Sources

Pollution sources include five main groups: traffic and cargo, pavement and embank­ment materials, road equipment, maintenance and operation, and external sources. Road and traffic pollutants having received the greatest attention include heavy metals, hydrocarbons, nutrients (mainly nitrogen), particulates and de-icing salt (Table 6.1). Recently, precious metals worn from catalytic converters have also been given attention. In addition to these pollutants, a range of gaseous pollu­tants is emitted as a result of fuel combustion. These are to a large extent aerially transported away from the road area, and this issue is beyond the scope of this overview.

The amount of pollutants originating in road and traffic depends on several aspects related to road design, road materials, road maintenance and operation, types of fuel used and traffic characteristics such as volume of light and heavy vehicles, speed and driving behaviour (Pacyna & Nriagu, 1988; Legret & Pagotto, 1999; Sarkar, 2002; Warner et al., 2002; Bohemen & Janssen van de Laak, 2003).

To a great extent, heavy metals, polyaromatic hydrocarbons (PAH) and, to a varying extent, other pollutants (e. g. sodium and chloride from de-icing) emitted from road and traffic sources accumulate in the soil in the vicinity of the road (WHO, 1989; Munch, 1992; Zereini et al., 1997). This continuous accumulation poses a long-lasting stress to vegetation, animals, soil microflora and other compart­ments of the ecosystems close to roads but seldom gives rise to acute toxic effects. On the contrary, acute toxic effects may occur following the infrequent events of traffic accidents involving dangerous goods such as petrol and diesel as well as acids and other chemicals, sometimes in large quantities. It should be noted here that both concentrations and load are of importance — instantly high concentrations may cause acute damage or may be lethal whereas the long-term performance of the ecosystem (component) may be more influenced by the total load of pollutants over a period of time. Die-off of roadside trees or twigs due to the use of de-icing salt is an example of damage being caused either by instantly high concentrations or the load over time, or both (Backman & Folkeson, 1995).

Table 6.1 Sources of contaminants originating in different road and traffic sources

5=

o3

£P

О