Many physical processes in the pavement, the embankment and the road environment influence the flow of water away from the road surface. Pollution transport is heavily influenced by the physical and chemical characteristics of the specific pollutants. It is also strongly influenced by the interaction between pollutants, and with materials making up the pavement and embankment. During their transport, pollutants interact with materials in the solid, liquid and gaseous form.
The physical processes of the movement of pollutants in and by fluids in roads and their environment can be described in terms of pollutant mass transport. Mass transport can take place in solution, in suspension or in the form of particulate matter. There are great differences between these processes, and they also vary between the unsaturated and the saturated part of the road construction. The majority of pollutant transport from the road surface is via surface runoff towards the soil, surface water and groundwater. Part of the precipitation falling on the pavement surface is infiltrated through the pavement or the soil adjacent to the pavement. In the beginning, the infiltrated water flows more or less vertically through the unsaturated zone. The nature of this flow mainly depends on the road geometry and the materials used in the road construction. Once having reached the groundwater, the infiltrated water will follow the direction of the groundwater flow that is usually more or less horizontal.
In porous pavements, in the embankment and in the soil adjacent to the road, the transport of pollutants is usually in water solution. Pavement cracks also allow transport of pollutants in the particulate form. Due to the clogging of pore spaces with particulates, however, this kind of transport is stopped some ten centimetres below the pavement surface (Brencic, 2007 pers. comm.; cf. Khilar & Fogler, 1998).
