Abstract
This paper integrates multichannel analysis of surface waves (MASW) and time-lapse electrical resistivity tomography (ERT) to define aquifer geometry and identify transient groundwater features of the Cascalheira Stream Basin Holocene alluvial aquifer (aquifer H), which contributes to the Santo André Lagoon, part of a coastal groundwater-dependent ecosystem (GDE), located in southwest Portugal. MASW measures shear-wave velocity (VS), allowing one to obtain steady geological models of the subsurface, and ERT measures subsurface electrical resistivity (ER), being subjected to ambient changes. MASW enables disambiguation of geological structures in low ER environments, such as coastal areas. This research covered one natural year and involved one MASW campaign, four ERT campaigns, and additional geological field surveys and groundwater monitoring to assist interpretation of results. In the area, the conjugate NW–SE and NE–SW strike-slip fault systems determine compartmentalization of geological structures and subsequent accommodation space for Holocene sedimentation. MASW and ERT surveys show how the NW–SE system deepens these structures toward the coast, whereas the NE–SW system generates small horsts and grabens, being one of these occupied by aquifer H. From upstream to downstream, aquifer H thickness and width increase from 10 m to 12 m and from 140 m to 240 m, respectively. Performance of VS and ER models was satisfactory, with a normalized error of the VR and ER models in the 0.01–0.09 range, meaning that a quantitative quota of uncertainty can be segregated from the overall uncertainty of groundwater models without substantially affecting its simulations accuracy. This methodology seeks to improve the design of shallow groundwater research in GDE preservation policies.
Highlights
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 the quantity and quality levels required for a good functioning of groundwater-dependent ecosystems (GDE) [4]
This is the case of the Santo André 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]
Summary
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 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 André 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
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