Spatial Characteristics And Environmental Risk Assessment Of Soil Heavy Metals In Urban Green Spaces

In recent years,due to the rapid development of urbanization and industrialization,the pollution problems existing in cities have an increasingly significant impact on people’s lives.Therefore,the research on heavy metal pollution of urban soil is one of the core contents in the current ecological environment research.The content of heavy metals in soil has a direct impact on animals and plants,so the pollution of heavy metals in urban soil has become particularly critical.The change of urban land use will have a significant impact on the soil structure,quality,nutrient cycle and function in the urban ecosystem,thus affecting the urban ecological environment.Urban green space plays a key role in protecting biodiversity and water resources,improving microclimate,isolating urban pollution,etc.,but the research of urban green space ecosystem still needs to be in-depth.Based on the sampling and indoor analysis of different types of green land,the contents and spatial variation characteristics of Cd,Cr,Cu and Zn in the green land soil were analyzed with the support of GS + and arcgis10.0,and the urbanization factors such as the type of green land,the age of green land,the distance from the city center and the building density were discussed The impact of heavy metal content in soil was analyzed from the point source and non-point source level by using the land accumulation index method and Nemero index method.The potential ecological risk index method was used to evaluate the regional heavy metal pollution and accumulation of heavy metal toxic factors in soil.The main research conclusions are as follows:(1)The content of green soil in Chengdu City belongs to the alkaline soil in a tight state,the average content of organic carbon is 15.69g/kg,the average content of cation exchange capacity is 5.31mg/kg,Cd content is 0.4mg/kg,Cr content is 63.86mg/kg,Cu content is 21.69mg/kg,Zn content is194.93mg/kg.The coefficients of variation were 52%,53%,22% and 71%,respectively.There is a significant positive correlation between the four heavy metal elements,the content of Cd and Zn has a strong spatial correlation,the content of Cr has a moderate spatial correlation,and the content of Cu has a weak spatial dependence.(2)The content of heavy metals in the soil of protective greenbelt in Chengdu is significantly higher than that of the other four types of greenbelt;the content of Cd in the soil of protective greenbelt is the highest,that in the park is the lowest,and that in the various types of greenbelt does not show significant difference;the content of Cr in the surface soil of different types of greenbelt is significantly different,among which the content of Cr in the road greenbelt is the highest,that in the park is the lowest,and that in the other types of greenbelt is the lowest Cr content of land type is relatively stable.The content of Cu is the highest in the park green space,slightly higher than other types of green space.The content of Zn in road green space is the highest,far higher than other land types,while the content of Zn in park green space is the lowest.In general,according to the classification of land use types,the heavy metal content of different land use types in Chengdu is arranged from large to small: shelter forest green space > Road Green Space > residential green space > unit green space > Park green space.(3)In the study area,all the heavy metal elements have a significant positive correlation with the age of green space.With the increase of age,the content of all the heavy metals has a slow upward trend.For some green spaces with longer age and higher vegetation coverage,the content of heavy metals has a slow upward and downward trend.There is a significant negative correlation between the distance from the city center and the heavy metal elements Cd,Cr,Cu,Zn in the soil.The heavy metal content decreases with the increase of the distance from the city center.The correlation analysis between the building density and the heavy metal content in the sample buffer zone shows that the correlation between the heavy metal elements Cd,Cr,Cu,Zn and the building density in the surface layer of green space is low,and the content of Zn is all low It has a significant positive correlation with the building density.(4)The results showed that Cd was the main pollution element,while Cr and Cu did not cause obvious pollution.However,after the evaluation by Nemero index method,the results show that road green space,residential green space,industrial green space and shelter forest green space all have moderate pollution,and park green space is slightly polluted.Through the potential ecological risk assessment,the results show that the heavy metal pollution in Chengdu shows a decreasing trend from the city center to the outside,and the trend of pollution suburbanization appears in the northeast and southwest directions.