| The pollution of soil heavy metals in plain agricultural areas is related to the safety of agricultural product quality and the health of farmland ecosystem.The pollution assessment of soil heavy metals has become the focus of attention in the world and has attracted extensive attention of governments and scientists.In recent years,the situation of soil heavy metal pollution in China is very serious,and soil pollution has attracted much attention.China has promulgated the "Law of the People’s Republic of China on the Prevention and Control of Soil Pollution" to regulate the prevention and control,and reasonably evaluate the situation of soil heavy metal pollution in different regions,which has realistic and long-term significance.Jiangning District of Nanjing City is the largest agricultural by-product logistics center in East China and the second largest in China.There are few reports on heavy metal pollution in soil in this area before,so it is necessary to investigate and evaluate heavy metal pollution in this area.Based on 29 surface soils(0-20 cm)collected in the study area from November to December 2017,the soil pH,organic matter and eight heavy metal elements,arsenic(As),cadmium(Cd),chromium(Cr),copper(Cu),mercury(Hg),nickel(Ni),lead(Pb)and zinc(Zn),were analyzed and determined.The latest published Standard for Soil Environmental Quality-Risk Control of Agricultural Soil Pollution(GB 15618-2)was adopted.018).A more scientific and reasonable evaluation method for this area was obtained by comprehensive evaluation with various widely recognized evaluation methods at home and abroad.Combined with the technical characteristics and spatial analysis functions of GIS,the spatial distribution characteristics of heavy metals were analyzed by using GIS geostatistics method.On this basis,two spatial analysis methods of soil type and land use type were further explored.Effects of auxiliary information on heavy metals in soil.In this paper,the spatial distribution characteristics and sources of heavy metals in the surface soil of the plain agricultural area in the study area were studied,and the assistant information of spatial analysis and pollution assessment were analyzed.The following conclusions were drawn:1.Through the analysis of spatial distribution characteristics of eight heavy metals,it can be seen that the spatial distribution of eight kinds of heavy metals in the study area is uneven.Among them,the distribution of Cr,Ni,Pb and Zn in the soil is similar,and there is a banded distribution from northwest to south in the same risk area.Combined with the correlation analysis of the four kinds of heavy metals in the soil,it can be preliminarily concluded that there are homologies among the heavy metals Cr,Ni,Pb and Zn in the study area.This may be related to the fact that the belt area is located in the residential concentrated area,and the specific sources need to be further analyzed.In the central part of the study area,there are machinery factories,textile factories,power companies and other enterprises.Transportation and transportation can enable heavy metals produced in industrial processes such as smelting to enter the surrounding environment with waste residue,waste water and dust,and heavy metals in the air are transferred to the surface through sedimentation.In the layer soil,the heavy metal content even exceeded the background value of the soil in the study area.Therefore,it is necessary to strengthen the prevention and control of industrial wastes produced by enterprises in the study area,so as to reduce the pollution of soil and the harm to human body.2.For the study of spatial variability of heavy metals in Plain Agricultural area,the explanatory ability of different auxiliary information is different:Comparing the contents of heavy metals in soil types in the study area,the average contents of heavy metals Cd,Cr,Hg,Ni and Zn in the order of high to low are:paddy soil>yellow brown soil>Neolithic soil>stony soil,which has the same regular distribution and high explanatory power of spatial variability.By comparing the heavy metal content of land use types in the study area,the average content of heavy metals Cd,Cr,Ni,Pb and Zn from high to low is paddy field>vegetable field>dry land>forest land>mountain area,which has a uniform regular distribution and high spatial variability explanatory power.Improving the prediction accuracy of soil heavy metals through auxiliary information is of great significance for accurately evaluating the grade of soil heavy metals pollution and identifying the sources of soil heavy metals pollution.3.The pollution risk of heavy metals in soils in this area was analyzed by single factor index method and Nemerow comprehensive pollution index method in traditional assessment model,and improved cumulative assessment model.According to the latest National Standard for Soil Environmental Quality-Risk Control of Soil Pollution on Agricultural Land(GB 15618-2018),in the traditional evaluation model,the Cd,Cu and Hg elements in the soil of the study area reached the warning line in some areas and had the risk of pollution;in the cumulative evaluation model,the Cd in the study area was polluted in some areas,and the Hg was polluted in some parts.There is moderate to strong pollution in the area,and the overall ecological environment of the study area is in a good state in the improved cumulative land assessment model.Therefore,in the future work of soil heavy metal pollution assessment,it is necessary to make specific analysis of the specific situation and make practical work of soil heavy metal pollution assessment according to local conditions.4.Through the comparison of three evaluation methods,the single factor index method is simple and intuitive,which has good applicability for the determination of excessive single heavy metals in soil survey and the classification of risk level;the risk distribution rule of Nemerow comprehensive index method and single factor index method based on different criteria is similar,the former combines the maximum value and the level of single factor index.The average value of comprehensive risk assessment will weaken the risk index,which may have a certain impact on the overall situation of some good,but there may be a high content of individual elements in the regional determination;and the latter can better reveal the regional distribution characteristics of any elements exceeding the standard through the maximum method,which has a heavy impact on the survey of soil risk and the statistics of heavy metal elements exceeding the standard.Importantly,the comprehensive index method can evaluate the soil heavy metal pollution more comprehensively and scientifically,and the improved cumulative evaluation method can reflect the characteristics of natural changes and human activities. |
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