Source Analysis And Vertical Transport Of Heavy Metals In The Impacted Zones Of Typical Mining Areas In Inner Mongolia

As one of China ’s energy security bases,Inner Mongolia Autonomous Region is rich in mineral resources and reserves.The development and utilization of non-ferrous mineral resources is a strong guarantee for high-quality economic development.However,industrial production activities such as mining,beneficiation,and smelting produce a large amount of wastewater,waste residue,and waste gas.This results in serious heavy metal pollution in soils around the mining areas,damage in the ecological environment and finally human health.However,the sources of heavy metals in the polluted soils in Northern China are much complex,and the heavy metal migration behaviors have been rarely explored.In this study,the soil in a copper mine area in Bayannaoer City and a lead-zinc mine area in Chifeng City,Inner Mongolia was taken as the research object.The spatial distribution characteristics,source analysis,and the heavy metal migration processes in the soils of the study areas at field conditions and certain futural scenarios were investigated by statistical analysis,GIS,numerical simulation and scene analysis.The research results will provide scientific guidance for soil management,restoration and safe utilization in the affected areas of northern mining areas.The main results are listed as following:(1)Bameng and Chifeng research areas are seriously polluted.The variations in average heavy metal concentrations gradually increase with soil depth.The heavy metal pollution level in Bameng soils decreases in the order of Cd > Cu > Pb > As > Zn.Concentrations of Cu,Cd and Pb in 90% of the soil samples and As and Zn in 60% of the soil samples are greater than the corresponding background values for soils in Inner Mongolia.In terms of spatial distribution,Pb and Cu are more concentrated around the mine area,while Zn and As exist as mostly in point form around the tailings and in the northern part of the study area.The heavy metal pollution level in the Chifeng study area is higher than that in Bameng,with the concentrations of Cd in all the soil samples and Cu,Zn,As and Pb concentrations in 90% of the soil samples being greater than the corresponding background values.The heavy metal pollution level in Chifeng soils decreases in the order of Cd > Cu > Zn > As > Pb.The spatial distributions of Pb,As and Zn are similar,with relatively higher values in soils around tailings and factories,while the distributions of Cu and Cd are more dispersed,with relatively more serious pollution around mines and on farmland in the south.(2)The correlation of heavy metals in Bameng soils,which have a single land use type,is significantly stronger than that in Chifeng soils.The correlation coefficient between heavy metals is 0.90 in all soil layers,except for that between Cd in 5-10 cm layer and As and Pb in 10-20 cm layer.This indicates that the sources or chemical properties of heavy metals in Bameng soils are similar.In Chifeng soils,the same heavy metal in different soil layers is significantly correlated,while different heavy metals in the same soil layer are poorly correlated.This implies that different heavy metals in the same soil layer may come from different sources.In the study area in Bameng,mining activities and transport are the main sources of heavy metal pollution,explaining 47.55% of the pollution sources.Additionally,tailings and soil-forming parent materials also contribute to the heavy metal pollution.Mining activities mainly contaminate the 0-5 cm soil layer,soil-forming parent materials mainly contaminate the 10-20 cm soil layer,and the intermediate soil layer is jointly polluted by the two sources.Heavy metal pollution sources in Chifeng are mainly from mining activities,industrial waste,soil-forming parent material,traffic and transportation and agricultural activities.The sources of pollution vary from one heavy metal to another,with mining activities being the source of all heavy metals.In addition,Cu comes from agricultural sources,Zn and Cd from waste,and As from parent materials,while Pb is mainly from natural sources,transport and industrial waste.(3)The migration pathways and characteristics of heavy metals in the Bameng and Chifeng study areas are different.Atmospheric deposition is the main heavy metal migration pathway in Bameng,contributing up to 80% to Cu,Zn,As and Pb migration,while surface runoff mainly works in the northern part of the mine area.This coincides with the sparse rainfall and strong winds in the area.In contrast,the migration pathways of different heavy metals very much in Chifeng.The contribution of surface runoff to Cu migration gradually decreases with increasing soil depth,and finally the contributions of atmospheric deposition and surface runoff are equal.Atmospheric deposition contributes more than 90% to the transports of Zn and Pb,while surface runoff is the main driving forces for As and Cd migration.(4)The Bameng soils have a high sand content of ～80%.Chifeng soils are composed of sand,slit and clay,each of which has a content of ～30%.Soil textures have great effect on heavy metal migration.In the case of current contamination in the0-20 cm soil layer in Bameng soils,the maximum vertical migration distance of heavy metals will be >120 cm,with maximum concentrations at 40 cm.In contrast,heavy metals in Chifeng soils can migrate to depths >80 cm,with peak concentrations occurring at 20 cm.The heavy metal concentrations in the soil profiles are high in the middle and low at the top and bottom ends.None of the heavy metals will contaminate the groundwater due to the retention effect of the soil particles.In the Chifeng soils,the concentration of Zn changes slowly while that of As changes faster,owing to their different retention affinities on soils.This clearly indicates that the affinity of heavy metals on soil particles is an important factor affecting the transport.Scenario analysis can predict the transport of heavy metals in different environments.The heavy metal transport behaviors in Bameng and Chifeng soils change differently when rainfall changes.When the amount or intensity of rainfall increases,the peak heavy metal concentration in Bameng will decrease and the migration distance will increase,the surface runoff will carry higher amounts of heavy metals,and the total heavy metal concentration in the soil profile will decrease.However,there is no significant change in the Chifeng soils,probably because of its high clay content or that the heavy metal concentrations have reached equilibrium.When the amount or intensity of rainfall decreases,the heavy metal migration distance in Bameng soils will decrease and the peak concentration will increase.Though As behaviors in Chifeng soils will change similarly,those of Zn,which has a high affinity coefficient on soil particles,will do not change significantly.Intermittent rainfall has no effect on the metal migration.When the groundwater level in Bameng becomes lower,soil water content decreases,the heavy metal concentration increases,but there is no significant change in the heavy metal migration depth.