Integrated Hydrochemical assessment and irrigation suitability of the groundwater of the Continental Terminal aquifer in the coastal sedimentary basin of Togo

Résumé

This study evaluates the chemical quality of the unconfined Continental Terminal aquifer in the Coastal Sedimentary Basin of Togo in order to characterize its hydrochemical functioning and support the sustainable management of groundwater resources under increasing anthropogenic and climatic pressures. A total of 246 groundwater samples collected between June and July 2025 were analyzed for the main physicochemical parameters, including pH, electrical conductivity (EC), total dissolved solids (TDS), total hardness (TH), HCO₃ − , Cl − , SO₄ 2− , NO₃ − , Ca 2+ , Mg 2+ , Na + , K + , F − , and Fe. The methodological approach combined hydrochemical analysis, the Piper diagram, correlation analysis, hierarchical cluster analysis (HCA), the Water Quality Index (WQI), the Sodium Adsorption Ratio (SAR), and GIS-based spatial mapping. The results reveal marked spatial variability in groundwater quality, controlled by natural mineralization, marine influence, and anthropogenic inputs. Chloride-dominated hydrochemical facies account for 82% of the samples, whereas calcium and magnesium bicarbonate facies (10%) are mainly distributed in the northern part of the basin. Correlation analysis indicates that EC, TDS, total hardness, Ca 2+ , Mg 2+ , Na + , Cl − , and SO₄ 2− are the principal variables controlling groundwater mineralization, with a pronounced saline influence evidenced by the strong Na + -Cl − association. Hierarchical cluster analysis identified six hydrochemical groups representing different stages of groundwater evolution, ranging from weakly mineralized waters to waters affected by marine salinity and anthropogenic inputs. WQI mapping identified priority monitoring areas, particularly Greater Lomé, the Zio Valley, and the coastal zone, where urban pressures increase aquifer vulnerability. These areas are affected by the two main degradation processes identified through the correlation analysis, namely natural salinization associated with seawater intrusion and anthropogenic contamination from agricultural and domestic activities. Accordingly, groundwater management should focus on (i) strengthening the monitoring of salinization and pollution indicators, (ii) delineating and protecting groundwater abstraction areas, and (iii) adopting an integrated management approach to ensure the long-term sustainability of the Continental Terminal groundwater resources.

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