| Due to its high toxicity and difficult degradation,heavy metals were widely found in the soil environment.They enterd organisms through the soil-vegetation ecosystem and accumulated in the body for a long time,ultimately endangerd human health.In this paper,the soil vegetation system of Tianlaochi catchment in Qilian Mountains was studied,the basin area was 12.8 km~2,280 soil samples(63 topsoil samples and 217profilesoil samples)of different vegetation types and 288 corresponding typical plant samples were collected.Heavy metal content was determined by laboratory experiments.Spatial interpolation,principal component analysis and pollution index were used to explore the distribution characteristics,source and ecological risk of heavy metals in the study area.The main research results are as follows:(1)The characteristics of heavy metal content in the catchment showed that the content of As,Cd,Pb,Mn,Cr,Cu and Zn in surface soil are respectively 3.00～12.5mg/kg,0.10～0.47mg/kg,35.75～103.28mg/kg,267.3～1060.1mg/kg,74.6～175.1mg/kg,21.4～74.9mg/kg,61.8～113.4mg/kg.Except As element,the content of heavy metal elements mostly exceeded the soil background value in Gansu Province,but did not exceed the screening value of soil pollution in agricultural land as the whole.The content of heavy metal elements in profile soils of different vegetation types and at the same depth varied significantly.The content of As,Cd,Pb,Mn,Cr,Cu and Zn in vegetation are respectively 0.01～7.60 mg/kg、0.13～0.36 mg/kg、30.34～99.73 mg/kg、145.0～474.91 mg/kg、68.3～248.09 mg/kg、4.08～20.95 mg/kg、60.09～100.62 mg/kg.It indicated that a certain amount of heavy metals have accumulated in vegetation.(2)The spatial distribution of heavy metals in the catchment showed that As,Mn and Cr was high in the northwest and southeast and low in the central region,Zn was low in the northwest and southeast and high in the central region,Cd was low in the northwest and high in the central and southeast,Pb and Cu were zonal and annular.Overall,the soil environment in the study area was complex and the distribution of heavy metal elements had significant heterogeneity.(3)Correlation analysis between soil physical and chemical properties and heavy metals in the catchment showed that there was a significantly negative correlation between soil p H and Cr in the catchment.Soil conductivity EC was significantly negatively correlated with Cd and significantly positively correlated with Mn.The content of soil carbonate was positively correlated with Pb.Soil organic matter content was negatively correlated with Pb.Soil physical and chemical properties had a certain effect on the distribution of heavy metal Pb in surface soil,but had little effect on the distribution of other elements.The altitude was positively correlated with Cd,Mn and Cr in the surface soil of the study area,and negatively correlated with Pb and Zn,indicating that altitude had a significant effect on the distribution of heavy metal elements in the soil of the study area.Altitude also had influence on the distribution of heavy metals in vegetation.(4)The correlation analysis of seven heavy metal elements in the soil of the catchment showed that As,Cr,Zn were significantly positively correlated,Pb,Cu,Cd were positively correlated.Correlation analysis of seven heavy metals in vegetation showed that As,Cd,Pb had extremely positive correlation,Mn,Cr,Cu,Zn were positively correlated.Three principal components were extracted from the principal component analysis of soil heavy metals in the catchment,which showed that the source were natural source,transportation source and agricultural source.Two principal components were extracted from the principal component analysis of the vegetation heavy metal elements in the catchment,which showed that the source were natural and agricultural.(5)The evaluation of pollution in the catchment:When the soil environmental background value of Gansu Province was taken as the evaluation standard,the soil heavy metal enrichment factor,geological accumulation index and Nemerow comprehensive pollution index showed that the overall pollution level of the study area was between slight and moderate.Taking the screening value of agricultural land soil pollution as the evaluation standard,each index factor showed that the whole study area belonged to the uncontaminated level.The potential ecological risk index showed that the ecological risk in the study area was slight.On the whole,soil quality in the study area was good.It was suggested that while ecological environment monitoring in the study area should be carried out,heavy metal content monitoring in soil and vegetation should be strengthened. |
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