Study On Pollution Characteristics Of PCBs, PAHs And Heavy Metals In Sediments From Dianchi Lake

Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and toxic heavy metals are considered as an important environmental issue that affecting human survival and health in the 21st century for their persistance, bioaccumulation and high toxicity, which has gradually become a research hotspots. Simultaneously, they are easily absorbed by suspended particulate matters when flowed into water environment because of their obvious characteristics of hydrophobic lipophilic, and then enter into sediments through deposition. The contents of PCBs, PAHs and heavy metals in sediments tend to be several orders of magnitude higher than that in the aqueous phase. Therefore, sediments are considered as indicator and can objectively reflect the pollution level of contaminants in the aquatic environment. As a consequence, it will be of significance to research the pollution characteristics of PCBs, PAHs and heavy metals in the sediments. However, the qualitative and quantitative analysis of PCBs and PAHs become a research emphasis and difficulties as the complexity of environmental sediment, leading to no standard analysis methods in domestic.Dianchi Lake is a shallow plateau lake that has always been classed as comprehensive restoration water areas in China, and the eutrophication was the research focus of water pollution control.However, the research on PCBs, PAHs and heavy metals in sediments of the lake are very limited. Therefore, it is difficult to reflect the overall pollution situation of Dianchi Lake, possibly resulting in lack of guidance for the pollution prevention of PAHs, PCBs and heavy metals. Given this, twenty-eight PCBs, sixteen PAHs in eight sediment cores and surface sediments collected from twenty-three sites of Dianchi Lake and eighteen sites of estuaries around this lake were simultaneous extracted, purified and detected using accelerated solvent extraction (ASE), solid phase extraction (SPE) and gas chromatography-triple quadruple mass spectrometry (GC-QqQ-MS/MS).And seven heavy metals were analyzed by atomic fluorescence spectrophotometer and graphite furnace atomic absorption spectrometer. The contamination level, distribution characteristic, source apportionment, ecological risk assessment and historical trend were researched for the purpose of determining the main pollutants and providing a scientific basis for the pollution treatment of PAHs, PCBs and heavy metals.(1)By optimizing the pre-processing conditions, a method for the simultaneous extraction, cleanup and determination of 28 PCBs and 16 PAHs in sediments has been developed by using GC-QqQ-MS/MS. The detection limits of PAHs and PCBs were in the range of 0.07-0.45 ng/g, 0.001-0.08 ng/g, and the limits of quantitation were 0.24-1.67 ng/g,0.003-0.25 ng/g, respectively. The average recoveries of the spiked samples ranged from 70.4% to 124.7% with RSD (n=6) of 1.1%-12.8% for PAHs, and from 95.6% to 125.7% with RSD (n=6) of 0.7%-6.4% for PCBs.It showed that the method could simultaneously detect PAHs and PCBs in sediment with good extraction and purification effect, high accuracy, good repeatability and less solvent consumption.(2) The contents of the total 28 PCBs (EPCBs) in surface sediments ranged from 160 to 78213 ng kg-1 dry weight (d.w.)(mean:5487 ng kg-1 d.w.) in Dianchi Lake, from 266 to 68183 ng kg-1 d.w.(mean:11012 ng kg-1 d.w.) in the estuaries around the lake. In general, the concentrations of IPCBs in Caohai section and its estuarieswere higher than those in Waihai section and its estuaries. Meanwhile, in the Waihai section, the highest PCB concentration was found in the north area, followed by that in the south and middle area. PCBs entered into Dianchi Lake through scour of the inflow rivers. And the distribution and basin of the rivers had largely impact on the level and distribution of PCBs in the sediments of Dianchi Lake. The TOC was not the main factor influencing the contents of PCBs. The content of EPCBs ranged from N.D. (Not detected) to 8487.95 ng kg-1 d.w.in sediment cores. The content of PCBs was highest in the surface of sediment, and the content decreased with the depth of sediment. Overall, the contents of PCBs were high between the depth of 0 and 30 cm, and the PCBs couldn’t be detected under the depth of 60 cm. The content variation and average content of PCBs in C1 cores were higher than the other samples. As in the surface sediments, the highest content were always found in the northern section, followed by southern and middle section. The congeners of PCBs containing 3-6 chlorine atoms accounted for 4.9%-97.9% of the total PCBs in surface sediments, and 85.60%-91.43% in sediment cores. Therefore, those less chlorinated aroclor mixtures containing 3-6 chlorinated atoms were the dominant compositions of PCBs. In the surface sediments, the contents of twelve DL-PCBs ranged from ND-15958 ng kg-1 d.w.(average:1013 ng kg-1 d.w.). The Caihai section and the north of Waihai section had the high contamination levels of DL-PCBs, which could pose a potential risk to aquatic organisms.Meanwhile, the content of TEQ of PCBs in the north of Caohai section was also high (R1:26.57 ng TEQ kg-1 d.w.; S1:18.38 ng TEQ kg-1 d.w.).This region was greatly influenced by the potential toxicity of PCBs, and the other regions of Dianchi Lake were less affected by PCBs. The ecological risk of PCBs in sediment cores was small.(3) The total PAH concentrations(EPAHs) in surface sediments varied from 210 to 11,070 ng g"1 dry weight (d.w.)(mean:1543 ng g-1 d.w.) in Dianchi Lake, from 230 to 12271ng g-1 d.w.(mean:3144 ng g-1 d.w.) in the estuaries around the lake. The PAHs content in the north of Caohai section was a relatively higher than that in other areas of this section, and Caohai section was higher than Waihai section. In the surface sediments of estuaries around Dianchi Lake, the highest content of ∑PAHs(R1:1227 ng g-1 d.w.) was found at the estuary of Laoyunliang River, while the lowest content of ∑PAHs(R13:230 ng g-1 d.w.) was found at the estuary of Baiyu River. And the average contents of ∑PAHs in the estuaries of Caohai and Waihai were 5468 and 2243 ng g-1 d.w., respectively. Obviously, the content of PAHs in the estuaries of Caohai section was higher than that of Waihai section. It indicates that the river runoff was the major input pathway of PAHs in Dianchi Lake. Correlation analysis results showed that the TOC played an important role in the distribution; transfer of LPAHs, while the influence on HPAHs was small. The content of ∑PAHs ranged from 38 to 2506 ng g-1 d.w. in the sediment cores. In general, the content of PAHs was highest at the surface sediment, and the content decreased with the depth. Similar to the distribution characteristics of PAHs in surface sediments, the highest of PAHs in the sediment cores were found at the north of Dianchi Lake, followed by south and middle. The high molecular weight PAHs (HPAHs) were dominant in sediments of Dianchi Lake, indicating the biomass(grass, wood) and coal combustion were the primary sourcesof PAHs. These results werebasically in agreement with the social and economic development of Dianchi basin. The contents of PAHs at the south of Caohai and its estuaries(S 1, R1, R3) and estuaries of Waihai (R8 and R9) in surface sediments exceeded the effects range low (ERL), indicating adverse biological effects would occur occasionally. However, the lower values of PAHs in the most of areas would probably not cause adverse biological effects. The toxic equivalent concentrations of potentially carcinogenic PAHs (TEQcarc) at the south of Caohai (S1), the estuaries of Caohai (R1、R2、R3)and Waihai (R11) exceeded 400 ng TEQ g-1 d.w.,suggesting a high potential ecological risk.BaA, BbF, BkF, IP and dBanA were the major contributors to carcinogenic potential of PAHs,they should be paid attention to reduce the potential risk for human health.The potential ecological risk of Waihai section of Dianchi Lake was small.(4) Seven heavy metals such as As, Pb, Cd, Cr, Cu, Zn and Hg in surface sediments from Dianchi Lake and its esturaies were analyzed. Index of geo-accumulation and potential ecological risk index were utilized to assess the pollution level and potential ecological risk of heavy metals. The results showed that the highest content of heavy metals in surface sediments was found in the north of Caohai section. The overall intensity of heavy metals pollution in Caohai section and its estuaries was higher than that in Waihai section and its estuaries. According to Environmental Quality Standard for soils, As and Cd were the main pollutants in sediments. The results of index of geo-accumlation assessment showed that Cd was the main pollutant with asevere pollution level, while other heavy metals were less than the moderate pollution level in surface sediments of Caohai section and its estuaries. However, all heavy metals were from uncontaminated to partial moderate pollution levels in Waihai section and its estuaries. The results of potential ecological risk assessment showed that the overall high potential ecological risk was found in Caohai section and its estuaries,while low risk levelwas found in Waihai section and its estuaries. The primary heavy metal pollutant with the largest contribution for the ecological risk indices (Rj) was Cd, followed by Hg, As, Pb and Cu. While slight potential ecological risks were caused by Zn and Cr which had a low contribution for Rj. The analysis of correlations and principal components showed that Cu, Pb, Zn, Cd, Hg and As in Dianchi Lake could have similar pollution sources.