Study On Spatial Distribution Mechanism Of Soil Heavy Metal Pollution Risk In Copper Smelting Affected Area

The pollutants discharged from copper smelting lead to excessive accumulation of heavy metals in the surrounding soil,threatening the ecological environment and the health of residents.The spatial distribution of heavy metals in the soil around smelters shows high heterogeneity because of the complex factors.In this research,literature data published in recent 20 years were collected,and a typical copper smelting influencing area in Hunan Province was selected as the study area.After sampling and local survey,concentrations of heavy metals and soil properties were detected in laboratory.Spatial interpolation,correlation analysis,multiple regression analysis and decision tree analysis were used in this study.The accumulation characteristics of heavy metals in soils of copper smelting influencing areas on a global scale,and the spatial distribution characteristics and influencing factors of heavy metals in typical copper smelting affected areas were studied.And the potential ecological and environmental risks of heavy metals in soil were analyzed.The main results are as follows:（1）The accumulation characteristics of soil heavy metals in copper smelting-affected areas were studied by integrating and analyzing the literature data on soil heavy metal concentrations in copper smelting-affected areas published in the past 20 years.The results showed that the average concentrations of As,Cd,Cu,Pb and Zn were 248,16.8,502,886and 250 mg kg^-1,respectively;The accumulation of Cd and Cu in soil was the most serious.High values of soil heavy metal Nemerow index（NIPI）mainly appear in smelting sites and surrounding soils with long smelting history,backward technology and insufficient environmental protection measures.Soil As,Cd,Cu,Pb and Zn have similar sources,and the concentrations decreased with the increase of sampling radius,and mainly accumulates within 3 km around the smelting sites.（2）A typical copper smelting affected area in Hunan Province was selected as the research object,and the distribution characteristics and key influencing factors of heavy metals were studied through sampling survey,combined with spatial interpolation,correlation analysis,and exponential decay fitting.The results showed that the average contents of As,Cd,Cu,Pb and Zn in the topsoil were 248.0,16.8,502.4,885.6 and 250.2 mg·kg^-1,which were 8.3,24.0,6.9,16.2 and 3.2 times the background value,respectively.is the most heavily accumulated element.The hot spots of heavy metal content are mainly distributed in the smelting production area and the runoff affected area.The content of heavy metals decreases with the increase of distance,and the pollution influence distance is 450-1000m.Analysis of soil profile samples showed that As,Cd,Cu,Pb and Zn contents decreased with increasing depth.The heavy metal content of different land use/cover types was grassland>farmland>forest land,and the heavy metal content of paired sampling was grassland>bare land.The soil heavy metal content of different slopes was in the order of flat land>gentle slope>steep slope.The increase of soil p H,the increase of organic matter（SOM）content,the increase of bulk density and the decrease of hydraulic conductivity will increase the accumulation of heavy metals in soil.（3）The results of the study on the spatial distribution and influencing factors of the available heavy metals in the soil of typical copper smelting affected areas show that the available content of As,Cd,Cu,Pb and Zn are0.10,1.00,4.24,5.20 and 7.28 mg·kg^-1,K_d,respectively.The values were32386,495,153981,16429 and 1310 L·kg^-1,respectively.Cd was the most polluted and most mobile element in the study area.Spatially,the high content of available heavy metals is mainly distributed in the smelting production area,the runoff-affected area,near towns,and downwind of the smelter;the low K_d value of heavy metals is mainly distributed in the low-lying area and the runoff-affected area around the smelting production area.Both the available state of heavy metals and K_d distribution were affected by land use/cover,total heavy metals,soil p H and SOM content.The available content of heavy metals in the soil of bare land and agricultural land was higher than that of grassland and forest land;The total amount of heavy metals increased,the soil p H and SOM content decreased,and the content of available heavy metals increased;the total amount of heavy metals,soil p H and SOM content increased,the content of available heavy metals decreased,and the K_d of heavy metals increased.In addition,the available content of heavy metals decreases with the increase of the distance from the smelting production area.（4）Through spatial interpolation,exponential decay fitting and decision tree analysis,the spatial distribution and key influencing factors of soil heavy metal ecological risk（RI）and polluted groundwater environmental risk（SGR）in typical copper smelting affected areas are studied.The results show that Cd is the main element of ecological risk,and the main elements of environmental risk are Pb and Cd.The ecological risks and environmental risks are mainly distributed in the smelting production area,the runoff affected area,the vicinity of the town and the downwind direction.The main factors affecting the distribution of ecological risk and environmental risk are distance,land use/cover type,slope,total amount of heavy metals,available state content,K_d,soil p H and SOM.Both RI and SGR values were the largest in the bare ground.The RI value is maximum in the flat region.The total and available content of heavy metals increased,the soil p H decreased,and the potential ecological risk increased;the total heavy metal increased,the K_d value decreased,the soil p H and SOM content decreased,and the environmental risk increased.With the increase of distance,the ecological risk and environmental risk of heavy metals were significantly reduced.There were extremely strong and strong ecological risks at 400 m and 400-800 m away from the smelting production area,respectively;and there was environmental risk within1700 m.Decision tree analysis shows that distance is the most important factor affecting the distribution of the two risks.The high ecological risk is in the area less than 455 m away from the smelting production area and the flat area in the non-upwind direction between 455 and 1005 m.Distance<1005 m and SOM content<6.4 g·kg^-1.