Road Equipment

Road equipment comprises crash barriers, road signs, sign-posts, lamp-posts, etc. Many of these structures are made of galvanized steel. Corrosion of these surfaces releases zinc to the environment (Barbosa & Hvitved-Jacobsen, 1999). Corrosion is promoted under moist conditions often prevailing as a result of splashing from the traf­fic during and after precipitation. Soiling and the use of de-icing salt further enhance the corrosion. Re-painting is usually preceded by the removal of old paint. The old paint may contain heavy metals. Regular washing of road equipment may contribute pollutants to the environment in cases where detergents are used (Folkeson, 2005).

6.2.2 Maintenance and Operation

Many measures taken within road maintenance and operation introduce pollutants into the highway environment. De-icing activities are among the most important of these. In countries with a cold climate, de-icing and snow clearing are important measures to reduce slipperiness and maintain the functionality of the road dur­ing periods with frost or snow. Ice and snow control is performed mechanically (ploughing) or with the use of chemicals. The most widely used chemical is sodium chloride (NaCl). As an anti-caking agent, a minute quantity of potassium-ferro — cyanide is often added to the salt. At places, other chemicals than NaCl are used, e. g. urea on some bridges, or calcium chloride or calcium magnesium acetate (Ihs & Gustafson, 1996; Persson & Ihs, 1998). Winter operation of high-class roads, usually heavily trafficked highways, in cold regions is accompanied with the use of large quantities of salt. De-icing chemicals can thus be a considerable source of contamination of soil as well as groundwater and surface waters (Blomqvist, 1998; Johansson Thunqvist, 2003). Moreover, de-icing salt has been shown to mobilise heavy metals accumulated in roadside soils (Norrstrom & Jacks, 1998). Dust­binding chemicals used mainly on gravel roads include inorganic salts such as cal­cium chloride (CaCl2) and magnesium chloride (MgCl2) (Alzubaidi, 1999).

Roadside vegetation and its maintenance also influence the transport of pollutants having entered the road environment. Dense and tall vegetation close to the road will trap pollutants and diminish their spread away from the road (Folkeson, 2005). Upon decay of the plant litter, the pollutants trapped or taken up by the shoots will enter the soil and contribute to pollutant accumulation in the roadside ecosystem. If the mown vegetation is collected, pollutants in the cut material will be exported from the roadside ecosystem. Increasingly, around the world, especially in parts of Europe, the USA and Australasia, vegetated swales (Fig. 1.10) at the side of roads are being deliberately employed as a part of the environmental management of the highway runoff water. They aim to reduce the quantity and improve the quality of runoff that enters groundwater (see also Chapter 12, e. g. Fletcher et al., 2002).

In countries where chemical vegetation control is still not banned, herbicides are directly released into the roadside environment. Unless rapidly degraded into less harmful substances, these toxins may contribute to groundwater or surface-water contamination.

Ditch clearing involves the handling of soils that can be heavily polluted with organic pollutants and heavy metals. Displacement of the material to the outer slope will lead to the accumulation of pollutants in the road area and eventually to the leaching of, e. g., heavy metals to the groundwater or surface water bodies. Where rehabilitation of roads is planned, any spreading of pollutants having accumulated in the road body or the roadside should be avoided.

Road-runoff water carries large amounts of pollutants away from the road sur­face. The amounts so transported vary greatly depending on a range of factors, the most important being traffic volume and characteristics and amount of precipitation. Pollutant concentrations in runoff have been widely studied. Concentration ranges commonly reported are collected in Table 6.3.

Care must be taken both in road design and in road operation so as to avoid con­tamination of surface waters and the groundwater. The Water Framework Directive aims at securing good quality in all natural waters, not only where sensitive aquifers or drinking-water abstraction points could be at risk (see Section 6.5 below). Some national road authorities have handbooks for the treatment of highway runoff, e. g. Sweden (Vagdagvatten, 2004).

At some places, runoff water is diverted to retention ponds or other facilities for handling of pollutants (Hvitved-Jacobsen & Yousef, 1991). Facilities for protection of the environment from pollutants should be properly maintained so as to secure the continuous effectiveness of the facility. For instance, sediments in retention ponds accumulate large amounts of pollutants and must be treated or disposed in such a way that pollutants do not enter into the environment (Hvitved-Jacobsen & Yousef, 1991; Stead-Dexter & Ward, 2004).

Likewise, water used for the washing of road tunnels (pavement, walls and roof) must be treated in a way that prevents the pollutants in the rinsing water from reach­ing the environment (Cordt et al., 1992; Barbosa et al., 2006).