Road and traffic pollutants are emitted in gaseous, solid or liquid form.
Materials used for the construction of pavements and embankments contain pollutants mainly in the solid state. However, pollutants initially in the solid state can be released into water. Two processes are at work:
• desorption — chemicals are detached from the solids to which they are loosely bound, and
• dissolution — chemicals are dissolved by adjacent water. Together these are known as leaching.
Pollutants will move from the solid phase to the dissolved phase until:
• The water cannot hold more (“solubility limit”); or
• There is no more solid phase to be desorbed or dissolved (“source limit”); or
• There is insufficient contact time for the processes of desorption or dissolution to complete (“availability limit”).
Road construction materials may contain a variety of potentially harmful chemicals. The quantity of pollutants leached depends on factors including the surface area exposed to leaching, the material history and the pH, redox potential and other chemical and physical characteristics of the leachate.
A presentation of a conceptual model of water fluxes from the road construction is presented in Fig. 2.1.
