Once having entered the road area, pollutants may start their transport to other ecosystem compartments. Any ecosystem compartment that may be affected by a pollutant can be considered a target. The pollutants will not be permanently trapped at these destinations but may stay there for a prolonged period of time.

Pollutants in the solid and in the liquid form are transported to the environment in various ways (see Fig. 6.5):

• infiltration into the road structure and further transport to the groundwater;

• pavement runoff;

• splashing to the road shoulders and ditches;

• spray.

The relationship between different sources of pollution and different targets will be a function of

• the “strength” of the source (i. e. the rate of the emission);

• the pollutant pathway from the source to the target;

• the physical and chemical processes affecting the pollutant during the transport; and

• the “vulnerability” of the target.

Water-borne transport of pollutants occurs on the road surface and on top of the adjacent soil but also in the interior of the road structure. Even if there is good knowledge of parts of these processes, there is insufficient knowledge to provide a quantitative appraisal of these pollutant fluxes on top of, inside and around the road structure towards the different targets. Therefore only a qualitative description of the possibly impacted targets can be provided.

The wearing course of a road is not an impervious layer. Under the influence of rainfall infiltration, pollutants previously settled on the surface course can infiltrate into the road structure. Pollutants included in the matrix of road materials can even­tually be made soluble. Then, the first target (A1[15]) is the soil underlying the road structure (the vadose zone). As the “road leachate” can go on percolating towards the saturated zone of the subsoil, the second possible target (B) is groundwater. The distribution of pollutants between targets A1 and B will vary depending on the pollutant, the nature of the underlying soil, the prevailing physical and chemical conditions in the soil, the thickness of the vadose zone and the dynamics of the aquifer.

Another part of the rainfall will be transported on the surface of the road. Runoff water can infiltrate into the road shoulder that is usually made of permeable mate­rial. In cold climates the winter precipitation as snow melts in one or more short periods of time during winter and spring. The polluted melt water may infiltrate into the road shoulder/ditches, or run away on the top of a still frozen soil. Even when

the runoff pathway is somewhat different from those described above, the targets remain the same. Where roads are equipped with an impervious collection system, the runoff water can be transported to a permeable ditch or to more sophisticated water treatment facilities such as infiltration basins and settling basins (retention basins) – see Chapter 13, Section 13.4.8. In the first case, the soil adjacent to the road structure (A2) is a target. In the second case (infiltration down to the saturated zone), the groundwater (B) will also be a target.

From roadside soil, pollutants can become available for plants (C1) or soil – inhabiting animals (D1). These plants and animals can act as sources of contamination of herbivorous (E1) and carnivorous (F1) organisms. Some hazardous substances can accumulate in the organisms and further be biomagnified in the food chain.

As groundwater (B) can be used for drinking water supply or for irrigation, the plant and animal targets can also be impacted through this target.

Runoff water, sometimes collected in treatment facilities, is eventually dis­charged into natural surface waters. The water in streams, lakes and ponds can thus be a target of pollution (G). In cases of heavy use of road de-icing salt, lake targets may become permanently stratified due to high-density salt water concentrating in the deep water layer. The result is stagnant hypolimnion water (the lower part of the lake volume) with oxygen depletion and biologically dead areas.

As runoff-water pollutants are often adsorbed to particles, the bottom sediments of lakes and slow flowing streams become significant targets (H). Lake sediments may become almost permanent traps for the pollutants. Eventually the water and sediments become sources of contamination of aquatic organisms being plants (C2), decomposers (D2), herbivores (E2) or carnivores (F2) (Bskken, 1994a; Bskken & Fsrovig, 2004). As humans are users of water resources, and often the top predator in the food chains, they are the ultimate target (F1).

Similar targets (C2, D2, E2 and F2) can be reached in cases where it is the impacted groundwater (target B) that feeds a surface water body. In streams and lakes, herbivorous (target E1) or carnivorous (target F1) terrestrial consumers can be impacted through the consumption of targets C2, E2 and F2. And finally, similarly to groundwater, surface water bodies can be used for drinking water supply and for irrigation and can therefore impact targets C1, D1, E1 and F1.

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).

Complexes are chemical compounds consisting of a central atom (metal) and ligands (consisting of a group, molecule or ion) tied to the central atom with at least one co-ordination bond. A chelate is a special form of complex where the ligand is attached to the central atom by at least two bonds. The most common ligand in water solutions is the water molecule itself but anions such as hydroxide, carbonate, hydrocarbonate, sulphate and organic acids also form ligands. The formation of a complex from a metal and a ligand is a balanced reaction characterized by a constant (Kc) that is often pH dependent. Some complexations can be considered as “surface complexation” reactions (e. g. of a metal with an iron oxide) as opposed to “aqueous complexation” reactions.

Organic and inorganic complexes are present in all natural waters. Organic acids such as humic acids (originating in humus formation upon decay of plant litter) make up one of the most important types of ligands in natural waters. Humic acids and other types of humic substances greatly affect the solubility and thus the availability of heavy metals to biota. In soil water, humic substances occur in dissolved form and in more or less insoluble aggregates. Compared to heavy metals occurring as insoluble aggregates, heavy metals occurring in the dissolved form are much more mobile and available and therefore more toxic to biota (Berggren Kleja et al., 2006).

Among the inorganic complexes, hydroxides of Fe and Mn are common in nat­ural soils. From a pollution point of view, it is of great importance whether the hy­droxides are present in dissolved or precipitated form because hydroxides regulate the mobility of heavy metals. The stability of the hydroxide complexes is greatly governed by the pH. Depending on the soil type, but also on the degree and charac­teristics of the pollution load, roadside soils vary greatly in pH. In many cases, pH is higher close to the road than further away (James, 1999).

Components present in the road/soil environment and likely to form complexes with heavy metals include hydroxides, carbonates, hydrocarbonates, sulphates and organic acids. They originate from deposition, road materials and infiltrating water.