| Soil is the base of agricultural production,plant growth,reproduction and biological production.It plays an irreplaceable role in the process of plant growth.It also plays a vital role for guaranting quality and safety of the soil environment,maintaining rapid development of economy and protecting human health.In recent years,more and more waste were discharged with the development of industry,increased using of agricultural pesticide and fertilizer application.Thus,soil heavy metal pollution has become a worldwide environmental problem which received widespread care and attracted a lot of attention of scholars all the world.Cities especially in the Yangtze River Delta(YRD)region in Eastern China face severe challenge of soil heavy metal pollution.For this reason,we selected a world famous port industrial city in YRD region as our study area.In this research pollution degree of soil heavy metal and spatial variation of soil heavy metal was analysised.And then the pollution range of soil heavy metal pollution was elineated and the uncertainty of the delineation of the range of soil heavy metal pollution was explored.Finally the accuracy of pollution area delineation was analysised.The results of this research can make great contribution to the work of monitoring and control of heavy metal pollution in soil.The main research contents and goals of this research are as followed:(1)Descriptive statistical analysis and spatial distribution characteristics of heavy metals in soilThe average content of Cr,Pb,Cd,Hg,As,Cu,Zn and Ni were 61.84 mg/kg,39.86 mg/kg,0.19 mg/kg,0.27 mg/kg,6.55 mg/kg,33.87 mg/kg,99.60 mg/kg,23.85mg/kg,respectively,all of which were lower than the corresponding value of national soil environmental quality standard value(GB15168-1995).All the coefficient of variation of 8 kinds of heavy metals in the study area was greater than 10%,of which Cu,Pb,Ni,Zn,Cr and As represent moderate variation,Cd and Hg were strong variability.The content of Cr,Pb,Cd,Hg,As,Cu,Zn and Ni represented different spatial distribution characteristics.Among it,area which has high content of Cr,Pb,Hg,Cu and Zn mainly distributes in the central region of the study area.Area which has high content of Cd was mainly distributes in the West and southeast part of the study area.The content of As and Ni in northern part of the study area and the southeast hilly region is relatively low,the content of other areas is relatively high,and the pollution areas are mainly concentrated in the central urban area.When it comes to the variable range eight trace heavy metals in soil,the variable range of Cr content was the lowest of 11.7 km.Followed by Cu,the variable range of 17 km,the variable range of Pb was biggest and reached a distance of 54.5 km.In the terms of coefficient of nugget(C/(C+C0)),the results showed that the sptial distribution of Cr,Hg and Ni is mainly affected by random factors induced by anthropogenic activities.The Pb,Cd,As,Cu and Zn were affected by both natural and random factors.(2)Analysis of the enrichment,pollution condition,analysis of spatial distribution of heavy metal pollution in soilThe average geo-accumulation index value of Cr,Pb,Cd,Hg,As,Cu,Zn,Ni were-0.02,0.06,-0.49,-0.43,-0.06,-0.08 and-0.21 in the study area,respectively.Among it,Cd and Hg belong to the degree of no pollution to moderate pollution,the average index value of other elements belong to the level of pollution.The evaluation results of single pollution index of heavy metals in soil showed that the single pollution index of Cr,Pb,Cd,Hg,As,Cu,Zn and Ni were 0.24,0.13,0.54,0.31,0.24,0.33,0.40 and 0.48,respectively.All of which belong to the security level.This represented that soil environment quality in study area stayed in a good situation in gereral.The average composite pollution index in soil was 0.59 in study area,which was in the safety level.In term of the characteristics of spatial distribution,the content of Cr,Pb,As,Zn,Cu and Ni were in the level of non pollution in most areas of the study area.The content of Cd in most area of study area remains in safe level except western part of the study area along the edge of the mountain Piedmont plains.There was large area which faces great risk of Hg pollution in research area,these areas mainly located in the central uraban area of the study area.Fortunately,most of it belongs to the degree of slight pollution.(3)Spatial distribution of soil heavy metal pollution probabilityIndicator kriging was used obtain the probability map of single heavy metal pollution index and composite heavy metal pollution index in soil in study area.The probability of exceeding threshold value of Cr and As were less than 1.0 in whole the study area.This illustrated that Cr and As posed a small threat to citizens in the study area;The probability of exceeding threshold value of Pb in most area were less than 1.0,but in some area this probability reached to 0.3 to 0.4;Pollution probability of Cd reached to 0.4 in central part of study area and posed certain threat to citizens in these places;Pollution probability of Hg was very big in central part of study area.In some places this probabily reached to 0.9;Pollution probability of Cu,Zn and Ni remained at a low level which indicated that the pollution risk stayed in very low level in most area.And these areas can be delineated as unpolluted area.But attention still need to be paid to some area of which pollution risk reached to 0.3 to 0.5,these areas may face great threat of Cu,Zn and Ni pollution.In the term of composite heavy metal pollution,high composite pollution probability areas were mainly distributed in core urban zone of study area.These areas were deeply affected by anthropic activities.So it is credible to delineate this aera as pollution area.(4)Uncertainty analysis of delineation of soil heavy metal pollution zoneIn this study 0.05 was setted as the step size and the probability values from the 0 to 1.0 were selected as the probability threshold for dividing pollution and non pollution area.Then the results are compared with the results of Nemero’s comprehensive pollution index,and the curve of classification error probability and probability threshold is plotted.We can determine the optimal probability of the pollution and pollution according to the misclassification rate of the scope delineation of soil heavy metal pollution.The results show that when the probability threshold is setted as 0.4,the classification accuracy was the highest and the classification accuracy was up to 92.88%.According to the results of defining pollution zone based on the spatial distribution of pollution probability,the pollution areas mainly distributed in the central part of the study area,which is a mass region goes from southwest to northeast.These places which were classified as contaminated area but actually belong to uncontaminated taken a proporation of 5.17%in total study region with area of 220.56 square kilmoeters,wihle these places which were classified as uncontaminated area but actually belong to contaminated taken a proporation of 2.09%in total study region with area of 89.26 square kilmoeters.On the whole,the classification accuracy is 92.73%and the area of 3954.18 square kilometer was classified correctly.It revealed that it is credible to delineate contaminated zone based on spatial distribution of contaminated probability via Indicaor Kriging.And the results highly consist with the reality.These places with high uncertainty of contamination mainly located in transitional zone between contaminated areas and uncontaminated areas,so we need to collect more samples to clarify the boundary of contaminated area. |
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