Geochemical Characteristics Of Soil Pb In Eastern Hezhou,Guangxi

Lead(Pb),a potentially toxic element,is a typical representative of soil heavy metal pollution.In recent years,the accumulation of Pb and other harmful elements in the soil environment is increasingly severe due to exogenous input and high geological background.Therefore,it is important for the classified management of land resources,the safe production of agricultural products,and the national health to identify the origin,study the distribution characteristics,and summary the enrichment and dilution rules of Pb and other harmful elements in the complex soil.This paper selects the Babu District and Pinggui District,Hezhou City,as the study area,and systematically collected surface soil samples,soil profile samples,rock samples,and crop seed and root soil samples to analyze some indexes,including heavy metal elements such as Pb,a large number of elements,pH,Corg and Pb isotopes.The main research results obtained in this paper are as follows:(1)The content of the Pb element in the surface soil of the study area ranges from 8.58 mg/kg to 1316 mg/kg,with an average of 43.42 mg/kg,which is higher than the national soil background value of the Pb element(30 mg/kg)and Guangxi soil background value of the Pb element(38mg/kg).In the surface soil of different parent material areas,the content of the Pb element ranks as follows: granite > Quaternary sediments > clastic rocks > carbonate rocks,indicating the outcropping areas of granite and the sediments of Quaternary have a specific pollution risk causing by Pb element.(2)The study of Pb isotopes in soil profiles shows that the primary sources of the Pb element in soils formed by granite and clastic parent materials are the parent materials.The Quaternary also suggests some characteristics of exogenous input.In different parent materials,the variation trend and influence factors of Pb content in soil profiles are slightly different.The accumulation of the Pb element in the granite soil profile is mainly controlled by the chemical weathering process of bearing rock minerals(potassium feldspar,biotite,etc.)of the Pb element.In the sediments of Quaternary,the lithology difference of different catchment areas and mining areas are the main factors for causing the spatial variation of the content of the Pb element.In the clastic rock area,heavy metal elements with low activity are largely enriched in the soil due to strong weathering and leaching,and the content of the Pb element in the soil profile is the highest.In carbonate rocks,the influence factors of carbonate rock include not only the secondary enrichment of Pb and other elements during the dissolution and acid-insoluble accumulation of carbonate rocks in the early stage but also the enrichment caused by the adsorption of soil iron/aluminum oxides,clay minerals,and organic matter during the weathering and soil formation in the late stage.(3)The content of the Pb element of rice,maize,and other crops in the study area is generally low,and it is manifested as rice root > stem and leaf > seeds.Only one sample in 169 rice seeds exceeds the permitted levels of the Pb element.Based on the biological enrichment coefficient of the Pb element in rice and the influence factors,a quantitative prediction model passing the validity test within the error range for the content of the Pb element in rice seeds is established.The model is applied to predict the content of the Pb element of rice seeds,and a land resource classification scheme based on soil risk level and rice seed security of the Pb element is proposed.The results show that cropland in the study area can be classified into priority protection,safe utilization and strict control,and their proportion is 99.69%,0.25% and 0.06%,respectively.The results can provide some bases for the safe production of crops and scientific management of land resources in the study area.