the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2026-1398', Anonymous Referee #1, 13 Jul 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1398/egusphere-2026-1398-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-1398-RC1 -
RC2: 'Comment on egusphere-2026-1398', Anonymous Referee #2, 17 Jul 2026
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General Comments / Overall Impression
The manuscript investigates the subsurface controls governing groundwater flow and salinity stratification across a microtidal estuarine wetland transect using a multimodal approach that integrates electrical resistivity tomography (ERT), shallow sediment analysis, and high-frequency hydrological monitoring. The integration of geophysics with core physical parameters and continuous hydraulic head measurements is highly commendable and addresses a critical knowledge gap in transitional supratidal forest environments.
However, while the dataset is valuable and the topic is highly relevant, the manuscript requires substantial revision before publication. The primary concerns lie in the geophysical processing, interpretation, and quantitative uncertainty of the hydrological data..
Given my expertise, I will focus primarily on the geophysical aspects of the manuscript. I recommend Major Revision to address the major and specific concerns outlined below.
Major Comments
- Geophysical Inversion Methodology & Temporal Inconsistencies
The authors state that individual inverted transects were spatially aligned and combined to provide a continuous representation of subsurface conductivity. However, these data were collected at different times of the year (February, June, and September 2024) and under fundamentally different hydrologic conditions—specifically, ERT-2 was acquired immediately following a storm surge event.
- Bulk electrical conductivity (ECB) is highly transient and dynamically influenced by temperature, soil moisture, and fluid salinity variations. Stitching profiles collected across different seasons and post-storm conditions into a single continuous representation without a robust normalization is geophysically unsound.
- Petrophysical Interpretation & Clay Surface Conduction
Throughout the manuscript, high bulk electrical conductivity () is interpreted almost exclusively as a proxy for "saline water" or "saltwater intrusion"
- This direct correlation is a significant oversimplification that ignores the litho-electrical properties of the substrate. The sediment cores confirm the presence of silty clay, clayey sand, and organic-rich clays in the intertidal zone. Clay minerals possess high cation exchange capacity (CEC) and produce substantial surface conduction (pore-surface conductivity). This matrix conduction can dominate the bulk response, potentially masking freshwater-dominated clay zones as highly saline. The authors must incorporate or at least discuss a petrophysical model (e.g., the Waxman-Smits equation or Archie's law with a clay-correction term) to separate interstitial fluid salinity from the clay matrix conduction.
- ERT Data Resolution and Depth of Investigation (DOI)
The authors present resistivity models extending to a depth of 30 m AHD and use these models to support the presence of a deep, freshwater-dominated aquifer.
- The sensitivity of electrical resistivity configurations decays exponentially with depth. For profiles with 2.5 m electrode spacing (covering ~145–147 m) and 5 m spacing resolving structure down to 30 m depth without assessing the Depth of Investigation (DOI) is mathematically unjustified. Because the inversions were conducted using ResIPy the authors should calculate and plot the DOI index map or a sensitivity matrix. Additionally, crucial technical details—such as the number of data levels, data points acquired, and reciprocal error statistics—must be tabulated.
- Hydraulic Head Uncertainty & Error Propagation
The derivation of continuous hydraulic head time series (Equations 1 and 2) involves several indirect measurements, including atmospheric pressure, water temperature, salinity, fluid density calculations, and manual dip
The authors compare very small hydraulic head differences (e.g., water table elevations of 0.025 to 0.085 m AHD across the supratidal and terrestrial zones) to infer vertical decoupling. However, given the vertical accuracy limitations of RTK-GNSS, barometric sensor tolerances, manual tape errors, and density approximations, the cumulative measurement error may exceed the reported hydraulic gradients. To validate these comparisons, the authors must implement a formal error propagation scheme and report the margin of uncertainty for the calculated heads.
- Spatial References & Temporal Comparisons
The spatial context of the geophysical lines is currently too abstract.
- The exact spatial tracks of ERT-1, ERT-2, and ERT-3 must be plotted as lines on Figure 1 rather than relying solely on the x-axis distances of Figure 4. Furthermore, the temporal overlap between ERT-1 (February baseline) and ERT-2 (June, post-storm surge) is a valuable asset of this study. Instead of qualitatively dismissing them as "very similar", the authors should generate a percentage difference plot or conduct a ratio inversion. This would allow them to quantitatively isolate and discuss the depth of storm-surge infiltration or seasonal salinity changes, even if the electrodes were not in the exact same positions.
Specific Comments
• Figure 1 Caption: The caption must be improved. It should explicitly define the meaning of the colored circles on the left panel (which are later understood to represent piezometer locations) . Are they rough locations, or do they represent precise boundaries between different vegetation units?
• Line 125: "This configuration enhances data redundancy and improves inversion robustness..." . The authors must add specific peer-reviewed references to geoelectrical literature to justify the choice of the gradient array over standard Wenner or Dipole-Dipole configurations. Additionally, state the exact number of levels and raw data points acquired for each of the three surveys .
• Line 136: "Changes in elevation across the transect were not considered during the inversion." . Given that the transect has a topographic relief of approximately 1.1 m, ignoring topography can distort the current flow pathways and introduce artifacts in the shallow, high-sensitivity zone of the inversion models. The authors must rigorously justify this decision or, preferably, re-run the inversions in ResIPy with the RTK-GNSS topography data incorporated.
• Line 138: "Individual inverted transects were spatially aligned and combined...". As highlighted in the major comments, explain the physical validity of combining profiles collected across different seasons and distinct hydrologic conditions (dry season vs. post-storm surge).
• Line 140: "Spatial patterns in ECB, together with vegetation community composition, were used as criteria...". Please specify exactly which spatial patterns of bulk conductivity (e.g., high-conductivity boundaries, vertical gradients) and which vegetation indices were utilized to choose the drilling locations.
• Line 145: Confirming the note on Figure 1: please explicitly state in the Figure 1 caption that the colored points correspond to the piezometer locations .
• Line 157: Please specify the exact depth of the screening interval relative to the ground surface (not just in AHD) .
• Lines 174–193: The hydraulic head is derived using multiple indirect measurements . In order to validate the fine head differences, a formal Taylor series error propagation scheme must be computed to establish the confidence intervals of the final hydraulic head values.
• Figure 4 Caption: The caption indicates that ERT-1 and ERT-2 were acquired at the same location but at different times. This spatial alignment should be clearly detailed in the methodology. Were the exact same electrode positions utilized? If not, how was the spatial offset handled?
• ResIPy DOI & Visualization: The assumed 30 m model depth is not justified . Please run a DOI analysis in ResIPy and mask out the low-sensitivity zones. Additionally, I recommend presenting the bulk conductivity scale in logarithmic format to better resolve high-contrast boundaries.
• Model Quality Control: To demonstrate the mathematical validity of the inversions, a comparison between the raw measured apparent resistivity data and the forward-modeled responses in a pseudosection format should be provided, at least as supplementary material .
• ERT-1 vs. ERT-2 Comparison: The storm surge occurred prior to ERT-2 . Comparing the differences between ERT-1 and ERT-2 would provide key insights into subsurface dynamics. The authors should exploit this to discuss short-term vs. long-term salinity retention.
• Discussion Literature Integration: To support the discussion on sediment-controlled hydraulic anisotropy, groundwater stratification, and the petrophysical interpretation of clay-rich estuarine sediments, the authors must consider integrating the findings of the following highly relevant works:
o https://doi.org/10.5194/hess-24-2121-2020
o https://doi.org/10.1016/j.jhydrol.2020.125050
Technical Corrections & References
• In-text citations: Ensure there is a comma separating the author and the year in all in-text references (e.g., lines 223–225).
• Bibliography Duplication: The references "Kumbier et al. 2021a" and "Kumbier et al. 2021b" in the bibliography refer to the exact same paper and journal issue . Please correct this duplication.Citation: https://doi.org/10.5194/egusphere-2026-1398-RC2
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