Traffic to trace metal fluxes

Road and street traffic is usually considered to be an important source of metal emissions to the environment. For example, in the series of monographs on metals published in the late 1990s, it was estimated (for the whole of Sweden) that the emission of copper from road and street traffic would be about 90 t/y, i.e. 44% of total copper emissions to the environment (L & L - Cu), the traffic emissions of zinc were about 500 t/y, i.e. 28% of the total (L & L - Zn), while the traffic emissions of chromium and nickel were 40 t/y (37% of the total) and 20 t/y (20% of the total), respectively (EW

Corresponding estimates of traffic-related metal emissions for 1995 in the city of Stockholm (Bergbäck et al., 2001; Table 3.9) resulted in the following percent fractions of the total metal emissions from goods accumulated within the city: Cu - 38%; Zn - 62%; Cr - 99%; and Ni -99%. It should be understood that a considerable part of the metal emissions from the road and street traffic in Stockholm originates from wear of road pavements.

Although the emissions from the various materials involved in street and road traffic may be rather accurately estimated, it is a considerably more difficult task to assess the fate of the metals emitted. This is due to the fact that the metal emissions primarily are in the form of dust particles, resulting from the wear of tyres, brake linings and road pavements and that a great (but variable) part of these particles may be transported away from the streets and roads as airborne particles and will settle either close to the roads or at longer distance depending on the actual weather conditions. Moreover, if the roads are surrounded by soft soil and various kinds of vegetation, the majority of the particles may be retained in this near-field zone. On the other hand, if street and road verges are sealed with concrete or asphalt pavements

- and street sweeping is not carried out regularly - the metal-containing particles will, sooner or later, be washed into the storm-water sewer.

Depending on the type of sewer system, combined or separated storm-water and sewage system, the traffic-generated metal-bearing particles will be transported to the STP or with the separate storm-water sewer directly to the nearby lake, river or coastal water. Thus, of the initial estimate of traffic emissions of copper, 90 t/y for the whole of Sweden in the mid-90s, only some 10 t/y were supposed to end up in the aquatic environment, the rest being retained in soils along roads, in sewage sludge or in landfills (L & L -Cu). In the city of Stockholm, however, due to the large surface areas covered with hard, impermeable materials, it must be assumed that a relatively greater fraction of the metal emissions from traffic activities will end up in the sewers and eventually in either the sewage sludge or in the receiving surface waters around the city.

0 0

Post a comment