In the road environment, contaminants are almost entirely moved by water-based or air-based processes. Air-based processes are, principally, by dust and spray, but these are not discussed further, being beyond the scope of this book. In water — based processes, contaminants are carried in and/or by the water through soil or aggregate pores, over the top of the pavement and through drainage systems. As water moves through the sub-surface, water that is fairly pure will have the ability to pick-up chemicals from the soil through which is flowing and to carry these elsewhere while runoff water that arrives from the pavement surface may percolate into the construction carrying impurities with it which are then “dropped” by one mechanism or another into the layer. The interaction between water and contaminants is the subject of Chapter 6 with methods of measurement being described in Chapter 7.
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Movement of contaminants is often considered using a “Source — Pathway — Receptor” framework (Fig. 1.7). In the context of the highway, the “source” would probably be the surface runoff water (although its antecedents of vehicle cargoes, atmospheric pollution, etc. could also be considered as the source) or the pavement
construction. The contaminated water then moves through the pavement and the drainage system which provides the “pathway” for the contaminant to move. Eventually it arrives at a place where it has a potentially deleterious impact — the “receptor”. In an ideal understanding the receptor is a human, animal or plant that is affected by the contaminant. In practice, policing of impacts on humans would be almost impossible to monitor and, anyway, too late to change an undesirable response. So, instead, it is normal, in most practical circumstances, to treat either the surface water body (river, lake, etc.) or the ultimate groundwater body (e. g. drinking water aquifer) as the receptor. Monitoring their quality is the subject of Chapter 7.
Chapter 12 gives information that, with the help of Chapter 13, can be used to mitigate problems due to contaminant movement in and with the water percolating in the near-pavement environment. Usually, these involve either interruption of the pathway or removal of the source or target.
