Polycyclic aromatic hydrocarbons in soils from the Central-Himalaya region: Distribution, sources, and risks to humans and wildlife

The Central Himalayas are not only a natural boundary between China and Nepal but also a natural barrier for transport of air masses from South Asia. In this study, 99 samples of surface soil were collected from five regions of Nepal on the southern side of the Central Himalayas, and 65 samples of surface soil were obtained from the northern side on the edge of the Tibetan Plateau, China (TPC). Concentrations of polycyclic aromatic hydrocarbons (PAHs) in soils were measured to determine their distribution, potential for accumulation, and sources, as well as risks to humans and the environment. Mean concentrations of Σ16PAHs were 2.4×102 and 3.3×102ng/g dry mass (dm) in soils collected from the TPC and Nepal, respectively. Significant correlations between concentrations of lower molecular weight PAHs (LMW-PAHs) in soils and altitude were found. Total organic carbon (TOC) in soil was positively but weakly correlated with concentrations of PAHs in the study area, which suggested little role of TOC in adsorption of PAHs. The cities of Kathmandu and Pokhara in Nepal and Nyemo (especially Zhangmu Port), Shigatse, and Lhasa on the TPC, were areas with relatively great concentrations of PAHs in soils. The main sources of PAHs identified by positive matrix factorization were emissions from motor vehicles and combustion of coal and biomass in the Central Himalayas. Calculated total benzo[a]pyrene potency equivalents of 0.23–44ng/gdm and index of additive cancer risk of 3.8×10−3–9.2×10−1 indicated that PAHs in almost all soils investigated posed de minimis risk of additional cancer to residents via direct contact and had no significant risk of additional cancers through consumption of potable water. Mean risk quotient values indicated that 39% of soils had a slight risk to wildlife and the ambient environment of the Central Himalayas.