松嫩盆地硝氮驱动的地下水铀富集机理OA

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 NO3-in groundwater,as well as its controlling mechanisms on uranium(U)migration and transformation in groundwater.This study comprehensively employed hydrogeochemical analysis,NO3-nitrogen and oxygen isotopes,and hydrogeochemical modeling to investigate the sources and transformation processes of NO3-,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 NO3-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 NO-nitrogen and oxygen isotopes suggest that NO3-in groundwater from the oxic zone mainly originates from the nitrification of soil NH4+and NH4+fertilizers,while it primarily comes from the input of NO3-fertilizers in the anoxic zone.In the oxic zone,high concentration of NO3-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 NO3-pollution.