Charrada et al. (2026) Assessment of groundwater salinization and nitrate pollution in a sub-humid to semi-arid agricultural area: a case study of the Sidi Smail plain in northwestern Tunisia

Identification

• Journal: Modeling Earth Systems and Environment
• Year: 2026
• Date: 2026-02-16
• Authors: Ghofrane Charrada, Brahim Askri, Wissem Hamdi, Salah Jellali, Boulbaba L’taief
• DOI: 10.1007/s40808-026-02719-6

Research Groups

• Higher Institute of the Sciences and Techniques of Waters of Gabès, University of Gabès, Gabès, Tunisia
• Research Unit of Modeling in Civil Engineering and Environment, National Engineering School of Gabès, University of Gabès, Gabès, Tunisia
• Center for Environmental Studies and Research, Sultan Qaboos University, Muscat, Oman
• Biology Department, College of Science, King Khalid University, Abha, Saudi Arabia

Short Summary

This study investigated the origins of salinity and nitrate in groundwater and the mechanisms governing its chemistry in the Sidi Smail plain, northwestern Tunisia. It revealed that 88% of groundwater samples were brackish/saline and 75% exceeded nitrate limits, primarily due to a combination of geogenic processes and anthropogenic factors like fertilizer overuse and evaporation.

Objective

• To depict the origins of salinity and nitrate in the groundwater of the Sidi Smail plain.
• To derive the principal mechanisms governing groundwater chemistry in the Sidi Smail plain.
• To appraise the groundwater quality in the Sidi Smail plain.

Study Configuration

• Spatial Scale: Sidi Smail plain, northwestern Tunisia, covering approximately 400 km².
• Temporal Scale: Groundwater and surface water samples collected in October 2021, at the end of the hot and dry summer season.

Methodology and Data

• Models used:
  • Nitrate Pollution Index (NPI)
  • PHREEQC (for saturation index calculations)
  • SPSS v26.0 (for Pearson correlation matrix and Principal Component Analysis - PCA)
  • ArcGIS 10.3 (for spatial distribution maps)
  • Piper diagram
  • Gibbs plots
• Data sources:
  • 28 groundwater samples (from 15 boreholes and 13 wells)
  • 2 surface water samples (from Medjerda River and Torch wadi)
  • Physicochemical properties (pH, electrical conductivity, total dissolved solids, major and minor ions: Mg²⁺, Na⁺, Ca²⁺, K⁺, Cl⁻, SO₄²⁻, Br⁻, NO₃⁻, F⁻, HCO₃⁻, NO₂⁻, NH₄⁺)
  • Stable isotope composition (δ¹⁸O and δ²H)
  • Geological maps (Bousalem and Oued Zarga, scale 1:50,000)
  • Hydrogeological data from CRDA Beja and DGRE (2020)

Main Results

• 88% of groundwater samples were classified as brackish/saline, with an average Total Dissolved Solids (TDS) value of 2,224 mg/L.
• 75% of groundwater samples exhibited nitrate concentrations exceeding the World Health Organization (WHO) threshold of 50 mg/L.
• Groundwater chemistry is primarily regulated by geogenic processes, including the dissolution of Triassic evaporite and carbonate minerals and reverse ionic exchange reactions.
• Anthropogenic factors, such as the overuse of fertilizers in agriculture and evaporation from surface water and groundwater under hot and dry climatic conditions, significantly influenced salinity and isotopic composition.
• The Nitrate Pollution Index (NPI) categorized groundwater samples into: clean (7%), light pollution (3%), moderate pollution (21%), significant pollution (14%), and very significant pollution (56%).
• The dominant water types identified were mixed Ca–Mg–Cl (75% of samples) and Na-Cl (18% of samples).
• Stable isotope analysis indicated isotopic enrichment in 81% of groundwater samples and both surface water samples, confirming the influence of evaporation and suggesting potential hydraulic connections between surface and groundwater.
• Cl⁻/Br⁻ molar ratios, alongside other geochemical indicators, suggested mixed origins of chloride, including halite dissolution and anthropogenic inputs from agricultural fertilizers and wastewater.

Contributions

• Provided the first comprehensive assessment of groundwater salinization and nitrate pollution in the Sidi Smail plain, a sub-humid to semi-arid agricultural area.
• Integrated a multi-method approach combining classical hydrochemical analyses, geochemical modeling, isotopic techniques, the Nitrate Pollution Index (NPI), and GIS-based spatial analysis to appraise groundwater quality.
• Offered crucial insights into the groundwater aquifer system, aiding users and decision-makers in developing effective management strategies.
• Highlighted the critical need for sustainable agricultural practices, particularly efficient fertilizer use, to protect groundwater resources from nitrate pollution in similar vulnerable regions.

Funding

• Small Research Project under grant number RGP1/145/46 from the Deanship of Research and Graduate Studies at King Khalid University.

Citation

@article{Charrada2026Assessment,
  author = {Charrada, Ghofrane and Askri, Brahim and Hamdi, Wissem and Jellali, Salah and L’taief, Boulbaba},
  title = {Assessment of groundwater salinization and nitrate pollution in a sub-humid to semi-arid agricultural area: a case study of the Sidi Smail plain in northwestern Tunisia},
  journal = {Modeling Earth Systems and Environment},
  year = {2026},
  doi = {10.1007/s40808-026-02719-6},
  url = {https://doi.org/10.1007/s40808-026-02719-6}
}