Layer-specific nitrate source apportionment in a high-permeability alluvial fan using dual isotopes, PMF and MixSIAR...

Nitrate contamination in high-permeability alluvial fan aquifers poses a significant challenge for groundwater resource management, due to the rapid migration of contaminants and complex interactions among multiple nitrate sources. This study employed an integrated approach, combining hydrochemical characterization, isotopic analyses, and multivariate statistical modeling, to investigate the spatial distribution of nitrate, identify transformation processes, and quantify the contributions of nitrate sources within an alluvial aquifer system in the North China Plain. Groundwater nitrate concentrations varied significantly (0.1-41.2 mg/L NO₃^--N), with the highest concentrations primarily observed in industrial areas. Principal component analysis (PCA) and positive matrix factorization (PMF) revealed substantial impacts of human activities, particularly domestic and industrial wastewater discharge, on nitrate accumulation and distribution. Dual-isotope analysis (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) demonstrated significant denitrification processes occurring in the shallow first aquifer, whereas nitrification was limited due to low ambient ammonium and nitrite concentrations. Furthermore, the Bayesian stable isotope mixing model (MixSIAR) showed that manure and sewage contributed 62.1% to nitrate in the first aquifer, while industrial wastewater dominated in the second and third aquifers (64.3% and 64.6%). This integrated approach provides a robust framework for nitrate source identification in heterogeneous aquifer systems, offering valuable insights for sustainable groundwater management practices in high-permeability alluvial aquifers.