Prolonged Natural Attenuation of N-Nitrosamines in Groundwater–Soil Systems: Insights into Kinetics, Pathways, and Degradation Mechanisms from Microcosm Experiments

Highly carcinogenic N-nitrosamines are frequently detected in groundwater, posing significant health risks to humans. However, knowledge of their attenuation characteristics and degradation mechanisms in the subsurface environment remains limited. Here, we conducted microcosm experiments to investigate the half-lives (T_1/2s) of five representative N-nitrosamines in groundwater–soil systems under varying redox conditions, environmental matrices, and carbon source additions. The results revealed that the T_1/2s of N-nitrosamines exceed 150 days in groundwater–soil systems with no contamination history and in groundwater environments without soil matrices. However, environmental matrices with long-term exposure to N-nitrosamine contamination demonstrated a shorter T_1/2, ranging from a few to several dozen days. Moreover, N-nitrosamine decay rates under aerobic conditions were approximately twice those under anaerobic conditions. Microbial cometabolism was the primary removal mechanism, producing secondary amines, nitrate, and nitrite observed as intermediates, and ammonium as the main product. Variations in microbial community composition were observed across experimental conditions, underscoring the role of specific microbial taxa such as Nitrospirota, Patescibacteria, and Zixibacteria in the degradation process. By integrating product profiling with microbial analysis, this study offers a comprehensive mechanistic understanding of N-nitrosamine degradation. These findings offer insights for developing targeted remediation strategies to mitigate health risks from groundwater contamination.