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

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 (δD, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-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 δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺. The redox conditions and the relationship between δ¹⁵N-NO₃^- and δ¹⁸O-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.