Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3 ) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3 content. Multiple isotopes (8D, 818O-H2O, 815N-NO3 , 818O-NO3 , and 815N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3 concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3 being the predominant form of nitrogen pollution. Moreover, the average NO3 levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3 accumulation in rivers and subsidence areas, which was further supported by the linear relationship between 815N-NO3 and 815N-NH4+. The redox conditions and the relationship between 815N-NO3 and 818O-NO3 , and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3 assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.