Hydrogeochemical and statistical analysis of leachate-impacted groundwater in an urban landfill setting

Groundwater contamination associated with legacy landfills is a major environmental concern in rapidly urbanising regions. This study investigates the seasonal hydrogeochemical characteristics of groundwater surrounding the Ghazipur landfill in East Delhi, India, using an integrated framework that combines compositional data analysis (CoDA), entropy-weighted water quality indexing (EWQI), multivariate statistics, hydrochemical facies analysis, and hydrogeological interpretation. Groundwater samples collected during the pre-monsoon (n = 14) and monsoon (n = 60) seasons of 2023 were analysed for major ions and physicochemical parameters. Hydrochemical facies analysis, Gibbs plots, bivariate relationships, and CLR-transformed PCA collectively indicate that groundwater chemistry is predominantly influenced by anthropogenic salinity enrichment associated with landfill leachate and urban contamination. Strong near-stoichiometric Na⁺-Cl^- relationships suggest conservative dissolved-ion transport within the shallow aquifer system, whereas NO₃^- exhibited spatially heterogeneous behaviour independent of the dominant salinity system, indicating mixed anthropogenic nitrogen inputs potentially associated with landfill-derived nitrogen transformation, sewage leakage, urban wastewater, and regional slaughterhouse-related organic waste activities. Monte Carlo sensitivity analysis confirmed that the nitrate dominant PCA structure remained statistically robust despite unequal seasonal sample sizes. Spatial EWQI mapping revealed substantial monsoonal expansion of groundwater-quality deterioration, particularly within downgradient southeastern sectors of the study area. The study demonstrates that monsoonal recharge enhances contaminant redistribution and modifies hydrochemical structure within shallow urban aquifers rather than simply diluting groundwater chemistry. The integrated CoDA-based framework implemented in this study provides an effective approach for evaluating complex groundwater contamination processes in landfill-affected urban environments.