Spatial Variability Characteristics Analysis Of Heavy Metals In Urban-rural Agricultural Soil Focusing On Anthropogenic Influence

Heavy metal pollution in agricultural soil is becoming conceemed for its affection on food security and sustainable development of agricultural economy.Spatial variation characteristics analysis has important value on the study of evaluation,prediction and management of heavy metals.With the rapid development of industry,agriculture and urbanization in China,anthropogenic sources are becoming main source of heavy metal pollution in agricultural soil.An accurate judgement of spatial variation characteristics of heavy metals determined by anthropogenic factors is important for pollution control.But there are not adequate researches about that.Firstly,many studies ignored pollution resulted from nature which was main source of heavy metals in soil.As a result,anthropogenic pollution may be overestimated and corresponding policies restrict local factories.Secondly,many scholars focus on improving accuracy of spatial distribution prediction of heavy metals,but if the samples were not representative,the spatial variation characteristic would not be revealed completely.Thirdly,in the complex anthropogenic factor influence zone,the urban-rural hinge for example,spatial variation analysis used geostatistical method would not be applied as espect.Lastly,auxiliary information may be effective for spatial variation,but how to choose proper information according to the study should be paid special attention.A total of 109 samples of Cu,Ni,Zn,Pb,Hg,As,Cr and Cd is collected in the paddy fields of Zhetang town,an urban-rural hinge area at the south of Nanjing,China.The study try to get explanatory power of the samples and to optimize the sampling seheme for different goals.Then contributions of the nature background and auxiliary informations to the distribution of the heavy metals are being discussed.And then influence of the anthropogeuic factors to the spatial variation of the heavy metals are being discussed.The result showed that:(1)Sampling optimization for improving prediction accuracy,which is carried out according to the priori knowledge,increases the density of sampling in the northwest,northeast and south region of the study area.At the 95%confidence level and the 5%relative error,the requirement of sampling number for each soil heavy metals in descending order:Cd>Hg>As>Pb>Cu=Ni>Cr?Zn.And the metal of Cd is chosen to determine later sampling number for its biggest variability.The result of validation of sampling optimization for improving later sampling efficiency shows that,the obtained sample could explan exactly the correlationship of Cr,Ni,Cu and Zn,and so do the metals of Pb-Zn and Pb-Cr.(2)Combined pollution of Pb and Zn locates at the northwest of the study area.Agricultural soil heavy metals of Cr,Ni,Cu and Zn show share common pollution source.In the middle east of the study area,Cr,Ni and C.u show combined pollution.The metal of Zn exists anisotropic and high correlationship spatial characteristics.The representative samples of As are mainly distributed in the main grain producing area in the northwest of the study area,and outliers of As are mainly distributed in the area of woodland.Hg shows spatial characteristic of isotropic and low spatial correlation,and shows random distribution.The metal of Cd shows share common pollution sources of both Cr-Ni-Cu-Zn and As,and its representative samples distributed in the northwest and middle of the study area.Pb,As and Cd are anisotropic and exist low spatial correlationship.(3)A total of 109 samples of Cu,Ni,Zn,Pb,Hg,As,Cr and Cd distributed in five map units(orthic yellow-brown soil,claypanic yellow-brown soil,hydromorphic paddy soil,gleyic paddy soil and Percogenic paddy soil)were collected in the Zhetang Town,a 71.2 kl2 paddy planting area of Nanjing city,China.The background value of the heavy metals in the study area was determined as Ni,23.5-29.Omg/kg;Cr,60.3-73.6mg/kg;Cu,33.6-44.5mg/kg;Zn,68.4-75.9mg/kg;Cd,0.139-0.362mg/kg;Pb,30.1-38.5mg/kg;As,10.6-18.Omg/kg;Hg,0.020-0.127mg/kg.Human factors have a significant impact of all the metals,especially for the metals of Pb,Hg and Zn.The regions of spatial variation affected highly of the metals of Cr,Ni and Cu by anthropogenic influence are located at the northwest region,and southeast region for As and southwest region for Cd.(4)In the research of spatial variation auxiliary information of soil heavy metals in the urban-rural hinge,the explanatory power ranges as soil type:0.044?0.118,DEM:0.104?0.426;land-use zoning:0.034?0.230;cluster analysis:0.101?0.335;cluster analysis based on principal component analysis:0.316-0.751.After comparation,the metod of cluster analysis based on principal component analysis shows better explanatory power to the spatial variation of heavy metals.Auxiliary information of soil types shows better understanding of Hg,but lower influence to the metal of Pb.And the explanatory powers of DEM for Cr,Cu,Zn and Ni were higher.The auxiliary information of land use shows a better understanding for As,but a lo?ver understanding for Cd in the urban-rural hinge.(5)In the study,the anthropogenic influence for the spatial variation was strengthened of soil metals after eliminating impact of the soil types factor.The factor of anthrogenic influence reduced the spatial variation of metals of Cr,Ni and Cu,and increase the range of high concentration area.For the metals of Zn?Pb?Cd and As,the influence of anthropogenic increase spatial variation of parts of the regions.For the metal of Hg,the anthropogenic influence show significant impact,and needed smaller scale sampling.In the main grain producing area northwest of the study area,combined pollution of Pb and Zn is mainly from farming operations,and the concentration level of the region is relatively low.The high concentration region has little relation with the combined pollution,and the high concentration regions of Pb and Zn resulted from different sources.