Study On The Migration Pattern And Environmental Effect Of Heavy Metal In The Soil Of The Metallic Tailings Area In

Plenty of heavy metal elements are left in metallic tailings and are likely to enter the soil environment around the tailings pond through overland runoff,seepage,wind-drifting sand,thus causing the environmental soil pollution of heavy metal.On the one hand,soil heavy metals,after long-time accumulation,severely endanger organisms by means of vegetation absorption,bio-concentration and biological amplification;on the other hand,soil heavy metals,influenced by complicated physical,chemical and biological factors,undergo constant changes in their space distribution in the soil,forms of existence and nature of harm.Hence,studies on migration paths of heavy metal contamination in the soil environment and its influencing factors not only are of instructive importance to soil remediation but also provide significant references for prevention of environmental pollution in the mines area.This paper combines the field survey and simulation study to analyze the migration pattern of heavy metal in the soil of metallic tailings area in South Shaanxi and its environmental effect,and obtains the following conclusions:(1)Space Distribution,Concentration and Categorization of Soil Heavy MetalsCu,Zn,Pb,Cd,Mn and Co are typical elements of pollution in the research area while Ni,V,Ge,Mo and Ba are only found in a small part near one or several tailings ponds.The concentration of Cu,Zn,Pb and Cd in the soil reaches a prominent pollution level,and Ni,Mn and Ba show the least obvious concentration.An apparent regional difference exists in the concentration of Ge and Mo.The rates of exchangeable fraction of Zn,Pb,Cd and Ni are quite large.The level of RAC ecological risk coding method is medium while it's low risk or zero risk for the ecological risk of other elements.The factor analysis of heavy metal contents in the soil is basically consistent with the clustering analysis.11 heavy metals can be divided into three categories:Cu-Zn-Pb-Cd-Co-V with universal and joint sources of pollution,Ge-Ba-Mo with an obvious difference in sources of pollution and Mn-Ni with natural pollution factors.(2)The Impact of Distance on Migration of Soil Heavy Metals According to the heavy metal contamination free zone defined by the potential ecological risk assessment results of Hakanson,within 400 to 1,000m from the tailings pond,the potential ecological risk level of heavy metals is low so it's with the ecological risk of mono element.As a result,the scope beyond 400m is the safety zone of the area.The migration of heavy metals is obviously influenced by distance:the main migration scope for Ba is 0-200m;0-400m for Ni and Co;0-600m for Cu,Zn,Pb,Cd,V and Mn;0-800m for Ge and Mo.If analyzed by the farthest migration distance,the migration ability ranking of 11 elements is as follows:Ge/Mo>Cu/Zn/Cd/V/Pb/Mn>Ni/Co>Ba.(3)The Impact of Height Difference on Migration of Soil Heavy MetalsThe dam surface of tailings is regarded as the 0 benchmark,and take the area with a height difference of 80-0m and the area where the contents of heavy metals are less than 0-(-80)m for research.According to the heavy metal contamination free zone defined by the potential ecological risk assessment results of Hakanson,within the height difference between 80-20m,the potential ecological risk of heavy metals is low and that of the mono element Cd is medium.Accordingly,the scope beyond 20m above the dam surface is the safety range.However,Cd contamination should be mainly prevented;within the scope of 0-(-80)m below the dam surface,the potential risk of soil heavy metals is above medium so the safety boundary cannot be yet determined.The metal migration in the soil is influenced by height difference in an extremely prominent way:the main migration scope for Cu,Cd,Zn,Pb and V is 0-20m and 0-(-80)m;0-(-40)m for Ba;0-20m and 0-(-40)m for Mn;0-20m and 0-(-60)m for Mo;0-20m and 0-(-20)m for Ni;0-(-40)m for Ge and 0-(-80)m for Co.If analyzed by the migration height difference scope,the upward migration ability ranking of 11 elements is as follows:Cu/Zn/Cd/V/Pb/Mn/Ni/Co>Ba/Ge/Mo;downward migration ability:Cu/Zn/Cd/Pb/V/Co>Mo>Ba/Ge/Mn>Ni.(4)The Impact of Types of Soil Utilization on Migration of Heavy MetalsThe analysis of contents of heavy metals in the soil shows that the nine elements of Zn,Cd,Ni,Mo,Ba,Cu,Pb,V and Co demonstrates greater migration in the grassland soil than in the farmland,forest land and abandoned land.Based on Hakanson's potential ecological RI,the harm level of grassland soil is high-level while it's medium for the other three elements.The potential ecological risk level of Cd mono element in four types of land utilization is relatively high,which means that Cd should be the key contamination factor that should be prevented in the research area.When analyzed in terms of chemical forms of metal elements,great differences exist among influences of soil types over the migration of 11 elements.For instance,the migration and potential ecological risk of Cu in the farmland are the greatest and Zn in grassland;the migration ability of Pb in the abandoned land is the strongest and its potential ecological risk is the greatest in the grassland soil;Cd suggests the greatest migration and potential ecological risk in the grassland;Ni and Co have the strongest migration ability in the grassland and almost equal ecological potential risk in four types of soil.The grassland is the land where multiple metals' migration ability or potential ecological risk is the most prominent.(5)A Simulation Study on Tailings LandfillIn terms of migration time,the obvious migration time of Cu,Zn,Pb and Cd is respectively 12,6,6 and 3 months,and the migration sequence is Cd>Pb/Zn>Cu.Regards to migration distance,the migration ability of Cu,Zn,Pb and Cd at the horizontal level is in line with Zn/Pb>Cu/Cd,and at the vertical level it follows Cd>Zn>Cu>Pb.As to the migration rate,the pattern is Cd>Pb>Zn>Cu at both vertical and horizontal levels.In terms of migration direction,influenced by Hanjiang River in the due south,SE,S and SW are the most apparent migration directions.The migration ability ranking results vary dramatically in such aspects as migration time,distance and rate.(6)A Mathematical Model of Soil Heavy Metal Migration in the Tailings Area in South ShaanxiThe heavy metal migration model in the simulation experiment:the migration amount of Cu,Zn,Pb and Cd are prominently correlated with the distance R,depth H and migration time t in a negative way.Hence,it serves as a supplement to the field survey which cannot reflect the patterns of "sequence of heavy metal migration" and "migration rate".The model showcases the initial stage of migration of heavy metals in the tailings to the soil,and represents a microscopic and ideal heavy metal migration prediction model.The heavy metal migration model of the field survey:plan B which takes into account the factor of land utilization types can simulate the migration trend of soil heavy metals in a more objective and exhaustive way than plan A does.The model in plan B verifies that land utilization types exert important influences over the emigration of soil heavy metals,and meanwhile that the contents of heavy metals manifest obvious correlations with the distance D,height difference H,sand S0 and S2.The group of models reflects the development stage of the tailing pond's heavy metals migration in the soil,and represents a more macroscopic,objective and practical heavy metal migration prediction model.