Research On The Distribution And Concentration Prediction Of Heavy Metals In The Soil Around Coal-fired Power Plants

A power plant which runs for many years in of Xuzhou in Jiangsu province was chosen as the research object.The objectives of this research were to detect the content of heavy metals in the soil,study the heavy metal distribution,evaluate the heavy metal enrichment,predict the soil heavy metal concentration and analyse the enrichment characteristics in the future,find out the main control factors and put forward relevant suggestions and measures to prevent and control pollution.The main conclusions are as follows:(1)Status evaluation of soil heavy metal environment:According to the standards of the national standard of soil environmental quality(GB15618-1995),the heavy metal in the study area is seriously exceeding the standard,and the monitoring point rate is 90%.Cd was seriously beyond standard,Hg is second.Using geometric mean and standard deviation to estimate the background value of the study area,the background values of Hg,As,Cd,Cr and Pb in the study area were 0.19mg/kg,5.65mg/kg,0.27 mg/kg,48.29 mg/kg and 29.84 mg/kg.Cd has a higher background value in this area.The result of index of geoaccumulation shows that presents area Igeo situation of 5 elements is light pollution.Pb and Hg is slightly higher than that of other elements.Among the 21 sampling points,9%of the sampling point Hg elements reached the partial pollution;14%of the samples were contaminated with Pb.The result of the Pollution Load Index shows that the area PLI is 1.36(mild contamination).Mercury and plumbum pollution index reached 14.3%of the total sampling points.(2)Analysis of distribution and and influencing factors:The elements of Hg,As and Pb with coal-fired flue gas as the main source of pollution will be centered on the chimney,with significant differences in different wind directions.Hg reaches the highest concentration in the dominant wind from the chimney between 1000 and 2000m.The maximum values of As and Cr all occur within 1000m.It shows that the soil in the range of 2km below the chimney has the most serious effect on the settlement of the heavy metal elements carried by the smoke from the power plant.(3)Analysis of soil heavy metal source:There are three main factors:the natural geology of the soil,the flue gas factor of the coal-fired power plant,and the automobile exhaust gas factor.The main source of Cd in the soil of the study area is the parent material of the soil,the main source of Hg element is the emission of coal-fired flue gas,and the main source of Pb element is automobile exhaust.According to the concentration contour map of each element,the study area is also affected by the mining,mainly affecting the concentration of Cd and Cr.(4)Prediction of concentration based on BP neural network:Five 3-layer BP neural network models were constructed to predict the soil concentration of heavy metals 5 in 5 years,10 years and 20 years.80 datas collected in the study of soil heavy metals in the surrounding soil of coal-fired power plants were collected as a learning training sample to improve the accuracy of the model.The accuracy of the model was 98.52%,98.22%,91.86%,90.70%and 88.31%.(5)Evaluation of heavy metal prediction results in two scenarios:Taking the study area as an example,the concentrations of Hg,As,Cd,Cr,and Pb in the soil after 5 years,10 years,and 20 years are predicted by optimistic scenario and no mutation scenario.Under the optimistic scenario,the over-standard rate of heavy metal Hg and Cd monitoring points continuously decreased.After 20 years,the over-standard rate of 21 points was 33.33%,which was obviously better than the current environmental quality status(over-standard rate 90%).The areas of As,Cd,Pb,and PLI(ZOnes)are all close to the cleanliness level.And the Igeo of Hg and Pb elements that are more severely enriched have improved significantly.The areas are only 0.14 and 0.03,and are nearly clean.Under the non-abrupt change scenario,the content of heavy metals in soil increased gradually.After 20 years,all Cd elements in the soil monitoring sites in the study area exceeded the standard,and the Hg exceeding standard rate reached 95.24%.The Hg Igeo region changed from mild pollution to partial pollution after 10 years,indicating that the power plant had a more serious impact on the soil quality in the study area.The PLI(zone)increased continuously,1.50,1.62,and 1.68,respectively.There was no pollution-free spot in the study area after 5 years.