Accumulation Characteristics And Pollution Assessment Of Heavy Metals In Soils Of Different Land Use Types In Tianjin

Soil is one of the most basic natural resources for human survival.The pollution of heavy metals in soil may affect the normal function of soil,cause pollution of food chain,affect food production,and pose a threat to human health and ecological environment.Therefore,it is of great significance to study the accumulation characteristics of soil heavy metals and scientifically evaluate the pollution level,to reduce heavy metal pollution,to rationally develop the economy,to improve people's quality of life,and to accelerate the construction of ecological civilization.In this paper,the topsoil of Tianjin is used as the research area and object,combined with the land use type,using various data analysis and multivariate statistical analysis methods to study seven heavy metal elements Cr,Ni,Cu,Zn,As,Pb,Cd in the soil.Content distribution and sources of pollution.The comprehensive accumulation index method,Nemero index method and pollution load index method were used to comprehensively evaluate the soil heavy metal pollution level in the study area.The main conclusions are as follows:(1)The soil in Tianjin is alkaline to strongly alkaline.The median contents of seven heavy metal elements are Cr(70.7 mg/kg),Ni(27.9 mg/kg),Cu(24.3 mg/kg),Zn(79.1mg/kg),As(11.9 mg/kg),Pb(24.3 mg/kg)and Cd(0.15 mg/kg).The median values of Cr and Ni elements are lower than the background values and do not exceed the screening values.The median values of Cu,Zn and Pb elements are close to the background values,and only a few(< 2%)points exceed the screening values.The median contents of As and CD elements are higher than the background values,and a few points exceed the screening values.As,Zn,Cu and Ni are moderate,Pb is strong and Cd is very strong.(2)Based on the background value of soil in Tianjin,there are more over-standard elements in Xiqing and Tanggu districts,less over-standard elements in Beichen,Baodi and Jizhou districts,but the variation coefficient of heavy metal content in Jizhou district is very large.In terms of land use types,the accumulation of heavy metals in grassland is the highest,and the variation coefficient of heavy metals content in garden land,industrial and mining storage land,residential land and transportation land is higher,which indicates that the heavy metals content in soil under these land use types is obviously disturbed by human factors.(3)Heavy metals in Tianjin soil are not related to Cd and AS,but the other heavy metals are related to each other.In the study area,the correlation between elements in different regions is different.Through cluster analysis and principal component analysis,the seven heavy metal elements are roughly divided into four groups: Cd,Pb and Zn as a group,human factors such as transportation and industry as the main pollution contributing sources(PC1);Cr and Ni as a group,soil parent materials as the main pollution contributing sources(PC2);Cu as a group,mining and metal smelting as the main pollution contributing sources(PC3);As as a group,presumed as agriculture.And surface runoff is the main pollution contributor(PC4).(4)The evaluation results by the method of ground accumulation index showed that all the seven heavy metal elements in Tianjin soil were pollution-free except Cd,and Cd was pollution-free to moderate level.Nemerow comprehensive pollution index evaluation method and pollution load index evaluation method show that the comprehensive pollution level of heavy metals in Tianjin soil is non-pollution level/safety level.The interpolation analysis and comparison of the indexes of the two methods show that the high value regions of the indexes calculated by the two methods are basically the same,and the specific high value points are different.