Madaula et al. (2025) Hypersaline recharge in Mediterranean coastal aquifers: The role of aquifer–lagoon connectivity

Identification

• Journal: Journal of Hydrology
• Year: 2025
• Date: 2025-11-19
• Authors: Eduard Madaula, Mario Zarroca, Carles Roqué i Pau, Gisela Gonzalvo-Henry, Warren Meredith, Xavier D. Quintana, J. Mencos, Anna Menció
• DOI: 10.1016/j.jhydrol.2025.134603

Research Groups

• External Geodynamics and Hydrogeology Group, Department of Geology, Universitat Aut`onoma de Barcelona, 08193 Bellaterra, Spain
• Grup de Geologia Aplicada i Ambiental (GAiA), Department of Environmental Sciences, Universitat de Girona 17003 Girona, Spain
• ICRA, Catalan Water Research Institute, Girona, Spain
• GRECO, Institute of Aquatic Ecology, University of Girona 17003 Girona, Spain

Short Summary

This study investigates seasonal salinity variations in the shallow aquifer of the La Pletera salt marsh using time-lapse electrical resistivity tomography (ERT) and continuous monitoring. It reveals that hypersaline lagoons are a primary source of aquifer salinization, particularly after rainfall events, often exceeding the influence of seawater intrusion.

Objective

• To determine the spatiotemporal variation in salinity in the shallow aquifer of the Baix Ter area, specifically in the La Pletera salt marsh, by comparing different seasons and assessing aquifer-lagoon interactions.
• To evaluate how severe drought and rainfall events control groundwater–lagoon flow exchanges.
• To identify and delineate the spatial extent and depth of high-salinity anomalies associated with lagoon water recharge.
• To characterize how lagoon–aquifer connectivity controls spatial and temporal salinity dynamics in the shallow aquifer, using time-lapse ERT observations.

Study Configuration

• Spatial Scale: La Pletera salt marsh (NE Iberian Peninsula). ERT profiles were 160 m long (FRA, L01-L04, BPI) and 400 m long (M03p, L04p), investigating depths up to 30 m for shorter profiles and up to 80 m for longer profiles. Piezometer depths were up to 8 m. Temperature correction was applied to the active zone down to 5.5 m depth.
• Temporal Scale: Continuous monitoring of water levels, temperature, and electrical conductivity (EC) from June 2022 to December 2024. Time-lapse ERT campaigns were conducted monthly from June 2022 to June 2023, with two additional measurements in May and November 2024.

Methodology and Data

• Models used:
  • Time-lapse Electrical Resistivity Tomography (ERT)
  • EarthImager 2D software package (version 2.4.4.) for ERT inversion using a smoothness-constrained least squares method.
  • Temperature correction of bulk EC using the Keller and Frischknecht (1966) equation (ECStd = ECT × (1 + α*(T −TStd)) with α = 0.025 and TStd = 17.6 °C).
  • Archie’s law for bulk to pore fluid electrical conductivity conversion (formation factor F ≈ 5, effective porosity = 0.3, cementation exponent = 1.3).
  • Non-parametric paired Friedman test for statistical analysis of bulk EC differences.
• Data sources:
  • Multi-electrode Terrameter LS2 system (ABEM) for ERT data acquisition.
  • Hand-held water level indicator (Geonica, KL010 model) for water heads in wells and piezometers.
  • TD-Diver data logger piezometers (vanEssen®) for continuous water level and temperature.
  • CTD-diver data loggers (vanEssen®) for continuous water level, temperature, and electrical conductivity (EC) in piezometers and lagoons.
  • Hand-held Crison CM35 conductivity meter for lagoon water EC during surveys.
  • Sea level records from l’Estartit meteorological station.
  • Stream water level data from Torroella de Montgrí (ACA).
  • Monthly accumulated precipitation records from l’Estartit meteorological station.

Main Results

• Aquifer water heads are strongly influenced by rainfall, showing a significant decline during severe drought periods.
• Hypersaline conditions were observed in FRA and M03 lagoons (mean EC up to 101.6 ± 18.5 mS/cm), often exceeding seawater EC (mean: 54.2 ± 15.8 mS/cm).
• ERT revealed high-EC anomalies (> 12 mS/cm bulk EC) in the shallow aquifer (0–12 m depth) adjacent to hypersaline lagoons, indicating lagoon water recharge.
• Temperature correction was critical for accurate interpretation, as seasonal atmospheric temperature variations caused EC changes of up to -30% in warmer periods and +22% in colder periods within the upper 5.5 m.
• A progressive increase in bulk EC was observed throughout the monitoring period (June 2022 - November 2024), attributed to prolonged drought conditions.
• Significant salinization peaks in the aquifer were recorded after rainfall events (>30 mm accumulated over 30 days), driven by transient hydraulic gradients where lagoon water levels temporarily rose above the adjacent aquifer.
• Converted pore-water EC values in sandy layers (3–12 m depth) reached up to 132 mS/cm, consistent with the hypersaline lagoon waters and nearly double the EC measured along the coastline.
• A conceptual model was proposed where hypersaline lagoon water recharges the aquifer when rainfall elevates lagoon water levels above the adjacent aquifer, creating a hydraulic gradient.

Contributions

• Provides the first quantitative evidence of lagoon-driven hypersaline recharge as a localized salinization mechanism in a Mediterranean coastal aquifer, distinguishing it from seawater intrusion or connate salinity.
• Highlights the critical importance of incorporating lagoon–aquifer interactions into salinization assessments, particularly in systems influenced by hypersaline lagoons.
• Develops a transferable hydrogeophysical framework and conceptual model for diagnosing and mitigating salinization in coastal aquifers, applicable to other Mediterranean and semi-arid coastal wetlands under future climate change scenarios.
• Demonstrates the effectiveness of time-lapse ERT, combined with continuous monitoring and rigorous temperature correction, as a powerful integrated tool for understanding complex salinity dynamics in vulnerable coastal systems.

Funding

• Project consortium TREASURE (PCI2024-153436), funded by the Joint Transnational Call of the Water4All European Partnership, 2022 (European Commission and Agencia Estatal de Investigaci´on (AEI) – Spanish State Research Agency).
• AquiPondSys grant (PID2023-147186OB-I00), funded by MCIN/AEI/10.13039/501100011033 and European Regional Development Fund (ERDF).
• AquiPondSys grant also funded the contract of E. Madaula (PAS-PR-171/24) and the Grant PREP2023-001170 awarded to G. Gonzalvo-Henry.
• Open Access funding provided thanks to the CRUE-CSIC agreement with Elsevier.

Citation

@article{Madaula2025Hypersaline,
  author = {Madaula, Eduard and Zarroca, Mario and Pau, Carles Roqué i and Gonzalvo-Henry, Gisela and Meredith, Warren and Quintana, Xavier D. and Mencos, J. and Menció, Anna},
  title = {Hypersaline recharge in Mediterranean coastal aquifers: The role of aquifer–lagoon connectivity},
  journal = {Journal of Hydrology},
  year = {2025},
  doi = {10.1016/j.jhydrol.2025.134603},
  url = {https://doi.org/10.1016/j.jhydrol.2025.134603}
}