Effects Of Mining Disturbance On Distribution Of Heavy Metals In Farmland Soil

In the process of coal mining,the overlying rock in the upper goaf is deformed and moved by gravity,tensile stress and compressive stress,and the strata bend and fall upward,and the surface collapse fissure is formed,which changes the original structure of the soil and leads to changes in its physical and chemical properties.Changes in soil physical and chemical properties affect the environmental chemical behaviors of heavy metals in soil environment,such as redox,complexation precipitation,adsorption and desorption,resulting in changes in the form and migration path of heavy metals,and then lead to the migration and spatial redistribution of heavy metals in soil environment.Therefore,to better understand the influence of coal mining disturbance on the distribution of heavy metals in farmland,it is pivotal to study the physical and chemical properties of soil,and research the distribution of heavy metals after the occurrence of main fissures in farmland as well.Which can not only help the underground mining to reduce the damage to the ecological environment,but also provide a practical reference for the ecological reconstruction of the mining area.The study took the farmland soil after mining disturbance in the typical mining area of Juye,Shandong as the research object,analyzed the changes of farmland soil p H and Eh after coal mining disturbance,evaluated the soil heavy metal pollution status and ecological risk.Combined the total amount of soil heavy metals in the area with its morphological distribution characteristics,to study the influence of mining disturbance on the distribution of heavy metals in farmland soil.The main research findings show as follows:(1)Soil properties change significantly after mining disturbance.The closer to the main fissure,the higher p H,the lower Eh,and the stronger soil reducibility.Only the total amount of Cd in all sampling points exceeded the background value of C layer soil in Shandong province,and the average value of 7 heavy metals did not exceed the risk screening value of farmland soil(GB15618-2018).The GI evaluation results show that Cd in the study area is not polluted to moderate degree,and the other 6 heavy metals are not polluted.The enrichment degree of heavy metals from large to small is as follows: Cd(0.24)> As(-0.88)> Pb(-1.01)> Cu(-1.37)> Zn(-1.51)> Ni(-1.71)> Cr(-2.55).(2)The spatial distribution of the total amount of 7 heavy metals in the region is relatively different: Cr mainly accumulated in the deep soil near and far from the fissure.Ni and As mainly accumulate in the whole section from F-0 to F-1,which is close to the fracture.The accumulation of Cu in F-2 and F-5 far from the fissure increases obviously.Zn mainly accumulates towards the main fracture,and the accumulation degree of Zn cuts down with the increase of profile depth.With the increase of the horizontal distance from the fracture,the total amount of Cd decreases.Pb accumulation is relatively small and evenly distributed at F-2,which is far from the fracture,and mainly accumulates at other locations in the regional range.(3)At different horizontal distances from fractures,Cd mainly exists in exchangeable form and reducible form;Pb and Cu in soil are mainly in residual and reducible forms;Cr,Ni,Zn and As mainly exist in the form of residue,among which the residue content of Cr is the highest,accounting for 83.48%-94.72%.With the increase of horizontal distance from the fissure,the potential migration capacity of Cd decreased gradually.The potential migration capacity of As in soil increased first and then decreased,and the potential migration capacity of As in F-2 was the strongest.The potential migration capacity of Cr,Ni,Zn and Pb in soil decreased first and then increased,and the potential migration capacity of these 4 heavy metals was the lowest at F-2.(4)The potential ecological risk assessment results showed that the average potential ecological risk of single metal is as follows: Cd(59.94)> As(8.30)> Pb(3.73)>Cu(2.97)> Ni(2.29)> Zn(0.53)> Cr(0.51).Cd had the strongest potential ecological risk of single element,and 5.26%,68.42% and 26.32% samples of Cd had serious potential ecological risk,medium potential ecological risk and low potential ecological risk respectively.And the other six heavy metals had low potential ecological risk grade in all samples.The average comprehensive potential ecological risk index of all heavy metals was 78.27,indicating low potential ecological risk to the surrounding environment.The comprehensive potential ecological risk index of these heavy metals in 0-10 cm soil profile of the main fissure was 247.74,which was medium potential ecological risk grade.The average individual pollution factor(ICF)evaluation results showed that the order of ecological risk was:Cd(5.91)>Pb(0.36)>Ni(0.35)>Cu(0.33)>As(0.27)>Zn(0.21)>Cr(0.15).Cd has a severe level ecological risk to the surrounding environment,while the other 6 elements have a low level ecological risk to the surrounding environment.The average global pollution coefficient(GCF)of 7 heavy metals was 0.15,indicating low-level ecological risk.(5)The effects of soil physical and chemical properties on the total distribution of different heavy metals in soil were significantly different in the study area.The p H and Eh were significantly correlated with the total distribution of most heavy metals(p<0.05),stepwise regression analysis showed that Eh was the main factor affecting the total amount of Ni,Zn,As,Cd and Pb.The effects of soil physical and chemical properties on the speciation distribution of different heavy metals were different,and p H,Eh and field water capacity were significantly correlated with the speciation distribution of most heavy metals(p<0.05),stepwise regression analysis showed that p H,Eh and field water capacity were the main factors affecting the speciation distribution of heavy metals in disturbed farmland soil.