Spatial Variability Analysis Of Soil Heavy Metal Pollution Based On PXRF

Urban soil is frequently disturbed by human activities,resulting in increasingly serious heavy metal pollution,which directly or indirectly affects urban environmental quality and human health.As a densely populated city,Nanjing is also an important industrial base in China and is rich in mineral resources,so it is easy to be polluted by heavy metals.Most of the existing studies on soil heavy metals in Nanjing focus on partial area or land for single use.In this study,the heavy traffic area,industrial area,leisure area and farmland area of Nanjing City were taken as research subjects,and the contamination status of heavy metals of the surface soil in Nanjing including Cr,Ni,Cu,Zn and Pb were analyzed.The spatial heterogeneity of these heavy metal elements in the region were evaluated by using geostatistics,so as to provide a basis for the prevention and control of soil heavy metal pollution in Nanjing.The major results were summarized as follow:（1）The mean concentrations of Cr,Ni,Cu,Zn and Pb in the soil of Nanjing were 112.8,39.2,49.1,126.3 and 37.3 mg/kg,respectively.Examined on the basis of the China Soil Environmental Quality Standard（GB15618—2018）,the excessive concentration rates of heavy metals decreased in the order of Pb>Cu>Zn>Cr>Ni,among which Pb pollution is particularly serious.The contents of Pb have exceeded the set standard by 44.95%,and the relatively high heavy metal values were found concentrated in industrial and traffic areas.Ni had almost no pollution,and only one sample in the leisure area exceeded the set standard.The normal test showed that the five heavy metals were all moderately variable,indicating that the content of each element relatively discrete,which is greatly affected by artificial random factors.（2）The contents of five heavy metals were isotropic.On the basis of structural analysis of semi-variance function,Cr、Cu、Zn and Pb are fitted by exponential model,while Ni with gaussian model as the best.Spatial variability indicator C₀/（C₀+C）of the five heavy metals are in the range of 0.501～0.901,of which Cr and Pb belong to moderate spatial auto-correlation,Ni,Cu and Zn belong to weak spatial auto-correlation.（3）Geostatistical module analysis revealed that the heavy metal content showed some spatial heterogeneity in the topsoil（0～20cm）of Nanjing City.The distribution of the five heavy metals differed greatly,it was high in the north and low in the south,and the high value areas of heavy metal were mainly distributed in traffic hinge and industrial cluster.The high value areas in the heavy traffic area were concentrated in the main urban area.The spatial distribution of five heavy metals of industrial area showed a decreasing trend from north to south,and the high value areas were mainly distributed in the industrial area with earlier pattern formation.Although Yeshan Iron Mine has been transformed into Yeshan National Mine Park,the legacy of heavy metals remains unsolved,and most of the high values in the farmland area occur at the intersection of the Nanjing border.There were significant differences in the spatial distribution of the five heavy metals within a single administrative region.For the same heavy metal,the spatial distribution among different administrative districts was also significantly different.Only Cu and Zn in Gulou District,Cu,Pb and Zn in Jianye District,Cu,Ni,Pb and Zn in Pukou District,Pb and Zn in Yuhuatai District and Pb and Zn in Lishui District show similar spatial distribution characteristics and high spatial correlation..（4）Performing pollution assessment by fuzzy comprehensive evaluation which based on AHP to recognize the state of soil pollution in Nanjing.The results indicated that most of the sites studied belonged to class I（71.1%）,56 sites occurred in classⅡ（25.69%）and 7 sites fell into classⅢ（3.21%）.Percentage of samples belonging to classⅠin each functional area were classified as follows:farmland area>leisure area>heavy traffic area>leisure area,among them,industrial area was more seriously polluted,and more than half of the sample sites（58.67%）belonged to the polluted classes（classes II and III）.In the farmland area,91.44%of the sites belonged to class I,and all of these sites exhibited membership degree values of class I higher than 0.85,suggesting that most of the soil in the farmland area was clean.Among the 11administrative areas,Jianye District is relatively clean.Except for Qinhuai District and Gulou District,which were slightly polluted（class II）,the other administrative districts belonged to class I,however,the samples belonging to class III were all located in Yuhuatai District,Qixia District,Pukou District,Luhe District and Lishui District,which as a whole were relatively clean.（5）The potential ecological risk showed that Pb was the major pollutant,which posed a high ecological risk,while Ni and Cu belonged to moderate ecological risk,Cr and Zn were low ecological risk.The mean value of the comprehensive potential ecological risk index（RI）was36.27 at the critical level of moderate and considerable ecological risk.The four functional areas belonged to moderate ecological risk,among which,there is no low ecological risk sample in the heavy traffic area and the industrial area,and the pollution is more serious.Each administrative district belongs to the moderate ecological risk.According to the spatial distribution map of RI values,the high value areas of the comprehensive ecological risk index were significantly affected by industrial clusters,and was similar to the spatial distribution pattern characteristics of heavy metal content and fuzzy comprehensive evaluation classification.