The Characteristics And Risk Assessment Of Soil Heavy Metal Pollution In Coal Mining Area Based On Hyperspectral Inversion

The large amount of coal mining has promoted the rapid development of China’s industrial economy.However,a large amount of coal gangue,coal ash dust and wastewater have been generated during the mining process.While occupying and destroying land resources,it has also caused the deterioration of the soil ecological environment around the coal mine area.The waste generated from coal mining continuously releases heavy metal elements into the soil,which causes the heavy metal pollution in the soil around the mining area to increase,thereby endangering the entire ecosystem and human health.Therefore,it is particularly important to carry out highly efficient and accurate monitoring and evaluation of soil heavy metal pollution in coal mining areas.In this study,a coal mining area in Jining City,Shandong Province was selected as the study area,and the study area was divided into subsided area,unsubsided area and reclaimed area,among which a total of 159 sampling points were set up.The spectral information of 159 soil samples was measured by ASD Field Spec4 ground object spectrometer in the laboratory.The soil spectra were pretreated by Savitzky-Golay convolution smoothing(SG),multiple scattering correction(MSC)and various combinations of different mathematical transformations.At the same time,60 soil samples were selected to determine the contents of heavy metals such as Ni,Cu,As,Zn,Cd and Pb.Based on the content of heavy metals and soil reflectance spectrum of the 60 sampling sites,a Regression model for estimating the content of six heavy metals was established by Partial Least Squares Regression analysis(PLSR).Using the content estimation model of each heavy metal element,the content of each heavy metal in the remaining 99 sampling points was inverted.Then,the pollution sources,spatial variation structure,spatial distribution characteristics,pollution degree and potential risk level of soil heavy metals in the mining area were studied deeply by combining the methods of local statistical analysis,multivariate statistical analysis and heavy metal pollution evaluation.The main results show that:(1)The average contents of the six heavy metals in the surface soil samples of the mining area were all lower than the screening value of soil pollution risk of agricultural land,but the average contents of Pb,Cu,Zn and Cd were all higher than the background value in the soil of Shandong Province.The average content of Cd was 1.59 times of the background value,and there was a high enrichment in the soil.The samples with Ni,Cu,Zn,Cd,Pb content exceeding the background value in the reclamation area were more than those in the subsided and unsubsided areas.And the contents of Cu,Zn,Cd and Pb in a few samples have exceeded the risk standard value.According to the coefficient of variation of heavy metals,Pb,Cu,Zn and Cd are highly variable,and the contents of Pb and Zn have high positive skewness,indicating that the contents of these four heavy metals are not evenly distributed,and may be affected by human factors and appear abnormal high values.(2)Partial least-squares regression modeling by hyperspectral method can effectively realize the quantitative inversion of heavy metals in soil of coal mine area to a certain extent.Taking the reciprocal logarithm of spectral reflectance treated by SG+MSC as the independent variable,the inversion model of heavy metal Ni content has high accuracy;The inverse spectral reflectance of SG+MSC transform was used as the independent variable to construct the optimal inversion model of As and Cd content;The estimation model of Pb content based on the spectral reflectance after MSC treatment has a good predictive ability;Taking the reciprocal logarithm of spectral reflectance treated by SG+MSC as the independent variable,the optimal estimation model of Cu and Zn contents was established.(3)According to the spatial variation and source analysis of heavy metals in the soil of the study area,there is a significant correlation between As and Ni in the soil,which preliminarily reflects that As and Ni are from the same source;There is also a good correlation between Cu,Pb and Zn,which indicates that the pollution sources of these three elements are consistent.Principal component analysis identified two principal components,Pb,Zn,Cd and Cu,which occupied a relatively high load on PC1 and were judged as human sources.Ni and As occupy high load on PC2,which is judged to be natural source.Ni and As have strong spatial correlation,which is mainly structural variation;The spatial correlation of Pb,Cu,Zn and Cd is weak,and the spatial variation is mainly random,which is greatly affected by human factors.Due to the influence of multiple factors,such as the parent material of soil,industrial,mining and agricultural production activities,transportation,and reclamation of subsided land,the spatial distribution pattern of the six heavy metals showed that the contents of the six heavy metals were higher in the central and eastern regions,while the contents of the six heavy metals were lower in the northern and western regions,and the high contents were mostly distributed in the soil reclamation areas.(4)According to the evaluation results of various heavy metal pollution,the heavy metal As in the surface soil of the study area is safe and clean,and the heavy metal Cd,Cu,Ni,Zn and Pb are enriched in different degrees,respectively.The average value of single factor pollution index of each heavy metal in reclaimed area is higher than that in subsided area and unsubsided area.The pollution level of heavy metal Cd was the highest,with mild pollution in local areas and moderate and heavy pollution in a few sample sites;97.45% of the sample sites in the study area were below the Nemero comprehensive pollution alert value,among which the unsubsided area and part of the reclaimed area showed mild and moderate pollution.The geoaccumulation index showed that the soil in the study area was not polluted by As,a few sample sites showed slight pollution of Ni,Cu,Zn and Pb,and a slight pollution of Cd in local areas.A few sample sites in the reclamation area also have severe pollution of Cd and moderate and moderate pollution of Cu,Zn and Pb.The overall potential ecological wind level in the study area is low,and only the western and east-central areas of the study area belong to the medium ecological risk level,among which element Cd is the main risk factor.Therefore,more attention should be paid to the areas and heavy metal elements with high pollution level.