Many previously published reports have shown that Cu, Zn, As, Cd and Pb are indicator heavy metals in contaminated soils near highways (Chen et al., 2010, Wang et al., 2012, Yan et al., 2013 and Werkenthin et al., 2014). Therefore it SM-164 was speculated that Cu, Zn, As, Cd and Pb (related to factor 1) were derived from a similar source and might be related to anthropogenic inputs.
3.3. Relationship between heavy-metal contents in roadside soils and distance from the highway
The factor 1 scores (corresponding to Cu, Zn, As, Cd and Pb) had significantly negative correlations with distance perpendicular to the highway, whereas the other two factors were not statistically significant (Table 4). The soil contents of Cu, Zn, Cd and Pb were among the five heavy metals significantly correlated with the distance from the highway (p < 0.01), indicating that the contents of these heavy metals in the topsoil along the Qinghai–Tibet highway were higher in samples taken nearer the road. Some studies have shown that the contents of traffic-related metals in topsoil decrease with increasing distance from the road edge ( Liu et al., 2009, Yan et al., 2013 and Werkenthin et al., 2014). These results support the suggestion that the contents of Cu, Zn, As, Cd and Pb in soils were closely related to traffic.