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3.IMPact evaluation

In road construction, there are a number of pathways by which occupational exposure and other impacts on human health may occur. As an example for powdery material during the construction phase, dusting may take place under dry and stormy conditions leading to possible inhalation issues. This problem is mostly solved by spraying water on the work site. Ingestion is generally not an exposure route in road construction as a road can not be regarded as a playing ground for small children The transport of dust during the operational phase leading to an impact on the surrounding is not a major issue, when measures are taken to minimize dusting during operation. Because of the time frame during which leaching occurs, leaching is probably the major source of environ-mental impact. Thus impact to surface water (run-off) and to subsoil and groundwater are the major routes to be addressed in the subsequent chapters, although the other aspects mentioned are addressed briefly.

In recent years the pressure on primary materials for road construction has increased. As a consequence, more and more alternative materials are used in Europe in different parts of the road. Although alternative materials may technically prove suitable, the long-term environmental implications of several alternative materials are still uncertain. Under the Construction Products Directive (CPD, see section 3.5.1) environmental criteria are addressed during the period of use of the products (road construction materials). Alternative materials may perform well in the primary application. However, uncertainty exists for subsequent cycles of use (recycling, reuse in other applications and “end-of-life”).

During their lifecycle, the materials used in road construction may give rise to various types of environmental impacts such as pollution of soil, groundwater and surface water (including subsequent effects on and/or uptake by flora and (micro) fauna) due to leaching and subsequent transport of contaminants with water, dust emissions and effects of direct contact between the materials and living organisms.

3.1Introduction to environmental issues

3.1.1General concepts related to environmental issues

This review will focus primarily on impacts associated with leaching and transport of contaminants from road construction materials with water during the active life of the road, since this is widely accepted as being the most important pathway by which these contaminants may enter the environment, particularly in the longer term. In addition, this is the main issue related to the general acceptance and regulation of the use of alternative materials for road construction. Dust emissions may occur during the construction (and demolition) of the road, but they can be minimised through the application of appropriate preventive measures. Impacts caused by emission of particles from the road surface due to traffic may be of some importance, but it will not be addressed in this context (see e.g. the European POLMIT study, 2002). Many of the studies and examples presented in this review consider only the impact on groundwater, but the results may easily be applied also to sub-surface soil and surface water bodies (both fresh water bodies and marine waters).

Since road and pavement design have more or less the same layers all over the world with minor differences in types of materials depending on availability and climate, the approach for evaluating the environmental impact from using alternative materials in construction could be a unified approach.

Terminology is an important part of communication in the environmental field as well. Technical references often have there own terminology and confusion between more technically and environmentally oriented scientist does occur sometimes. A glossary of terms is therefore very helpful to clarify the meaning of certain expressions. In Appendix 7, a glossary of terms is given relating to environmental terms.

In several European member states and through EU funded projects knowledge on leaching issues for a wide range of materials has grown significantly in the last decade. This relates to different fields - soil, sludge, sediments, waste, construction, recycled materials - as environmental impact is relevant in all of these areas. In the framework of a European project on Harmonisation of leaching/extraction test (van der Sloot et al, 1997), it was found that there are more common aspects than there are fundamental differences in test methods that have been developed for different material types.

The need for methods that provide insight in the underlying release processes is growing. Too simple methods, like a single step extraction, lack the finesses needed to make proper judgements, given the complexity that surrounds an environmental impact evaluation. A key element in all areas is the wish to obtain results that reflect as much as possible a measure of true impact on both short and long term. A combination of characterisation of material behaviour, with more simplified testing for verification and quality control purposes, can provide the necessary understanding. At the same time, this approach limits the need for testing when the level of knowledge is sufficient and/or the variability in quality needs to be assessed. In CEN TC 292 the development of characterisation methods has started and is now expanding into the soil domain (ISO TC 190/ CEN TC 345).

From an environmental perspective, the scenario of a road base or an embankment focuses on the height of the application, which determines to what extent the material needs to be considered at its own pH or at imposed pH values. Another important aspect to consider is to what extent the material has release properties that place it in a percolation or a diffusion controlled release regime. This will largely determine which tools and models are applicable for testing the materials and interpreting the test results in order to estimate the (potential) environmental impacts.

3.1.2General concepts related to eco-toxicity issues

Realism of eco-toxicity tests

An eco-toxicologist is defined as an person who uses ecological variables to assess toxicity (Cairns, 1989). Effects of pollutants can be observed at various levels in the biological hierarchy ranging from molecules (DNA, enzymes...) to ecosystems: molecules  individuals  populations  communities  ecosystems. A population is defined as a group of individuals of the same species. A community refers to a group of interacting populations in a ecosystem.

Depending on the biological organization, three classes of laboratory tests are commonly distinguished:

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  single species tests (isolated species) ;
  
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  multi-species tests (more than one species) ;
  
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  community tests or model ecosystems.
  

Whereas effects at molecular and individuals levels can be easily measured in small-scale laboratory bioassays, effects on populations and communities are much more difficult to test. These are usually carried out in microcosms or mesocosms systems (pilot scale).

Moving down the biological hierarchy (ecosystems to molecules), systems become easier to control and reaction time is reduced, so reproducibility, reliability, robustness and repeatability should increase. On the other hand, since ecological processes largely involve individuals, populations and communities, relevance increases in the opposite direction, from molecules to ecosystems. The relevance is defined as the "ecological realism" (Calow, 1993).

Advantages of single-species tests are the following: they are easy to handle, easier to perform, cheaper and easier to interpret both statistically and theoretically. The cause-effect relationship is usually obvious.

Multi-species or community toxicity tests are more currently used to develop and validate new theory on ecosystems structure and function because they are more realistic. They provide the opportunity to simultaneously investigate responses at many different levels of biological organization and identify direct and indirect effects of toxicants. To conclude, decisions to select the appropriate type of tests are closely related to the expected performance (reproducibility, reliability) and relevance (realism).

Hazard and risk

Before giving a scientific definition of risk, we will simply imagine the following situation: to cross a big traffic road is quite hazardous for human health. If you watch left and right before crossing this road, the hazard remains the same but the risk becomes more limited. Now, if you do not cross this road, the hazardous properties of the road are still unchanged but the road represents no risk for you. What is the difference? It deals with the degree of vulnerability. The risk includes the exposure scenario to potentially negative effects (hazard).

So, it appears necessary to distinguish an hazard assessment method from a risk assessment method. The latter one is a combination of the consequence of a negative effect and the probability of its occurrence (Caracas, 1998).

The hazard assessment of alternative materials in re-used scenarios does not examine :

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  The fate of the pollutants released in the environment : dispersion, adsorption by solid particle matters, repartition and accumulation in biota, change or persistence in composition or speciation...
  
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  The bio-availability, bioaccumulation, bio-magnification of the pollutants.
  
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  ...
  

The term of intrinsic eco-toxic characteristics of material refers to potential hazard towards ecosystems.

Biological data for hazard assessment are usually provided from single-species tests. It allows to determine the eco-toxic properties of materials.

Choosing among eco-toxicological tests

In the framework of this program, compromise decisions encourage the use of standardized tests. Single-species tests are the most common ones. They are useful and successful to characterize and discriminate substances, products, by-products, waste or waste-based material towards their eco-toxic intrinsic properties. Tests are carried out in laboratory systems under very high controlled conditions to ensure a better accuracy, repeatability and reproducibility.

It has been extensively reported and discussed that the sensitivity of different species exposed to the same pollutant varies widely. Thus, it is better to implement a set of tests involving different species from several trophic levels. Trophic levels are defined by the biological food interactions: organisms from the higher trophic level feed upon certain others representing the lower trophic level. This is also designed as the food chain. It includes the following main classes:

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  primary producers : autotrophic plants and autotrophic bacteria
  
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  consumers: herbivorous animals
  
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  predators: carnivorous animals
  
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  decomposers: consumers of dead biomass (heterotrophic bacteria, fungi...).
  

Depending on the expected target ecosystem both tests on terrestrial organisms and aquatic organisms should be included.

It is also well known that a more complete information will be achieved if both acute and chronic effects are examined. Depending on the time regeneration, i.e. the life cycle of the organism, a short term effect (acute) or a long-term effect (chronic) is observed. For example, if the reproduction of the species is examined this is a chronic test. We also refer to genotoxic tests. These are tests implemented to observe the effect of pollutants such as mutations on genome (Calow, 1993). Traditionally these tests are considered as an other category because both acute and chronic effects could be observed.

General principles of single-species tests

Various concentrations in leachates are tested and the toxicity is then stated as a "concentration-effect" or "dose-response" relation (Cairns and Pratt, 1993).

Tests consist in the measurement of the biological responses (effect) of the organisms exposed to various concentration of pollutant. This is a relative effect because the response of exposed organisms is compared with the response of standard preparation (absence of pollutants).

The biological response is measured after a given time duration. The observed effects are sub-lethal or lethal (survival/death). Variables used to measure sub-lethal biological effects are usually behaviour (mobility...), body growth (weight and/or size), population growth (increase in biomass or in cell number), reproduction (cumulative numbers of offspring).

The traditional description of toxic effects is based on the computer-assisted calculations of the:

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  Effective concentration (EC): this is the concentration of leachate (or effluent, substance...) that is estimated to be effective in producing a given percentage of sub-lethal response. e.g. : EC50=10% means that 10% of leachate causes 50% of effects on the measured biological variable such as the mobility. In some cases the endpoint could refer to the EC25 calculation instead of the EC50.
  
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  Inhibition concentration (IC): this is the concentration that would cause a given percentage of inhibition. It is not a "yes/no" response as for the effective concentration but an inhibition of a biological variable such as growth. IC50 is the concentration for which 50% of inhibition is observed on the biological variable. In some cases the endpoint could refer to the EC25 calculation instead of the EC50.
  
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  Lethal concentration (LC): this is the concentration that would cause a given percentage of death. LC50 is the concentration which causes 50% of death.
  
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  No-observed effect concentration (NOEC): This is the concentration which does not differ significantly from the control.
  
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  Lowest observed effect concentration (LOEC) : This is the lowest concentration of the leachate that causes a statistically adverse effect.