Distribution Characteristics and Ecological Risk Assessment of Heavy Metals in Roadside Soil of Important National Highways on the Qinghai-Xizang Plateau

Abstract

To clarify the impact of transportation on the sensitive and fragile ecosystems of the Qinghai Tibet Plateau and major ecological safety barrier functions, soil samples within 0-25 m on the roadside were collected from sections of national highways such as G214, G213, G345, G109, G316, and G317, and the contents of six heavy metals were analyzed. Then, the degree of heavy metal pollution and the risk of ecological hazards were evaluated using the single-factor pollution index method （Pi）, Nemero comprehensive index method （PN）, and potential ecological risk index method （RI）. The results showed that the heavy metal contents of As, Cd, Hg, Ni, Pb, and Zn in the soil of important transportation national roads on the Qinghai Tibet Plateau ranged from 5.65 to 176.00, 0.04 to 0.27, 0.01 to 0.14, 9.52 to 113.00, 9.16 to 54.50, and 24.70 to 109.00 mg·kg-1, respectively, showing high variability. In some sections of the soil, the values of the elements As, Cd, and Hg were higher than the local soil background values. The single-factor pollution index of heavy metals in roadside soil was Pi （As） > Pi （Hg） > Pi（Cd） > Pi （Pb） > Pi （Ni） > Pi （Zn）. The Nemero comprehensive pollution index ranged from 0.41 to 9.20, with an average value of 1.53, indicating clean and mild pollution. Some areas showed a moderate or severe pollution. The average potential ecological risk index of the research section was 106.2, and the soil was generally in a state of no pollution and light pollution. Only two road sections had soil heavy metal enrichment reaching moderate and strong ecological hazards. The comprehensive potential risk of the G213a road section indicated moderate to severe ecological risk, mainly contributed by Hg, As, and Cd. The comprehensive pollution risk of the G317 road section indicated mild to moderate ecological risk, mainly contributed by Hg and Cd. The heavy metal content in the soil of the Qinghai Tibet Plateau road area was not significantly correlated with the roadside distance and soil depth but was significantly positively correlated with the annual average temperature （P < 0.05）. In all, there was a trend of heavy metal input into the soil environment in areas with intense human activities and high traffic flow during road construction on the Qinghai Tibet Plateau.