Hydrogeochemical characterization and modeling vulnerability assessment using SINTACS methods: the case of the Hennaya aquifer (NW Algeria)

This study explores the interactions between geological, hydrogeological, and human influences on groundwater quality and vulnerability within the Hennaya aquifer in Northern Algeria. Located in the Tafna watershed, this 24 km2 area is characterized by a semi-arid climate with cold winters, receiving an average annual precipitation of 477.7 mm and actual evapotranspiration of 422.7 mm. The region experiences distinct hydrological seasons, with flooding from November to May and low water levels from June to October. The aquifer geological features include Tortonian sandstone, conglomerates, travertines, gravel, and clay, underlain by Helvetian marls. Piezometric data reveal a groundwater flow from southwest to northeast, with springs emerging in conglomerate-rich zones. The hydraulic gradient and transmissivity vary, with conglomerates showing higher values than sandstone. Groundwater samples were collected during both wet and dry seasons, and analyzed for physical and chemical parameters, including pH, EC, TDS, major ions, and nitrates. The hydrogeochemical analysis indicates alkaline-earthy water with a chloride-calcium facies. Rock weathering is the dominant geochemical process, with under-saturation in carbonate and evaporitic minerals. Chloro-alkaline indices and base exchange patterns suggest direct cation exchange, classifying the groundwater as Na-SO4 type from shallow meteoric sources. High nitrate levels, linked to agricultural activities, reflect shallow water table depths and anthropogenic impacts. The SINTACS vulnerability assessment identifies medium to high vulnerability zones, with nitrate concentrations correlating with vulnerability indices, highlighting the significant role of land use in groundwater contamination.