Road-related pollution sources include traffic and cargo, pavement and embankment materials, road equipment, maintenance and operation, and external sources. Road and traffic pollutants having received the greatest attention include heavy metals (e. g. from vehicle corrosion, cargo spills and road equipment), hydrocarbons (from fuels, lubricants and bitumen), nutrients (generated from motor exhausts), particulates (from pavement and exhausts) and de-icing salt. Runoff, splash/spray and seepage through the road construction and the soil are major transport routes of pollutants from the road to the environment.
Pollutant transport through road materials and soils in the road environment is governed by the same physical processes as those occurring in soils elsewhere. During their downward transport, contaminants in the aqueous phase interact with the solid phase. For mass transport in saturated media, diffusion, advection and dispersion are the major processes. Mass transport in unsaturated soil strongly depends on soil-moisture distribution inside the pores. After prolonged dry periods, the first flush of runoff often contains large quantities of pollutants accumulated on the road surface. Long-lying snow close to roads accumulates traffic pollutants.
In road soils, like elsewhere, the most significant chemical processes governing the transport of substances including pollutants are sorption/desorption, dis — solution/precipitation and exchange reactions. Sorption of substances in the liquid form on soil particles greatly influences pollutant solubility and transport in soils. Redox conditions and acidity (pH) largely regulate the solubility and thus the mobility of heavy metals. Many heavy metals are more mobile under acidic conditions.
Roadside vegetation influences the transport of traffic contaminants through air, water and soil. Plants close to heavily trafficked roads accumulate traffic pollutants such as heavy metals. Heavy metals, organics, de-icing salt and other toxic substances disturb biological processes in plants, animals, micro-organisms and other biota and may contaminate water bodies and the groundwater.
European legislation puts increasingly strong demands on the protection of water against pollution. Road-keepers are responsible for ensuring that the construction and use of roads is not detrimental to the quality of natural waters.
Strategies for the protection of the environment from road and traffic pollutants should primarily be directed towards limiting the generation of pollutants. As a complement to source-based measures, mitigation measures aim at reducing the dispersal of pollutants to the roadside environment and detrimental effects on soil, water and biota. Principles of road and traffic pollution prevention and mitigation include both technical and biological methods some of which are briefly outlined in Chapter 12.
Including consideration of measures for environmental protection at an early planning stage is much more cost efficient than retrofitting measures and installations afterwards. To judge the need for prevention and mitigation measures, chemical and biological characterization of soil and water is often required. Principles for the sampling and analysis are briefly described in Chapter 7.
The issue of contaminants in the environment is a very large subject and it is not possible within a few chapters to fully address the issues, even limiting the coverage to highway-related topics. Readers who want to explore further will find no shortage of reading material and can readily study the underlying science in much more detail than has been possible in this chapter (e. g. Fetter, 1993; Rand & Petrocelli, 1995; Charbeneau, 1999).
