Assessment of groundwater nitrate contamination and its origin in the Bousaada Region, Algeria

Groundwater is the principal water resource in the semi-arid region of Bousaada. This study assesses hydrochemical characteristics, temporal evolution, and sources of nitrate contamination using more than 200 groundwater samples, including current sampling and archived dataset. Groundwater is characterized by the dominance of Cl⁻ and SO₄²⁻, together with Ca²⁺ and Mg²⁺, indicating contributions from evaporitic dissolution and carbonate processes. Most samples exceeded guideline values for of NO₃⁻, and a large proportion surpassed chloride and sulfate standards. Temporal analysis (1967-2022) reveals a sustained increase in nitrate concentrations, with mean values reaching about 128 mg/L, and 50% of newly analysed samples exceeded 100 mg/L. Pollution indices and health risk assessment indicate that the majority of groundwater samples are highly contaminated and pose significant health risks, particularly in agricultural zones and near Wadi Bousaada. Hydrochemical ionic ratios, and multivariate (PCA) analysis, indicate that nitrate contamination primarily originates from agricultural fertilizers and septic tank effluents, with additional influence from Wadi Bousaada. Spatial modeling using kriging approaches improved prediction accuracy and highlighted hotspots in the Maadher agricultural area and near Wadi Bousaada. The findings demonstrate significant groundwater quality deterioration over five decades and emphasize the urgent need for improved agricultural and sanitation management.