The Spatial Distribution Of Heavy Metals In Farmland Soil And Ecological Risk Assessment Around The Galvanizing Plant

With the development of the industry,a large number of waste water,residue and gas containing heavy metals of without treatment enter into the farmland environment,leading to the farmland soil suffering varying degrees of pollution.Through the field research,arranged the sampling sites reasonably and measured the physico-chemical properties and the content of the heavy metals in the soil.Based on the geostatistical analysis methods and took the GIS as platform,analyzed the heavy metal characteristics of spatial variability in the area.By using the single factor pollution index,Nemerow composite index,evaluated the soil environment quality in the area.Combining with toxicity coefficient of the heavy metal and potential hazardous index method,analyzed the potential ecological risk.The main conclusions can be divided as follows:Through the analysis and measuring of the content of soil heavy metals in area,it was found that the range of change of Zn,Cd,Cu,Pb,Mn and Ni respectively was 114.17-7190.0 mg·kg^-1,0.95-49.48 mg·kg^-1,25-1355.83 mg·kg^-1,3.67-251.83 mg·kg^-1,142.78-1248.0 mg·kg^-1?11.89-744.33 mg·kg^-1;the mean of them were 533.83 mg·kg^-1,0.95 mg·kg^-1,91.89 mg·kg^-1,27.29 mg·kg^-1,491.44 mg·kg^-1and 58.93 mg·kg^-1.In addition to Mn and Ni,the contents of Zn,Cd,Cu and Pb exceeded the local background value.The accumulation of heavy metal in soil was relatively common.According to the correlationship between the six heavy metals in soil,analysis shows that:Zn,Pb,Cu,Cd,and Ni are present in the extremely significant positive correlation with each other,and the correlation coefficients were 0.740,0.770,0.791,0.808 respectively.It shows the close relationship among these elements.Therefore,we could speculate these heavy metals may be from the same source or industrial activity.The correlation between Mn and other several heavy metals is not apparent,and presumably Mn may come from a variety of other sources.Using geo-statistical methods to study spatial variability characteristics of six kinds of soil heavy metals in the studied area,we found that spheroidal model is the best choice for Cd,Zn,Cu and Ni,while Mn and Pb fit exponential model better.The Co/?C+C_O?values of Zn,Pb,Cu and Ni were 59.58%,31.15%and 32.13%,shows that these three heavy metals has a medium spatial correlation.In addition to the structure factor,human factor was a source which lead to the spatial distribution difference.The Co/?C+C_O?values of Cd,Mn and Ni were 10.97%,6.29%and 11.86%,shows that these several heavy metals has a strong spatial correlation.The strong spatial correlation of soil was usually controlled and determined by internal factors,indicating that random factors had little influence on the distribution of these three heavy metals,while structural factors play a leading role in their spatial differences.Choosing the best semi-variogram,using ordinary Kriging method to measure the concentration of heavy metals in soil,shows the spatial distribution in the studied area.Zn,Ni and Cu had a similar spatial distribution.Near the galvanized factory and in the north of it,there were high concentrations.High content area of Cd in the central were mainly distributed in the central section,the southwest and northern,and the low concentration areas were mainly located in both the east and the west sides.High concentration of Pb mainly concentrated in central and the northeast areas.The pollution level of Mn was minimized,and the distance was relatively far from the galvanized factory.The assessment of heavy metal pollution shows that:Cd in the whole area was in heavy pollution levels.The pollution of Zn was also in pollution status except the western edge of the area which showed clean level.The contamination of Cu was centered on the northern of the factory,and there were mainly light pollution and heavy pollution in the area.The distribution of high value of Pb content was located in the middle part of the study area which reached moderate pollution and severe pollution level,but it had not yet to reach polluted level.The pollution area of Ni was mainly concentrated in the central and the northern edge,which was similar to Cu,most of all in a state of mild pollution or clean except the areas where near the factory reached serious pollution.In addition,the content of Mn in the whole area was not exceed the content of soil background value,which was in a clean state.Using Hankanson method to research the potential ecological risk of heavy metals in the soil,the result indicates that:the regional ecological risk was mainly concentrated in the southern and central regions,and the eastern and western region ecological risk were relatively weak,but also reached the medium ecological harm level.