Study On The Distribution Characteristics Of Surface Soil Magnetic Susceptibility And Heavy Metal Pollution

With the acceleration of urban modernization construction,soil heavy metal pollution is gradually receiving attention.Soil magnetic susceptibility as the research system of soil heavy metal pollution beneficial urban soil heavy metal pollution.This study takes Nanjing City as the research area,with the city as the center,extending to the surrounding areas,and dividing it into seven sample areas.The topsoil 0-6cm soil was collected and divided into three layers of 0-2,2-4,and 4-6 cm to compare with the mixed soil samples.Based on basic statistical analysis,Arc GIS geostatistical analysis,soil pollution assessment,and geographic detectors,data analysis was carried out to detect the change characteristics of different layers between soil magnetic susceptibility and heavy metal element content in Nanjing.The research results indicate that;（1）The data of soil magnetic susceptibility in the study area are discrete,and regional differences are obvious.Most areas contain Superparamagnetism particles.There are four situations in space:double high for x_lfand x_fd%,high for x_lfand low for x_fd%,double low,and low for x_lfand high for x_fd%.The first,sixth and seventh sample areas have less soil pollution,the sixth sample area has magnetic anomaly areas,and the second,third,fourth and fifth sample areas have obvious magnetic anomaly areas,most of which are industrial sources,traffic sources and areas with high soil development.After stratification,there are differences in the spatial distribution between each level and the mixed sample spatial distribution.The higher the x_lf,the more significant the interlayer differences.The interlayer difference of x_fd%is stronger than the mass magnetic susceptibility.（2）The concentration data of eight soil heavy metal elements in the study area have a high degree of dispersion,and the degree of variation belongs to medium or strong variability.The average value is higher than the background value of Nanjing city,and the peak value is2-5 times the background value.Spatial analysis showed that the second sample area Baguazhou formed surface pollution of heavy metals in soil.The main sources of heavy metal pollution in the soil of the region are the three wastes from upstream lead zinc mines and steel smelting enterprises,agricultural fertilization,and plastic film.The fourth sample area is contaminated with Fe elements at the Hongshan Zoo,and the Zhongshan Scenic Area has formed varying degrees of Cu,Pb,Mn,and Zn enrichment.Cr is enriched at 4-6 cm,while Mn is enriched at 0-2 and 4-6 cm.Fe,Ni,and Pb elements are enriched in all three levels.The overall evaluation of heavy metal pollution in the soil of the research area belongs to light to moderate pollution,with some areas experiencing severe Mn pollution.There is moderate to severe pollution of Cd,Cr,Pb,Fe,and Zn in the 0-2 cm topsoil.Ni and Fe elements belong to a lightly polluted state.The cross-sectional study shows that the I_geotrend in the study area can be roughly divided into three types:increasing first and then decreasing,gradually decreasing,and decreasing first and then increasing.The average I_geovalues of 8 soil heavy metal elements are basically the same as those of the 2-4 cm layer.（3）Correlation analysis shows that the magnetic susceptibility x_lfof soil quality is negatively correlated with the frequency magnetic susceptibility x_fd%.The content of heavy metals in soil is positively correlated with mass magnetic susceptibility,and negatively correlated with frequency magnetic susceptibility.The correlation between Mn,Zn,Fe elements and soil xlf is strong,while the correlation between Cd,Cu and x_lfis weak.The correlation between soil x_fd%and 8 soil heavy metal elements is weak to strong,with Zn（-0.51）,Cd（-0.41）,and Cu（-0.47）having a strong correlation.89%of the correlation results between heavy metal elements in soil are positive.There is a significant correlation between Zn Cd,Zn Cu,Fe Ni,Ni Cr,and Cu Cd.The spatial correlation analysis showed that the spatial distribution of soil quality magnetic susceptibility and soil heavy metal element content was different.The spatial distribution trend of soil frequency magnetic susceptibility in the second sample area is similar to that of heavy metal elements Cd,Cu,Fe,Ni,Mn and Zn in six soils.The magnetic susceptibility of soil quality in the fourth sample area has spatial correlation with Fe and Mn.The frequency magnetic susceptibility of soil is negatively correlated with Fe and Mn in space.（4）The factor detector based on geographical exploration shows that the single factor of8 kinds of soil heavy metal elements has weak explanatory power q to the two indicators of magnetic susceptibility,and the influencing factors affecting the spatial distribution of soil magnetic susceptibility x_lfand x_fd%are not single soil heavy metal elements.The first and second explanatory power factors of soil quality magnetic susceptibility are Mn（0.24）and Cr（0.14）,respectively.7 kinds of soil heavy metal elements Cd,Cr,Cu,Ni,Pb,Fe,Zn have strong explanatory power to frequency magnetic susceptibility（q>0.5）.The explanatory power of heavy metal elements in different layers of soil to magnetic susceptibility varies greatly.Cd,Cu,and Pb have weaker explanatory power at all three levels.Cr and Ni have the strongest explanatory power at 2-4 cm.Fe and Zn have the strongest explanatory power at 0-2 cm.Mn has a stronger explanatory power for xlf in the 2-4cm layer,while its explanatory power for x_fd%is lower in the three layers（q<0.2）.（5）The interactive detection based on geographical detectors shows that the interaction of soil heavy metal element factors on magnetic susceptibility is strong.The interaction explanatory power of soil heavy metals on xlf is strongest in the 2-4 cm layer,and the interaction explanatory power of Zn∩Pb,F∩Cr,Fe∩Ni,Pb∩Cu,and Mn with any element is greater than 0.6.The 8 heavy metal elements in soil have an enhanced interactive explanatory effect on x_fd%at 0-2,2-4,and 4-6 cm（q>0.5）.The ecological survey showed that the significant difference between sub regions of the relative frequency magnetic susceptibility of eight heavy metals in soil was stronger than that of the relative mass magnetic susceptibility.Among the three levels,Mn has the strongest significant difference.In the C layer（4-6 cm）,the significant differences in the sub regions of eight soil heavy metal elements are stronger than those in the A（0-2 cm）layer and B（2-4 cm）layer.（6）The study of sampling interval shows that the peak value of magnetic susceptibility of surface soil of four samples is between 0～7 cm.Compared with 30 cm topsoil mixed sample,the increase of sampling interval is conducive to obtaining the accurate change characteristics of soil magnetic susceptibility of urban forest land.Effectively revealing subtle differences between different profiles and conducting pollutant tracing work.In the study of magnetic susceptibility of topsoil,a more precise sampling interval should be selected in combination with the soil conditions.