Evaluation Of Heavy Metal Pollution In Soil Of A Mining Area In Hunan Province

As an important part of the earth’s terrestrial ecosystem,soil is the most basic natural resource for human survival.It can be continuously regenerated and recycled.With the development of China’s industry and agriculture,the problem of soil pollution is growing increasingly serious,with heavy metals in the soil causing serious harm to the soil structure,affecting the ecological environment and human health.In some areas of Hunan,mining and mineral smelting activities have led to serious heavy metal pollution in the surrounding areas.The treatment of heavy metal pollution has become the focus of national pollution prevention and control.The study area is a mining area with years of mining history in Hunan,which is rich in various metal and non-metallic mineral resources.It has important reference significance for the follow-up local land use,environmental treatment,pollution prevention and control countermeasures to reduce the soil heavy metal pollution in this area,and assess soil heavy metal pollution and the risks to ecology and human health.Through the previous field investigation and comprehensive analysis,this paper formed a set of scientific comprehensive layout scheme,collected soil samples from 660 sampling points,determined the contents of soil heavy metals Zn,Pb,Cd,Hg,As and Sb,carried out correlation analysis and principal component analysis by SPSS,and drew the regional pollution spatial distribution maps after spatial interpolation by ArcGIS.The single factor index and Nemerow comprehensive pollution index were used to evaluate the overall pollution of the mining area.The potential ecological risk index and human health risk assessment were used to evaluate the risk of heavy metal pollution.Finally,K-means and SVM were adopted to explore a new method of pollution evaluation and classification.The main conclusions are as follows:(1)Zn,Cd and Pb in the mining area may have similar sources.As,Sb and Hg may have similar sources.Zn,Pb and Cd in the first principal component have high positive load,which mainly represents the pollution caused by mineral mining and smelting.In the second principal component,Hg,As and Sb have high positive load.These three metals belong to volatile heavy metals,which may be caused by the deposition of air pollutants produced by regional smelting.Except Pb,the average contents of the other five elements are greater than the soil background value in Hunan.There is a considerable degree of enrichment.The contents of six kinds of heavy metals in soil are very uneven.There are very high values with great differences in some areas.In particular,the variable coefficient is far greater than the critical value of 0.36,that is,the soil heavy metals in the mining area are seriously affected by human beings.(2)According to the calculation of single factor pollution index method,most of the soil in this area has been polluted by heavy metals in varying degrees.The pollution degree of Pb and Zn is the lightest,97.73%of the areas are not polluted by Pb and 90%of the areas are not polluted by Zn;The percentages of Cd in all sites were clean,slightly polluted,moderately polluted and heavily polluted were 63.48%,16.36%,3.84%and 16.21%respectively.The pollution of Hg,As and Sb is relatively serious.Only 28.64%of this area has Hg pollution evaluation grade of clean,and the proportions of mild pollution,moderate pollution and severe pollution are 23.64%,11.82%and 35.91%respectively.43.03%of the sampling points were heavily polluted by Sb.The proportion that is still clean is 36.52%.The As’s distribution under single factor pollution index is similar with Sb.35.45%of the sampling points are heavily polluted.According to the Nemerow comprehensive pollution index,only 5%of the sampling points belong to clean soil that has not been polluted,5%of the areas are fairly clean soil,12%and 8%of the areas have been slightly and moderately polluted,and 70%of the areas have been serverely polluted.(3)The potential ecological risk coefficient Eri of each metal is obtained by the potential ecological risk evaluation method:Zn is 0.06-39.6,Pb is 0.01-73.57,Cd is 0.50-29750,Hg is 0.33-13933.33,As is 0.37-3340 and Sb is 0.01-8120.The average Zn is 0.83,Pb is 1.04,Cd is 164.69,Hg is 298.08,As is 53.93 and Sb is 226.67.From the average value of potential ecological risk coefficient of six heavy metals,the potential ecological risk degree of Hg is the highest,followed by Sb,Cd,As,Pb and Zn.The percentages of light risk,medium risk,strong risk,very strong risk and extremely strong risk in the whole mining area are 37%,20%,17%,12%and 14%respectively.There are great differences in potential ecological risks.(4)Among the three exposure routes of oral intake,oral and nasal inhalation and skin contact,oral and nasal inhalation of soil heavy metal pollution has the greatest impact on health,which is the main route.The impact of skin contact can be ignored.In the non-carcinogenic risk assessment,the order of risk from large to small is:Sb ≥As>Pb>Hg>Cd>Zn.The risk of human carcinogenesis caused by As and Cd should be worthy of attention.(5)After preprocessing the data,a set of evaluation model of soil heavy metal pollution is obtained by using k-means and SVM based on the results of potential comprehensive risk index.The accuracy of the model in the training set is 0.9778 and the accuracy of the test set is 0.9556.It has a good classification and evaluation function.