Multivariate analysis of groundwater quality using PCA and HAC: Geochemical controls on mineralization, nitrification, and pollutant dynamics in southeastern Arid Region of Algeria

Water supply for various uses in the Eloued area is provided by groundwater. The objective of this study is to understand the mechanisms that govern the hydrochemistry of these waters. Principal Component Analysis (PCA) and Hierarchical Ascending Classification (HAC) were applied to the chemical data of 113 sampled water points. The average water temperature is 21.23°C with an average pH of 7.38. The average electrical conductivity is 4748 μS/cm with extreme values between 2678 and 18076 μS/cm. These waters are highly mineralized in all water points. Half of the nitrate concentrations are above WHO standards. PCA and HAC identified five principal components explaining 83% of the total variance across eight hydrochemical variables. Three key processes were identified: (1) mineralization driven by geological weathering, long water residence time, and anthropogenic inputs (associated with EC, TDS, NO₂⁻); (2) acid-base equilibrium influencing pH and NH₄⁺ levels; and (3) nitrification processes linked to temperature and NO₃⁻, suggesting temperature-enhanced microbial activity. Elevated nitrate levels are mainly attributed to human activities, such as fertilizer use, wastewater discharge, and organic matter decomposition. These findings highlight the combined influence of natural and anthropogenic factors on groundwater quality in arid environments.