Traffic-related trace elements in soils along six highway segments on the Tibetan Plateau: Influence factors and spatial variation

- In this study area, Cu, Zn, Cd and Pb in roadside soils were identified to be mostly related to traffic.
- Cd and Pb showed greater levels of accumulation than Cu and Zn.
- The maximum distance of influence ranged from 16 m to 144 m perpendicular from the road edge.
- Of six highway segments studied, the highway from Putonquan to Lhasa with higher traffic volume, greater proportion of high-emission vehicles, and higher altitude showed greater levels of accumulation.

The accumulation of traffic-related trace elements in soil as the result of anthropogenic activities raises serious concerns about environmental pollution and public health. Traffic is the main source of trace elements in roadside soil on the Tibetan Plateau, an area otherwise devoid of industrial emissions. Indeed, the rapid development of tourism and transportation in this region means it is becoming increasingly important to identify the accumulation levels, influence distance, spatial distribution, and other relevant factors influencing trace elements. In this study, 229 soil samples along six segments of the major transportation routes on the Tibetan Plateau (highways G214, S308, and G109), were collected for analysis of eight trace elements (Cr, Co, Ni, As, Cu, Zn, Cd, and Pb). The results of statistical analyses showed that of the eight trace elements in soils, Cu, Zn, Cd, and Pb were primarily derived from traffic. The relationship between the trace element accumulation levels and the distance from the roadside followed an exponential decline, with the exception of Segment 3, the only unpaved gravel road studied. In addition, the distance of influence from the roadside varied by trace element and segment, ranging from 16 m to 144 m. Background values for each segment were different because of soil heterogeneity, while a number of other potential influencing factors (including traffic volume, road surface material, roadside distance, land cover, terrain, and altitude) all had significant effects on trace-element concentrations. Overall, however, concentrations along most of the road segments investigated were at, or below, levels defined as low on the Nemero Synthesis index.

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