Pollution Characteristics And Risk Assessment Of Topsoil Heavy Metals In Different Functional Areas In The Main Urban Area Of Lanzhou

Urban soil is an important carrier and resource to ensure the quality of the urban ecological environment and the health of the urban population.However,as the process of urbanization accelerates,a large number of heavy metal pollutants generated by unreasonable human activities will also enter the urban environment.Pollutants such as heavy metals are mainly concentrated in the surface soil of the city.When their concentration exceeds the background value of the soil,it will affect the normal function of the soil,resulting in pollution of the food chain,which in turn affects crop yields and poses a threat to the ecological environment and human health.Current research on heavy metals in urban surface soils mainly focuses on individual urban functional subjects such as roads,parks and campuses,but the evaluation method is single,and the analysis of the factors affecting soil heavy metal pollution is not comprehensive,and the various functional areas of the city are not combined.Therefore,scientifically evaluating the pollution status of heavy metals in urban topsoil and researching the sources of pollution will contribute to the development of social economy and the construction of ecological civilization.In this study,the surface soils of different functional areas in the main urban area of Lanzhou City were used as the research object.X-ray fluorescence spectrometer was used to determine the total amount of 8 heavy metal elements（V,Cr,Co,Ni,Cu,Zn,As and Pb）and the main elements of soil-forming parent material（Fe₂O₃and Mg O）in 269surface soil samples in the study area.Using descriptive statistics,geo-accumulation index and geo-statistical analysis methods to study the content,enrichment and spatial distribution characteristics of surface soil heavy metals in different functional areas of Lanzhou City.At the same time,correlation analysis,principal component analysis and Unmix receptor model were used to determine the main sources of heavy metals in the surface soils in the study area,influencing factors,and their relationships with soil parent materials.The single-factor pollution index method and Nemeiro comprehensive pollution index method were used to evaluate the pollution level of heavy metals in the surface soils of different functional areas in the main urban area of Lanzhou.The potential ecological risk index method was used to analyze the ecological risk level of heavy metals in the topsoil of the study area.Finally,the USEPA health risk assessment model is used to evaluate the health risk status of children and adults in the study area.The analysis draws the following main conclusions.First of all,among the 8 heavy metal elements in the surface soil of the main urban area of Lanzhou City,the average content of V and Cr are lower than the background value of Gansu Province,Ni and As are close to the background value,and Co,Cu,Zn and Pb are higher than the background value,respectively.4.69,1.71,1.58 and 1.94times the value.The average content of V and Cr in each district did not exceed the background value,the Co、Cu、Zn and Pb exceeded the background value,Ni exceeded the background value in Xigu District and Chengguan District,and the As was equivalent to the background value.The mean content of V and Cr in each functional area is lower than the background value,As except that the industrial area is lower than the background value,the mean content of Co、Ni、Cu、Zn and Pb in each functional area is higher than the background value.Second,the geo-accumulation index method reveals that V,Cr,Ni and As are basically non-enriched in the study area,Co is mainly enriched in a moderate degree,and Zn and Cu are mainly enriched slightly.The average accumulation index of Co in the main urban area is Anning District>Xigu District>Qilihe District>Chengguan District,all of which are moderately enriched,Pb element performance is Qilihe District>Xigu District>Chengguan District>Anning District,Pb is slightly enriched.Third,multivariate statistical analysis shows that the two pollution sources of heavy metals,namely Co,Cu,Zn,and Pb,are controlled by man-made sources,and V,Cr,Ni and As are controlled by natural sources.The Unmix receptor model source resolved 4 pollution sources,namely,source 1（Zn-Pb）is an industrial and traffic source,source 2（Cu）is a mixed source,one part comes from the auto repair industry,and the other part comes from cables and hardware equipment.Source 3（Co）is the construction industry.source 4（Cr,Ni and As）is a natural source.Fourth,the average single-factor pollution index of the eight heavy metals is Co>Pb>Cu>Zn>As>Ni>V>Cr,and the average Nemeiro comprehensive pollution index in the main urban area is Anning District>Xigu District>Qilihe District>Chengguan District,all districts have severe pollution levels.The mean value of the Nemeiro Comprehensive Pollution Index in each functional area is in order of commercial area>industrial area>cultural and educational area>traffic area>park>residential area.All functional areas are also heavily polluted,and Co and Pb are the main pollution factors.Fifth,the average value of the individual potential ecological risk indexes of the eight heavy metals is Co>As>Pb>Cu>Ni>Cr>V>Zn,and the average value of the comprehensive potential ecological risk index in the main urban area is Xigu District>Qilihe District>Anning District>Chengguan District,each district has reached a medium ecological risk.The average value of the comprehensive potential ecological risk index of each functional area is business area>industrial area>cultural and educational area>traffic area>park>residential area.Each functional area also has a medium ecological risk,and Co and As are the main ecological risk factors.Sixth,the non-carcinogenic exposure doses of the 8 heavy metals are Co>Zn>V>Cr>Cu>Ni>Pb>As,and the daily average exposure dose of children under the non-carcinogenic exposure route is skin contact>hand-oral intake>Breathing inhalation,adult is hand-oral ingestion>breathing inhalation>skin contact.The non-carcinogenic risk index of children in different functional areas is residential area>cultural and educational area>commercial area>leisure area>transportation area>industrial area,which is 9.63 times that of adults.There is no non-carcinogenic risk for adults.The carcinogenic risk index of children in each functional area exceeds 10^-4,which is 1.22 times that of adults.The carcinogenic heavy metal elements in each functional area are all expressed as Cr>Co>As>Ni.Cr,Co and As are the main carcinogens.It has a high risk of carcinogenesis and needs to be vigilant.