Spatial Distribution Characteristics And Influencing Factors Of Heavy Metal Accumulations In Soil And Rice Along Expressway

This paper discussed the impacts of heavy metals pollution on farm soil and Rice (Oryza sativa L.) along the Shanghai-Nanjing Expressway in different sections according to the traffic volumes, topographies, climate characteristics. We took soil and rice samples various representative sections (i.e. Jurong, Danyang and Suzhou). Then we determined available contents of Pb, Cd, Cr and Zn in soil rice. GIS technology and space interpolation method were utilized to carry on the spatial distribution characteristics analysis on available content, total amount of heavy metal. During rice seedling stage, soil and seedlings were collected synchronizing in the south of the Danyang road at different distance to be carried on cultivation management in experimental greenhouse. The heavy metal contents of the different organs of rice potted in greenhouse as well as along the expressway were measured after rice harvesting. By the contrast, main sources of heavy metal contained in the rice along road and the proportion between heavy metal from the air and that from the soil were analyzed. Following conclusions were achieved:1. The traffic activities caused different degree of heavy metal accumulations in farmland soils along this expressway. The concentrations of Pb, Cd, Cr, and Zn in the roadside soils of different sites increased significantly with the traffic density. Contents of Cr and Zn in rice staid below the residue limits, contents of Pb and Cd of partial rice grains were out of limit. It could be concluded that Pb and Cr caused by expressway traffic had polluted rice along the highway to a certain degree. As traffic flow increases, Pb contents of all these sample dots rose, and yet respective contents of Cd, Cr and Zn of these sample dots did not show rising tendency.2. The spatial interpolation results of heavy metal contents in the roadside soils showed that the concentrations of Pb in different side of the expressway initially increased to a maximum value with the distance to the road, and then gradually decreased to the soil background value. While the correlation of the contents of Cd, Cr, Zn with the distance to the road were inconsistent. The spatial distributions of available contents of Pb, Cr, and Zn weren’t consisted with that of their total contents. There was a significant correlation between the available contents, total contents, soil pH and organic matter, indicating that soil pH and organic matter were important factors affected the spatial distribution of the available contents of heavy metals in the roadside soils. Pb and Cd contents in rice grain on both sides of highway rose initially, and then decline gradually with increasing distance away from highway. But contents of Cr and Zn do not agreed with them. Pb content in rice grain had a significant positive correlation with soil Pb amount, available content and soil organic content separately. Cd content of rice grain was significant positively correlated to Cd available content of soil. And Cr content of rice grain correlated significant negatively to Cr available content of soil, and significant positively to soil organic content.3. The content divergences of Cd and Pb in the rice between the sideway and the greenhouse were obvious, while the contents divergence of Cr or Zn was not. It might be owing to the fact that rice on sideway assimilates Pb and Cd by means of root uptaking as well as of foliar absorbing. Meanwhile rice in greenhouse assimilates Pb and Cd only by root uptaking. There were significant differences in Pb and Cd contents of glumes among various sample dots, whereas Cr or Zn content showed no significant differences. Pb, Cd and Cr contents of stem leaf showed respectively significant differences among various sample dots, whereas there was no significant difference in Zn contents. We worked out the different metal source proportionalities of different plant organs. In rice, Pb consisted of34.29%from soil and65.71%from atmosphere; Cd consisted of30.40%from soil and68.22%from atmosphere. In glumes, Pb consisted of65.92%from soil and34.08%from atmosphere; Cd consisted of28.70%from soil and71.30%from atmosphere. In stem leaf, Pb contents from atmosphere and soil were separately34.28%and65.72%; Cd consisted of66.83%and Zn57.06%from soil, the rest was from atmosphere.4. It could be concluded that traffic volumes on both sides of expressway was still one of the major environmental problems affecting the safety of soil and agricultural products, especially the Pb pollution. The concentrations of Pb, Cd, Cr, and Zn in the roadside soils of different sites increased significantly with the traffic density, the Pb content in rice was significantly higher than those in Danyang, Jurong because of the largest traffic volumes. Direction of wind is the major factor of affecting on both sides of heavy metals contents in soil and rice grain. In the whole rice growing season, the near southerly wind dominates. There is more atmospheric deposition on the north road, therefore the pollution degree of road north was higher than that in the road south. In addition, along the expressway heavy metals in agricultural soils and rice, soil pH and organic matter such as the basic physical and chemical indicators of disturbance and human activities combined effect of a variety of factors.