Groundwater nitrate responses to extreme rainfall in alluvial–diluvial plain aquifers: Evidence from hydrogeochemistry and isotopes

Rainfall promotes the infiltration of surface materials into aquifers, but its role in nitrogen enrichment in the aquifers of the North China Plain remains unclear. This study collected surface water and groundwater samples after heavy rainfall and analyzed hydrogeochemical, isotopic, and nitrate properties. Results showed that groundwater nitrate concentrations were significantly higher than surface water, with levels far exceeding standards. Groundwater was primarily high-nitrate HCO₃-Ca type, controlled by carbonate and silicate dissolution. After rainfall, nitrate concentrations of groundwater increased in the northern part and decreased in the southern part, showing spatial variations. NO₃^-/Cl^- ratios identified agriculture, reclaimed water, and sewage as primary nitrate sources, while tritium analysis confirmed ongoing pollution from agricultural activities and domestic sewage. Industrial activities impacted surface water nitrate more significantly, while agriculture and sewage dominated groundwater pollution. Groundwater nitrate concentration changes were predominantly influenced by the unsaturated zone and soil permeability. In rapid groundwater response zone, rainfall rapidly transported surface nitrate into groundwater, increasing its concentration. In delayed groundwater response zone, limited infiltration and dilution from return flow reduced nitrate levels. This study elucidates the mechanisms governing nitrate variations after rainfall, providing scientific guidance for the protection and restoration of water quality in the North China Plain.