Usually, roadside soils are or become covered by vegetation. Especially where the plant cover is large or the vegetation dense, the vegetation as a physical body influences the air-borne transport of pollutants from the road and traffic to the surrounding environment. Usually, however, the vegetationis kept low by mowing andbushcutting. To some extent, pollutants deposited on leafy surfaces enter the interior of the plant.
Under good growing conditions, plants will produce a more or less dense root system. Their root mass will greatly influence the movement not only of water but also of pollutants in the soil. Root uptake can withdraw large quantities of water from percolation. Root uptake also forms an important pathway of pollutants into the plant. The tendency to be taken up by roots differs greatly between contaminants and also between plant species. Once absorbed, the pollutants become trapped within the plant and they are therefore removed from the soil system until either the plant is consumed or decomposed.
The vegetation is also a producer of organic matter. Upon death, the plant with its shoot and root parts will form plant litter which will eventually be decomposed to form soil-organic matter. Soil-organic matter is an important factor in a range of biological, chemical and physical processes in the soil.
The ability of plants to take up pollutants, especially through their roots, is actively or passively utilised for run-off treatment. This form of bio-remediation can be an efficient means of treating pollutants accumulating in the road environment. Ditches are often vegetated, and plants such as tall-growing grass or sedge species often absorb and retain heavy metals and other pollutants to a considerable degree. In the case of organic pollutants, the plants help degrade at least some of the compounds. In the case of heavy metals, the pollutants will stay in the roadside environment unless cut vegetation (or the ditch mass) is removed and transported elsewhere. Heavy metals in themselves are not degradable. The use of plants for either absorption or biodegradation (organism-mediated breakdown of substances) of contaminants in soil is known as phytoremediation.
Roadsides are inhabited or otherwise utilised by a variety of animals. Through grazing, animals will ingest pollutants present in or on the biomass. Likewise, animals of prey will ingest any pollutants present in their prey. In the case of mobile animals, this will form a pathway of pollutants to the environment away from the roadside.
Roadside soils also accommodate a range of animals exploring the soil resources. Burrowing organisms such as earthworms and arthropods ingest large quantities of soil. Soil ingestion and excretion is an important means of contaminant transport within the soil. This process may also mobilise contaminants that had previously been bound to soil particles by sorption processes. Tunnelling will also create channels for water flow, which increases soil permeability to water. This will result in any future intrusion of contaminated water passing through the soil more rapidly, which reduces the ability of the soil to adsorb the contaminants.
Every soil is also inhabited by micro-organisms. Micro-organisms are highly involved in the turnover of organic matter in the soil. In natural soils, a wide range of complicated microbial processes involving enzymes participate in the prolonged process in which organic substances from plant, animal and microbial matter are decomposed into simple compounds. Some of these constitute nutrients necessary for biomass build-up with the help of photosynthesis. Organic exudates produced by micro-organisms also greatly influence soil structure.
Bacteria, algae and fungi are highly involved also in the transformation of soil pollutants. Many organic pollutants are gradually degraded to less harmful compounds by the action of micro-organisms. Also heavy-metal pollutants are influenced by micro-organisms. Chelating agents exuded by micro-organisms greatly influence the chemical form and mobility of heavy metals in the soil. Unlike organics, heavy metals, which in themselves are elements, are not decomposed, even if they are transformed into chemical compounds which may render them either less or more available to plant and animal life. The availability and toxicity of heavy metals to plants, animals and micro-organisms is greatly influenced by the heavy-metal speciation. Often, the free hydrated form is the most prevailing form, and also the most available and toxic to biota.
In the vicinity of roads, road pollutants accumulate in soil, water and other ecosystem compartments. There is a wealth of literature documenting various types of detrimental effects of road and traffic pollutants on plants, animals and microorganisms (see, e. g., Scanlon, 1991). Even if micro-organisms are especially sensitive to toxic substances, plants and animals are also sensitive. The sensitivity differs greatly between various plant, animal and microbial groups, and between toxic substances.
Of the substances occurring in elevated concentrations in road environments, heavy metals, PAH and de-icing salt are the most relevant and most studied. Generally, biological processes involving enzymes are known to be especially prone to disturbance from heavy-metal pollutants. Micro-organism-mediated processes such as organic-matter breakdown, humification, and nitrogen and phosphorus mineralization are largely susceptible to disturbance from elevated heavy — metal concentrations. Such effects have been reported from the vicinity of roads (Tyler, 1974). Reduced photosynthesis rate, growth and reproductive ability are among the most commonly reported effects of heavy-metal exposure to plants and animals (Bazzaz et al., 1974; Rolfe & Bazzaz, 1975; Sprague, 1987; Holdway, 1988; Weis & Weis, 1991; Sarkar, 2002).
Contaminants, especially those with high mobility, often reach surface waters and the groundwater. De-icing-salt contamination of groundwater and surface water bodies is often a problem in countries using de-icing salt (Johansson Thunqvist, 2003).
