| 08 Apr 2026
Beneath the tide: Sediment-controlled groundwater and salinity stratification across an estuarine wetland transect
Abstract. Coastal vegetated wetlands occur at the interface between marine and terrestrial hydrological processes, yet the subsurface controls governing salinity persistence and vegetation zonation remain poorly constrained, particularly in settings where wave energy is suppressed, such as barrier estuaries. We investigate seawater–groundwater interactions along a transect spanning mangrove, saltmarsh, supratidal, and terrestrial vegetation zones using electrical resistivity tomography (ERT), water-level and salinity monitoring, and sediment analysis. Results reveal a pronounced lateral and vertical salinity structure characterised by a shallow saline wedge extending inland within fine-grained intertidal sediments, while deeper groundwater remains comparatively fresh. Vegetation zonation closely mirrors this subsurface composition: mangrove and saltmarsh communities are associated with persistent shallow salinity in low-permeability silt–clay substrates, whereas supratidal and terrestrial vegetation occurs where sandier sediments and less saline water dominate. Although rainfall and tidal forcing dynamically influence shallow groundwater, salinity variability attenuates rapidly with depth, indicating limited vertical connectivity and sediment-controlled hydraulic anisotropy. The persistence of fresh groundwater beneath saline shallow sediments departs from the classical homogeneous, density-driven coastal aquifer model. Instead, the system reflects a hydraulically stratified composition in which fine-sediment accretion promotes vertical permeability contrasts and shallow saline retention, while deeper freshwater-dominated zones (indicated by low bulk electrical conductivity, potentially reflecting terrestrially recharged groundwater) extend towards the estuary. This vertically differentiated configuration influences vegetation distribution and contributes to carbon-rich intertidal sediment accumulation. We propose a refined conceptual model for microtidal, wave-dominated barrier estuaries with similar settings, emphasising sediment-controlled hydraulic anisotropy over density-driven stratification.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
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