松嫩盆地硝氮驱动的地下水铀富集机理

高志鹏; 郭华明; 丁正; 姜春平; 胡学达; 徐达

doi:10.16030/j.heg.202503051

松嫩盆地硝氮驱动的地下水铀富集机理

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

摘要

摘要:
目的氮素污染导致的地下水重金属次生污染是当前研究的重点和热点。论文旨在探究地下水中硝氮的分布、来源及其对地下水铀迁移转化的控制机制。
方法本研究综合采用水化学、\mathrmNO_3^- 氮氧同位素及水文地球化学模拟的方法，探究了松嫩盆地西部山前倾斜平原地下水中\mathrmNO_3^- 来源、转化及其驱动的地下水铀富集过程。
结果研究区地下水沿流向逐渐由氧化环境演变至还原环境，氧化区地下水铀质量浓度（平均14.2 μg/L）显著高于还原区（平均4.1 μg/L）；氧化区地下水\mathrmNO_3^- 平均质量浓度高达188 mg/L，还原区普遍低于10 mg/L。\mathrmNO_3^- 氮氧同位素结果表明：氧化区地下水\mathrmNO_3^- 主要来自土壤\mathrmNH_4^+ 及\mathrmNH_4^+ 氮肥的硝化作用；而在还原区则主要来自\mathrmNO_3^- 氮肥的输入。
结论氧化区高浓度的\mathrmNO_3^- 一方面将固态U（IV）氧化为溶解态U（VI），另一方面通过驱动碳酸盐岩风化导致Ca-U-CO₃三元络合物的形成，进一步促进了地下水铀的富集；还原区主要发生硅酸盐岩风化及Ca²⁺与Na⁺离子交换过程，地下水氧化还原电位和Ca²⁺浓度均较低，不利于地下水铀的富集。本研究对于理解地下水\mathrmNO_3^- 污染导致的次生铀浓度超标具有重要的理论意义。

Abstract:
Objectives 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 \mathrmNO_3^- in groundwater, as well as its controlling mechanisms on uranium (U) migration and transformation in groundwater.
Methods This study comprehensively employed hydrogeochemical analysis, \mathrmNO_3^- nitrogen and oxygen isotopes, and hydrogeochemical modeling to investigate the sources and transformation processes of \mathrmNO_3^- , and the relevant driving mechanism on U enrichment in groundwater from the western piedmont plain of the Songnen Basin.
Results 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 \mathrmNO_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 \mathrmNO_3^- nitrogen and oxygen isotopes suggest that \mathrmNO_3^- in groundwater from the oxic zone mainly originates from the nitrification of soil \mathrmNH_4^+ and NH₄ fertilizers, while it primarily comes from the input of \mathrmNO_3^- fertilizers in the anoxic zone.
Conclusion In the oxic zone, high concentration of \mathrmNO_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 (ORP) 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 \mathrmNO_3^- pollution.

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