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<p>Groundwater resources are a major concern in the Sahel, particularly in the Lake Chad Basin, which covers about 2.5 million km<sup>2</sup> in sub-Saharan Africa. Despite several studies conducted in this basin, knowledge of many aspects crucial for managing and utilizing water resources remains limited. This is the case for the Massenya floodplain (Chad), which is a hydrological transition zone where significant exchanges occur between surface water and groundwater, thereby helping to mitigate the effects of extreme events. This study used a combined approach based on hydrodynamics (piezometric levels), stable isotopes (δ<sup>18</sup>O, δ<sup>2</sup>H), and hydrochemistry to assess potential groundwater recharge dynamics and water quality using 63 boreholes sampled in March 2024. Water table depth varied from 4.31&#xa0;m below ground level (m bgl) to 56.51&#xa0;m bgl, with an average depth of 25.78&#xa0;m. These variations are primarily driven by surface topography and the complex hydrostratigraphic architecture of the multilayered aquifer system (Continental Terminal, Pliocene, and Quaternary), deposited during successive transgressions and regressions of Lake Chad. Groundwater isotopic signatures reveal contrasting recharge sources, ranging from recent inputs from precipitation and floodwaters, to older waters enriched by evaporation. Observed trends of increasing regional rainfall and floodplain expansion suggest enhanced aquifer recharge. This hypothesis is supported by satellite-derived evidence of flood pulse dynamics, and the presence of modern isotopic signatures in groundwater, indicating a significant contribution of surface water to aquifer replenishment. Hydrochemical analysis shows that Ca–HCO<sub>3</sub><sup>−</sup> facies dominate, while elevated nitrate concentrations were found in some boreholes located in densely populated areas. Based on major ion concentrations, groundwater quality appeared excellent for irrigation and generally suitable for drinking, except in localized areas affected by nitrate contamination. These results highlight the critical role of floodplains in sustaining aquifer recharge under changing climatic and anthropogenic pressures.</p>

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Hydrogeological functioning of the Massenya floodplain, Lake Chad Basin: insights from stable isotopes and hydrochemistry

  • 期刊论文

  • 发表日期：

    2026年3月10日

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  • Abdallah Mahamat-Nour,
  • Bichara Djimet D. Bichara,
  • Florence Sylvestre,
  • Bertil Nlend,
  • Christine Vallet-Coulomb,
  • Abderamane Moussa

摘要

Groundwater resources are a major concern in the Sahel, particularly in the Lake Chad Basin, which covers about 2.5 million km2 in sub-Saharan Africa. Despite several studies conducted in this basin, knowledge of many aspects crucial for managing and utilizing water resources remains limited. This is the case for the Massenya floodplain (Chad), which is a hydrological transition zone where significant exchanges occur between surface water and groundwater, thereby helping to mitigate the effects of extreme events. This study used a combined approach based on hydrodynamics (piezometric levels), stable isotopes (δ18O, δ2H), and hydrochemistry to assess potential groundwater recharge dynamics and water quality using 63 boreholes sampled in March 2024. Water table depth varied from 4.31 m below ground level (m bgl) to 56.51 m bgl, with an average depth of 25.78 m. These variations are primarily driven by surface topography and the complex hydrostratigraphic architecture of the multilayered aquifer system (Continental Terminal, Pliocene, and Quaternary), deposited during successive transgressions and regressions of Lake Chad. Groundwater isotopic signatures reveal contrasting recharge sources, ranging from recent inputs from precipitation and floodwaters, to older waters enriched by evaporation. Observed trends of increasing regional rainfall and floodplain expansion suggest enhanced aquifer recharge. This hypothesis is supported by satellite-derived evidence of flood pulse dynamics, and the presence of modern isotopic signatures in groundwater, indicating a significant contribution of surface water to aquifer replenishment. Hydrochemical analysis shows that Ca–HCO3− facies dominate, while elevated nitrate concentrations were found in some boreholes located in densely populated areas. Based on major ion concentrations, groundwater quality appeared excellent for irrigation and generally suitable for drinking, except in localized areas affected by nitrate contamination. These results highlight the critical role of floodplains in sustaining aquifer recharge under changing climatic and anthropogenic pressures.