Hydrogeochemical assessment of groundwater quality in the middle Seybouse sub-basin (North-Eastern Algeria)

Abstract

This study evaluates groundwater potability in the Middle Seybouse sub-basin (northeastern Algeria) using the Groundwater Quality Index (GWQI) based on eleven key physicochemical parameters. A total of 17 groundwater samples were collected during the dry season of 2021 and analyzed for major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻), pH, electrical conductivity (EC), and total dissolved solids (TDS). The calculated GWQI values ranged from 23.33 to 81.18, with a mean of 46.03. Results indicate that 94.1% of groundwater samples are suitable for drinking purposes (excellent to permissible quality), while the remaining 5.9% fall into the doubtful quality category. The spatial distribution of water quality reveals significant mineralization in specific areas, particularly at sampling point F BS2, where elevated chloride (398.2 mg/L), sodium (81 mg/L), and electrical conductivity (2090 µS/cm) reflect localized salinization. Hydrochemical analysis and Piper diagram classification indicate that groundwater chemistry is primarily controlled by water-rock interaction, particularly the dissolution of carbonate formations and evaporitic minerals. The temporal analysis (2017-2021) shows a steady increase in nitrate concentrations, suggesting an increasing anthropogenic influence from agricultural activities. This study provides an updated, spatially explicit evaluation of groundwater potability in the Middle Seybouse sub-basin, supporting evidence-based water resource management in one of Algeria's increasingly stressed basins.