Spatial Distribution And Source Identification Of Cadmium In Typical Polluted Areas Of Guizhou Province

Cadmium（Cd）is a toxic and harmful heavy metal element.The situation of soil Cd pollution in China is very serious.Wanshan Mercury Mine is a typical high Cd area in Guizhou Province.Soil Cd sources and migration pathways have not been reported.In this paper,41 sampling sites were selected in Wanshan area.Rocks,forestland soil,farmland soil,mining waste rock,slag samples and background samples were collected systematically.Trace elements and cadmium isotopes were analyzed by ICP-MS and MC-ICP-MS.The spatial distribution trend of cadmium in Wanshan area was predicted by spatial interpolation method.The spatial distribution trend and main influencing factors of cadmium in Wanshan area were discussed.The following main understandings were obtained:1.The arithmetic mean Cd content of rocks in Wanshan areas is 0.44mg/kg.The Cd content of Aoxi limestone outcropped in the west is the highest with an average of4.58 mg/kg,while that of Qingshuijiang limestone outcropped in the East is the lowest with an average of 0.10 mg/kg.The spatial distribution of cadmium in rocks tends to be higher in the West and lower in the east.The variation ranges of Cd content in mining waste rock and smelting waste slag are 0.95-1.32 mg/kg and 0.77-1.08 mg/kg,respectively.The variation range of rocks in Wanshan area is-11.13-0.06ε,and the variation range of Cd isotope composition of limestone is consistent with that of rocks.ε^114/110Cd in Black shale is-1.27ε,ε^114/110Cd in mining waste rock and smelting waste is-0.71εandε^114/110Cd=2.04ε,respectively,which indicates that distinct Cd isotope fractionation can be produced during ore smelting,and heavy Cd isotope is enriched in the waste slag.2.The average cadmium content in the soils of Wanshan district is 0.43 ug/g,which is 4.78 times of the national background value（0.09 ug/g）.The spatial distribution of cadmium in soils showed a trend of high in the West and low in the east.The average Cd content in soils of high cadmium areas in the West was 0.83 mg/kg,twice as much as that in the east.ε^114/110Cd of soil were-5.76¹.34ε,Wanshan Town was-1.15±2.75ε,forest soil was-5.76⁰.55ε,farmland soil was-2.64⁰.53ε.The soil near the mining waste heap is-2.75-1.94ε,and the soil near the slag heap is 1.34ε.The spatial distribution of soilε^114/110Cd showed a trend of decreasing from abandoned slag heap to surrounding area.The regional soil of Wanshan town was heavierε^114/110Cd,and the background soilε^114/110Cd of northern Wanshan was less than-1.75ε.The change trend of soil Cd is consistent with that of rocks,indicating that rock weathering affects the spatial distribution of Cd.The distribution of Cd in woodland soil is point source distribution,while that in farmland soil is point source distribution as well as non-point source distribution.Because the application of phosphorus fertilizer usually contains high U,the distribution of Cd in farmland soil is consistent with that of U,indicating that the application of phosphorus fertilizer has obvious effect.The main influencing factors of cadmium distribution in Wanshan soil are rock weathering,stacking of waste mercury slag and application of phosphorus fertilizer.3.According to the spatial distribution of cadmium content and cadmium isotope,Wanshan can be divided into three areas:the area affected by rock weathering is most likely to be controlled when soilε^114/110Cd<-1.75εand the content of Cd is less than0.29 mg/kg;the area were affected by slag stacking whenε^114/110Cd Cd>-1.95εand the content of Cd is higher than 0.40 mg/kg.and other areas affected by rock weathering,slag stacking and application of phosphorus fertilizer and other areas of interaction.The range of potential ecological risk index in Wanshan area is 14-367.40.1%of the areas have a mild ecological risk range;over 20%of the areas have moderate to high ecological risk.Among them,the potential ecological risk index of Wanshan Town is 115,which is moderate ecological risk.The spatial distribution trend of potential ecological risk index is higher in the West and lower in the east,which is consistent with the spatial distribution trend of Cd content in rocks and soils.