The Study On The Enrichment Mechanism Of Cd And Other Elements In The Xijiang River Basin

The high content area of heavy metals in the Pearl River Delta is highly consistent with the Quaternary distribution area,and the environmental effect of Cd element is the most significant.The Pearl River Delta is mainly controlled by Xijiang River and Beijiang River.Therefore,the floodplain of Xijiang River Basin is the research area in this paper.The element content analysis,morphological analysis and mineralogical analysis are carried out on the suspended solid samples from different locations in the upper and lower reaches of Xijiang River Basin,the natural surfacies and deep soil samples in the floodplain,and the natural soil profile samples in different sedimentary phases.Based on these results,this paper summarizes and discusses the content,spatial distribution characteristics,enrichment degree and influencing factors of suspended solids and soil samples in Xijiang River Basin.The geochemical behaviors of heavy metals and their influencing factors in the process of soil formation in different sedimentary facies are also discussed in this paper.Compared with the sediments in the whole country,the Pearl River system,and the suspended solids in the Yangtze River and Liaohe river basins,the suspended solids show the higher degree of the content enrichment in the heavy metal elements.Four heavy metals in the suspended solids are all polluted in different degrees.The pollution level from high to low is Cd > Cu > Zn > Pb,and the pollution level of Cd in the whole basin is serious.The heavy metals in the soil of the floodplain are significantly enriched compared to the background values of the whole country and Guangdong.The pollution of Cd which exceeds the risk screening value is serious and Cu,Pb and Zn also show the pollution risk.Compared with the suspended solids in Xijiang River Basin,Cd and Zn in the surfacies soil show the enrichment,which indicates that the deposition and soil forming process cause the further enrichment of Cd and Zn elements.The contents of heavy metal elements in different sedimentary facies are significantly higher than the background values and standard values,showing the characteristics of heavy metal enrichments.The enrichment degree of each heavy metal element and its potential active state is different in different depositional environments.The enrichments of Cd,Pb and Zn in the delta plain facies deposition environment are the most significant,which are greater than those in the river facies.The enrichments of Cd,Pb and Zn are the lowest in the delta-front facies.Different depositional environments also lead to different factors affecting the enrichment of heavy metal elements in different sedimentary phases.The heavy metal elements in suspended solids,surface and deep soil and profile soil all show the characteristics of enrichment,indicating that they mainly come from geological background.In other words,the carbonate rocks and sulfide deposits in the upper reaches are widely distributed.The fine-grained particles which are rich in heavy metal elements in the weathering materials are preferentially eroded and deposited in the downstream low-lying area after long-distance moving,so the soil formed on the basis of this material has a high content of heavy metals.