Integrating hydrochemical evolution, contamination sources and stable isotopes (δ¹⁸O and δ²D) behaviour in a river-fed alluvial aquifer, Saharanpur, India

NO 3 ⁻ has become a major contaminant in groundwater within Central Ganga Plains seeing a significant rise in NO 3 ⁻and heavy metal contaminants, particularly Mn, which often acts as its intermediary. This study investigates the hydrochemical characteristics by analyzing major ions, trace metals, and spatial distribution of δ¹⁸O and δ²D isotopes while collaborating with hydrographic features and water level dynamics. Gibbs plot and principal component analysis identify rock-water interaction as the primary process governing hydrochemistry. Various bivariate scatter plots infer silicate weathering, carbonate dissolution, and ion-exchange reactions and anthropogenic pollutions. The Chadha diagram confirms Ca-HCO₃ as the dominant species, though localized variations towards Na-Cl are linked to anthropogenic influences. Specifically, NO 3 ⁻ and Mn concentrations are closely tied (attributed) to anthropogenic sources like fertilizer use, livestock waste, manure, and sewage. However, their concentration is shaped by seasonal recharge dynamics and fluctuating redox conditions. Seasonal variation in natural background levels (NBL) of NO 3 ⁻ and Mn also reflects recharge dynamics, with concentrations varying from 7.81 to 21.87 mg/L and from 15.7 to 10.88 µg/L from dry to wet season, respectively. Isotopic data shows shallow groundwater with a wide range of δ¹⁸O values (-10.1‰ to -6.5‰), indicating mixed recharge and evaporation, while deep groundwater aligns more tightly with the local meteoric water line (LMWL) δ¹⁸O values (-9.2‰ to -5.1‰), suggesting consistent paleo-recharge. The study emphasizes how anthropogenic pollutants are enhanced in groundwater due to the combined influence of geogenic and hydro-dynamic factors in the aquifer. The study recommends controlled fertilizer use, adequate waste management and improved sewerage systems for safe and sustainable groundwater management. • Silicate and carbonate weathering are dominant geogenic control on hydrochemistry. • Groundwater is over saturated (SI>0) with chalcedony, quartz, calcite and dolomite. • Topography and recharge govern local redox conditions. • δ¹⁸O traces shallow recharge from high-altitude canal and in-situ recharge sources. • NO 3 - - Mn stem from fertilizer, livestock and human wastes. • NO 3 - and Mn exhibit regionally antagonistic and locally coherent behaviour.