Mahamat-Nour et al. (2026) Hydrogeological functioning of the Massenya floodplain, Lake Chad Basin: insights from stable isotopes and hydrochemistry

Identification

• Journal: Aquatic Sciences
• Year: 2026
• Date: 2026-03-10
• Authors: Abdallah Mahamat-Nour, Bichara Djimet D. Bichara, Florence Sylvestre, Bertil Yvon Nlend, Christine Vallet-Coulomb, Abderamane Moussa
• DOI: 10.1007/s00027-026-01284-5

Research Groups

• Laboratoire Hydro-Géosciences et Réservoirs, Université de N’Djamena, Campus de Farcha, N’Djamena, Tchad
• Aix Marseille University, CNRS, IRD, Collège de France, INRAE, CEREGE, Technopôle Méditerranéen de l’Arbois, Aix-en-Provence, France
• Faculté des Sciences, Université de Douala, Po Box 24157, Douala, Cameroun

Short Summary

This study investigates the hydrogeological functioning and water quality of the Massenya floodplain in the Lake Chad Basin using hydrodynamics, stable isotopes, and hydrochemistry. It reveals a dual groundwater recharge system, with recent precipitation and floodwater replenishing shallow aquifers and older, fossil waters in deeper horizons, highlighting the critical role of floodplains in sustaining water resources despite anthropogenic pressures and climate variability.

Objective

• To assess potential groundwater recharge dynamics and water quality in the Massenya floodplain.
• To define the physico-chemical properties and geochemical facies of groundwater.
• To analyze the mechanisms of interaction between surface waters (Chari and Logone rivers) and the Quaternary aquifer.
• To assess the influence of flood dynamics on the sustainability and renewal of water resources.

Study Configuration

• Spatial Scale: Massenya floodplain, southern Lake Chad Basin, Chad, covering an area of 45,000 km².
• Temporal Scale: Fieldwork conducted March 19-25, 2024 (dry season). Isotopic data from Chari River (July 2015 - November 2016) and N’Djamena GNIP station (1961–2018). Rainfall data (1971-2015).

Methodology and Data

• Models used: Drinking Water Quality Index (DWQI), Irrigation Water Quality Index (IWQI), Sodium Adsorption Ratio (SAR).
• Data sources:
  • Field measurements from 63 boreholes (piezometric levels, pH, electrical conductivity, temperature, alkalinity, surface elevation via GPS).
  • Groundwater samples analyzed for major dissolved chemical elements (Hach DR3900 spectrometer).
  • Stable isotopes (δ¹⁸O, δ²H) analyzed on 31 samples (Picarro L2140-i spectrometer).
  • Additional isotopic data for Chari River from Mahamat Nour (2019).
  • Precipitation isotopic data (δ¹⁸O, δ²H) from N’Djamena Station (1961–2018) of the Global Network of Isotopes in Precipitation (GNIP).
  • Satellite-derived evidence of flood pulse dynamics.

Main Results

• Water table depth varied from 4.31 m to 56.51 m below ground level, with an average of 25.78 m. Piezometric levels ranged from 267 m to 332 m above sea level, showing high spatial variability influenced by topography and hydrostratigraphy.
• Groundwater isotopic signatures indicate a dual recharge system: recent inputs from precipitation and floodwaters (predominant in shallow aquifers, < 30 m) and older, evaporated waters (preserved in deeper horizons, > 50 m) reflecting past wetter climates.
• The Massenya Plain is identified as a main recharge zone, with floodwaters from the Chari River significantly contributing to shallow aquifer replenishment, particularly during July-August.
• Hydrochemical analysis shows a predominance of Ca–HCO₃⁻ facies (93% of samples), indicating meteoric origin and limited mineralization. Secondary facies (Ca-Mg-Cl-SO₄ and Na-K-HCO₃) reflect local evaporative influence or cation exchange.
• Groundwater quality is generally excellent for drinking (98% excellent, 2% good based on DWQI), but localized elevated nitrate concentrations in densely populated areas suggest anthropogenic contamination.
• For irrigation, 33% of water points are unrestricted, 60% have low restrictions, and 7% have moderate restrictions (based on IWQI), mainly due to higher electrical conductivity or local cation exchange processes.
• The study highlights a strong hydraulic and geochemical coupling between surface water and groundwater, with increased regional rainfall and floodplain expansion suggesting enhanced aquifer recharge.

Contributions

• Provides a comprehensive, multidisciplinary assessment of hydrogeological functioning and water quality in the under-explored Massenya floodplain, integrating hydrodynamics, stable isotopes, and hydrochemistry.
• Quantifies the dual nature of groundwater recharge (modern vs. fossil water) and the critical role of floodplains in sustaining shallow aquifers in the Sahelian context.
• Offers insights into the vulnerability of groundwater resources to both climatic variability and increasing anthropogenic pressures, providing a baseline for sustainable management strategies.
• Emphasizes the need for integrated management combining monitoring, tracers, and modeling for long-term resource preservation.

Funding

• European Union (grant no. DCI-PANAF/2020/420-028), through the African Research Initiative for Scientific Excellence (ARISE), pilot program.

Citation

@article{MahamatNour2026Hydrogeological,
  author = {Mahamat-Nour, Abdallah and Bichara, Bichara Djimet D. and Sylvestre, Florence and Nlend, Bertil Yvon and Vallet-Coulomb, Christine and Moussa, Abderamane},
  title = {Hydrogeological functioning of the Massenya floodplain, Lake Chad Basin: insights from stable isotopes and hydrochemistry},
  journal = {Aquatic Sciences},
  year = {2026},
  doi = {10.1007/s00027-026-01284-5},
  url = {https://doi.org/10.1007/s00027-026-01284-5}
}