Traffic and Cargo

Road traffic and cargo produce a range of compounds that pollute the environment. Corrosion of vehicle compartments is a source of heavy metals. Tyre wear gives rise to particles containing zinc, cadmium and iron (literature cited by Fergusson, 1990 [p. 420]; Landner & Lindestrom, 1998; Sarkar, 2002). Brake pads and brake linings emit copper, zinc and lead (Weckwerth, 2001). Fuel, fuel additives and lubricants are sources of hydrocarbons. Lead (Pb) is no longer allowed in the EU states but in countries where leaded petrol is still used, e. g. many African countries, this metal is emitted in the exhausts. Wear of catalytic converters gives rise to emission of plat­inum, palladium and rhodium, though in minor amounts. Spills and littering from cargoes also release a wide range of contaminants. Car-polish and windscreen clean­ing agents give rise to the spread of organic detergents. Snow banks along roads ac­cumulate the pollutants over time and may become highly polluted. Through petrol and diesel spillages and other contamination, petrol-filling stations, often situated adjacent to roads, continuously contribute a range of contaminants, notably organic compounds from petrol products, to road runoff and the road environment.

6.2.1 Pavement and Embankment Materials

Pavement and embankment materials can be sources of contaminants that reach the environment either through leaching, runoff transport or aerial transport. The amount reaching the environment varies to a great extent with the type of material used in the various layers, the type, condition and wear resistance of the surface layer, the influence of water and traffic, and a range of other factors.

Pollutant leaching from modern types of bitumen used in asphalt pavements is usually low (Lindgren, 1998). As a substitute for or compliment to natural aggre­gates, various kinds of secondary materials may be used in road constructions. Some of the most commonly used secondary or manufactured materials include:

• crushed asphalt, concrete and brick (from old road pavements and demolished buildings);

• rock or soil associated with mining activities;

• by-products from metallurgical processes, such as slag;

• pulverised and bottom fuel ash — particularly “fly ash” from coal burning elec­tricity generation; and

• other industrial by-products such as bottom ash from municipal solid waste in­cineration.

The re-use of materials can be considered advantageous from a natural resource — management point of view. The content of hazardous compounds must, however, be considered. A range of heavy metals and other pollutants such as oil and or­ganic micro-contaminants (e. g. PAH, PCB) may be contained in such alternative materials. The concentrations and leaching ability vary greatly between materials and should be tested to ascertain feasibility for road-construction usage (Baldwin et al., 1997; Lindgren, 1998; Apul et al., 2003; Hill, 2004; Olsson, 2005; Dawson et al., 2006).

Pollutant leaching from road-construction materials containing potentially harm­ful chemicals has been subject to a Czech field study (Jandova, 2006). Water seeping down from the road surface through the pavement and embankment was collected 1.5 m beneath the road surface using the device described in Chapter 7 (Section 7.4.5 and Fig. 7.8), having passed through a pavement foundation formed of slag. The data of Legret et al. (2005), for water having passed through an asphalt containing recycled components, are given for comparison in Table 6.2. Significant PAH concentrations in the soil beneath the asphalt were observed also by Sadler et al. (1999) due to water entering the environment through leaching from asphalt surfaces. Results from leaching tests on standard hot-mix asphalt have been reported by Kriech (1990, 1991). Except for naphthalene, all PAH were below the detection limits. The same fact was observed for metals — only chromium was found in con­centrations above the detection limit. Legret et al. (2005) analysed percolating water through two core samples containing 10% and 20% of reclaimed asphalt pavement. They also described leaching of selected heavy metals and PAH from reclaimed

asphalt pavement in samples from an experimental site that were tested in both static batch tests and column leaching tests.

Where allowed, the use of studded tyres causes substantial pavement wear, typ­ically in the range of 2-10 g/km/vehicle for modern pavements of high quality (Jacobson, 2005). The wear is higher from pavements of lower quality. Pave­ment wear results in high aerial concentrations of particles. Onto these parti­cles, other pollutants such as heavy metals become adsorbed (Dahl et al., 2006; Lindbom et al., 2006). Aggregates of different mineralogical origin vary in their heavy-metal content. Granite/gneissic aggregates, e. g., have been shown to contain higher concentrations of heavy metals than does porphyry (Lindgren, 1996). This concentration difference in combination with lower resistance of granite/gneiss to studded-tyre wear results in higher release of Cu, Cr and Zn from this type of aggregate than from porphyry (Lindgren, 1996). The build-up of tyre-generated pavement-wear dust on the street surface and along streets during the winter often results in greatly elevated aerial particle concentrations during dry winter and spring days (Gustafsson, 2002). Dust generation from the unbound surface layers of gravel roads is a well-known problem (Oscarsson, 2007).