The Pollution Of Heavy Metals And PAHs In Sediments Of Nansha Mangrove Wetland

This thesis presents the assessment results of6heavy metals and more than28polycyclic aromatic hydrocarbons(PAHs) in20surface sediments and33columnar sediments, including the content of heavy metal in some community dominant species, such as pagatpat, water hyacinth and cyperus malaccensis. The thesis also includes the study of the content distribution, source, degree of enrichment, ecological risk assessment of heavy metals and PAHs; the interannual change of pollution of sediment columns; the absorption and distribution of heavy metals in plant community dominant species. The conclusions are as follows:1. To varying degrees, mangrove wetlands in Nansha has been polluted by6heavy metals and the heavy metal pollution near the Pearl river mouth is more evident than that in the river, which might relate to the transition and deposition of Pearl river system. The concentration of heavy metals in columnar sediments is characterized by evident vertical distribution and the trend of surface layer enrichment, which proves there has been strong heavy metal pollutants in Pearl river valley in recent years, reflecting the deteriorating trend in ecological environment in Nansha mangrove wetlands.2. In general, the correlation of6heavy metal elements is not evident. Thus, it is concluded that the heavy metal pollution in Pearl river valley belongs to the mixed source. The levels of the enrichment of heavy metal elements are moderate and serious. Among them, the elements reaching the level of serious enrichment are Pb> Cd、Cu、Cr and Mn.3. Among the dominant species in mangrove community, water hyacinth> cyperus malaccensis> pagatpat in terms of the coefficient of heavy metal enrichment. Water hyacinth’s enrichment ability towards heavy metals is much higher than that of other two dominant species; therefore, water hyacinth is the major enrichment plant for absorbing heavy metals in wetlands.4. In the surface sediments, the content of PAHs ranges from332.05ng/g to1651.61ng/g, which shows the average concentration is on the higher level of pollution:the PAHs mainly exist in the5th and6th ring and it increases from upstream to downstream. PAHs is a compound pollution and it mainly comes from the combustion of petroleum and coal. In the columnar sediments, there are two to three peaks of PAHs in different depths. After reaching the peak, the statistics tend to stabilize.5. The high molecular weight of PAHs accounts for59.55%to87.64%, which shows a high carcinogenic risk of Pearl river water. The results of the ecological risk assessment showed that of7PAHs (altogether15items) ecological risk assessment indexes are greater than ERM, indicating the fact that PAHs pollution often has a negative effect on organisms and it may have a serious ecological risk.