The implementation of mitigation and prevention measures from roads and road traffic should follow the pollutant fate in the environment. Before planning and designing of protection against pollutants from road and road traffic, a conceptual model of the pollutant fate in the particular environment should be established. This should help to estimate potential risks and hazards to water bodies’ pollution. The model usually consist of three main parts that are represented by definition of pollutant sources, pathways of pollutants through the environment and targets that receive pollution from the sources in road environment. The concept of pollutant fate in the environment is described in greater detail in Chapter 6.
As in every environmentally-driven decision, care must be taken that the benefit to the water body is not offset by an equal or worse disbenefit to another environmental compartment. For example, the higher fuel consumption of cars using a longer road could be evaluated in terms of non-renewable energy impact and
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Table 12.1 Classification of pollution mitigation approaches and methods
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greenhouse gas emissions and these compared with the lower risk achieved to the groundwater. How to make such a comparison is beyond the scope of this book, but interested readers are referred to (Falcocchio, 2004).
It will never be possible to prevent all deleterious impacts of the road on the hydro-environment. However, there are many actions that can be taken to significantly reduce impacts. In general, the pollution management policy is that protection of the environment should be preformed in a way that the source concentrations of contaminants are reduced as much as possible and to limit, or prevent completely, the appearance of contaminants in the targets. To reach this goal several mitigation approaches and mitigation methods can be adopted (Table 12.1).
Mitigation approaches are divided according to the pollutant fate model: source — pathway — target.
Mitigation at source can be performed with:
• prevention methods;
• avoidance methods; and
• reduction methods.
Prevention methods are in general applied to stop emissions of pollutants in the environment or at particular environmental sensitive areas (e. g. on Natura 2000 areas of sensitive water habitat). A typical general prevention approach is banning of leaded fuel or banning the use of road de-icing agents on the environmental sensitive areas.
Avoidance methods, in general, can be defined as special design procedures, mainly connected with road alignment, that avoid crossing environmentally sensitive areas. They seek to prevent a problem from arising in the first place (or minimize the problem). Often these design options are very costly and they very often interfere with the goals of the road. For example, a road may be longer in order to avoid a particularly sensitive groundwater body leading to greater construction costs and ongoing fuel consumption costs.
Reduction methods are those that are implemented when emissions from roads and the road environment cannot be stopped. They can be implemented by various traffic restrictions such as travel velocity reductions (e. g. on groundwater safe-guard zones) or reduction of traffic flow (e. g. embargo of dangerous goods transport over environmentally sensitive areas). Also, among reduction methods, the proper selection of construction materials can be included (e. g. alternative material use for sub-grade that do not interact with the soil environment).
Mitigation along the pathways can be achieved with
• interception methods; and
• reorientation methods.
Reorientation methods divert water that was polluted at the road surface, or inside the pavement, out of the area sensitive to water pollution perhaps to runoff treatment facilities where water is intercepted and treated. A watertight drainage system that diverts runoff water is a typical example of this.
Interception methods are technical measures that enable interception of pollutant flux. These interceptions can be defined as run-off treatment facilities (e. g. detention ponds) or absorption barriers (e. g. reactive barriers).
Mitigation at the target is achieved when pollutant reaches the target and its deleterious impact is reduced by
• remediation methods; and
• compensation methods.
Remediation methods are only feasible when some deleterious and adverse effects appear at an environmental target (e. g. damage to local fish habitat as the consequence of leakage from alternative material built into a sub-base). A typical remediation method in the pavement and embankment domain could be the replacement of contaminated granular base and sub-base materials by earth works. These methods would be extreme and only used in situations when previous mitigation measures were not successful. The use of these methods should not be implemented as an integral method for permanent pollution protection. However, in environmentally sensitive areas they could be planned as a part of the intervention measures.
Compensation methods are economic measures or replacement measures. The latter are applied in the case that road construction, and all the consequences of it, damage a particular habitat or water body. In this case a new, substitute, habitat or water body is included as part of the construction cost in the area where previously the zone was of lower ecological value. As an economic measure, compensation methods are applied as indemnity to the owners of the land crossed by the road or who are influenced by it. From the environmental point of view, compensation methods for pollution mitigation should be avoided. This approach implements the principle that loss of environmental values can be compensated by economic measures. Remediation and compensation mitigation methods are usually applied outside of the embankment zone, so they are not covered in full detail here.
Mitigation methods can be further divided into:
• ex-situ methods. Ex-situ methods are implemented externally as non — technical measures or as technical measures performed in places that are not part of the near-road environment.
• in-situ methods. In-situ methods can be defined as mitigation methods implemented on the road or in the near vicinity. These methods are further divided into:
o intervention measures, о non-intervention measures.
Intervention measures are those that involve intervention by human action, either when a problem is detected or on a regular basis (e. g. to maintain a pumping system). Active approaches are the least desirable for a number of reasons:
• Their success depends on continued human attention… which is often difficult to guarantee;
• They continue to require funding after construction, both in terms of payment to the personnel involved and, in many cases, in terms of the running costs of electrical or other energy consuming equipment. In the future there may be pressure on funding and a lack of appreciation of a problem that is in focus at the present time. This can lead to less attention at some future date than is necessary; and
• Detection of a problem is necessary in many cases for the active approach to be implemented. Some problems will be readily detected — e. g. those resulting from a spillage during a traffic accident — but many will not be easily detectable in which case it is difficult to incentivize the search for a problem which could conceivably (but probably doesn’t) exist.
Non-intervention measures rely on the installation of some constructed element that continues to function over a large part, or all, of the life of the project in which it is installed. They are often more costly than active ones if expenditure is only considered over a year or two. However, in the long term the ongoing costs of providing active control will usually make passive approaches seem more economic.
The constructed element is designed to achieve one or more of the following:
• that any actual or potential contamination pathway is blocked;
• that there is a purpose-installed receptor for the any potential or actual contaminant that will prevent the contaminant from reaching a natural receptor to which it would present a hazard; and
• that the water regime adjacent to the road is maintained in an acceptable manner.
It is always best to attempt to avoid pollution problems rather than to intervene
after the event. Data published by the UK’s Highways Agency (2006) for 5 British
roads reveals the high variability of success of different techniques. Sometimes 99%
reduction in contaminant concentration was achieved, sometimes there was even an increase in concentration after use of a “clean-up” technique due, presumably, to remobilization of previously arrested contaminant.
No road construction can ever have a zero impact on the environment in which it is placed. The materials of which it is constructed will yield a different response to the hydrological situation than did the soils that they have replaced. The construction interrupts the preceding natural flow regime (Fig. 12.3). The traffic on the road generates various pollutants that fall on the road (Chapter 6, Section 6.2). For these reasons the road designer must assess the potential impact of each aspect, compute the risk of unacceptable pollution and put in place mitigation measures that will address each unacceptable risk in a technically and economically satisfactory manner — this is a major challenge, especially as regulatory regimes become more and more demanding.
