Temporal Variation Trend And Ecological Risk Assessment Of Heavy Metals In Sediments Of Dishui Lake And Its Inflowing Rivers

Located in Lingang new city of Shanghai, Dishui Lake is one of the largestartificial lake in China. The total area of Dishui Lake is about5.56km2. Watercoming from Dazhi river runs through Suitang river, Fram center river, Luchaoriver and many branches into the Dishui Lake. Heavy metals have importantpractical significance in lake pollution control due to their non-biodegradableproperties, toxicity and accumulative behavious. So we should their pollutioncondition, spatial and temporal distribution characteristics and sources in DishuiLake.From February2011to October2012, the sediments from the Dishui Lakeand its inflowings were collected once each quarter. The total number ofacquisitions was8times. Concentrations, spatial and temporal distribution,mode of occurrence of heavy metals (Cu, Cr, Pb, Zn, Cd, Hg and As) in the tenwell-distributed surface sediment samples points of Dishui Lake and itsinflowing rivers were analyzed by various domestic and overseas researchmethods. In addition, several evaluation methods which based on total contentand sequential extraction systems, were applied to assess the ecological risk ofheavy metals in Dishui Lake. The study can provide the basis of detailed data forwater quality management. And it is also an important part of the pollutionprevention and environmental protection. The main results were summed up asfollows.1. The heavy metal content in sediment in each kind of sampling metals was measured by Atomic Absorption Spectrophotometry. From2011to2012,the average concentration of7heavy metals as following: Zn (202.719mg·kg-1)>Cr (24.806mg·kg-1)> Pb (22.586mg·kg-1)> Cu (20.924mg·kg-1)> As(14.448mg·kg-1)>Hg (0.494mg·kg-1)>Cd (0.475mg·kg-1). The heavy metals inDishui Lake and its following rivers has the spatial and temporal distributionbecause of the different levels effect of artificial factor. As a whole, theconcentrations of heavy metals in sediments of the inflowing rivers were higherthan those in the lake (outside sluice river> inside sluice river> the lakedistrict). According to the Pearson correlation analysis, there was a remarkablepositive correlation among the heavy metals in the sediments of Dishui Lake. Atthe same time, the significant correlation between TOC and heavy metalsindicated that these elements combined with organic matter closely in thesediments. However, negative correlation between the TN and the heavy metalswere detected. So it declared that they have different source and antagonismeffect. The result of Principal component analysis(PCA) showed that the sourceof heavy metals could be reflected by the first two components that were F1(Cd,Pb, Cr, As, Cu, Hg, Zn, TOC,68.97%) which was controlled by human activityand atmospheric precipitation. Based on the second principal component, onlyTN displayed higher positive-load(11.810%). So the first principal componentshowed on behalf of the main pollution source in this study.2. The BCR(Community Bureau of Reference) sequential extraction wasemployed to research the different chemical forms of heavy metals in thesediments of Dishui Lake and its inflowing rivers. The results showed that the average values of Hg, As, Cr, Zn which the main fraction were residual phasewere0.296mg·kg-1、10.724mg·kg-1、15.706mg·kg-1、81.594mg·kg-1,and thenthey have weakly bioavailability. Cu gave top priority to residual phase (theannual average content was12.522mg kg-1) following by iron/manganeseoxide combination fraction (the annual average content was6.029mg·kg-1).What’s more, the value ofthe extraction fraction was hinger than residual phaseof Cd. And the main fraction were exchangeable and carbonate. Pb were majorin iron/manganese oxide combination fraction and residual phase in eachsampling points. Based on Pearson correlation analysis to research the five kindsof chemical forms of heavy metals,TOC and TN. The resulted that there werethe significant correlation between chemical forms and TOC. The reductionstate and oxidation state among all metals had remarkable correlation betweenTOC which influenced by organic matter expect Hg. It was detected negativecorrelation among TN, water-soluble fraction, weak acid fraction, reductionfraction and oxidation fraction of Cd, Hg.3. Based on the total concentration of metals in sediments, two differentmethods were applied to evaluate the pollution and ecological risk of metals ofDishui Lake. The Hakanson Potential Ecological Risk Index for heavy metalsshowed that serious potential ecological risk lied in the lake and inflowing riversediment with the average potential toxicity response index for various heavymetals (RI) of616.344. The potential ecological risk index of Hg (395.306) wasthe highest, followed by Cd (158.226), then As, Cu, Pb, Zn and Cr. Cd and Hgreached serious ecological risk. Therefore, Cd and Hg were the main potential ecological risk factor among the7heavy metals. On the spatial distribution, thepotential ecological risk index of heavy metals in sediments of the inflowingrivers were higher than those in the lake (outside sluice river> inside sluiceriver> the lake district). According to the indices SQG-Q (mean sedimentquality guideline quotient), the average index of SQG-Q for heavy metals was0.242-0.903which between0.1to1, so that it was characterized by moderatepotential adverse biological effects. The index of SQG-Q(0.903) which weshould pay attention was the outside sluice river(Sample point1).The result of RSP showed that the RSP contamination index of Cd was thehighest which average value was1.268in the study area while others wereappeared mild contamination. But sampling site1present moderate pollution.The pollution indices of these7heavy metals elements were as followsCd>Pb>Zn>Cu>Cr>Hg>As.The ecological risk assessment of heavy metals in Dishui Lake showed thatthe ecological risk based on the total amount of evaluation method was strongerthan thos based on the metal’s form. From the spatial distribution, the results ofthe two methods was basically same that outside sluice river> inside sluiceriver> the lake district. The evaluation method based on the total amount ofheavy metals have different results in different research areas dute to thescientific and rationality of environment background value, sediment qualitycriteria and other evaluation were difficult to guarantee. However, the evaluationof heavy metals based on the form can analysis the pollution situation of heavymetals, while also a effectively understand the transport characteristics, pollution sources, bioavailability and so on. Therefore, the ecological risk ofheavy metals should be comprehensive analysis through different evaluationmethods.