| The expressway plays an important role in modern traffic. It promotes economical development and social progress of the adjacent areas. But some kinds of harmful substances produced from expressway would have a pollution effect on the surrounding environment. Among harmful substances, heavy metals are brought mainly from the combustion of vehicle fuel and have a dispersion to near soil by atmospheric deposition,rain runoff etc. This will not only affect directly the quality and safety of agricultural products, but also endanger indirectly human health through food chain. Therefore, the environmental effect of expressway is a very worthwhile concern. In addition, the branches and leaves of tree can filter and adsorb the heavy metals, which is in company with dust particles. So plant barriers on both sides of expressway could hinder the proliferation of heavy metals and will reduce the area of contaminated soil.Study aim: Considering the above two factors, we studied on three sections of Lian-Huo expressway in ZhongMou county. Each sampling section is of different plant barriers character. Our aim is to examine the capacity difference of each plant barriers in prevention adjacent soil from heavy metal pollution. We intend to make some proposals to agricultural land protection and expressway shelterbelts design.Methods: We took some experiments to examine soil properties and soil heavy metals by national standard methods and applied the software of Excel 2002, Spss10.0, CorelDWAW11 and Mapinfow6.0 etc. to analyze datum and illustrate results.Results:(1)The content of six kinds of heavy metals in WanSheng section, which is no plant barriers nearby expressway, is higher significantly than other two sections (excepting Cd and Cu in individual Section).(2) Section LinChang (being of thick plant barriers and big forest closure) is better than section LuGang (being of narrow plant barriers and small forest closure) in prevention adjacent soil from pollution of Ni, Cr and Zn. But for Pb, Section LinChang has no more efficient effect than LuGang on prevention Pb dispersion.(3) Section LuGang shelterbelt appears to be as efficient as LinChang in prevention adjacent soil from pollution of Zn, Ni and Pb, but for Cr, Lu Gang section has basically no apparent protective effects.(4)For section WanSheng, Zn (10 to 20 m away from the expressway with a polluted peak, until 300 m away Zn concentration of soil decrease gradually) and Pb (polluted peak of soil Pb concentration appeared at 75 m away from the expressway, and the Pb concentration at 300 m is still significantly higher than contrast point 1000 m away from the expressway) diffusion capacity is far higher than other results studied on non-expressway .Conclusion (1) Heavy metals of soil in this region might be derived from soil parent material, agricultural activity (utilization of fertilizer and pesticide) and the impact of expressway. PCA (Principal Component Analysis) show that soil parent material and agricultural activity play the most effective role in heavy metals concentration of soil in this area. The second factor is of expressway, it is of larger impact on Pb and Cd.(2) The behaviors of heavy metal migration and accumulation are affected by a variety of factors. Correlation analysis showed that pH is most relative with heavy metal concentration of soil (excepting Cu and Cr), the following is CEC and clay content and soil organic carbon is weakest relation with heavy metals concentration (excepting Pb).(3) Heavy metal concentration of soil is varied with the different distance from the expressway .It shows that the expressway has a noticeable impact on adjacent soil. To make a polluted evaluation basing contrast sample, datum says that the near soil has been contaminated lightly by this expressway.(4) To void soil being polluted heavily, we should take the following measures: Plant barriers had better to consist of indeciduous species, including mainly arbor and shrubbery. Barriers width should beyond 75 m at least and forest closure have got to be larger possibly. Those kinds of crops, which are less sensitive to heavy metal pollution, are suitable for cultivation in the area along expressway. Problems and Prospects:(1) Sampling density is a little inadequate in the scope from 100 to 300 m away from the expressway, 50 or 25 m is better possibly to sample spacing.(2)The water capacity, clay mineralogy characters, base saturation, nutrient cycling rate and other physical or chemical properties of soil have not studied, what relationship is indistinct between these properties with heavy metals of soil.(3)This paper is studied basing on the total heavy metals in soil, if we can study farther different forms of heavy metals and its accumulation feature in biosphere, trying to reveal heavy metals migration and accumulation in the expressway-soil-biosphere system will be next aim.
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