Abstract: The mangrove wetlands at the sewage outfalls in Futian of Shenzhen, and Guanhai Corridor of Zhanjiang were selected as the research objects. The potential ecological risk index method and the accumulation index method were used to analyze the pollution of heavy metals in the surface mangrove soil at the sewage outlet from land sources, and the pollution sources of heavy metals were discussed by correlation analysis and principal component analysis. The results showed that the mean values of heavy metals in the two study areas were within the range of national soil environmental quality standards. The mass fractions of six heavy metals in Futian exceeded the background values of soil in Guangdong province. Only Cd, Cu and Zn in the Guanhai Corridor exceeded the background values of soil in Guangdong province. The comprehensive potential ecological risk assessment showed that Futian mangrove had medium potential ecological risk, while the Guanhai Corridor had slight to strong ecological risk. According to the results of geoaccumulation index, Cu in mangrove soil of Futian reached medium strong pollution, Cd, Ni and Zn reached medium pollution, and Pb and Cr were slightly polluted. However, Cd, Zn and Cu in mangrove soil of Guanhai Corridor were slightly to moderately polluted, and Ni, Pb and Cr were uncontaminated. The correlation between heavy metals in mangrove soil of Futian was not significant, mainly from industrial sewage and traffic pollution. There was a significant positive correlation between heavy metal elements in the Guanhai Corridor, and the pollution mainly comes from urban domestic sewage.

Key words: sewage outlet into the sea, heavy metal pollution, ecological risk, mangrove forest