Distribution Characteristics And Risk Assessment Of Heavy Metals In Contaminated Soil Particle Size Fraction In Typical Pb-Zn Mining Area

Soil aggregates are the most basic structural units of the soil,and the characteristics of different particle sizes for the adsorption of environmental pollutants are also significantly different.The behavior of pollutants in the soil microenvironment is largely affected by the distribution of aggregates,thus affecting its mobility and biological effectiveness.At present,the study on soil aggregate at home and abroad mainly concentrated in the classification characteristics and organic matter,etc.There are rarely report studing on heavy metal fraction,enrichment in plants and risk assessment on soil around the minng area.This paper studies the migration and transformation of heavy metals(Cd,Pb and Zn)in soil-agricultural systems from the perspective of vertical soil aggregates.It will provide a scientific basis for understanding the environmental effects of heavy metals in soil.The basis is instructive that the results are used as environmental risk assessment and targeted remediation of contaminated soils.In order to understand the vertical distribution characteristics and pollution status of the contaminated soil in the mining area,we took the contaminated soil(0-100cm)in the Pb-Zn mining area in Duyun,Guizhou Province and the soil in the non-mining area was used as the control point.Sampling sites were set up in two Pb-Zn mines and three types of crops(leaf vegetables,fruit vegetables and grains)were collected.Through the analysis and determination of heavy metals(Cd,Pb,Zn)and their fraction in soil aggregates,the distribution characteristics of heavy metals in soil aggregates at different depths were studied,and the contents of surrounding crops were analyzed.The enrichment coefficient method was used to study the accummulation characteristics of soil aggregate heavy metals in soil-crop systems.The distribution factor method,geological accumulation index method,potential ecological hazard index method,and potential health risk index method was used to evaluate the condition of heavy metal contamination of aggregated particles in the soil vertical section of the study area.The main research results are as follows:(1)The soil heavy metals Cd,Pb,and Zn in the study area were all contaminated to a certain extent,and existed in the residual state in all soil layers.The average contents of heavy metals Cd,Pb and Zn in the soil samples of the two mining areas were 23.50,615.4 and 6027 mg/kg,respectively,which were higher than the background values of Guizhou soil.The reference to the soil background value of Guizhou was as follows:Zn<Cd<Pb.In the two mining areas,Cd exists mainly in the residual and reducible forms in the depth layers of each soil;Pb is in the residue and oxidizable extractable states in each soil depth,followed by the reducible state;and Zn is mainly existence in the form of residuals and its proportion is extremely high,all reach above 80%.(2)There are significant differences in the distribution characteristics of heavy metals in soils with different depths in the two mining areas,and the 0-100cm of vertical profile of the soil aggregates shows that metals Cd,Pb,and Zn suffer different degrees of ecological damage.Cd is enriched in 0.25-0.5 mm particle size and Zn is enriched in0.25-0.5 mm,0.053-0.25 mm and Pb content mainly accumulates in 0.5-1mm in 0-100cm soil of A mining area.The distribution factors of Pb and Zn in soil aggregates were all lower than 1.The highest potential ecological risk index of aggregates was Cd(E_r~i?240),indicating that the ecological hazards of Cd was extremely strong in the region.The potential ecological hazard indices for Pb and Zn are greater than 30 in all particle sizes at soil depths of 0-100 cm,indicating a moderate potential ecological hazard or above.(3)The three different types of crops(leaf vegetables,fruit vegetables,and grains)planted around the two mining areas have different enrichment capabilities for heavy metals(Cd,Pb,and Zn)of soil aggregates.For leafy crops,the average bioaccumulation(BCF)of metal elements in different soil aggregates at the soil depth of 0-100 cm in the A mining area was in the order of Zn>Cd>Pb,and the order of BAF in the B mining area was Cd>Zn>Pb;For fruit crops in A area,BAF is the order of Cd>Pb>Zn;the average BAF order of B mines is Cd>Pb>Zn for grain crops.The average BAF of metal elements in two soils in different areas was Cd>Zn>Pb.(4)Long-term consumption of the three types of crops(leaf vegetables,fruit vegetables,and grains)produced around the two mining areas in the study area have chronic toxicity effects on the human body.The average contents of Cd,Pb and Zn in the edible parts of the three types of crops in the two mining areas were in the following order:Zn>Pb>Cd.The Cd and Pb concentrations of the edible parts of different types of crops were as follows:fruit and vegetable>grain>leafy vegetables.The trend of Zn concentration was:grain>leafy vegetables>fruit and vegetable.The total target hazard coefficient(TTHQ)of agricultural crop heavy metal(Cd,Pb and Zn)was fruit>grain>leafy vegetable,and TTHQ value was more than 1,that can be seen that long-term consumption of these three types of crops produced in this area may present a certain degree of health risk to the human body.In summary,the three heavy metals(Cd,Pb,and Zn)in the two Pb-Zn mines in Duyun are all contaminated to a certain extent,and there is a significant difference in the distribution characteristics of heavy metals of aggregates at different depths of soil.Long-term consumption of the three types of crops produced around the two mining areas in the study area have chronic toxicity effects on the human body.