Magnetic properties of surface soils in Beijing area and their environ-mental implications

Convenient and fast techniques for detecting pollution are urgently needed in modern environmental studies. The ability to identify effectively environmental baseline is required for any of such new techniques. In recent years, the magnetic technique has been gradually applied to monitoring the urban environmental pollution. As the compositional origin of surface soil (0—10 cm, topsoils) from urban and suburb areas is complex, the key point for the magnetic technique to be used as an effective way of urban environmental monitoring is whether it can distinguish the natural processes from the human-induced effects. In this study, environmental magnetic measurements on the topsoils of all administrative districts and counties in the Beijing area were carried out. The characteristics of topsoils’ magnetic properties in Beijing are present, along with their relationship to the location, parent material, type of human activity. The results indicate that the magnetic properties of the topsoils in the northwestern and southwestern mountainous regions are controlled mainly by the natural environmental (including the topography and parent material). The magnetic susceptibility values of most topsoils here are larger than 4×10 −6 m 3 /kg −1 and the coarser (larger pseudo-single domain or multi-domain) ferrimagnetic minerals dominate, which are determined by the parent material. The magnetic properties of the topsoils in the central and eastern intermountain valleys and plains are relatively weak. Yet the values of χ ARM /SIRM and χ ARM / χ lf of the topsoils near the industry sites and roads with heavy traffic in those regions are sharply increased due to the impact of human activities. The main magnetic minerals are finer (single domain-smaller pseudo-single domain) ferrimagnetic minerals. The results also show that the χ ARM /SIRM and χ ARM / χ lf are more effective for discriminating the heavily polluted topsoils from those developed on the strong magnetic parent material.