Category WATER IN ROAD STRUCTURES

Sampling can aim at documenting contaminant concentrations and fluxes during and after storm events (or other rain or snow melt), at mirroring the load and flux of contaminants over an extended period of time, or at characterizing an accidental discharge.

Road runoff presents specific characteristics that change significantly from place to place, depending on site characteristics and many other factors (Barbosa & Hvitved-Jacobsen, 2001). Therefore, the monitoring programme should also include the characterization of the most important factors such as: traffic characteristics (volume, speed, type of vehicles, fuel types, etc.), geographic location, climate, topography, drainage area and road design, pavement characteristics, right-of-way characteristics and adjacent land use. Although the quality of water is being in­fluenced by several external characteristics, it is the duration and intensity of the pavement washing that determines the degree of dilution and transport of pollutants. The site selected for runoff sampling should be a drainage area (with significant and representative size) where the runoff from the paved and unpaved areas can be isolated from other sources within the selected highway system.

Runoff sampling may be used to characterize the influence of the mean daily traffic of a road section. For that purpose samples must be collected in a discrete way, in a surface drainage pipe (e. g. Fig. 7.1), during rain or snow thawing events, and be associated with a specific road drainage area. The runoff flow measurement devices are usually installed transversally in the outflow pipe from the system drain­ing the area of pavement that is of interest and must include a calibrated component that links the observed water level to the corresponding runoff volume by knowing the shape and dimension of the pipe. Runoff sampling depends on the occurrence of precipitation, and therefore, sampling is facilitated by the use of sampling equip­ment that is activated by a sensing device which automatically starts the sampling whenever the flow rate exceeds a previously defined value. Such sampling equip­ment also has the benefit of allowing the sampling of the first flush of a storm event.

This type of monitoring gives discrete information about the changes in runoff pollution during a certain period and allows the determination of the Event Mean Concentration (EMC) and the Site Mean Concentration (SMC). An EMC is

calculated for an individual storm event as the total mass load of a pollutant parameter (CV) divided by the total runoff water volume (V) discharged during the storm (Hvitved-Jacobsen & Vollertsen, 2003):

EMC = ZCV/ZV (7.1)

A SMC is a characteristic runoff annual pollution loads for a specific site, typically defined by the arithmetic mean value of the EMC’s measured at one site (Hvitved – Jacobsen & Vollertsen, 2003).

To evaluate the EMC it is advisable to sample the whole event in a way that is time constant and volume proportional. In this method, discrete samples are col­lected at equal time increments and composed proportional to the varying flow rate during the sampling period (FHWA, 1987, 1988). In order to mirror seasonal varia­tions, a monitoring programme should, ideally, include several discrete storm events for a one-year period. Authors differ in their opinions about the minimum number of storm events needed. Some authors consider 10, others 6 storm event episodes, each one characterized by a minimum of 5 samples (Barbosa, 1999, Burton & Pitt, 2002). Probably, the choice is influenced by storm severity and other climatological factors that may vary widely between countries and regions.

To allow the characterization of road runoff throughout a specific period, runoff monitoring devices should be:

• automated;

• include a rainfall device;

• include a runoff flow measurement device; and

• include sampling and recording equipment.

All these devices should operate in phase with each other in order to allow the determination of lag time from rainfall to runoff and the determination of pollutant load in each sample.

Figure 7.2 illustrates an alternative approach, common for measuring lower flow volumes than observed in runoff collection pipes – e. g. rainfall and seepage water. Water is fed into a bucket (Fig. 7.2a) that tips alternately to left and right when it fills. The number of tips may be counted electronically. Figure 7.2b shows a typical installation, in this case with the possibility of some of the water being sampled for later analysis. By integrating the output of the tipping buckets with the automatic water sampling equipment (as shown in Fig. 7.3) it is possible to sample water on a volumetric basis rather than a time basis.

Where discrete sampling is not feasible, runoff sampling can be accumulative. For that it is possible to arrange special sampling devices, in the form of a gutter installed along the sealed pavement’s edge to capture runoff water and then to con­duct it to a type of tank, such as a rainwater settling tank or a retention tank installed close to or away from the road. Afterwards the tank can be sampled by abstracting a proportion into a vessel installed in a chamber dug into the roadside soil. These devices can give a broad idea about the road pollution but are not appropriated for load calculations if their draining section is not easily delineable. These sampling
methods are less informative since they give information about the road pollution integrated over a period of time. Also, other sources than the road and its traffic might contribute to the contaminants collected in the tanks. Furthermore, change in water condition can take place between the time that water arrives at the tank and the time at which a specimen is collected. Sampling from tanks, therefore, only gives a qualitative overview of road pollution. In the case of an overall sample taken in

Fig. 7.3 Automatic water sampler with refrigerated cabinet (Courtesy of Hach. ©Hach Company, 2007)

a settling tank or retention tank, water collection must be made at several locations and depths of the tank so as to provide a composite sample representative of the water in the entire tank.

7.4.1 Introduction

The procedure for sampling is primarily influenced by the source of the water (e. g. in a borehole, in a pipe) and by the equipment available with which to sample it. The equipment, itself, is largely controlled by the sampling location. Once collected by the sampling device, water samples must be quickly processed before changes in make-up occur due to various physical and biological processes. The following sections discuss the collection and immediate treatment of surface and sub-surface waters and of soil. They provide a general overview of the techniques and pro­cedures for application in pavements and the ground around highways. There is insufficient space to cover all aspects of sampling techniques and practice, so inter­ested readers are directed to other texts where these aspects are fully described. In particular, reference may be made to the following sources : [18]

In establishing a monitoring programme and data collection schedule, the following points will need addressing. This is not a comprehensive list for every eventuality, but most monitoring programmes will need to consider this list as a minimum:

• The equipment, skills, storage and transportation facilities (and, if required, power to the site):

о Sampling, transport and storage protocols should be obtained or prepared to promote good practice and consequently to yield reliability of readings ob­tained from tests.

о A responsible person should be identified for sample collection and instru­ment readings and made available over the life of the programme. о Health and safety plans are required for personnel engaged in the sampling.

• Timing of sample collection:

о Sometimes a frequency that reduces over time will be satisfactory if adequate behaviour is demonstrated by early analysis, especially where some new ma­terial or construction is being tried or where a singular event has occurred (such as a cargo spillage). Otherwise regular sampling will usually be pre­ferred. There are some cases where only a single sample or one “before” and one “after” sample are necessary at any particular point. о Sampling needs to take account of the weather conditions in which the me­dia will be sampled / measured (rain, wind, heat, etc.). Sometimes collection should be at a fixed time of day, a certain temperature or in a certain season when, otherwise, there wouldn’t be (or would be doubts about) comparability between specimens.

о Sometimes samples are to be collected from a discrete, sometimes from a continuous, source. In some low flow situations, sampling may exhaust the source until slow seepage provides the next specimen. This may have an in­fluence on sampling frequency and/or volume.

• Location of sampling:

о There is usually a conflict between the desired number of points to be sampled and the budget available. As far as possible, the locations should aim to ensure spatial reliability and representativeness. In particular the design should give confidence that all “hot-spots” will be located and that invalid and unreliable readings can be easily distinguished from genuine extreme values. о Vertical and horizontal positions in the ground should be chosen depending on the expected source and route of contaminant flow and the receiving media. For example, where traffic factors are thought to be influential, closeness to the wheel path may be important; where runoff is important, measurements in the verge may be important; where a developing pollutant front is to be detected (or refuted) then positions laterally and beneath the source point may be required. There are similar issues for water bodies – at what depth(s)

and water velocity(ies) should specimens be collected? In the context of road construction, such a question will be irrelevant if water is collected from seep­ages.

о Consideration should be given to the accessibility of sampling locations once the road is open to traffic. Access through the carriageway will often require lane closures that are expensive or difficult to arrange. The access point may also be difficult to keep sealed under traffic loading thereby compromising the quality of specimens collected.

• Protection:

о Sample collection points and instruments should be designed to be replace­able if they become damaged or aged. Appropriate protection of collection points and instruments against vandalism, traffic over-run, grass-cutting and other maintenance/rehabilitation will often be needed.

• Materials to be sampled:

о The sampling programme needs to assess all relevant material, i. e., ground­water, surface water, soils and aggregates as well as these media in their ref­erence condition(s).

о Adjacent surface water bodies may need assessment if an affect by polluted water from the road is suspected.

о Soil or construction material may need monitoring as well as groundwater. If the subgrade or a construction material will (or may) transport or sorb contaminants, it may need to be sampled from time-to-time to check for any alteration (e. g. permeability value or sorbed contaminant level). Similarly, if construction material, in-place, is thought to be a source of ongoing contami­nation, the plan should consider sampling it and testing its leachability as use continues.

о While water samples are only tested for their chemical properties, soil speci­mens may need testing for total solids make-up, organic content, mineralogy, particle size distribution and specific surface area. The last two are important in understanding sorption behaviour. Water can also be extracted from soil samples, e. g. by using a centrifuge.

• Selecting parameters for analysis:

о It is expensive to analyse for all species all of the time. Therefore, it may be necessary to identify key analytes that may act as indicators of change in the seepage/transport process, and to concentrate monitoring on these.

о Often, a regular frequency of assessment can be maintained at modest cost if full chemical analysis is sometimes replaced by surrogates, e. g. pH, Eh, electrical conductivity, etc. by electrical means.

о Where “trigger” values have been set for some intervention, the concentra­tions of the “trigger” species will require specific, ongoing, analysis.

о It can be a false economy only to analyse collected specimens for the analytes of known concern. A record of the contamination of other species may yield important information on an underlying chemical process (e. g. ion exchange) or may give rise to unexpected values that will lead to the identification of some unexpected problem or benefit.

о The water or soil should be sampled in amounts large enough to permit all the desired testing to be performed on it.

Some duplicated sampling will be needed, usually from the same location. The aims are:

• to ensure evidence of a reading’s representativeness;

• to adequately define statistical scatter in readings;

• to monitor genuine fluctuations in source concentration (e. g. as a consequence of flow levels, season, traffic, etc.);

• to ensure that both mean and “worst-case” values are available; and

• to allow repeated analyses in case of dispute.

To ensure best practice in this area most environmental regulatory authorities issue guidance on sampling (see reference list at end of Section 7.4.1).

An appropriate database / record keeping system must be provided (or constituted):

• To hold the data.

• To have data extracted/interpreted in a manner that has meaning. There is no point in collecting data that cannot be successfully accessed.

• To allow it to be interrogated in a way that permits the likely users (owners, regulators, researchers, etc.) to apply the method of interpretation that meets their needs. As most data storage is archived electronically, consideration should also be given to providing secure network access.

• To be easily maintained and amended.

• To have some semi-automatic processing capability that will alert the database owner to take some investigative action if the data contained seems to indicate a problem. There are more than a few examples of a record system holding the data

that would have indicated a potential problem long before it became an issue

if only someone had looked at the data!

• The database / system must be properly documented and backed-up. Accepted archival systems are required for both electronic and paper records.

• The database should hold all the data that passes certain pre-defined quality levels. The quality levels should not be set too high otherwise too many useful data points will be excluded. The disadvantage is that some invalid or unreliable readings will be stored. Therefore, sufficient data points should be stored in the database so that later data analyses can differentiate genuinely high or low values from those readings that are unreliably high or low.

An ongoing budget should be secured to enable monitoring and database main­tenance to continue over the full time-scale required by the probable contaminant transport behaviour. If it cannot be ensured, a sustainable “fall-back” programme should be incorporated into the plan.

Modern software systems are readily, and economically, available to provide se­cure, accessible data storage and retrieval capability. Data entry to these systems is also considerably more user-friendly than in the past. The standardized data format of the Association of Geotechnical and Geoenvironmental Specialists (AGS, 2004) is one such system that has the great advantage of being non-proprietary. Thus, data stored in this manner is readily interchangeable between different users. Field data can be collected by “palmtop” (“PDA”) computer and combined with applications developed using open source software (e. g. Walthall & Waterman, 2006; Chandler et al., 2006).

7.2.1 Data Collection

It is a general rule that data must always be collected with a specific purpose in mind. This rule is as much true concerning contaminant levels in soils and groundwaters as it is in any other field. Therefore, before any sampling regime is contemplated and before any specimens are analysed, it is essential to decide the purpose for which samples and analyses might be required. Then a systematic sampling programme and network needs to be designed and planned with the purpose and constraints clearly in view. It is usually a non-routine task to identify the questions that need answering by the sampling and analysis programme. Perhaps these will be suggested because of standards set by an environmental regulator. Perhaps they will arise be­cause of a client’s desire to establish or maintain a reputation for environmentally responsible behaviour. Perhaps they will be set as a consequence of the desire to establish a benchmark for future work. Probably there will be some combination of these prompts together with others. It is the monitoring agency’s job to ensure that the data collected is that which is really needed and not that habitually collected. There is no point in collecting data that is never used, nor will it usually be possible to go back and collect missing data.

If a road is to be constructed of an unusual material, then a programme of labora­tory assessments of the leaching potential of the material might be established long before construction is planned. The purpose here would probably be to understand the possible yields of contaminants in time, and in concentration, under conditions that are expected to pertain in-situ once construction commences and, also, after construction is completed. Initially a wide range of elements might be studied to de­termine what species are of concern within the runoff contaminants (see Chapter 6). Later studies might take place on a reduced number, only addressing those elements previously identified as potentially giving concern. Probably some kind of computer modelling will be intended to extend the in-isolation laboratory results to in-situ conditions. In that case, it is necessary to obtain data in a form suitable for use in the selected modelling program. Once construction commences, data collection in-situ will most probably be required. At that time the aim would be to confirm that the interpretation made from the laboratory assessments and numerical modelling are, indeed, valid.

In examples where a potentially contaminating material is to be used, for which construction experience and contaminant behaviour is already understood, labora­tory studies might be restricted to confirming that the material is, indeed, similar to previously used examples. In-situ investigations might then be limited to confirming similar behaviour to that previously experienced. In practice, no two installations are identical, so some sample collection and analyses are likely to be required to explore the specific application.

For many studies of contaminants in the road environment, a “base-line” study is required. This investigation has the aim of characterizing the in-situ ground and water quality condition prior to a planned action. Thus, the naturally occurring conditions of a “greenfield” site would be recorded before a new road was constructed in the vicinity, or a contaminated site would be characterised prior to activity designed to improve water quality. In both cases the aims are, first, to be able to determine the effects of the activity on the surrounding ground and water quality and, secondly, to assess whether some intervention is, therefore, needed.

Teresa Leitao*, Andrew Dawson*, Torleif Bakken, Mihael Brencic, Lennart Folkeson, Denis Francois, Petra Kunmska, Roman LiCbinsky and Martin Vojtesek

Abstract This chapter presents a general overview of procedures and methods for sampling and analysis of contaminants in water and soil in the road environment. The chapter concerns the water and seepage in road structures under the influence of traffic loading, and in the adjacent ground extending to the water table where contaminant seepage is of concern. The text gives an introduction to this subject and guides the reader to relevant literature with detailed information about practices of sampling and analysis. The chapter in divided into five main sections: principles of data collection and storage, sampling design, water and soil sampling procedures, and in-situ and laboratory measurements and analyses.

Keywords Road contaminants ■ data collection and storage ■ sampling design ■ in-situ and laboratory analysis ■ water ■ soil

7.1 Introduction

The purpose of contaminant sampling and analysis is mainly to characterize a specific road in terms of its runoff characteristics and the water percolating vertically through the road structure, as well as the existing state of quality of the adjacent water and soil. Sampling and analysis can also be performed to identify a specific pollution episode. The environmental compartments, usually considered as being potentially affected, comprise surface waters, groundwater, soil and soil water. To­gether they can give a global picture of contaminant dispersion and pathways after entering the soil (see Chapter 6). [16] [17]

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).

Across Europe, the legislation on the influence of road and road traffic on water and water bodies and associated ecosystems is wide and complex. European legislation in general prohibits water pollution and limits influences on the water biotopes. These general rules are transferred into national legislation in very different ways. Realization of these rules depends on the country’s prevailing natural conditions (e. g. climatic regime), uses of water and technical regulations concerning road plan­ning, design, construction and maintenance. In general, water pollution from roads is regulated in two main groups of legislation: environmental law and construction law. This section refers to environmental law.

Water is one of the most comprehensively regulated areas of the EU environmen­tal legislation with directives regulating quality and standards for, e. g., dangerous substances in water, fishing water, drinking water and groundwater. The Water Framework Directive (WFD) of 2000 (EU, 2000) is the most important directive under the group of environmental law that regulates water pollution. The Ground­water Directive (EU, 2006), on the protection of groundwater against pollution and deterioration, is also a feature of the WFD.

The purpose of the WFD is to establish a framework for the protection of inland surface waters, transitional waters, coastal waters as well as groundwater. It aims at enhanced protection and improvement of the aquatic environment, and ensures the progressive reduction of pollution of water, based on a long-term protection and prevention of further pollution. Common environmental quality standards and emission limit values for certain groups or families of pollutants should be laid down as minimum requirements in Community legislation.

At latest 15 years after the date of entry into force of the WFD, i. e. 2015, Member States shall have protected all their water bodies with the aim of having a good water status. Good water quality is such that the concentrations of pollutants do not exceed the quality standards applicable under other relevant Community legislations.

Furthermore, the WFD presents an indicative list of what in general is considered the main groups of pollutants in water. Some of these are toxic while others are nutrient salts or substances causing oxygen depletion. In particular a number of priority substances have been listed and given special attention (the List of Priority Substances in the field of water policy). The List contains 33 substances [http:// europa. eu. int/comm/environment/water/water-framework/priority _substances. htm]. Some of these are typical traffic and road pollutants.

The substances on the list are already controlled, to varying degrees, by EU and national legislation. Further controls, independent of the WFD, are expected for a number of substances as a result of European and other international regulations. The European Parliament and the European Council will adopt specific measures against pollution of water by individual pollutants or groups of pollutants presenting a significant risk to or via the aquatic environment, including such risks to waters used for the abstraction of drinking water. For those pollutants, measures will be aimed at the progressive reduction and, for priority hazardous substances, at the cessation or phasing out of discharges, emissions and losses.

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).