The risk posed by heavy metals to food safety and the environment are of great concern to governments and society in many countries. Heavy metal pollution in agricultural soils is becoming serious with the rapid industrialization and urbanization in developing countries [1
]. This is a typical environmental issue due to anthropogenic activities in the countries in the Third Pole region which consists of the Tibetan Plateau and surrounding mountains. The Third Pole region covers parts of eight countries, consisting China, India, Russia, Bhutan, Myanmar, Nepal, Pakistan and Afghanistan, in which a fifth of the World’s population live [2
]. Though the Third Pole region is relatively underdeveloped, it has undergone rapid economic growth and booming tourism in recent years, which have brought an increase of transportation activities and related pollution. The pressure on the unique ecosystem in the anthropogenic living areas of the Third Pole region is predictably increasing [3
]. Traffic activities are one of the major sources leading to heavy metal contamination in roadside soils due to their long-term accumulation. Therefore, the local contamination resulting from transportation activities is receiving increasing attention in the Third Pole countries.
As one of the developing counties in the Third Pole region, Nepal’s agriculture has been the mainstay of the economy, accounting for 40 percent of GDP and 60 percent of the labor force. In Nepal, overland transportation is the major transportation mode but road density is low at 11.4 km of road per 100 km2 and 0.71 km per 1,000 population, mainly because of the country’s complex mountainous topography and insufficient resources. Although traffic volume is generally low in the remote rural areas in Nepal, the cumulative contamination effect of long-term exposure to traffic activities cannot be neglected. More importantly, the roadside farmland soil is associated with the food chain and public health.
Heavy metals can directly harm public health by entering the body via soil and dust, dermal contact or breathing [4
]. The typical elements Cd, Pb, Zn, and Cu in the roadside soils coming from traffic activity can be transported through the food chain into the human body and thus be very toxic to people. In agricultural areas, intake of heavy metals through the soil-crop system could play a predominant role in human exposure to heavy metals [5
]. In general, heavy metals with high concentrations in the environment result in health problems adversely affecting the nervous, blood forming, cardiovascular, renal and reproductive systems. The consequences of heavy metal pollution include reduced intelligence, attention deficit and behavioral abnormality, as well as contribution to cardiovascular disease in adults [6
]. Some trace metals (such as Cu and Zn) are harmless in small amounts, but the others (mainly Pb, As, Hg and Cd), even at extremely low concentrations, are toxic and are potential cofactors, initiators or promoters in many diseases, including increased risk of cancer [7
]. However, it is not easy to remove heavy metals from the soils because of their irreversible immobilization within different soil components [9
The mechanisms of heavy metal emission from vehicles consist of fuel consumption, engine oil consumption, tire wear, brake wear, and road abrasion [10
]. Engine oil consumption is responsible for the largest emission for Cd, tire wear contributes the most important emission for Zn, and brake wear is the most important source of emissions for Cu and Pb [12
]. Though the use of unleaded gasoline has caused a subsequent reduction in fuel emissions of Pb, it may still occur in exhaust gas and come from worn metal alloys in the engine [12
]. Bitumen and mineral filler materials in asphalt road surfaces contain different heavy metal species, including Cu, Zn, Cd, and Pb [12
]. Heavy metals can be transported into the roadside soils by atmospheric precipitation or road runoff [13
Monitoring studies have been conducted in many cities and regions to investigate the roadside heavy metal contamination, including China’s Hong Kong [15
], Beijing [18
], and Shanghai [19
], Mexico City [20
], Turkey’s Elazig [21
], England’s Yorkshire [22
], Jordan’s Amman [23
], Greece’s Kavala [24
. It was found that roadside heavy metal concentration is influenced by multiple factors, including traffic properties, highway characteristics, roadside terrain, roadside distance, wind direction, etc
Generally, the longer the highway usage history, the higher the concentration in the roadside soil [25
], because it is positively related to traffic volume [18
]. Normally, the heavy metal content in roadside soils has a belt-shaped distribution in terms of distance to road edge, decreasing exponentially with distance from road [26
]. Compared to the background nature value of heavy metal content, the influential space of traffic pollution can be up to 50 m far from road but within 100 m [27
]. In addition, most of the deposited metal particles remain in the 0–5 cm of the roadside surface soil depth [29
]. The plants along roadside also have higher heavy metal content and can effectively lower the concentration of heavy metals in soil [30
]. Few studies with factorial design analyses were focused on the heavy metal contamination in rural roadside farmland soils due to traffic activities. More importantly, no previous research was conducted to investigate the roadside soil heavy metal pollution in mountainous areas.
This research aims to investigate the influence of transportation activities on farmland soils along a highway across mountainous areas in Nepal. Corresponding to the complex geographic feature of mountain highways, a study with Multivariate Analysis of Variance (MANOVA) design was conducted to analyze the effects of highway altitude, roadside distance, roadside terrain, and tree protection on the concentrations of Cu, Zn, Cd, and Pb in the farmland soil in mountainous areas and thereby indicate potential mitigation strategies for realization of human-nature harmony in the Third Pole.
This study involving factorial design analyses was focused on heavy metal accumulation in roadside farmland soils along a highway across mountainous areas in Nepal. It was observed that although the average concentrations of Cu, Zn, Cd, and Pb at the sampling sites are lower than the tolerable levels, a few maximum Zn, Cd, and Pb concentrations are close to or higher than the tolerable level. It indicates that some roadside spots may be severely polluted. Furthermore, the correlation analysis showed that either Cu or Cd content is significantly correlated with Zn and Pb content but no significant correlation between Cu and Cd was identified. The pattern can be reasonably explained in terms of vehicle emission mechanism, which suggests the heavy metals’ homology of the traffic pollution source.
Due to the complex terrain characteristics of the representative environments, it is a challenging task for the experimental design to identify the most important factors associated with heavy-metal contamination from traffic activities. Based on a careful investigation on the roadside features of the Trishuli Highway, altitude, roadside distance, terrain, and tree protection are selected as four typical factors for this experiment. The MANOVA results indicate that the factors are complicatedly associated with the concentrations of Cu, Zn, and Pb, except for Cd. The heavy metal concentrations vary at the different altitude levels but display a similar variation pattern. Although altitude is not meaningful for explanation of heavy metal contamination, as a block factor it plays an important role in identifying the other factors effects. It was found that Pb concentration in the downgrade roadside soil is significantly lower than that in the upgrade soil, but Zn concentration in the downgrade roadside soil is marginally higher than that in the upgrade soil. This opposite trend might be due to the difference in deposition forms between Pb and Zn contaminants, where Pb is mainly transferred through air deposition while Zn is more likely to be carried through water runoff.
The analysis indicates that trees growing linearly along roadways can effectively reduce the heavy metals’ concentrations in the roadside farmland. Therefore, planting trees may be considered as an effective countermeasure for existing crop plots that are close to roadways. Furthermore, in this study it was not found that the heavy-metal concentrations in the rural farmland soils are consistently decreasing with the increment of roadside distance. This conclusion differs from most of the finding in the previous urban roadside soil studies, primarily because of the lower traffic volume on the Trishuli Highway. On the other hand, the spatial distribution of heavy-metal contaminants in the roadside farmland topsoil could be disturbed by the frequent farming activities, crop growth distribution, and complex local terrain and environments. Therefore, it is suggested for the future roadside farmland studies to do more detailed sampling in the first 10 m from road edge for capturing the spatial heavy-metal distribution pattern.
Finally, the findings of this study would be useful for understanding how the heavy-metal content in rural farmland roadside soil is influenced by traffic activities and helpful in making policies for avoiding hazardous heavy metal contaminants in agricultural soils in mountainous areas of the Third Pole region.