Polycyclic aromatic hydrocarbons (PAHs) in surface soils of tropical reef islands in China under external plant and soil introduction: Occurrence, sources, risks, and relationships with soil properties, vegetation cover, and soil source

Abstract

This study explored the levels, sources, and risks of PAHs in soils from Yongle Atoll (YLA) and Xuande Atoll (XDA) of the Xisha Islands (XSIs) in the South China Sea, China, under different vegetation cover types and soil sources. The results clearly showed that the levels of 16 US EPA priority PAHs (Σ16PAHs) are relatively low in XDA and YLA, with concentrations ranging from not detected (ND) to 151 ng/g (average 15.7 ng/g) and ND to 5.8 ng/g (average 2.1 ng/g), respectively. Three- and four-ring PAHs (62.3% and 53.8%) were widely distributed in YLA and XDA. The average concentration of Σ16PAHs in soils with shrub cover was 1.4, 1.8, 4.8, and 5.0 times higher than that in soils with herbaceous cover, vegetable cover, arbor cover, and no plant cover, respectively. Source analysis using binary diagnostic ratios and the positive matrix factorization (PMF) model suggested that PAHs have similar sources (gasoline/coal combustion, coke production, and biomass combustion), but different contributions in native soil and introduced soil. Moreover, diesel-related vehicular emission was identified to be an additional source of PAHs in native soil. Pearson's correlations revealed strong relationships between PAHs and organic matter or total organic carbon. The cancer risk of PAHs varied among different vegetation cover types and soil sources, following the orders herbaceous cover > vegetable cover > shrub cover > arbor cover > no plant cover and introduced soil > mixed soil > native soil. Nevertheless, the risk remained lower than the risk threshold (10−6), suggesting low carcinogenesis risk in the two atolls. Our findings provide new evidence for the introduction of external vegetation/soil acting as a driver of changes in the characteristics of PAHs in islands, and also underline the negligibility of the PAH increase in soils in the South China Sea, China, from the perspective of health hazards.