Spatial Distribution,Sources And Pollution Assessment Of Heavy Metals In Small Scale Farmland Soil

Heavy metal pollution in farmland soil has become one of the important problems in agricultural environment research.Therefore,heavy metals in farmland soil is investigated and analyzed,and the spatial distribution and Simulation,source and pollution assessment of heavy metals were researched by using multivariate statistics,spatial analysis and geostatisties methods.Which is practical significance to prevent deepening of heavy metals pollution and ecological hazards,to protect human health and scarce land resources.Topsoil(0-20 cm)samples from 45 sites in a 96000 m2 farmland were collected in Bijiang River watershed of Yunnan province and the contents of total Cd,Zn,Pb,As,Cu,Cr and Ni were measured by ICP-MS,Elemental C,N,etc.were determined by elemental analyzer.Firstly,the methods of statistical analysis,semi-variance function structural analysis,directional analysis and trend analysis are respectively used to analyze the contents of seven heavy metals in farmland soil.Then,spatial interpolations of seven heavy metals' contents were made by using certainty spatial interpolation and kriging interpolation.At the same time,the precision of every kindsof spatial interpolation models were computed and contrasted,then the optimal spatial interpolation model was selected to made the distribution maps of these seven heavy metals.Secondly,combined with multivariate statistics and spatial analysis techniques to analysis the pollution sources of seven heavy metals.Finally,Pollution Assessment of seven heavy metals by using Nemerow comprehensive pollution index method and potential ecological risk index method,and using the risk factors of the two assessment methods for spatial interpolation to analyze the spatial distribution characteristics of risk factors.The main research contents and conclusions are shown as the following:(1)According to the statistical analysis,the results showed the average contents of Cd,Zn and Pb in farmland soil exceed the local background values.It indicated Cd,Zn and Pb have been concentrated in the farmland soils.Examined on the basis of the National Soil Environmental Quality Standards(Class ?),the contents of Cd and Zn have respectively exceeded the set standards,other five heavy metals are not exceeded.Cd was exceeded also associated with its higher background value(1.70 mg/kg)in addition to the possible human pollution.Examined on the basis of the Environmental Quality of Edible Agricultural Products,the contents of Cd,Zn,Pb and As have respectively exceeded the set standards by 100%,95.6%,100.0%and 4.5%.The total variance of the seven heavy metals is moderately variable and the total variances trend of them were:Zn>Cd>Pb>As>Cr>Cu>Ni.The normality test showed that except for Cd could not pass the test and the other six heavy metals were normal distribution through transformation.(2)The contents of seven heavy metals were anisotropic.Therefore,it is necessary to consider the direction effect when using Kriging interpolation.(3)According to the trend analysis,surface trends of Cd,Zn,Pb and As are obviously third-order polynomial trend.Cr and Cu are obviously second-order polynomial trend.Ni is obviously first-order polynomial trend.(4)On the basis of structural analysis of semi-variance function,the semi-variance functions of seven heavy metals,Cd,Zn and Cu are fitted by Spherical model,while Pb and As are fitted by exponential model,Ni is fitted by Gaussian model.Spatial variability indicator o/(C+C0)of Cd,Zn,Pb and As are in the range of 0.38?0.51 and all belong to moderate spatial auto-correlation.Co/(C+C0)of Cu and Ni are respectively 0 and 0.15 and both belong to strong spatial auto-correlation.(5)Interpolation precision is compared by different interpolation methods including certainty spatial interpolation and kriging interpolation.The results showed among all the interpolation methods,the optimal interpolation methods of Cd and Ni are respectively the third-order polynomial trend of ordinary kriging and the second--order polynomial trend of ordinary kriging,Pb,Zn and As are all the third-order polynomial trend of disjunctive kriging.Cr is the first-order polynomial trend of global polynomial.Cu is the first-order polynomial trend of disjunctive kriging.(6)Cd,Zn,Pb and As are highly correlated and Cu,Cr and Ni are highly correlated based the statistics analysis results of SPSS software,which indicated that Cd,Zn,Pb and As are from the same pollution source and Cu,Cr and Ni are from the same source.There was a significant positive correlation between pH and Cd,Zn,Pb,but weak correlation with Cu,Cr and Ni,and no significant correlation between C,N,O and other heavy metals in farmland soil.The results showed contents of heavy metals in soil was less affected by soil physical and chemical properties.The results of regression analysis showed that the contents of Cd,Pb and Zn were extremely related.However,As is not only affected by Cd,Pb and Zn,but also affected by other heavy metals and random factors.Cr,Cu and Ni were extremely related.However,Cr is not only affected by Cr and Cu,but also affected by other heavy metals and random factors.The principal component analysis showed that the first principal component(C1)is related with the group of factors set Cd,Zn and Pb,which could be classified as "traffic factor" and is mainly affected by transport.The second principal component(C2)is Cu,Cr and Ni,which could be classified as "natural factors" and is mainly affected by the role of geochemistry.(7)According to Nemerow pollution index(NPI)method,37.78%of the study area was moderately pollution and 62.22%were heavily pollution.NPI spatial interpolation diagram shows heavily pollution area is distributing along the provincial road 227.The moderate pollution area distributed in the middle of the study area and near the river.The proportion of different pollution levels to the study area:Strong risk(57.78%),strong risk(13.33%)and strong risk(2.2%).Cd is the main ecological risk factor of study area.The strong risk areas are mainly distributing along the provincial road 227 and part of eastern of residential area,the rest of the region are moderate risk according to ecological risk index.We studied on spatial distribution,sources and pollution assessment of heavy metals in farmland soil based on spatial analysis techniques,geostatistics methods and multivariate statistics.We make attempt to reveal the spatial distribution and variation,sources and pollution assessment of seven heavy metals in the study area.The results will provide scientific basis for soil heavy metal pollution prevention,effective guidance for land use choice and ecological risk assessment in a small scale farmland.