Contamination Of Surface Soil On Both Sides Of The Highway Around Xi'an Heavy Metals

With the development of transportation industry and rapid increase of motor vehicles, a lot of studies suggested that highway traffic had been one of the main sources of heavy metals pollution in soil along the highway. Many scholars at home and abroad have paid more attention to highway road for resulting in more severe pollution with the characteristics of large traffic flow, rapid mobility and wide range. It was of great theoretical significance and practical significance for providing scientific basis for highway road construction, the layout of agriculture production along highway road and pollution prevention to analyze the pollution, distribution and influencing factors of heavy metals in soil along the highway.This research chose three different operating life highway roads (Xibao highway,15years; Xihu highway,9years; Xizha highway,5years) to study heavy metals pollution in soil along different operating life highway roads around Xi’an. The physical and chemical characteristics of soil along the highway roads were analyzed, and the major constituents and the level of main heavy metals in soil were measured by using X-Ray fluorescence spectrometer. Analyzed the chemical speciation of heavy metals through the modified BCR four-step sequential extraction method. The particle size composition of soil and the heavy metals distribution situation in different particle size of soil were discussed through the screening experiment.The main conclusions of this study are as follows:(1) The soil of the study area are all alkaline soil, standard deviation and coefficient of variation of soil conductivity, organic matter, calcium carbonate and magnetic susceptibility on both sides of the highway are all Xibao> Xihu> Xizha, it is showed that the influence of operation life long highway on the roadside soils is more severe. On both sides of the highway soil is clay and silt mainly, soil cohesiveness and adsorption are strong. Three areas of major elements in the soil on both sides of the highway are consistent with the crustal abundance, the soil constant element of the study area distributed evenly.(2)The average contents of soil heavy metals As, Co, Cr, Cu, Mn, Ni, Pb and Zn on both sides of Xibao highway were10.86,17.22,88.94,27.48,759.8,74.56,29.18and34.02mg/kg; The average contents of soil heavy metals As, Co, Cr, Cu, Mn, Ni, Pb and Zn on both sides of Xihu highway were9.66,17.63,89.75,29.96,739.33,78.26,29.63and33.21mg/kg; The average contents of soil heavy metals As, Co, Cr, Cu, Mn, Ni, Pb and Zn on both sides of Xizha highway were11.81,19.11,90.90,29.81,796.26,75.45,30.28and36.89mg/kg. Overall, the element As is less than or equal to the soil background of Shaanxi Provincial. The coefficient of variation of the elements Cu, Co, Zn and Pb in Xibao and Xihu highway were significantly higher than Xizha, and the coefficient of variation of elements Mn and Ni were Xibao highway significantly higher than Xihu and Xizha, but the coefficient of variation of element Cr was no difference in the three highways. The results further illustrate that the influence of operation life long highway on the roadside soils is more severe.(3)Highway as a linear pollution source, the diffusion of pollutant does not reduce simplex with the increasing distance from roadbed, some smaller particles can float to the areas that are far from the highway, most high value areas locate at the places where are apart from the roadbed about5～20meters,50～75meters or100～200meters. The high value areas of the soils along three highways are coincident, heavy metal content change rule in the200meters range. As a result of being far from the highway or affected by the surrounding human activities, the content of heavy metals appears irregular fluctuations or the content is overtop beyond200meters. Comparing Xizha highway with Xibao highway and Xihu highway, the diffusion scope of heavy metals is smaller. Relating to landform, both Xibao highway and Xihu highway roadbed are above the ground about3meters, which promotes the pollutions diffusion, but Xizha highway roadbed belows the ground then it isn’t conducive to the diffusion of pollutants. The spatial distribution of heavy metals content in the soil and pollution on both sides of highway have no obvious difference, that means the effect of wind direction is small.(4) The analysis of the three highways roadside about soil morphological results show that the proportion of the unsteady state is:Mn>Pb>Co>Ni>Cr>Zn>Cu, unsteady state content of Mn, Pb and Co is higher, and the S1weak acid extraction condition and S2reduction extraction condition are the main conditions, accounted for40～66%of the total. They can be absorbed by the plant directly, then transport and transform to become toxic elements which have a higher risk to the environment. Residuel condition content of Ni, Cr, Cu and Zn is the major condition, which is close to or more than80%, the direct use of conversion and migration is less, and have a low potential hazards to the environment. (5)The evaluation result showed that the soil heavy metals of both sides along the three highways is belong to mild contamination, but the evaluation of Co and Cr has reached the level of moderate pollution in some sampling points.(6)According to the results of correlation analysis, factor analysis and cluster analysis, the source analysis results of heavy metal elements As, Co, Cr, Cu, Pb, Zn, Ni and Mn in the surface soil on both sides of highway are not the same. The surface soil on both sides of the highway can be divided into four categories:the first category is Cu, Pb and Zn, which mainly due to the impact of the highway operators; the second category is As, Ni and Mn, mainly subjected to the parent material, and also due to the impact of the highway operators; the third is Co, affected by the highway, the impact of which is small, and has other sources of pollution; the fourth is Cr, which is more serious polluted and affected by the highway operators and agricultural production activities;(7) The service time of highway is related to the enrichment of heavy metals Cu, Ni, Pb and Zn, but is not related to the enrichment of heavy metals As, Co, Mn and Cr. The variation range of heavy metal content and enrichment factor of highway which has long service time is greater than that of the highway having shorter service time. Cu, Ni, Pb and Cr were mainly accumulated in the soil which has smaller particle size<50μm and63-50μm, Zn mainly enriched in the soil which has smaller particle size<50μm,and As, Co and Mn mainly enriched in the soil which has bigger particle size>63μm. Heavy metals enrichment factors (except As and Mn) are enriched in the major enrichment particle size higher than other particle sizes.