| The excessive level of heavy metals in soil has always been a concern of the academic community.Cd and Pb have once become the focus of research due to the characteristics of high excessive level of soil and high toxicity.In recent years,more and more attention has been paid to the significant enrichment of Cd and Pb during the weathering of carbonate rocks,especially to the ecological environment and food security risks caused by the migration of Cd and Pb elements in karst areas.However,the migration mechanism and material source of Cd and Pb are still unclear,which has limited soil pollution control and land management in karst areas.In order to systematically study the migration process and driving mechanism of heavy metals Cd and Pb during soil erosion in karst areas,this study took typical carbonate rocks and clastic rocks in southern Guizhou as the research object,investigated the contents and morphology of Cd and Pb,soil physical and chemical properties,soil particle size and other indexes of test samples.The influence mechanism of soil erosion on the migration characteristics of Cd and Pb in karst areas was discussed,and the following conclusions were drawn:1)The results of soil grain size analysis showed that chemical migration was the key factor leading to the significant increase of soil element content in alluvial layer,while mechanical migration did not significantly change the spatial pattern of element content.The mechanical migration of Pb in slope soil was mainly studied during the erosion process.In addition to mechanical migration,chemical migration of soil Cd is also an important mode of migration.Therefore,soil erosion changes the spatial distribution pattern of the original Cd and Pb content in the study slope to varying degrees,and the variation range of Cd content in the study slope is greater than that of Pb.2)Cd content in soil is obviously controlled by the type of parent rock,so the migration characteristics of Cd in soil developed in different lithologies are different.The weathering and dissolution of carbonate rocks in karst area can release a large amount of Cd,so that the content of Cd in the soil of alluvium in karst area is generally higher than that of the residual layer.However,although the alluvium of clastic rock area receives Cd from the eluvial layer,the accumulation of Cd in the alluvium is not significant due to insufficient Cd source.In addition,topography is also an important factor controlling Cd migration.Even in the same karst area,the soil Cd content of alluvium in the exposed slope is significantly higher than that in the completely covered slope.This is mainly due to the fact that weathering dissolution of exposed carbonate rocks can continue to provide active Cd to the alluvium.3)Soil erosion affected the occurrence and morphological characteristics of Cd and Pb in soil to varying degrees.The morphology of Pb in soil at different positions on slope was dominated by residual state,and the proportion of soluble,reducible and oxidizing states of weak acid in soil did not fluctuate significantly.From the top of the slope to the foot of the slope,the proportion of Cd soluble state of weak acid increased significantly,and the ecological risk increased accordingly.The activity of Cd in soil is affected by the type of parent rock,and the activity of Cd in weathering carbonate rock is stronger than that in weathering clastic rock.Soil erosion may lead to the increase of Cd activity in alluvial soil at the foot of slope,and Cd activity and migration and transformation ability are stronger than Pb during soil erosion in karst area.4)The results of risk assessment index method(RAC)and secondary and primary phase distribution ratio method(RSP)showed that Pb was a low risk to the ecological environment,while Cd was a medium risk to the ecological environment.The ecological environment of Cd is affected by the type of parent rock.The ecological environment risk of Cd in carbonate soil is higher than that in clastic soil.Topography and geomorphology are also important factors affecting the environmental and ecological risk of Cd.The environmental and ecological risk of Cd on the exposed bedrock slope is significantly higher than that on the completely covered bedrock slope.Compared with mechanical migration,the ecological risk of element chemical migration is higher.For the area with abnormal Cd geological background and a large amount of exposed bedrock,it is suggested to return farmland to forest,conserve water and soil,and strengthen slope management. |
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