Study On The Characteristics Of Heavy Metal Migration And Enrichment And Environmental Risk Assessment During The Weathering Of Carbonate Rocks

Carbonate weathered soil is an important soil type in the karst area of southwest China.The phenomenon of excessive accumulation of heavy metals in the weathering and formation of carbonate rocks and the problems of excessive accumulation of heavy metals in the soil have gradually attracted academic attention.However,the cause of heavy metal accumulation in the soil in the karst area is not very clear at present.To explore the carbonate rock in weathered soil heavy metal elements in the process of migration and enrichment characteristics,this paper trait area as the main research area,choose a different formation(Cambrian,Permian and carboniferous)of carbonate rocks are typical soil profile as the research object,using the extraction of carbonate acid insolubles experimental study on the occurrence characteristics of heavy metals in the bedrock,And combined with the feature of mass balance method to explore the migration of heavy metals,through comparing the different formation carbonate rocks and weathered soil heavy metal element geochemical characteristics,preliminary discussed the characteristics of migration and enrichment of heavy metals in carbonate rock weathering profile,that influences the carbonate weathering process of soil heavy metal migration enrichment factors and their impact on the environment,the main results were as follows:(1)There are some differences in the heavy metal elements in carbonate rocks from different strata,but the distribution characteristics of heavy metal contents in carbonate rocks are opposite to those of acid insoluble matter.The distribution characteristics of heavy metal content in soil were similar to that of acid insoluble matter.During the weathering process of carbonate rocks,the heavy metal content in the soil may be greatly affected by the heavy metal content of acid insoluble substances in carbonate rocks to some extent.(2)Various heavy metal elements in weathering profiles of carbonate rocks show strong leaching characteristics from top to bottom.There are significant differences in the degree of heavy metal migration in the weathering profiles of carbonate rocks in different strata.The degree of heavy metal migration is stronger in the Carboniferous and Permian,followed by the Cambrian.The migration characteristics of elements in different sections show a certain regularity,the migration degree of Ni and Cr is stronger,the migration degree of As and Pb is relatively weak,which indicates that the unique geochemical properties of some elements affect their migration ability in the supergenetic environment to a certain extent.At the same time,all kinds of heavy metals do not tend to occur in a single mineral phase of carbonate rocks.The difference of occurrence of such heavy metals in carbonate rocks leads to the difference of weathering rate,which makes the heavy metal elements with a larger proportion in the acid soluble of rocks easier to be leachated in large quantities during weathering.(3)The weathering and soil-forming process of carbonate rocks in the study area has the characteristics of low background and high enrichment.The phenomenon of excessive enrichment of heavy metals in weathered carbonate soils relative to parent rocks is probably due to the huge loss of volume and mass caused by the leaching of Ca and Mg elements during the weathering of carbonate rocks,which leads to the relative enrichment of heavy metals in weathered carbonate soils.At the same time,part of heavy metals released during the weathering of carbonate rocks will remain in the residual soil in various forms,which is conducive to the enrichment of heavy metals in the soil.(4)There is a significant positive correlation between the content of heavy metals in most acidinsoluble minerals and the content of heavy metals in the soil,which indicates that the content of heavy metals in the acid-insoluble minerals of carbonate rocks may play a key role in controlling the content of heavy metals in weathered soil.However,the correlation between the content of heavy metals in some soils and the acid insoluble minerals is low,which indicates that the weathering process of carbonate rocks is affected by other factors,which may be related to the release of heavy metals in the acid soluble mineral phase and the secondary enrichment in the soil.(5)The content of heavy metals in some weathering profiles of carbonate rocks in the study area is seriously out of standard,and the extent of heavy metals in weathering soils of carbonate rocks in different strata varies greatly,especially in the Permian and Carboniferous weathering soils.In terms of element types,Cd exceeded the standard of the most serious.In view of the unique weathering process and heavy metal enrichment characteristics of carbonate rocks,the government and relevant departments should pay more attention to the excessive heavy metals in weathered carbonate soils,especially in special strata,and strengthen the monitoring of heavy metal pollution in surface runoff and groundwater in karst areas.