Contamination of polycyclic aromatic hydrocarbons in surface sediments of mangrove swamps

The concentrations of total polycyclic aromatic hydrocarbons (∑PAHs) and 15 individual PAH compounds in 20 surface sediments collected from four mangrove swamps in Hong Kong were analysed. ∑PAH concentrations ranged from 356 to 11,098 ng g^-1 dry weight with mean and median values of 1992 and 1142 ng g^-1, respectively. These values were significantly higher than those of marine bottom sediments of Hong Kong harbours, suggesting that more PAHs were accumulated in mangrove surface sediments. The concentrations of ∑PAHs as well as individual PAH compound varied significantly among mangrove swamps. The swamps heavily polluted by livestock and industrial sewage, such as Ho Chung and Mai Po, had much higher concentrations of total PAHs and individual PAH than the other swamps. The PAH profiles were similar among four mangrove swamps, and were dominated by naphthalene (two-ring PAH), flourene and phenanthrene (three-ring PAH). The mangrove sediments had higher percentages of low-molecular-weight PAHs. These indicated that PAHs in mangrove sediments might originate from oil or sewage contamination (petrogenic input). Ratio values of specific PAH compounds such as phenanthrene/anthracene and fluoranthene/pyrene, were calculated to evaluate the possible source of PAH contamination in mangrove sediments. These ratios varied among samples, suggesting that mangrove sediments might have a mixed pattern of pyrolytic and petrogenic inputs of PAHs. Sediments collected from Ho Chung mangrove swamp appeared to be more dominated by pyrolytic input while those from Tolo showed strong petrogenic contamination.