Assessment of groundwater quality using GWQI and an AHP-Developed Health-Risk-Oriented Index (HR-GWQI): evidence from the Mitidja Plain (Algeria)

Abstract

Groundwater quality assessment in the alluvial aquifer system of the Mitidja Plain during the 2019 low-water period was conducted using an integrated approach combining the conventional Groundwater Quality Index (GWQI) with an innovative Human Risk-Groundwater Quality Index (HR-GWQI) developed through Analytic Hierarchy Process (AHP) weighting and GIS-based spatial analysis. Based on eleven physicochemical parameters and data from 23 monitoring wells, the study revealed that natural mineralization driven by carbonate and evaporite dissolution together with seawater intrusion controls the general ionic composition, whereas nitrate contamination of agricultural origin constitutes the main health concern, with 43.5% of samples exceeding the WHO guideline value of 50.0 mg/L. Although the conventional GWQI classified most wells as suitable for drinking purposes, it underestimated nitrate-related health risks due to the uniform weighting of parameters. In contrast, the HR-GWQI, which assigns greater importance to nitrate concentrations through AHP-derived weighting, provided a more realistic and health-protective classification of groundwater quality by reclassifying several contaminated wells into lower quality categories. A sensitivity analysis further confirmed that the HR-GWQI is most robust in the wells with the highest nitrate burden, an emergent property of the AHP health-risk-oriented weighting scheme. GIS-based spatial distribution mapping of the monitoring points further revealed that the most significant groundwater quality deterioration is concentrated in the northeastern coastal zone and the western sectors of the plain, areas strongly affected by agricultural pressure and saline intrusion. These findings demonstrate that the HR-GWQI represents a more effective tool for groundwater quality evaluation and health-risk-oriented management in nitrate-contaminated aquifer systems across North Africa and the Mediterranean Basin.