Abstract:

Mining of ion-adsorption rare earth elements (REEs) has caused severe nitrogen pollution in water and soil. However, the distribution, migration and transformation, and pollution sources of nitrate (${\mathrm{NO}}_{3}^{-}$-N) in water bodies affected by such drainage remains insufficiently studied. This study investigated the origins and transformations of ${\mathrm{NO}}_{3}^{-}$-N in surface water and groundwater downstream of the Zudong ion-adsorption REE mining area in southern Jiangxi, China, using hydrochemical analysis and a multi-isotope (δ¹⁸O-H₂O,δ¹⁵N-${\mathrm{NO}}_{3}^{-}$ and δ¹⁸O-${\mathrm{NO}}_{3}^{-}$) approach. The contribution of various sources was quantitatively assessed using the MixSIAR model. The results showed that surface water and groundwater were weakly acidic and were characterized by low TDS values. Surface water was predominantly of the SO₄-Ca type, whereas 80% of the groundwater samples were of the HCO₃-Ca type. The concentrations of total nitrogen (TN), ${\mathrm{NO}}_{3}^{-}$-N, and ${\mathrm{NH}}_{4}^{+}$-N in surface water were significantly higher than those in groundwater, indicating that nitrogen pollution in surface water was closely related to mining activities. Spatially, nitrogen sources were closely related to mining activities that utilized ammonium-nitrogen-rich brine, which posed a significant nitrogen pollution risk to surface water. Distributions of land-use types indicated that nitrogen sources in surface water differed from those in groundwater. Nitrogen in surface water primarily originated from mine drainage in forested areas, whereas nitrogen in groundwater was mainly derived from agricultural activities on cultivated land. The compositions of δ¹⁸O-H₂O, δ¹⁵N-${\mathrm{NO}}_{3}^{-}$, δ¹⁸O-${\mathrm{NO}}_{3}^{-}$ and fractionation coefficients revealed that nitrification was the dominant process in both surface water and groundwater. End-member analysis based on measured and reconstructed δ¹⁵N-${\mathrm{NO}}_{3}^{-}$ and δ¹⁸O-${\mathrm{NO}}_{3}^{-}$ values suggested that ${\mathrm{NO}}_{3}^{-}$-N in SW1 was primarily derived from ammonium nitrogen discharged from mining activities. In contrast, ${\mathrm{NO}}_{3}^{-}$-N in SW2 was influenced by both mining and agricultural activities, with contributions from ammonium nitrogen, soil nitrogen, and sewage manure. ${\mathrm{NO}}_{3}^{-}$-N in groundwater mainly originated from soil nitrogen and sewage manure. Quantitative assessment using the MixSIAR model indicated that mine drainage contributed over 50% (mean value) of the ${\mathrm{NO}}_{3}^{-}$-N in SW1 (surface water nearest to the mining area). Of this contribution, 65% to 94% originated from the native ${\mathrm{NO}}_{3}^{-}$-N present in the mine drainage; while in surface water farther from the mining area (SW2), mine drainage contributed approximately 30% of the ${\mathrm{NO}}_{3}^{-}$-N. Uncertainty analysis (UI₉₀) showed that the contribution of atmospheric precipitation was the most stable, while the contributions of mine drainage, sewage and manure, and soil nitrogen exhibited significant uncertainty. This research elucidates the formation mechanism of ${\mathrm{NO}}_{3}^{-}$-N pollution in ion-adsorption REE mining areas and provides a scientific basis for the precise prevention and control of nitrogen pollution.

Key words: rare earth mine, nitrogen pollution, nitrification, Bayesian isotope mixture model, fractionation and enrichment