Seasonal dynamics and interfacial exchange of parent and alkylated polycyclic aromatic hydrocarbons in a subtropical lake: Overlooked alkylated derivatives involved a shift in ecological risk assessment.

Traditional research on polycyclic aromatic hydrocarbons (PAHs) focuses on 16 parent compounds (P-PAHs), overlooking alkylated PAHs (Alk-PAHs), which may be more stable and toxic, potentially leading to a substantial underestimation of ecological risks. This study conducted a comprehensive seasonal survey (2023-2024) in Wanghu lake, a subtropical shallow lake, analyzing 22 P-PAHs and 20 Alk-PAHs in dissolved and suspended particulate phases (DP and SPP), surface sediments, and surrounding soils. Concentrations, compositional profiles, seasonal trends, interfacial exchange, and ecological risks were systematically assessed. Alk-PAHs dominated the aquatic phase (61% in DP), while P-PAHs were more abundant in soils and sediments (approximately 60%). PAH exhibited a clear "higher in winter, lower in summer" pattern, driven by temperature, precipitation, hydrology, and anthropogenic inputs, with compositional profiles indicating mixed sources. Interfacial exchange revealed seasonal partitioning: low molecular weight PAHs (LMW-PAHs) tended to volatilize from water in summer, whereas high molecular weight PAHs (HMW-PAHs) settled into sediments via particle deposition in winter. Ecological risk assessment demonstrated that although HMW-P-PAHs contributed the most to the Benzo[a]pyrene toxic equivalency (BaP-TEQ) due to their high toxic equivalency factors, the toxic contribution of Alk-PAHs was non-negligible (averaging 20% in the SPP), especially 1,8-DMNap, 1-MFlu, and 2-MFlu. Winter sediments were identified as the key sink for ecological risk convergence. These findings demonstrate that current monitoring frameworks substantially underestimate ecological risks by excluding Alk-PAHs, highlighting the necessity of including them in routine risk assessment and expanding priority pollutant lists beyond the traditional 16 priority PAHs.