Effect Of Pearl River System On Environmental Quality Of Alluvial Plain Soil

According to multi-purpose geochemical survey,Cadmium and other heavy metal elements in the soil of Pearl River Delta(PRD)Quaternary alluvial plain are enriched in the upper reaches of the Pearl River and the coastal plain area,which is controlled by the regional geological background and land-sea interaction.The Pearl River system plays an important role in the formation and development of the delta.River system,as an important carrier and storage of heavy metals,makes a difference in the environmental quality of alluvial plain.Focussing on the effect of the Pearl River system on soil environment,a series of geochemical analysis and interpretation jobs including the Quaternary core,modern floodplain,stream sediments and natural forest soil profiles have been done in this thesis.The environmental evolution history,river chemical composition and land-sea interaction facies in the PRD have been clearly understood.The results show that the geochemical features of the soil in PRD Quaternary region is mainly controlled by its provenance and land-sea interaction.The respective contribution rates of these two factors are different in different sedimentary facies.Provenance contributed most in delta plain facies,and land-sea interaction involved much in deposition process under the prodelta facie.The mobile fraction of Cd is high in the Quaternary,and its ecological risk is the highest.The geoaccumulation index,potential ecological risk index and sediment guidlines have been applied to evaluate the risk of the PRD sediments,which showed the minimum risk in East River sediments and the highest potential ecological risk of heavy metals in sediment of North River.Influenced by mining activities,Cd,Pb,Zn in the upper areas of North River is highly enriched.The distribution of heavy metals in the sediments of the floodplain profile showed a common pattern.The heavy metals in the section show the highest peak at 38.5 cm,which may record a period of environmental degradation in the basin.The contents of heavy metals in the sediments of the floodplain are mainly affected by organic carbon,clay components and iron oxides.Barium content in soil is regarded as an indicative of land-sea facies in this study.Land-sea interaction has an important influence on the distribution of Cd in soil.With the change of sedimentary facies from land to sea to marine,the content of Cd in soil increases.In the soil profile of continental forest land in the middle and upper reaches of West River,Cd is enriched in top layer,but the content of Cd increased rapidly in deep layer of soil profile under the marine facie.Effect of Pearl River system on soil environmental quality is mainly reflected as follows:(1)Carried by Pearl River,stream sediment and suspended solids can be treat as the parent material of soil in Quaternary plain,whose composition characteristics can be an important indicator to predict future changes in watershed environment;(2)River-ocean interaction occurred during PRD formation,which leaded to diverse sedimentary facies in plain,and the physical and chemical properties of soil changed alonge with this interaction,which further influence the enrichment and activity of heavy metals in each sedimentary facies zone.