Distribution Characteristics,Pollution Evaluation And Source Analysis Of Heavy Metals In Urban Greenland Soils

The rapid development of urbanization leads to the expansion of urban area and the change of land use mode,which affects the distribution of heavy metals in the soil of urban green space,causes the accumulation of heavy metals in the soil to different degrees,and directly or indirectly affects human health through the atmosphere,water body and food chain,and has a huge impact on the urban ecological environment.Thus,it is of great significance to analyze the distribution characteristics,pollution and sources of heavy metals in urban green land soil in the gradient zone of urbanization.In the current study,we selected the central city-urban fringe-new urban area satellite gradient belt in Chengdu,China as our study area,and then explore the distribution characteristics of soil heavy metals,and combine the distance from the city center,building density and road density to explore the impact of urbanization on the spatial distribution of soil heavy metals.Geoaccumulation index,potential ecological hazard index and risk assessment coding method(RAC)were used to evaluate the pollution of heavy metals in urban green land soil.Using principal component analysis,Pb isotope ratio,and positive matrix factorization(PMF)to investigate the source of heavy metals in the soil of the study area.The main research results as following:(1)The total average values of Cu,Pb,Zn,Cd,Cr,Mn,and Ni in the study area were49.97,43.62,96.25,0.33,62.30,605.40,and 35.94 mg / kg,of which the average values of Cu,Pb,Cd,Mn,and Ni It exceeded the background value by 1.4 to 2.8 times.The content of Zn and Cr in 33% and 36% samples exceeded the background value,but neither exceeded the screening value of soil pollution risk for construction land.Cu is a high-level mutation and Ni is a low-level mutation,and the other six heavy metals are medium-level.Heavy metals in urban green space soils are affected by human activities and accumulate to a certain extent.(2)The heavy metals Cu,Pb,Zn,and Mn in the greenbelt soil in the "central urban area-urban fringe-new urban area" gradient zone showed a trend of decreasing first and then increasing,but Cd,Cr,and Ni did not show obvious spatial distribution rules.The content of Cu,Zn and Cd is highest in industrial green space,and the content of Pb in traffic green space is significantly higher than other green spaces.Cu,Pb,Zn,and Cd decreased significantly with increasing distance from the city center,and increased with increasing building density and road density within a certain range.Cr,Mn,and Ni have no significant correlation with green space type,distance from city center,building density and road density.(3)Different heavy metals have different occurrence patterns.The highest proportion of weak acid solution is Cd,which indicates that it has high ecological toxicity and is the main heavy metal pollution element in the study area.Cr and Ni account for the highest proportion of residual state,which is relatively safe for the environment.The extractable state(sum of F1,F2,and F3)of each heavy metal element is Cd(72%)> Pb(60%)> Mn(59%)> Zn(44%)> Cu(39%)> Ni(24%)> Cr(21%),the potential hazards of Cd,Pb,Mn,Zn and Cu are high,and they have high secondary release capacity.(4)Geo-accumulation index evaluation results show that Mn,Cd,and Pb are mildly polluted,and the geo-accumulation index values show a decreasing trend from the city center to the city edge.The potential ecological risk assessment results show that the study area is generally a slight ecological risk,but the Cd is a medium ecological hazard,and it has a significant contribution to the potential ecological risk of the environment.The risk evaluation coding method evaluation results show that Cd is a high-risk pollution,Mn and Zn are medium risks,Cu,Ni,and Pb are low risks.The Cd and Mn evaluation indexes have high index values in residential and industrial green areas and the lowest in traffic green areas,which may be related to the total heavy metal content,soil p H,organic matter,and CEC.(5)There are four types of heavy metal sources in the study area,including natural sources,atmospheric deposition sources,industrial activity sources and traffic activity sources.Mn and Ni are mainly controlled by natural sources,Cu by industrial activities and traffic activities,Cd by industrial activities and atmospheric deposition,Cr by atmospheric deposition and traffic activities,Pb and Zn by industrial activities,traffic activities and atmospheric deposition.