Mechanism of nitrate-driven groundwater uranium enrichment in the Songnen Basin

Nitrogen pollution-induced secondary contamination of heavy metals in groundwater is a current focal point and hot topic of research. The study aims to investigate the distribution and sources of ${\mathrm{NO}}_3^- $ in groundwater, as well as its controlling mechanisms on uranium(U) migration and transformation in groundwater. This study comprehensively employed hydrogeochemical analysis, ${\mathrm{NO}}_3^- $ nitrogen and oxygen isotopes, and hydrogeochemical modeling to investigate the sources and transformation processes of ${\mathrm{NO}}_3^- $, and the relevant driving mechanism on U enrichment in groundwater from the western piedmont plain of the Songnen Basin. The results indicate that the groundwater in the study area gradually evolves from an oxidizing environment to a reducing environment along the groundwater flow path. The average U concentration (14.2 μg/L) in the oxic zone is significantly higher than that in the anoxic zone (4.1 μg/L). The average concentration of ${\mathrm{NO}}_3^- $ in the groundwater of the oxic zone is as high as 188 mg/L, while it is generally below 10 mg/L in the anoxic zone. The results of ${\mathrm{NO}}_3^- $ nitrogen and oxygen isotopes suggest that ${\mathrm{NO}}_3^- $ in groundwater from the oxic zone mainly originates from the nitrification of soil ${\mathrm{NH}}_4^+ $ and ${\mathrm{NH}}_4^+ $ fertilizers, while it primarily comes from the input of ${\mathrm{NO}}_3^- $ fertilizers in the anoxic zone. In the oxic zone, high concentration of ${\mathrm{NO}}_3^- $ not only oxidizes solid U(IV) to soluble U(VI) but also promotes the formation of Ca-U-CO3 ternary complexes through the weathering of carbonate rocks, thereby further promoting the enrichment of U in groundwater. In the anoxic zone, the main processes are the weathering of silicate minerals and the cation exchange between Ca2+ and Na+. The groundwater redox potential and Ca2+ concentrations are both low in the anoxic zone, which are not conducive to the enrichment of U in groundwater. This study poses considerable theoretical importance for understanding the secondary U contamination in groundwater caused by ${\mathrm{NO}}_3^- $ pollution.