Investigation And Risk Evaluation Of Heavy Metal In The Soil Of Sunken Green Space

In recent years, the problem of urban fresh water resource shortage is becoming more and more serious. Sponge city is becoming more and more popular as an answer to this problem in urban's new area construction and rural urbanization of China. The use of urban open space for rainwater utilization in different forms can complement the shortage of urban water resources. Sunken green space is a typical kind of rainwater utilization method of groundwater supplement. However, the rainwater runoff often contains pollutants which are formed by the scour of confluence surface. In the process of rainwater infiltration, some if these pollutants are trapped by the surface soil and latter degrated by soil creature. Heavy metal, as a kind of hard for degredation chenmicl element, is also trapped in the soil. Different from degredation of organic matter, heavy metal acceumulates. So there's a potential risk of sunken green space soil pollution from heavy metal.The study object of this paper is 14 sunken green space areas in Beijing. These areas contain resident community, park, public service center, and parking lot. Surface soils in these areas were collecyed by 0-10 cm, 10-20 cm and 20-30 cm length. The main method applied in this study congtains plum shaped site arrangement, microwave digestion of soil sample and flame atomic absorption spectrometry for the analysis of Pb, Zn, Cu, Cr. Based on the second level of soil environment quality standard, the risk of soil pullution in these 14 areas were estimated by single factor index, Nemerow comprehensive pollution index and potential ecological risk index. The conclusion are as follows.1. The overal content range of heavy metal is, 7.7-86 mg/kg for Pb, 72.6-225.1 mg/kg for Zn, 9.0-82.4 mg/kg for Cu, 61.8-122.9 mg/kg for Cr. The average content range of heavy metal is, 32.2 mg/kg for Pb, 125.9 mg/kg for Zn, 38.2 mg/kg for Cu, 84.2 mg/kg for Cr.2. The contents of Pb, Zn, Cu, Cr in sunken green space soil of Tianxiu community and Shuangzi community decrease by the depth of surface soil. But in other studied areas, it seems that there's no certain connection between heavy metal depth and surface soil depth.3. None of the content value of soil heavy metal exceds the second level limit value of soil environment quality standard. All of the single factor indexes for the samples are less than 1, and all of the Nemerow comprehensive indexes are less than 0.7, indicting that the soil is in a clean condition. Correspondingly, potential ecological risk indexer of soil at each depth in all the areas are less than 150, showing that the ecological risk is slight. But pollution exits when the background value is chosen as the standard.4. Correlation analysis result shows that the content of Zn and Cu are significantly associated with each other, with a coefficent value of 0.95. This shows there is the possibility that Zn and Cu come from the same source. Result also shows that distribution Zn has strong relation with Cr, and the same is true between Cu and Cr. There is no significant relation between other metals.