Influence of Hydrogeochemical Processes and Soil–Urbanization Interactions on Groundwater Quality in the Ganga Alluvial Aquifer, India

This study investigates seasonal hydrogeochemical variations and anthropogenic influences on groundwater quality in Azamgarh district, India, analyzing 76 samples (38 post-monsoon 2021, 38 pre-monsoon 2022). Post-monsoon water quality shows significant deterioration, with 55% of samples unsuitable for drinking (based on Water Quality Index) and 10.5% exceeding nitrate limits, indicating health risks. Elevated total dissolved solids (39% of samples) revealed salinity concerns, particularly in southeastern areas, though Sodium Adsorption Ratios suggested irrigation water remained generally safe. Urban zones exhibited heightened contamination from sewage infiltration and inadequate waste management. Multivariate statistical analyses identified distinct contamination sources, sewage inputs correlated with $${\text{NO}}_{3}^{ - }$$ and pH variations, while agricultural fertilizers influenced $${\text{NO}}_{3}^{ - }$$ and K+ concentrations. Groundwater depletion shows strong correlation with urban expansion (1991–2014), driven by 41.3% population growth and reduced recharge from impervious surfaces. Irrigation suitability assessments demonstrated critical limitations; all samples exceeded permissible Magnesium Hazard values, 28.9% were unsuitable according to Kelly’s Ratio, and 86.8% failed Permeability Index standards, indicating a substantial risk to soil health and agricultural productivity. Seasonal hydrogeochemical processes exhibited distinct patterns, pre-monsoon hardness correlated with sandy soil textures enhancing mineral dissolution, while bicarbonate levels showed negative correlation with soil organic carbon density. Post-monsoon conditions revealed increased magnesium and bicarbonate concentrations with aquifer depth, reflecting prolonged water-rock interactions. The findings highlight the need for adaptive management, including regulated fertilizer use, improved wastewater treatment, and integration of hydrogeochemical insights into land-use policies for sustainable groundwater management in rapidly urbanizing alluvial regions.