Nitrogen Sources and Transformation Pathways in a Highly Urbanized Shallow Aquifer: Insights from an Integrated Hydrochemical and Isotopic Approach Incorporating δ15N-DON

This study investigates nitrogen sources and biogeochemical pathways in a highly urbanized shallow aquifer in Shinagawa Ward, Tokyo, using an integrated approach combining hydrochemical analysis, multivariate statistics (PCA and K-means cluster analysis), and stable nitrogen isotopes (δ15N-NH4+, δ15N-NO3−, δ15N-DON, and dual δ15N–δ18O-NO3−). K-means clustering (K = 2, silhouette = 0.54) partitioned all 41 samples into a background group (n = 34) and an ion-enriched group (n = 7; wells sbi 1, 2, 3, 4, 5, 13, and 19), with the latter exhibiting hydrochemical signatures consistent with localized sewage leakage. The convergence of hydrochemical, multivariate, and isotopic evidence suggests that soil organic matter may represent the dominant diffuse background source of nitrogen across the study area. DON constitutes the dominant fraction of total dissolved nitrogen (TDN), while the linear correlations between TDN and DON concentrations (r = 0.77, p < 0.001) and between δ15N-TDN and δ15N-DON (r = 0.88, p < 0.001) indicate a common primary source. The dominance of DON combined with the theoretical inverse relationship between δ15N-DON and DON concentration is consistent with active soil DON mineralization, supported by an isotope fractionation factor (ε = −4.4 ± 0.78‰). Dual isotope analysis of NO3− (δ15N–N–δ18O slope = 0.51) points towards denitrification as an ongoing process in the aquifer. Taken together, the isotopic variations among nitrogen species suggest a transformation sequence from soil organic nitrogen → DON → NH4+/NO3− → N2, though each step in this sequence is supported to varying degrees of confidence. These findings highlight the value of δ15N-DON as a tracer for nitrogen source attribution and cycling in urban groundwater systems, and underscore the importance of considering all dissolved nitrogen fractions in contamination assessments.