Soil Heavy Metals Pollution Characteristics And Ecological Risk Assessment Of Minshan Small Watershed

Heavy metal pollution,which has attracted much attention in china,has become more and more increasingly serious and pose a serious threat to human health and ecology environment. The current study of heavy metal pollution mainly focused on the soils in sewage irrigation areas,mining areas and the typical suburban areas,but that in the small watershed was few.In this research,the top soil in the about 12 km2 of Minshan small watershed was chosen as the research object and four heavy metals(plumbum, cadmium, copper and zinc) was as the index,the total quantity and the contents of every forms of the heavy metals were analyzed,and based on this analysis,the heavy metals pollution and ecological risks were assessed in the area.Based on geostatistics and GIS as the technological means,this paper had probed into the present pollution situation,reason and route of the four heavy metals and reasonably assessed the present heavy metal pollution situation and ecological risks in this area,which could provide basis and early warming for the prevention of the degradation of soil quality and explore the rational utilization of land.(1)The average content of Pb,Cd,Cu,Zn was 7.75mg/kg,0.297mg/kg,13.04mg/kg, 73.20mg/kg,respectively,with a normal overall content compared with the background values of Chinese soil and Yaan garden soil in mingshan small watershed. Soil pH,available phosphorus content,organic matter content and other physical and chemical properties had an important impact on the heavy metals content.(2)Because of the different affinity between the heavy metal ions and the components in the soil,the form distribution and the percentage of the heavy metals were in great difference in this area,among them,the order of the exchangeable percentage of the heavy metals:Zn>Pb>Cd>Cu,that of the carbonate bound form:Zn>Pb>Cd>Cu,that of Fe-Mn oxide form:Zn>Pb>Cd>Cu;that of the organic matter bound:Zn>Pb>Cd>Cu.The residual bound took most part of the every forms of the 4 heavy metals.Correlation analysis of the 5 forms of every heavy metal was carried out respectively,and the correlation existed among them.(3)After the original data conversion of the available heavy metal contents,the logarithmical transformation of the Pb,Cu,Zn data was better,while power transformation of Cd was better.The variogram model and parameters of the available contents of the 4 heavy metals were used in the ArcGIS platform with the method of ordinary kriging for spatial interpolation,the result showed:the available high values of the four heavy metals were distributed in the terraces of minshan river;it was the better farming soil:farmland soil,urban soil affected by human activities and industrial park soil.This showed that the available contents of heavy metals had a close relation with the human activities.(4)Through the assessment by pollution index,it showed:the average single pollution index of Pb,Cd,Cu and Zn was 0.22,1.48,0.37,0.73, respectively.From the synthesis pollution index,33.33% of the synthesis pollution index of the soil samples was among 0-1, no pollution;66.67% of those was amongl-2,light pollution.The highest pollution contribution rate was Cd,with average single pollution index 1.48.Through the assessment by index of geoaccumulation the index of pb and Zn was less than 0,and the geoaccumulation index of Pb,Cd,Cu,Zn wasl.88,1.47,0.51,-0.64, respectively.Therefore,Cd was the moderate pollution;followed by Cu,19.61% of soil samples of Cu pollution was from no pollution to the moderate pollution; but no Zn and Pb pollution in the soil.(5)From the single coefficient of potential ecological risk(Ei):the single pollution index of Pb,Cu,Zn was far less than 40,the slight ecological hazard;but that of Cd was among 64.37～196.79,and the average was 129.02,68.63% of the soil samples were intensive ecological hazard,21.57% were strong ecological hazard,and 9.80% were slight ecological hazard.From the single coefficient of potential ecological risk in the whole area,the samples whose index was less than 150 were 35 amples,68.63% of soil samples were the slight ecological hazard;and the samples whose index was among150-300 were 16,31.37% of the soil samples were moderate ecological hazard.So,the potential hazard of Cd should be paid attention to.Through the assessment by risk encoding method, all of the soil samples of Cd were in low risk;84.31% of the soil samples of Pb were in low risk and the rate of no risk and moderate risk was7.84%;76.47% of those of Cu were in low risk followed by no risk, accounting for 21.57%,and the moderate risk only accounted for 1.96%;and 88.24%of those of Zn were in the low risk,11.76% moderate risk.Therefore,the ecological risk of soil heavy metal in mingshan small watershed was in low risk,but 11.76% of Zn,7.84% of Pb and 1.96% of Cu were moderate risk.