[Nitrate Sources and Key Transformation of River Water in a Piedmont Agricultural Area]

Water nitrate （NO₃^-） pollution is a pressing global environmental issue. Accurately identifying the diverse sources and quantifying the proportional contributions of NO₃^- in surface river water remains challenging， especially under complex hydrogeological conditions. This study investigated the surface river water in a typical piedmont agricultural area by integrating hydrochemistry， multiple isotopes （δ²H-H₂O， δ¹⁸O-H₂O， δ¹⁸O-NO₃^-， and δ¹⁵N-NO₃^-）， and dissolved organic carbon （DOC）， in combination with two quantitative models （i.e.， Bayesian stable isotope mixing， MixSIAR； positive matrix factorization， PMF）. It aimed to elucidate the distribution patterns， sources， contribution rates， and transformation processes of NO₃^- in river water. The results indicated that NO₃^- concentrations in the piedmont agricultural area [（35.6±12.5） mg·L^-1] were significantly higher than those in the middle and lower urban reaches [（1.12±1.16） mg·L^-1]， with nitrification identified as the primary process regulating NO₃^- levels. In urban river sections， the excretion by algae and aquatic plants led to the accumulation of DOC [（13.5±3.9） mg·L^-1]， which promoted the dissimilatory nitrate reduction to ammonium （DNRA） process， thereby inhibiting NO₃^- accumulation. Although there were some discrepancies between the quantitative results of the MixSIAR and PMF models， both models consistently indicated that the high NO₃^- concentrations in the piedmont agricultural river sections were primarily attributed to manure and sewage discharges （MixSIAR： 92.0%， PMF： 84.1%， contribution rate）. These findings provide targeted scientific support for the prevention and control of NO₃^- pollution in surface water within the study area， while also offering methodological references for other NO₃^--polluted regions worldwide with similar conditions.