[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.