| Heavy metals,polycyclic aromatic hydrocarbons(PAHs),polychlorinated biphenyls(PCBs)and organochlorine pesticides(OCPs)have attracted extensive attention due to their refractory,difficulty to volatilize and extremely persistent toxicity,and become a hot issue in environmental science research in the 21 st century.Artificial activity leaves them in the environment for a long-term existence and eventually converges into rivers,lakes and oceans through surface runoff.In the water body,these pollutants are combined with the deposited particles through a series of physical and chemical effects,and deposited at the water bottom,so the sediments become a reservoir of these pollutants.And under certain conditions,the sediments can be hydrodynamically suspended,transported over long distances,and become a source of pollution again.Therefore,sediments are not only the carrier of pollutants,but also the source of pollutants.Its characteristics make it suitable as an “indicator” for pollutants,which could better reflect the pollution situation in the area.Due to its special landform,the Subei shoal can be considered the largest silt shoal in Asia.Meanwhile,aquaculture in Meretrix meretrix L.,Bullacta exarata,Mactra chinensis and seaweed has been developed in the area.The development and utilization of tidal flats increase economic growth,meanwhile,it can threaten the fragile tidal flat ecology.The previous researches on the pollution status of heavy metals,PAHs,PCBs and OCPs in Subei Shoal were limited due to smaller investigation areas and kinds of pollutants.Therefore,this study comprehensively evaluated and analyzed various pollutants in the surface sediments of the Subei Shoal in order to understand the current ecological environment of the region,which would provide a scientific basis for the prevention,control and treatment of the ecological environment in the Subei Shoal.The concentrations of heavy metals Ni,Zn,Cr,Cu,As,Pb,Cd and Hg were 47.88?±?8.93,38.18?±?8.86,19.22?±?5.14,11.32?±?5.07,6.97?±?2.45,0.13?±?0.72,0.56?±?0.77 and 0.06?±?0.02?mg/kg,respectively,in surface sediments of the Subei Shoal.The contents of total organic carbon(TOC),clay,silt and sand were 1.7?±?0.8%,3.3?±?3.2%,13.6?±?14.2% and 83.1?±?17.4%,respectively.The spatial distribution of heavy metals Cu,Pb,Zn,Cr,Cd,Ni and As showed similar spatial distribution,with the highest in the northern parts,the second in the central parts,and the lowest in the southern part.The spatial distribution of TOC,clay,and sand were similar to these heavy metals,but the distribution of Hg was just inverse.Principal component analysis suggested that Cu,Zn,Cd,Cr and Ni were mainly derived from natural sources,whereas Pb,Cd,As and Hg from industrial and agricultural sources.Results of geo-accumulation index,potential ecological risk index(RI),pollution load index(PLI),toxic risk index(TRI)and contamination severity index(CSI)demonstrated that pollution levels of Cd and Hg were moderate,which should attract more attention as main pollution factors.The pollution was mainly distributed in the central and northern parts,and the southern part had a good ecological environment.Moreover,the contaminated stations accounted respective for 33.4%,25.9%,33.3% and 70.4% of RI,PLI,TRI and CSI,of which 70.4% of the contamination severity index stations contained 66.7% of much lower severity stations.In this study,sixteen United States Environmental Protection Agency(USEPA)priority controlled polycyclic aromatic hydrocarbons(PAHs)were detected in surface sediments of the Subei Shoal.The detection results indicated that except for Acy,Ace,Ant,BkF,and DBA,which were not detected in the samples,the other 11 PAHs were detected.Total PAH concentrations ranged from 0.0 to 25.2 ng/g with an average of 5.9 ng/g.The concentrations of PAHs appeared in the high value area in the middle parts of Subei Shoal,followed by north.The PAHs were dominated by high molecular weight PAHs(4-5 rings),and the light-weight ratio,light molecular weight/heavy molecular weight(LMW/HMW),indicates that the PAHs in the Subei Shoal were mainly from high-temperature combustion sources.Principal component analysis(PCA)showed that PAHs in the sediments came mainly from coal combustion,transportation(oil combustion),coking,and petroleum sources.Both effect range low/effect range median(ERL/ERM)values and mean effects range–median quotient(M-ERM-Q)showed that the ecological risk from PAHs was low toxicity risk level in the Subei Shoal.In this study,ten PCBs and 17 OCPs were detected in the surface sediments of the Subei Shoal,but only 5 of them were detected,respectively.The concentrations of ?PCBs and ?OCPs were n.d.~18.247 ng/g(average 2.117 ng/g)and n.d.~1.840 ng/g(average 0.080 ng/g),respectively,which the concentrations were at a low level,and their distribution was unevenly distributed.The results of ecological risk assessment showed that the residues of PCBs and OCPs in the surface sediments of the Subei Shoal had little effect on the toxic and side effects of organisms. |
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