Contamination Of Heavy Metal And Magnetic Characteristics In Mangrove Sediments Of Fujian Province

Mangroves are mainly distributed in the estuary and intertidal zone of the bay in tropical and subtropical coastal areas.Due to their special habitat,heavy metals tend to accumulate in the sediments of the mangroves.Heavy metals have the characteristics of strong toxicity,wide sources,and non-degradability.They can accumulate in the food chain and cause serious harm to animals and humans.Therefore,this paper selects mangrove ecosystem in Fujian Province as the experimental area.Firstly,study the content and occurrence of heavy metals V,Cr,Ni,Cu,Zn,As,Pb,Cd,Hg in sediments,and the heavy metal accumulation capacity of different tissues of mangrove plants;secondly,study the magnetism of mangrove sediments,By analyzing the characteristics of the magnetic minerals in the sediments,combining with the multivariate statistical analysis methods,the sources of heavy metal pollution and the reasons for their changes can be discovered;tertiary,through the method of 210Pb to determine the deposition rate of mangrove sediments,and analyze the temporal and spatial distribution of heavy metals,pollution history and potential hazard situation;Finally,through the heavy metal pollution risk assessment method to evaluate the current situation of heavy metal pollution,providing a theoretical basis for mangrove wetland protection.The main conclusions of this study are as follows:(1)The contentration of Hg、V、Cr、Ni、Cu、Zn、As、Cd and Pb in the surface sediments of mangrove wetland in Fujian are 0.17±0.12 mg/kg、99.56±9.03 mg/kg、71.99±9.81 mg/kg、30.42±7.76 mg/kg、34.80±13.58 mg/kg、161.77±59.28 mg/kg、13.00±1.95 mg/kg、0.42±0.06 mg/kg、63.54±31.44 mg/kg respectively,all of which exceeded the elemental background value of coastal sediments in Fujian Province.The V、Cr、Ni、As and Zn are mainly exist in the fom of residual state,Cd is mainly exist in the weak acid soluble state,and Pb is mainly present in the form of reducible state in the sedimentary environment.The contentration of heavy metals in mangrove plants(Aegiceras corniculatum&Kandelia obovata)generally shows a tendency that Cr>Zn>Cu>Ni>As>Pb>V.The comprehensive enrichment ability of Aegiceras corniculatum&Kandelia obovata for different heavy metal elements is the stem>Leaf>propagule,and the content of heavy metals in seedlings is higher than that of stems,leaves and propagules of mature plants.(2)The χLF of mangrove surface sediments in Fujian Province vary from 4.30×108 m3/kg to 15.85×10-8 m3/kg,and the χFD%vary from 4.26%to 15.60%.The χLF of each region on the whole is Jiulongjiangkou>Fuding>Quanzhou Bay>Meizhou Island>Zhangjiangkou.The range of χLF,χFD%,SIRM,and χARM of columnar sediments is 8.66～11.72×10-8 m3/kg,2.33%～3.66%,26.57～43.30×10-4 Am2/kg,2.32～2.69×10-6 m3/kg respectively.There are many kinds of magnetic minerals in the sediments,such as,ferrimagnetic minerals,antiferromagnetic minerals.The magnetic mineral particles in the mangrove sediments of Quanzhou Bay are mainly SD and PSD,but there are also a few SP particles.Heavy metal pollution in mangrove sediments mainly comes from industrial wastewater、domestic sewage、pollution from aquaculture、pesticide pollution、traffic pollution and pollution from fishing ports.(3)Through relationship of the decay of 210Pb and the depth,the deposition rate of mangroves in Quanzhou Bay is 1.74 cm/a.As the depth increases,the content of heavy metals decreases.(4)According to the marine sediment quality standard(GB1868-2002),the heavy metals,Hg、Cr、Cu、As and Cd are low pollution in the mangrove wetland sediments,it conforms the the first category quality standards of marine sediments,while Zn and Pb meet the second category.By the evaluation of geo-accumulation index pollution,to assess the heavy metals in mangrove sediments in Fujian Province,the study found the the heavy metals,Cd and Hg are non-polluting,Ni and Cu are moderately polluted,and V,Cr,Pb,and Zn are serious pollution.Through the evaluation of the potential ecological hazards of mangroves in Fujian Province,it was found that various regions have certain ecological risks,especially the heavy metals Hg and Cd.Although the concentration in the environment is low,they are highly toxic and harmful.In summary,the regional ecological risk level of Quanzhou Bay and Jiulong River Estuary is levelⅢ,which represents a strong ecological hazard,the ecological risk level of Fuding Yaojiayu is level Ⅱ,which represents a medium ecological hazard,and the ecological risk level of Meizhou Island and Zhangjiang Estuary is Level Ⅰ,manifested as a slight ecological hazard.Finally,in the light of the distribution ratio method of the method of RSP,it is found that the mangrove wetland in Fujian Province are polluted in varying degrees.