Concentrations and characteristics of polycyclic aromatic hydrocarbons (PAHs), phenols and chlorobenzenes in deep seawater near South China Sea.

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Polycyclic aromatic hydrocarbons (PAHs) and other aromatic hydrocarbons are persistent environmental contaminants with diverse origins, posing significant risks to marine ecosystems. This study investigates the concentrations, vertical distribution, and sources of these pollutants in the South China Sea near the Xisha Islands, a region with distinct oceanographic conditions and anthropogenic influences. A total of 23 aromatic hydrocarbon compounds were analyzed, including 17 PAHs, 3 phenols, and 3 chlorobenzenes. Of these, 3 PAHs (fluorene, benzo[k]fluoranthene, and dibenz[a,h]anthracene) were below detection limits. Seawater samples collected at four depths (200, 400, 1000, and 1200 m) revealed distinct depth-dependent patterns. Lower-molecular-weight PAHs (2-3 rings) exhibited higher concentrations at shallower depths (200 m and 400 m), likely influenced by atmospheric deposition and surface runoff. High-molecular-weight PAHs (4-6 rings) were found in lower concentrations but persisted at greater depths due to their higher stability and slower degradation rates. Methylated PAHs, such as 2-methylnaphthalene and 1-methylnaphthalene, were concentrated at intermediate depths, suggesting ongoing degradation processes and historical inputs. Phenolic and chlorobenzene compounds displayed relatively consistent concentrations across depths, indicating widespread anthropogenic influence. These results highlight the combined impact of natural and anthropogenic sources, including industrial discharges, shipping activities, and atmospheric transport, on the distribution of aromatic hydrocarbons in the South China Sea. The study provides critical baseline data for understanding pollutant behavior in deep-sea environments and emphasizes the importance of targeted monitoring and effective management strategies to mitigate their ecological impacts. PRACTITIONER POINTS: PAHs and methylated PAHs - Concentrations vary with depth, suggesting diverse sources. Shallow waters -High anilines, phenols, and chlorobenzenes indicate recent inputs. Deeper waters - Complex PAHs persist, highlighting long-term ecological risks. Degradation - Methylated PAHs degrade uniquely, affecting health risk assessments. Management - Findings inform effective monitoring and mitigation of marine pollution.