The accurate identification of NO₃^--N pollution sources are pivotal to the assessment of groundwater pollution risks. However, current studies on NO₃^--N source apportionment simplifies complex multi-layer aquifer systems into single-layer models without accounting for the impact of occurrence conditions. In this study, we quantitatively identify distinct NO₃^--N pollution sources in unconfined and confined groundwater, highlighting the critical role of occurrence conditions for management.