Source Discrimination Of Potentially Toxic Elements And Cadmium Ecological Risks In Soils From Pearl River Delta Economic Zone,China

Delta plains have been the cradles of civilization,and priority development areas for industrialization and urbanization for centuries.Therefore,the alluvial soils have been exposed to potentially toxic elements(PTEs)owing to strong anthropogenic activities as well as the riverine sediments derived from upstream parent materials.The crops planted on the soils of delta plains also exhibited certain potential ecological risks.Although anthropogenic contamination has been regarded as the most important source of PTEs in the soils of large river delta plains,the contaminant status or sources of PTEs in alluvial soils continues to be controversial.The extent to which human activities affect PTEs in soils,and the effects of different parent materials on the bioavailability of PTEs in soils are worth exploring.Based on high density geochemical data of soils and stream sediments of the Pearl River Delta Economic Zone,this study used enrichment factor,discriminant analysis,principal components analysis,cumulative distribution function,and positive matrix factorization to identify the sources of PTEs in soils,and distinguish the different contributions of the Pearl River system to soils and stream sediments.The data of rice grain and corresponding rhizosphere soil samples developed in different parent materials were used to discuss the influence factors of Cd bioavailability,and establish the prediction models of prediction models of the content and proportion of active state and potential active state of Cd in soils,as well as the content and bioaccumulation factor of Cd in rice grains.Based on the prediction models,the zoning of land safe usage was carried out.The results indicated that parent material,rather than anthropogenic contamination sources,was the most significant factor affecting the geochemical characteristics of PTEs in soils.Soils and stream sediments developed in quaternary sediments had high concentrations of Cd,Cr,Cu,Hg,Zn,Fe,Mn,and Ca.Chemical weathering and erosion of carbonate rocks,acid magmatic rocks,and polymetallic deposits distributed in the Pearl River watershed were the main sources of PTEs in deep and surface soils.The enrichment degree of PTEs in soils developed in the downstream of West River tributary and North River tributary showed significant difference.Stream sediments and soils developed in downstream of West River tributary had relatively high concentrations of Cd,Cr and Cu.While stream sediments and soils developed in downstream of North River tributary were more enriched in Pb.This difference is owing to the difference of the main parent materials distributed in West and North River tributary.Diffuse contamination and many small local contamination sources were the most significant anthropogenic sources of PTEs in surface soils.However,intensive industrial,agricultural,and urban activities failed to change the soil geochemical characteristics derived from the parent material at the regional scale.p H and Cd concentration were the main factors affecting the content and proportion of active state of Cd,while CEC,clay and Cd content were the main factors affecting the potential active content of Cd in rhizosphere soils.The influence factors were slightly different in soil developed in different parent materials.According to the prediction model of Cd content or enrichment coefficient of rice grains,the zoning of land safe usage was carried out using the geochemical data collected from the National Multi-Purpose Regional Geochemical Survey in the study area.The results showed that lands of the study area under preferential reached approximately 89.58%.Lands under safe exploitation accounting for 9.71% of the whole region.Lands needed structural adjustment was mainly distributed in soils developed in terrigenous clastic rocks.The Pearl River Delta showed high Cd concentrations in soils but relatively low concentrations in rice grains,and presented low ecological risk.This study represents a valuable reference for source apportionment of PTEs in soils in the large river delta,which is undergoing intense agricultural and industrial activities and urbanization.It also provides scientific basis for controlling and prevention of Cd pollution,as well as the safety management in farmland soils in delta plains.