Gaiolini et al. (2025) Salt migration and export via subsurface irrigation in a saline reclaimed landscape of the Po River lowland (Italy)

Identification

• Journal: Journal of Hydrology
• Year: 2025
• Date: 2025-11-07
• Authors: Mattia Gaiolini, Chiara Sbarbati, Abraham Ofori, Patrick Habakaramo Macumu, Maria Pia Gervasio, Elena Tamisari, Fabio Vincenzi, Elisa Soana, Giuseppe Castaldelli, Vincenzo Piscopo, Micòl Mastrocicco, Nicolò Colombani
• DOI: 10.1016/j.jhydrol.2025.134563

Research Groups

• Department of Ecological and Biological Sciences (DEB), University of Tuscia, Viterbo, Italy
• Department of Environmental, Biological and Pharmaceutical Sciences and Technologies (DiSTABiF), Campania University “Luigi Vanvitelli”, Caserta, Italy
• Department of Materials, Environmental Sciences and Urban Planning (SIMAU), Marche Polytechnic University, Ancona, Italy
• Geochemistry and Environment Department (GED), Goma Volcano Observatory, Goma, DR Congo
• Department of Environmental and Prevention Sciences (DiSAP), University of Ferrara, Ferrara, Italy

Short Summary

This study investigates the causes and quantifies the sources of dissolved salts in a saline reclaimed landscape of the Po River lowland, Italy, focusing on the impact of subsurface irrigation via tile drains. It reveals that sub-irrigation significantly accelerates salinization and salt export, with peaty lenses and decomposing halophytes acting as major salt sources, leading to high surface water salinity.

Objective

• To identify the causes of salinization and quantify the key sources of dissolved salts in a reclaimed area of the Po River lowland.
• To evaluate groundwater dynamics through time series analysis of integrated field measurements.
• To deepen the understanding of coastal aquifer and aquitard lenses and their connections with agricultural practices and meteorological drivers, particularly the role of sub-irrigation and tile drainage on shallow aquifer systems.

Study Configuration

• Spatial Scale: Two adjacent agricultural fields (A1 ≈5 hectares, A2 ≈2 hectares) near Berra village, Po River lowland, Italy, approximately 24 kilometers from the Adriatic Sea. The fields are equipped with tile drains (5 cm internal diameter, 0.70-0.90 m below ground level depth, 10 m spacing).
• Temporal Scale: Continuous monitoring from 16 April to 26 November 2024 (over 6 months). Remote sensing data were collected every 5 days during this period. Three field campaigns were conducted on 27 May 2024, 09 July 2024, and 11 September 2024.

Methodology and Data

• Models used:
  • Doodson filter (for time-series alignment and smoothing)
  • Low Pass 33 (LP33) filter (for frequency domain analysis)
  • Pearson, Kendall, and Spearman correlation coefficients (for statistical analysis of variable associations)
  • Third-order polynomial regression curve (for estimating outflow rates from drainage canals)
  • Hilhorst (2000) relationship (for converting soil bulk electrical conductivity to porewater electrical conductivity)
• Data sources:
  • Continuous Field Monitoring: Diver® dataloggers (groundwater levels, temperature, electrical conductivity in piezometers and surface canals), multi-level samplers (MLS) for water samples and hydrogeological measurements, HANNA Instruments® multiparameter probe (dissolved oxygen, pH, oxygen reduction potential, electrical conductivity, temperature in ditches), 5TE® Meter probes (volumetric water content, temperature, soil bulk electrical conductivity in vadose zone), Sentek–Drill&Drop® probe (same parameters in vadose zone), Zentra Advanced Cloud Data Logger (ZL6) for data recording.
  • Remote Sensing: Sentinel-2 L2A Soil Adjusted Vegetation Index (SAVI) and Normalized Difference Moisture Index (NDMI) data (10 m and 20 m pixel size, respectively) from Copernicus Browser.
  • Meteorological Data: Hourly precipitation data from Avanzarola meteorological station (ARPAE), cross-checked with a manual pluviometer. Monthly average temperature and yearly average precipitation from Climate Reanalyzer.
  • Flow Measurements: Portable hand flow-meter or bucket and chronometer for discharge rates during low flow periods.
  • Laboratory Analysis: Standard analytical methods (APHA, 2022) for electrical conductivity-salinity relationship.

Main Results

• Capillary rise was identified as the primary driver of soil salinity in the mildly saline, maize-cultivated field (A1).
• In the saline, uncultivated field (A2), capillary rise was exacerbated by additional salt release from root decomposition after mowing, leading to porewater electrical conductivity (ECpw) increases up to 20 mS/cm.
• Groundwater flow was generally directed towards surface drains, which act as the main source of salinization for surface waters. EC in the ditch draining A2 abruptly increased to 6.5 mS/cm, while in the ditch draining A1, it rose more gradually from 0.55 to 2 mS/cm.
• Frequency domain analysis highlighted internal salinity dynamics, including increased porewater salinity after mowing in A2 and strong correlations between volumetric water content (VWC) and ECpw at 60 cm below ground level.
• The total salt export during the sub-irrigation period was 67.2 ± 6.5 tonnes, accounting for approximately 80 % of the cumulative salt exported (84.5 ± 8.0 tonnes) over the entire monitoring period.
• Sub-irrigation significantly accelerates salinization in shallow groundwater systems under saline conditions, and freshening of shallow groundwater via tile drains was not observed.
• Peaty lenses and decomposing salt-tolerant weeds were identified as significant long-term sources of salinity, contributing to elevated groundwater and surface water concentrations even after flushing.

Contributions

• This study provides a comprehensive, integrated monitoring approach to quantify water and dissolved salt exchange fluxes in the vadose zone, groundwater, and surface water of a tile-drained agricultural field in a saline reclaimed landscape.
• It quantifies the significant impact of sub-irrigation on accelerating salt export, demonstrating that 80 % of total salt export occurred during sub-irrigation periods.
• It identifies and quantifies previously underestimated sources of salinity, specifically the long-term release of salts from peaty lenses and the decomposition of mulched salt-tolerant weeds, which can temporarily increase porewater EC up to 20 mS/cm.
• The research integrates continuous field measurements with remote sensing data and frequency domain analysis, offering deeper insights into complex hydrogeological processes and their temporal variability.
• It highlights the ineffectiveness of tile drains in achieving shallow groundwater freshening in such saline environments and provides crucial data for developing differentiated management strategies to mitigate salinization risks in similar lowland areas.

Funding

The research received financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 2 February 2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU. The project title is NO3EXCESS – Nitrogen Origin, EXport and Cycling in coastal irrigatEd SettingS, with CUP F53D23002160001 and Grant Assignment Decree No. 2022AKA3HL adopted on 30 June 2024 by the Italian Ministry of University and Research (MUR).

Citation

@article{Gaiolini2025Salt,
  author = {Gaiolini, Mattia and Sbarbati, Chiara and Ofori, Abraham and Macumu, Patrick Habakaramo and Gervasio, Maria Pia and Tamisari, Elena and Vincenzi, Fabio and Soana, Elisa and Castaldelli, Giuseppe and Piscopo, Vincenzo and Mastrocicco, Micòl and Colombani, Nicolò},
  title = {Salt migration and export via subsurface irrigation in a saline reclaimed landscape of the Po River lowland (Italy)},
  journal = {Journal of Hydrology},
  year = {2025},
  doi = {10.1016/j.jhydrol.2025.134563},
  url = {https://doi.org/10.1016/j.jhydrol.2025.134563}
}

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