## 1 Introduction  
Aquifers play a critical role in sustaining the economy and the environment of coastal areas [1]. These areas are often subjected to high rates of groundwater drawing to meet the increasing urban, tourism, industrial, and agricultural demands, adding stress to groundwater bodies and dependent ecosystems [2,3]. The combination of global climate forces underlying human pressures threatens the fragile balance between freshwater and saltwater and therefore the quantity and quality levels required for a good functioning of groundwater-dependent ecosystems (GDE) [4]. These effects are especially visible in small unconfined aquifers because their greater exposure to human impacts and smaller storage capacity may limit a regular water provision to ecosystems during drier periods [5,6]. This is the case of the Santo Andre Lagoon (SAL), which, together with the Sancha Lagoon, form a coastal GDE space in southwest Portugal whose hydrological functioning depends on regular inputs of fresh surface water and groundwater [7,8]. This GDE space was catalogued in 1993 as a Special Protection RAMSAR Area due to its ecological value for wildlife preservation [RAMSAR website: [[https://www.ramsar.org/es/humedal/portugal](https://www.ramsar.org/es/humedal/portugal)]([https://www.ramsar.org/es/humedal/portugal](https://www.ramsar.org/es/humedal/portugal))]. This type of ecosystem is environmentally protected by the European Water Framework Directive [9], which also establishes the obligation to permanently characterize its hydrological functioning. Preliminary findings from the Portuguese R&D Groundscene Project [10,11] showed that freshwater contribution from tributary streams, which in turn are groundwater-dependent, regulates the water salinity balance of the SAL ecosystem. The Groundscene Project left a blank in what concerns the detailed hydrological functioning needed for predictive modeling under scenarios of climate change and subsequent new land and water uses for human adaptation. This is a large task in progress that is being boarded through implementing and coupling well suited rainfall-runoff and groundwater modeling tools [12, 13, 14]. Over the base of detailed and existing or compiled weather, land use, and water fluxes datasets, implementation of groundwater modeling tools involves three general stages for aquifer conceptualization [15, 16, 17]: (1) geometry definition, (2) acquisition of hydraulics data, and (3) evaluation of water balance components. This paper is aimed at advancing the first stage, i.e., aquifer geometry definition, and at qualifying some transient groundwater features subordinately. In flat coastal areas, near-surface geophysical techniques have been widely used in groundwater research to acquire this basic information [8, 18, 19, 20, 21, 22, 23, 24]. These techniques are non-invasive, usually cheap to apply, and useful when geotechnical sounding data are sparse or not able to provide detailed subsurface information required for groundwater modeling, such as aquifer geometry and some transient groundwater features. This paper integrates multichannel analysis of surface waves (MASW) and time-lapse electrical resistivity tomography (ERT) to define aquifer geometry and qualitatively identify some transient groundwater features of the alluvial aquifer contributing to the SAL. Integrated MASW and ERT results were intentional for this purpose because the MASW technique responds to the steady shear modulus of subsurface materials, expressing seismic shear-wave velocity (VS) [25, 26, 27, 28], whereas ERT responds to the electrical conductivity (EC) of subsurface media, which is subjected to transient ambient changes [21, 29, 30, 31]. The use of MASW in groundwater research is incipient [32, 33, 34] and has mostly addressed disambiguation of the subsurface geological structures deduced from ERT in high EC environments, such as coastal areas [33, 35, 36, 37].