Nitrogen inputs, fate, and associated health risks in an intensive agricultural district: The role of irrigation-induced nitrogen

Irrigation significantly impacts the hydrogeochemical cycle but is often overlooked in agricultural nitrogen budgets. This study innovatively considers river irrigation as a nitrate source and integrates multiple methods (hydrochemical, nitrate isotopes, water isotopes, Bayesian mixing modeling (MixSIAR), and Monte Carlo simulation). Using this approach, it assesses nitrate concentrations and sources in the lower Yellow River, aiming to elucidate the role of irrigation-induced nitrogen in groundwater of the Shandong section irrigation districts. Results show a declining nitrate trend in the lower Yellow River from 2011 to 2021, primarily derived from soil nitrogen (SN), manure and sewage (M&S). The contribution of Yellow River water (YR) to groundwater nitrate in cropland (16.7 %) is significantly higher than residential areas (4.5 %) and livestock & poultry breeding areas (8.5 %), aligns with the fact that Yellow River diversion is primarily used for cropland irrigation in 2019. From 2009 to 2019, groundwater nitrate concentration declined progressively in the irrigation area. The nitrate concentration in inland areas decreased, while that in coastal areas showed a slight increase. The nitrate sources in groundwater are closely related to land use types. Manure and sewage were the main nitrate source in groundwater. Human health risk assessment with Monte Carlo (HHRA) showed that infants faced the most severe threats from nitrate exposure, although the overall health risk decreased in 2019 compared to 2009. These findings highlight the importance of incorporating irrigation water into nutrient management. Integration of MixSIAR and HHRA was first applied in the Yellow River irrigation district.