late Triassic up to the present [44; 45]. Locally, the CSB includes the following synthetic succession from bottom to top [40; 46; 47; 48; 49]: (1) Variscan crystalline bedrock includes (i) Devonian low-grade metapelites and quartzites, and (ii) Carboniferous low-grade metapelites, vulcano-sediments, and tuffites underlying to metapelites, greywakes, and conglomerates; (2) Mesozoic sedimentary cover includes (i) Triassic continental sandstones intruded by basaltic and doleritic dikes, and (ii) Lower Jurassic platform dolomites, carbonates, and marks; and (3) Upper Miocene to present sedimentary filling includes (i) Pliocene marine sandstones, biocalcarnites, and marks, (ii) Pleistocene continental fluvial terraces and clay-rich colluvials underlying to coastal sand dunes, and (iii) Holocene alluvial, lagoon, and beach sands sediments. In this extensional geological domain, the conjugate NE-SW and NW-SE strike-slip fault systems affecting underlying formations since the Neogene determine the accommodation space for Holocene sedimentation [43; 47], the subject of this paper. As a result, the Holocene sedimentary record may reach 20 m in thickness in the CSB-SAL boundary (Figure 1b) with the following detailed succession from bottom to top (1) alternating marine muds and tidal sands below the SAL, (2) organic matter-rich lagoon muds with more than 20% of bioclasts below the SAL, (3) fluvial coarse sands and sandy loam with some bioclasts in the old (currently inland) lagoon space, and (4) coarse-grained deposits occupying the current flood plain of the Casalheira stream valley [40; 46; 47; 48]. In hydrogeological terms, geological formations can be classified into four groups according to the permeability type and the storage capacity reported by [46; 50]: (1) Devonian and Carboniferous metapelites are low-permeability formations forming the impervious basement of local aquifers; (2) Triassic sandstones and Lower Jurassic carbonates form moderately to highly permeable aquifers with karst features and thickness up to 200 m; (3) Pliocene and Pleistocene sediments are low-to-moderate permeable formations forming an unconfined multilayer aquifer of about 30 m in thickness hydraulically connected to the uppermost Holocene formations; and (4) Holocene alluvial deposits comprise sand, gravels, and silt of 10 m to 20 m in thickness filling the Casalheira Stream valley, an unconfined aquifer (hereafter aquifer H) of 1.46 km\({}^{2}\) at the CSB outlet that contributes to the SAL (Figure 1b). The hydrogeological functioning of the CSB depends on water exchange between aquifer H and underlying Pliocene and Pleistocene formations, the extent of weathered metapelities slopes upstream, and thickness and extent of aquifer H downstream (Figure 2). Recharge to aquifer H comes from direct rainfall and runoff infiltration, interflow runoff from fissured and weathered metapelities upstream, transferences from underlying formations, and some irrigation return from agriculture. Discharge from aquifer H depends on actual evapotranspiration and the scarce groundwater pumping in the area. The resulting positive net groundwater balance from aquifer H sustains the freshwater-saltwater balance of the SAL. However, groundwater contribution to the SAL is temporally retained at the aquifer H outlet because the existence of low-permeability (clay-rich) interbedded lagoon sediments diminishes the hydraulic connectivity between aquifer H and the SAL, and saltwater-freshwater interface within the sediments underlying the SAL forces fresh groundwater to be discharged above it [7; 51]. The result is that the piezometric level of aquifer H rises above topography after rainfall-recharge events and a significant fraction of groundwater flooding the CSB outlet is evacuated to the SAL as surface runoff [7; 8].