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<p>Groundwater serves as the primary supply source for socioeconomic activities in the Casamance region (south of Senegal), yet hydrogeochemical and water quality characteristics as well as flow patterns of groundwater at a regional scale are largely undocumented. This study aims to contribute to the understanding of hydrogeochemical processes of mineralization and to the hydraulic functioning of the Continental Terminal (CT) and Oligo-Miocene (OM) aquifer systems. Field campaigns were conducted in December 2021 (beginning of the dry season) and May 2024 (end of the dry season) to collect water samples from dug wells (n = 93), boreholes and piezometers (n = 46), and surface water (n = 3) for major and minor ions chemistry, water isotopes (δ<sup>2</sup>H, δ<sup>18</sup>O, <sup>3</sup>H) and carbon isotopes (δ<sup>13</sup>C and <sup>14</sup>C) analyses. The results reveal that silicate weathering and carbonate dissolution, together with ion exchange, are responsible for the groundwater mineralization in both aquifers with dominating Ca–HCO₃, mixed-HCO₃, and Na–Cl water types. The Comprehensive Pollution Index (CPI) method used to evaluate anthropogenic influence reveals that the pollutant levels in the CT aquifer are moderate to high, while the OM aquifer contains high-salinity (TDS = 2346&#xa0;mg/L) groundwater derived from residual marine waters. Stable isotope distributions in the OM aquifer reveal two distinct patterns: enriched values (from −6.68 to − 4.67 ‰ δ<sup>1</sup>⁸O) in the confined part of the aquifer and depleted values between − 6.85 and − 5.14 ‰ δ<sup>1</sup>⁸O in the unconfined part. Variations in stable isotope values indicate recharge under different climatic conditions and mixing with seawater, while <sup>3</sup>H content indicates recent recharge in the CT aquifer and sub-modern water in the OM aquifer. δ<sup>13</sup>C and <sup>14</sup>C correlation reveals a rise in carbon activity from the confined to the unconfined aquifer and an active isotopic exchange with the carbonate fraction of the OM aquifer matrix. These findings provide the first regional-scale insight into the functioning of the CT and OM aquifer systems. They will guide future interventions in water supply projects as well as in securing reliable and high-quality water for local communities and supporting water resource management in the Casamance region.</p>

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Hydrogeochemical and isotopic characterization of the upper and intermediate aquifers of the casamance (Senegal)

  • 期刊论文

  • 发表日期：

    2026年5月11日

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  • Mouhamet M. Diaw,
  • Isai Bassène,
  • Ibra D. Gueye,
  • Djim M. L. Diongue,
  • Mathias Diedhiou,
  • Rim Trabelsi,
  • Kamel Zouari,
  • Serigne Faye

摘要

Groundwater serves as the primary supply source for socioeconomic activities in the Casamance region (south of Senegal), yet hydrogeochemical and water quality characteristics as well as flow patterns of groundwater at a regional scale are largely undocumented. This study aims to contribute to the understanding of hydrogeochemical processes of mineralization and to the hydraulic functioning of the Continental Terminal (CT) and Oligo-Miocene (OM) aquifer systems. Field campaigns were conducted in December 2021 (beginning of the dry season) and May 2024 (end of the dry season) to collect water samples from dug wells (n = 93), boreholes and piezometers (n = 46), and surface water (n = 3) for major and minor ions chemistry, water isotopes (δ2H, δ18O, 3H) and carbon isotopes (δ13C and 14C) analyses. The results reveal that silicate weathering and carbonate dissolution, together with ion exchange, are responsible for the groundwater mineralization in both aquifers with dominating Ca–HCO₃, mixed-HCO₃, and Na–Cl water types. The Comprehensive Pollution Index (CPI) method used to evaluate anthropogenic influence reveals that the pollutant levels in the CT aquifer are moderate to high, while the OM aquifer contains high-salinity (TDS = 2346 mg/L) groundwater derived from residual marine waters. Stable isotope distributions in the OM aquifer reveal two distinct patterns: enriched values (from −6.68 to − 4.67 ‰ δ1⁸O) in the confined part of the aquifer and depleted values between − 6.85 and − 5.14 ‰ δ1⁸O in the unconfined part. Variations in stable isotope values indicate recharge under different climatic conditions and mixing with seawater, while 3H content indicates recent recharge in the CT aquifer and sub-modern water in the OM aquifer. δ13C and 14C correlation reveals a rise in carbon activity from the confined to the unconfined aquifer and an active isotopic exchange with the carbonate fraction of the OM aquifer matrix. These findings provide the first regional-scale insight into the functioning of the CT and OM aquifer systems. They will guide future interventions in water supply projects as well as in securing reliable and high-quality water for local communities and supporting water resource management in the Casamance region.