Hydrogeochemical assessment of groundwater quality and controlling processes in the quaternary aquifer of Oued Souf, Southeastern Algeria

In southeastern Algeria, the Quaternary phreatic aquifer of Oued Souf serves as the primary groundwater source due to the lack of surface water. This groundwater is vital for drinking, domestic use, and agricultural and industrial activities. The present study assesses its quality using the Entropy-Weighted Water Quality Index (EWQI), comparing results to Algerian and WHO drinking water standards, and examines the hydrogeochemical characteristics and processes influencing its chemistry. To achieve these aims, thirty groundwater samples were collected in August 2023 and analyzed for key physicochemical parameters. Spatial variations were assessed using GIS, correlation analysis, and geochemical modelling. Results showed pH values between 7.04 and 8.08, indicating neutral to slightly alkaline groundwater, and EC ranging from 4586 to 18680 µS/cm, indicating high salinity. Most samples exceeded Algerian drinking water standards. Ca–Cl–SO₄ facies dominated in the south, shifting to Na–Cl–SO₄ northward, reflecting increased mineralization along flow paths. Correlation analysis identified Na⁺, Cl⁻, Mg2⁺, SO₄2⁻, and TDS as main mineralization factors. Saturation indices indicated undersaturation with respect to gypsum, anhydrite, and halite, and precipitation of aragonite, calcite, and dolomite, which enhanced the dissolution of evaporite minerals, as confirmed by Gibbs plots. Dominant processes included evaporite and cation exchange, alongside anthropogenic impacts from domestic, agricultural, and industrial activities, leading to rising salinity and declining water quality. The EWQI results showed that all samples were unsuitable for drinking, underscoring the need to implement integrated groundwater management strategies to protect this resource sustainably.