Magnetic Properties Of Urban Topsoil In Xinyang City And Their Indicative Implications For Heavy Metal Pollution

Urban topsoil is not only an important part of the urban ecosystem,but also a powerful carrier of pollutants in the urban environment,which has a profound impact on the ecological environment and residents’health.In this paper,130 topsoil samples from the urban area of Xinyang City were used as the research objects,and the methods of environmental magnetism,geochemical elements and chroma were used to firstly analyze the magnetic properties（content,type and magnetic domain of magnetic minerals）and material sources of urban topsoil,and secondly analyze the concentration characteristics,pollution status,and pollution sources of 8 heavy metal elements（arsenic（As）,cobalt（Co）,copper（Cu）,chromium（Cr）,lead（Pb）,nickel（Ni）,zinc（Zn）,and vanadium（V））in the urban topsoil,and finally explore the response relationship and indicative significance of magnetic and chroma parameters on heavy metal pollution.The results of the study show that:（1）The anthropogenic magnetic mineral input was strong in the urban topsoil of Xinyang City.The magnetic mineral content was about 2.5 times that of the background sample.The magnetic mineral type was mainly the low coercivity ferrimagnetic mineral magnetite.Moreover,the magnetic particles were coarse,mainly pseudo-single domain（PSD）and multi-domain（MD）.（2）The concentrations of heavy metals Cu,Zn,Pb and As were strongly enriched in the urban topsoil,while the concentrations of Co,Ni,Cr and V were weakly accumulated.1)The single-factor pollution index evaluation indicated that heavy metal elements Cu,Zn and Pb were heavily polluted,while As,Co,Ni,Cr and V were less polluted.2)The evaluation results of the Nemerow composite pollution index showed that the integrated pollution of heavy metals in the urban topsoil samples was relatively heavy,with 39%,51%and 40%of the samples being lightly,moderately and heavily polluted,respectively.3)The results of multivariate statistical analysis showed that the heavy metal elements in the urban topsoil were mainly from three pollution sources,among which,Co,Ni,V and Cr were mainly from natural sources,Pb and As were mainly from vehicle emissions,and Cu and Zn were mainly from traffic emissions（excluding vehicle emissions）.4)The results of the principal component analysis-multiple linear regression（PCA-MLR）model analysis showed that the contribution of natural sources was 6.3%,the contribution of vehicle emissions was 22.7%,and the contribution of traffic emissions（excluding vehicle emissions）was 71%.（3）The magnetic and chroma parameters were sensitive to the concentration of heavy metal elements and could effectively indicate the concentration of heavy metal elements from different sources.The parameters magnetic susceptibility（χ）,saturation isothermal remanent magnetization（SIRM）and soft isothermal remanent magnetization（SOFT）,which reflect the absolute content of magnetic minerals,could positively indicate the concentrations of heavy metals Cu,Zn,Pb and As,which were dominated by anthropogenic sources,and the parameters percentage frequency-dependent susceptibility(χ_fd%)and ratio parameters(χ_arm/χandχ_arm/SIRM),which reflect the magnetic domains of magnetic minerals,could positively indicate the concentrations of heavy metals Co,Ni,Cr and V,which were dominated by natural sources.Meanwhile,the chroma parameters lightness（L^*）,redness（a^*）and yellowness（b^*）could negatively indicate the concentrations of heavy metals Zn,Pb and As,which were mainly from anthropogenic sources,and a^*and b^*could positively indicate the concentrations of heavy metals Co,Ni,Cr and V,which were mainly from natural sources.