Study On Heavy Metal Content And Pollution Of Soil Near Bailuyuan Highway In Xi'an Area

With the rapid development of the transport industry,the number of motor vehicles and the traffic mileage are increasing,which has led to the heavy metals pollution in the soil and crops around two sides of the road,the pollution is a serious threat to the ecological environment and people's health,which has aroused people's great attention.Base on these,this paper choose the Bailuyuan road,the wheat field of Milu village and the peach field of Bai village of Bailuyuan as the research targets,the surface soil samples on both sides of the east,middle and west sections of the Bailuyuan road and the loess-paleosol soil profiles of the wheat and peach field in Bailuyuan with the depth of 2.5m are collected.The contents of heavy metal elements,pH value,conductivity,magnetic susceptibility and grain size composition of the samples are measured by X-Ray fluorescence spectrometer,pH meter,conductivity meter,magnetic susceptibility meter and laser grain size analyzer.The spatial distribution and enrichment characteristics of heavy metal elements As,Co,Cr,Zn,Cu,Ni,Pb,Mn on surface soil along both sides of Bailuyuan road and soil profiles with the depth of 2.5m are studied,meanwhile,the pollution degree is evaluated and the sources of heavy metals are discussed.The main conclusions are as follows:(1)The variation range of pH value of the soil in this paper is 7.0-8.5,which belongs to alkaline soil.The grain size distribution of the surface soil on both sides of the road and the loess-paleosol soil profile showed that the coarse silt has reached the highest content,the content of clay is medium,and the fine silt has the lowest content,and the soil which studied in this paper are all silty sand soil with a certain amount of clay.(2)The average contents of eight kinds of heavy metal elements in east,middle and west sections of the surface soils on both sides of the Bailuyuan road are higher than the background value of soil in Shaanxi province.In the loess-paleosol soil profile with the depth of 2.5m,except for the content of Zn in the peach field of Bai village is lower than that the soil background value of Shaanxi province,the contents of other heavy metals are higher than those of the soil background value of Shaanxi province.(3)In the range of 200m from the outer edge of the roadbed,there are three types of distribution characteristic of heavy metal contents in the surface soils on both sides of the road:Firstly,the exponential distribution which gradually decreases with the increase of the distance from the road.Secondly,the skewed distribution which increase first and then decrease with the increase of the distance from the road.Thirdly,the distribution started with a decrease,then an increase and a decrease at the end.The average content of heavy metal on the south side is higher than north side due to the north wind,which leads to an easy enrichment of heavy metals to the south side.The high contents of heavy metals in the east section are located in the range of 5?50m from the subgrade,especially at 25m.The peak content of the middle section appears at 0?35m,especially 0m and 10m.The high value of the west section of the road is concentrated at 0?15m,especially 0m and 15m.In all,the distribution of pollutants is that the eastern section>middle section>western section,because there are no protective forest on the east section and the protective forest on the middle,western sections can prevent heavy metal elements.The content of a few elements around the road is still high in the range of 100?200m,may due to human agricultural activities.(4)In the loess-paleosol profile with the depth of 2.5m,the contents of heavy metal elements perform as stratified enrichment.As,Co,Ni and Mn are mainly enriched in the surface layer of soil profile with the depth of 0?0.35m in Milu village,and Zn is enriched in the surface layer of 0-0.5m depth,Pb is mainly concentrated in the upper layer of soil profile with the depth of 0-0.9m,Cr and Cu are enriched in the upper layer of soil profile with the depth of 0?1.1m.In the peach field of Bai village,As?Cu?Mn?Cr?Pb?Zn?Ni?Co are all mainly enriched at the middle and lower layer of 1.2?2.2m depth.In general,the heavy metal elements mainly enriched in the surface and the upper layers in the wheat field of the Milu village,while it mainly enriched in the middle and lower layers in the peach field of the Bai Village.The enrichment depth of heavy metal elements in loess-paleosol profile is mainly determined by the adsorption of clay layer in the holocene paleosol,which is a natural accumulation in the middle holocene.(5)Based on the results by enrichment factor method,the Nemerow comprehensive pollution index method and the potential ecological risk index method showed that the surface soil on both sides of Bailuyuan road and the loess-paleosol profiles with the depth of 2.5m of Bailuyuan all belong to the mild pollution level,while a few sections or profiles with As and Co has reached the moderate pollution level.The results evaluated by the Geo-accumulation pollution index method indicate that most of the heavy metal elements in the surface soil of the roadside and loess-paleosol profiles of Bailuyuan are not polluted,while the Co in the wheat field of the Milu village has reached the level of mild pollution.(6)Based on the results of correlation analysis,principal component analysis and cluster analysis,the sources of eight heavy metal elements in the surface soil on both sides of the Bailuyuan road can be divided into three categories:?Natural source elements.As,Cr,Ni,Mn are influenced by natural factors,mainly from soil parent material.? Agricultural and traffic source elements.Zn,Cu and Pb are mainly influenced by the combination of agricultural sources and transportation sources.?Natural and traffic source element.Co is affected by traffic pollution sources and natural sources,especially natural sources.