Source apportionment and risk assessment of polycyclic aromatic hydrocarbons in surface sediments from tributaries of Dongzhai Harbor

Mangrove wetlands, as typical ecosystems at the land-sea interface, serve as significant sinks for polycyclic aromatic hydrocarbons (PAHs). In April 2023, surface sediment samples (0–5 cm) were collected from 29 sites across five tributaries of the Dongzhai Harbor National Nature Reserve, Hainan, and analyzed for 16 priority PAHs by gas chromatography-mass spectrometry (GC-MS). Diagnostic ratios, principal component analysis (PCA), and total organic carbon (TOC) data were integrated to characterize the pollution profiles, apportion the sources, and evaluate the associated ecological and health risks. Key findings include: ① total PAHs concentrations ranged from 12.66 to 132.44 ng/g (mean 48.27 ng/g), with notable spatial heterogeneity—Sanjiang River (77.23 ng/g) and Yanzhou River (66.73 ng/g) exhibited the highest pollution due to proximity to aquaculture zones, docks, and residential areas, while Hegang River (15.75 ng/g) showed the lowest levels owing to dense mangrove coverage; ② medium- and low-ring PAHs (2–4 rings) dominated (77.3%), indicating petroleum leakage and low-intensity combustion as primary sources, with TOC significantly correlating with PAHs concentrations (strongest for 3-ring, 5-ring, and total PAHs); ③ source apportionment revealed that combustion sources contributed over 80% of PAHs, with significant variability among tributaries: Hegang River was dominated by petroleum combustion (48.14%), Yanfengdong River and Qunlong River by coal combustion (54.86% and 42.85%, respectively), and Sanjiang River exhibited a mixed source of petroleum and coal combustion (72.93%). Minor petroleum leakage was detected only at a few sites in the Qunlong and Sanjiang Rivers. ④ the overall ecological risk was relatively low, with only naphthalene (NAP) and dibenz(a,h)anthracene (DahA) exceeding the rare effect level (REL) at isolated sampling points. However, health risks were prominent, exhibiting a spatial pattern of “localized high-risk areas within an overall potential risk context”. The core risk zones were identified at Sanjiang River SJR-2 and Yanfengdong River YFDR-18, dominated by the ingestion pathway. In comparison with domestic and international mangroves, the concentration of PAHs in Dongzhai Port is lower than those found in Zhanjiang, Guangdong, the Tamsui River, and industrialized areas (such as Tumaco Bay in Colombia), but is higher than that in the Beilun River estuary in Guangxi, with only a few locations exceeding the REL. The study highlights that mangrove coverage and human activity intensity jointly drive spatial distribution of PAHs. Priority should be given to controlling combustion emissions and mitigating ingestion exposure at high-risk sites to balance ecological conservation and health protection.