Accumulation Characteristics And Historical Inversion Of Heavy Metal In Reservoir Sediments Of Pb-Zn Smelting Area

Northwestern Guizhou,located in the karst region of southwestern China,is a typical area of high geological background of heavy metals with superposition of pollution from lead and zinc smelting.Heavy metals in sediments of the regional water system are not only affected by natural weathering but also by the historical legacy of clay-based zinc smelting and agricultural activities,which have induced severe accumulation of heavy metal.Therefore,it is urgent to study the accumulation characteristics of heavy metals in reservoir sediments,analyze their sources,and evaluate associated ecological risks.In this study,Maoshui reservoir（MS）,located in typical soil-refinery zinc pollution area,is set as the research object,and Yangwanqiao reservoir（YWQ）,located in the geological high background area（no pollution sources）,is regarded as the control.Furthermore,enrichment coefficient,ground accumulation index,and continuous chemical extraction are used to analyze the sediment heavy metal accumulation characteristics and evaluate the ecological risk.Then PMF model combined with isotope tracing are used to identify sources of heavy metals.Finally,isotope dating is utilized to inverse the sedimentation history.The results are shown as follows:（1）The contents of heavy metals Cd,As,Pb,Cr,Cu,Ni,and Zn in the surface sediments of MS reservoir are 10.30～27.30 mg/kg,41.90～97.70 mg/kg,515.10～1318.60 mg/kg,19.10～142.00 mg/kg,87.20～223.00 mg/kg,36.90～67.80 mg/kg,and 1723.80～3735.70 mg/kg,repectively.Thereinto,the average contents of Cd,Pb,Zn,As and Cu are 4.62,13.97,11.36,2.74and 2.43 times of that in YWQ reservoir,respectively.While the average contents of Cd,Pb,Zn and As in sediment column are 18.11,8.2,9.1 and 2.75 times of that in YWQ,respectively.Both results of EF and I_geoevaluation show that the accumulation of Cd,Pb and Zn in the sediment of MS reservoir were much higher than that in YWQ reservoir,and all sample sites are belong to strong accumulation.（2）The bioavailability of Cd,Pb and Zn in MS reservoir are relatively high,and their major chemical speciation are weak acid extracted state（68%）,oxidizable state（38%）,and reducible state（42%）,respectively.The evaluation results of RAC and SQGs show that the Cd,Zn and Pb in MS reservoir are characterized with high risk,medium risk,and low risk,respectively,and they are able to produce potential biotoxicity for all sample sites in MS.In comparison to YWQ reservoir,heavy metals in MS reservoir are relatively higher.While for YWQ reservoir,chemical speciation of heavy metals are primarily residual form-based except for Cd and Pb,indicating that these heavy metals have better chemical stability,which manifests that both their environmental risk and biological toxicity are relatively low.（3）The results of the analysis based on PMF model combined with correlation analysis show that heavy metals in MS reservoir sediments are influenced by multiple factors,including smelting activity-related sources（46.6%）,geological sources（20.7%）,atmospheric deposition&industrial activity sources（13.1%）,and agricultural sources（19.6%）.Comparing with YWQ reservoir,the sources of heavy metals in MS reservoir are feasible to be impacted by Pb and Zn smelting and agricultural activities,which is consistent with results of Pb and Cd isotopic tracing that contribution of smelting activities for Pb and Cd are 43.9%and 43.7%.（4）The dating results show that MS reservoir sediments have recorded sedimentation history over 40 years,and it`s average sedimentation rate is 1.0 cm/a.The change characters of heavy metals in sediment are corresponding to the time of mining and smelting activities.Furthermore,the most obvious accumulation of heavy metals appeared around 1994,a peak season of indigenous zinc smelting activities,and low values of content in the initial period（around 1984）and after prohibition（after 2004）of indigenous zinc smelting.Heavy metals in reservoir sediments in the Pb-Zn smelting area are characterized with high concentration（e.g.Cd,Pb and Zn）,high mobility,and high ecological risks.Inversion results show that heavy metals mainly originate from historical Pb-Zn smelting activities and geological sources.Therefore,it is necessary to treat pollution of regional reservoir sediments and take measures of risk controlled.