土壤剖面的磁学特征及其对交通污染的指示意义——以北京首都机场高速公路为例

MINERAL MAGNETIC PROPERTIES OF SOIL PROFILE AND THEIR APPLICATION IN TRAFFIC POLLUTION MEASUREMENTS——A CASE STUDY FROM THE CAPITAL AIRPORT EXPRESSWAY IN BEIJING

对北京首都机场高速公路旁采集的土壤柱状T01剖面的磁学参数和金属元素分析，探讨了研究区内现代交通导致的土壤磁学性质的变化及其对环境污染的响应。结果表明，磁参数（χ，ARM和SIRM）与重金属含量呈同步垂向变化趋势。来源于交通运输排放的污染物是土壤剖面上部（0～8cm）磁性和金属含量增强的主要原因，8cm以下，土壤基本未受到污染，磁性矿物和重金属含量较低，磁性颗粒变化稳定，基本代表了该地区土壤的自然背景。尽管土壤岩石磁学分析表明剖面上下部受污染和未受污染样品的磁载体均是粒度较粗的多畴磁铁矿，但是结合磁参数比值曲线，说明底部样品的磁颗粒的粒度较顶部偏细。磁性矿物的含量变化没有影响磁颗粒的粒度特征。指标聚类等相关分析表明，土壤磁参数（χ，ARM和SIRM）与重金属元素（Pb，Zn和Cu）含量显著相关；结合模糊聚类分析，磁参数可用于追踪、识别交通污染物质在土壤剖面中的富集、迁移状态，揭示不同深度土壤的污染程度。

Magnetic method, a fast and economic measuring technology, has been successfully applied in environmental pollution measurements. However, most case studies were related to surface measurements integrating signal from certain soil volumes, which cannot provide the details of each layer, especial the magnetically enhanced layer. To better understand the magnetic parameters and their links to heavy metals for screening different migration of pollutants with depth, a highway side soil profile of 22cm depth was sampled from the grass land of the expressway linking Beijing City and the Capital International Airport. Magnetic measurements, geochemical analysis, and multivariate statistics were performed for the soil samples. The high resolution results reveal that there is a big difference of magnetic proxies and other parameters with different depth. Magnetic proxies show similar trends compare to heavy metal elements with higher concentrations at the top and lower concentrations in the lower part. The soil profile can be divided into two parts. The uppermost soil horizon （0-8cm）represent the pollution-rich layer with a high concentration of ferromagnetic phases and metallic elements. The values of χ are very high with average 141.60×10-10m^3/kg in this layer. The anthropogenic dust input from traffic activity is the predominant cause for strong signals of magnetic phases and heavy metals in this layer. Below 8cm, there is little pollution in soil with lower concentration of magnetic minerals and heavy metals comparable to natural background values.χ are quite stable and relatively low and reduced to the average 11.5 × 10^-8m^3/kg with much lighter color, representing pedogenic or geogenic signals with little anthropogenic pollution. S-ratio also generally decreases with depth, it decreases from 0.93 in the layer of 0 - 8cm to 0.87 below 8cm, which indicates that the collected samples are overwhelmingly predominated by ferro（ i） magnetic minerals, while the contribution of a few imperfect antiferromagnetic components is a little more in the low part. Rock magnetic experiments show that MD magnetite was the main magnetic carrier both in the upper and the lower part. But integrated with magnetic parameters it reveals that the magnetic grain size in the upper part is a little coarser than the lower, which indicates that grain size characteristics of magnetic particles are not affected by the changes of magnetic mineral concentration. Index clusters and linkage analysis show a positive correlation of χ, ARM, SIRM and Pb, Zn, Cu. It can be concluded that magnetic concentration parameters can be used as a proxy for pollution investigation as it is fast, sensitive, lowcost, and high-efficient to conduct screening heavy metal pollution. Fuzzy C-means cluster analysis can clearly divide the soil profile into different intervals and help to distinguish their characteristics. It can characterize the accumulation depth of pollutants, their enrichment, and their migration.