Multi-Isotope-Based Tracing of Drainage Nitrogen Behavior and Surface and Groundwater Pathways in High-Nitrogen Rare Earth Mines

High-nitrogen mining drainage (HNMD) is a significant source of watershed nitrogen pollution, influencing the nitrogen distribution in streams through various pathways, including surface runoff (HNMDs) and subsurface runoff (HNMDg). In this study, the nitrogen contributions of HNMDs and HNMDg were characterized by using water chemistry analysis, isotope analysis, and a Bayesian stable isotope mixing model. The combined effects of HNMDs, HNMDg, and domestic sewage (DS) were found to substantially impact nitrogen dynamics in the study area. On average, HNMDs and HNMDg contributed 60.5 ± 8.8% and 19.8 ± 12.5%, respectively, to riverine nitrogen. After accounting for the exclusion of DS, the dominance of HNMDs became more pronounced, contributing 67.0 ± 4.1% and 81.9 ± 0.1% of the HNMD nitrogen in the Chakeng and Caiyang Rivers, respectively. HNMDs and HNMDg displayed distinct nitrogen discharge behaviors within the watershed, which influenced the observed variations in nitrogen fluxes. Precipitation had a stronger influence on nitrogen discharge from HNMDs compared to HNMDg. Furthermore, NH4+-N from HNMD was more likely to enter streams via surface runoff, while HNMDg served as a critical and relatively stable source of nitrogen discharge.