Nitrate source apportionment and risk assessment: A study in the largest ion-adsorption rare earth mine in China

Nitrate (NO₃^-) pollution in water bodies has received widespread attention, but studies on nitrogen transformation and pollution risk assessment are still limited, especially in rare earth mining areas. In this study, surface and groundwater samples were collected from the largest rare earth mining site in southern China, and analyzed for the hydrochemical and stable isotopic characteristics. The results showed that the NO₃^- concentrations ranged from 1.61 to 453.11 mg/L, with 35% of surface water and 53.3% of groundwater samples exceeding the WHO standard (i.e., 50 mg/L). Health risk assessment showed that 31.4% of the water samples had a moderate to high non-carcinogenic risk, and the high-risk areas were concentrated in rare earth mining regions. Additionally, adults were more vulnerable to the non-carcinogenic health risks than children. The high variability of δ¹⁵N-NO₃^- (from -6.43 to 17.09‰) and δ¹⁸O-NO₃^- (from -7.91 to 22.79‰) showed that NO₃^- was influenced by multiple nitrogen sources and transformation processes. Hydrochemistry and isotopic evidence further indicated that NO₃^- was primarily influenced by nitrification and hydraulic connection between surface and groundwater. The results of the Bayesian mixing model showed that about 70% of NO₃^- originated from mine drainage and soil N in the rare earth mining area, while more than 90% of NO₃^- originated from fertilizer, soil N, and manure and sewage in rural and urban areas in the middle and downstream. This study suggests reducing anthropogenic nitrogen discharge (e.g., leaching agents and fertilizer inputs) as the primary means of NO₃^- pollution control with biogeochemical processes (e.g., denitrification) to further reduce its pollution.