Study On City Soil Environmental Quality Evaluation Method With Heavy Metals

City has been the product of human civilization and indispensable to the survival of the human being. In the rapid advance of urbanization process, urban ecological environment has attracted a widespread of attention. An important part of the urban ecosystem, urban soil has played a crucial role in the growth of green plants. The importance in the quality of urban soil needed for the growth of green plants, the city’s ecological environment and human health too. In recent years, haze has affected the green soil quality. The objective of this case study, we divide Beibei District into four functional areas, a total sum of42samples, within each functional area, a cross-section was dug, which is divided into3-4layer sand according to the profile development. The author tested part of the physical and chemical properties in the soil (pH, bulk density, organic matter and mechanical composition), the total and available contents of soil Cd, Pb, Cu, Zn and Pb chemical forms. The results are as following:(1) In urban soil of Beibei District, most contents of Cd, Pb, Cu, Zn were more background values, but not exceeding the national soil environmental measurement standards.The average of urban soil Cd, Pb, Cu, Zn concentration in Beibei District were0.23mg/kg,22.08mg/kg,25.60mg/kg,104.57mg/kg, which of available content were0.08mg/kg,2.45mg/kg,2.37mg/kg,5.95mg/kg; and which of the activation rate were0%-92.32%1.27%-31.48%1.39%-30.88%1.04%-27.38%. There are differences in the level of whole amount, activation rate and available content of heavy metals of urban soil in Beibei District. From the background value, urban soil Cd, Pb, Cu, Zn have a certain degree over the background value in Beibei District. The proportion of the CD are more than the background values,reached a maximum of80.95%, but the reference to the State Environmental Measurement Standard (GB15618-1995) point of view, most of the soil belongs to a limit value of the soil, a small part belongs to the limit values of the two soil, suggesting that the environmental quality of urban soil is basically to protect human health in Beibei District.(2) There is a certain degree of accumulation for heavy metals in the urban soil surface of Beibei District, this indicates that human activities are an important source of heavy metal in urban soil. And soil parent material as endogenous is a part of heavy metal sources in the urban soil. In the majority, heavy metals concentration are more evenly distributed in the cross-sectional of samples, but part of the distribution are quite different. The distribution of heavy metals available content and activation rate in the soil surface is more regular than the heavy metals total concentration in the soil profile distribution. The available content and activation rate in surface aggregation phenomenon are more obvious. The available content and activation rate is very small and close to zero in the lower layer. That is affected by the surface biological and organic matter content, clay content, which adsorbs more effective heavy metal elements to make it available contents increasing. Thus the only difference in total content suggested that is difficult to explain the impact of the heavy metals in the environment, the effect of heavy metals available contents are more significant for the urban soil environmental quality. The revention and control of heavy metal pollution in urban soils should increase in the control of an external source of heavy metals.Heavy metals concentration and available content in urban soil Pb. Cd. Cu. Zn of Beibei District is good relationship. And there is a extremely significant positive correlation between heavy metals concentration and available content. An illustratation shows that there are some consistency in source and better associated between the four heavy metals, and it is the possibility of combined pollution that will enhance with synergy.(3) four heavy metals element content difference is smaller and more well distributed, in most of different soil layers in the urban soil profile cities of Beibei District. The vertical distribution of heavy metals is less regularity between the urban functions. Especially difference is more obvious in residential areas then other functional areas. This indicates that urban soil is effected by foreign soil and human disturbance soil and moved over shorter development time. And it is broken that the regularity of the urban soil profile development is. The contact of genesis in different soil layers is disrupted and even not existing.(4) The sequence of different forms of Pb in urban soil surface of Beibei was bount to residual> carbonate fraction> Fe-Mn oxide bound> exchangeable fraction> bound to organic matter. Various functional areas in Beibei District, The proportion of the various chemical forms vary greatly in various functional areas soil of Beibei, but the sequence is the same with the sequence of different forms of Pb in urban soil surface of Beibei. From the Path Analysis, Pb available content of urban soil in Beibei District is from the contribution of exchangeable Pb and bound to organic Pb, followed by carbonate-bound Pb and residual Pb, and minimum contribution of Fe-Mn oxide bound Pb. Looking at the relationship of chemical forms of Pb and physical, chemical properties of urban soil in Beibei District, chemical forms of Pb is increasing with the total concent increasing, especially the exchangeable fraction increasing clearly; with the pH of the soil increasing, exchangeable form is very decreasing significantly but the residual fraction is increasing; it is more complex that the effect of soil organic matter on the form of Pb is, Pb organic bound is very significantly increasing with organic matter increasing; it is more complex that the effect of soil texture on the morphology of Pb.It is relatively low that biological activity coefficient and migration coefficients of urban soil Pb in Beibei District are, indicating Pb relatively stable in the soil and the smaller impact on human health and ecological risk. But because heavy metals in the soil is a dynamic system, it will be effected by the heavy metal total content, soil pH, organic matter, clay content, and changes in human activities, it may even form a haze into the city air with dust, so we should pay more attention to potential ecological risk of heavy metals in urban soils to prevent the human health and ecological destruction.(5) The urban soil environmental quality evaluation result of a single pollution index, Nemerow Pollution Index (API) and the potential ecological index are broadly consistent with the conclusions in Beibei Distric. The Pb, Cu and Zn pollution indexes are in the security level and ormild potential ecological risk level, which is consistent with the Pb biological activity coefficient and the migration coefficient results. It is the maximal of the Cd pollution levels in the soil and potential ecological risk level, including the evaluation of the Cd total content were slightly polluted, but based on the evaluation of the effective state is that97.62%of Cd is in the security level. In contrast, the evaluation based on effective state can more accurately reflect the extent of environmental and ecological risk of heavy metals in the soil. Soil available pollution starting value as an evaluation criterion is not only feasible for carrying out urban soil quality evaluation of the area, but more precise evaluation of the soil heavy metal pollution degree and ecological risk, in order to develop appropriate measures to protect the soil environment.Through the evaluation of the environmental quality of urban soils, Beibei District is generally in a safe and pollution-free state,it is in line with the requirements of urban ecological environmental protection. But Chongqing is in a subtropical humid monsoon climate and more precipitation. The soil is easily leached and caused acid that heavy metals activity will increase with reduced pH. So we should improve the degree of attention of urban soil potential ecological risk of heavy metals and take measures to prevent the human health and ecological destruction.