Role of height and position in the vertical distribution pattern of urban surface-deposited sediments and associated heavy metals

Abstract

Urban surface-deposited sediments (USDs) with different spatial positions and heights are a mixture of various pollutants with complex sources and are widely distributed in urban environments. However, the influence of urban surfaces on the vertical distribution pattern of USDs and associated pollution characteristics due to their spatial position and height is still unclear. We investigated the vertical distribution pattern of USDs and associated heavy metals in the Haidian District of Beijing. There was an obvious gradient in mass per unit area (unit: g m⁻²) for the bulk USDs as follows: low (platform, 3 m, 2.3 ± 1.0; overbridge, 5 m, 4.0 ± 3.2), medium (rooftop, 20 m, 17.7 ± 5.7), and high (road surface, 0 m, 28.0 ± 17.7; sidewalk, 0 m, 38.3 ± 25.3). The concentrations of six heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in USDs on road surfaces, sidewalks, platforms, overbridges and rooftops were 3.4, 3.5, 7.1, 7.5 and 6.9 times higher on average than in the corresponding Beijing background soil, respectively. The potential heavy metal contribution loads per unit area for the bulk USDs were in the following order: low (platform and overbridge), medium (road surface and sidewalk), and high (rooftop), implying that a flat rooftop was the “hot spot” for heavy metal contamination. Particle size had an important effect on pollution distribution and their output. Furthermore, the accuracy of classification of USDs by height using linear discrimination analysis (LDA) was also affected by particle size due to their sources and ability to stay on different urban surfaces. LDA suggests that the spatial height could best distinguish the USD types when their particle size is less than 149 μm. The dynamic variation of the grain size composition of road deposited sediments (RDS) was deduced based on a schematic model of the USDs input–output relationship. These findings provide new insights into the heavy metal geochemical cycle in urban surface particulates.