Occurrence, fate, and risk assessment of N-nitrosamines in groundwater and river water in an agricultural area

N-nitrosamines have been frequently detected in natural waters as a kind of nitroso compound with significant carcinogenic effects on humans. In remote agricultural areas, groundwater is often consumed directly due to inadequate water supply systems, necessitating the investigation of the occurrence, sources, and cancer risk of N-nitrosamines in the groundwater of agricultural areas. This study identified eight N-nitrosamines in groundwater and river water in the Jianghan Plain, a popular agricultural area in central China. The total concentrations of N-nitrosamines in the groundwater and river water were <2.3~61 ng/L and 3.2~10 ng/L, respectively. N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosomorpholine (NMOR), N-nitrosopyrrolidine (NPYR), and N-nitrosodi-n-butylamine (NDBA) were detected in groundwater, with the main compound being NDMA (up to 52 ng/L). These N-nitrosamines were also detected in river water at comparable concentrations. The significant negative correlations between N-nitrosamines and water temperature/DO indicated that aerobic biodegradation had an influence on the N-nitrosamine distribution. Ammonium (NH₄⁺) was proven not to be a N-nitrosamine precursor, and it rarely reacts with the precursors to form N-nitrosamines. NH₄⁺ and NDBA have similar sources in the JHP. Nitrite (NO₂^-) could consume active hydroxyl radicals, which played a decisive role in forming N-nitrosamines from the ozonation of secondary amines, rather than react with secondary amines when the concentration of NO₂^- was low. In the case of high NO₂^- concentrations, NO₂^- reacts with amines in the environment to form N-nitrosamines. The concentrations of NDMA, NDEA, and NDBA precursors were higher in groundwater than in river water, as suggested by the formation experiment. Redundancy analysis and multiple linear regression analysis results showed the primary source of N-nitrosamines by applying nitrogen fertilizer and specific N-nitrosamines such as NPYR carried by pesticides in groundwater. The average (1.08 × 10^-5) and maximum (8.18 × 10^-5) total cancer risk values of detected N-nitrosamines were higher than the accepted risk level (10^-5), suggesting a potential carcinogenic risk of contaminated groundwater. To minimize N-nitrosamine contamination in the groundwater of agricultural areas, further research on selecting pesticides and fertilizers that are heavily used is urgently needed.