The level of pollution originating form roads depends on several factors that can also have an influence on pollution mitigation and prevention. These factors can be divided into two general groups:
• natural factors that depend on the environmental characteristics of the road’s surroundings; and
• technical factors that are connected with road design, construction and traffic characteristics.
Together with air, water is the main transport media for pollution dispersal from roads to the environment. Water is a very efficient solvent and, during its path through the road construction and surrounding environment, it dissolves and transports many pollutants — some of them in large quantities and over long distances. Pollutants are transported in water by the mechanisms described in Chapter 6 with solution and suspension of solids being important. Preventing pollution impacts through technical measures represents a considerable challenge to planners, designers and operators of roads.
Road construction is an activity that very directly affects the hydrologic and geoenvironment. Due to their linear nature, roads characteristically divide the hydrologic and geo-environment into two or more parts. The degree to which these parts are separated from one another depends on
• the road category;
• the road’s topography related to the surrounding area;
• the kind and density of traffic; and
• the existence of connecting structures between the two sides of the road (hydraulic or for biota).
The influences of road operation on water bodies may be classified as direct and indirect. Direct influences include water pollution that can be treated by various treatment methods. Also, the road construction may disrupt sub-surface and/or surface water flow paths. Most of these direct influences are measurable if one is prepared to monitor the impact of the road. Indirect influences are not usually detectable at first sight being mainly connected with activities that are induced by the road operation — for example, new industrial zones development or new residential areas. Ultimately, these consequences can be more severe than direct pollution of water body from road run-off. Nevertheless, we are addressing only the former, the hydrological impact of new developments being far beyond the scope of this book.
During planning, construction, operation and maintenance of roads, protection and mitigation measures for the water environment play an important role. These activities have to be established on the basis of conceptual models that enable the proper implementation of the appropriate technical measures. Natural conditions such as sensitivity and vulnerability of water bodies are among the most important influences to be considered by these conceptual models.
Various constraints can influence the selection of procedures to mitigate the threat of pollution from roads and traffic to many of the environmental compartments. For example, water bodies are very important as they represent an important habitat, may provide a water resource for public water supply and last, but not least, water is one of the most important transport agents for spreading pollutants. These constraints are the consequence of natural site characteristics, usually defined as the sensitivity and vulnerability of water bodies, they depend on geotechnical and hydro-technical conditions in the environment and on road traffic characteristics, especially the share of heavy vehicles in the total daily traffic flow. Legislation is another important constraint to pollution mitigation. To protect the environment as much as possible, there is an increasing trend for laws and other legal instruments to impose upon planners, designers, operators and users of roads many obligations that should be carefully studied and implemented. The knowledge of all constraints can help establish proper criteria for preventing and mitigating harm from road pollution.
In the past extensive monitoring of road infrastructure and its influence on various environmental compartments has been performed. Among other results of monitoring that have been extensively reported are those affecting the hydro — and hydro-geological environment (e. g. Hamilton & Harrison, 1991; Bruen et al., 2006). The most profound and known effects of roads on the water environment, especially in northern European countries, are the influences of winter maintenance practice where salts and some other agents are used as de-icing agents (Amrhein et al., 1992; Oberg et al., 1991). Also reported are the influences of heavy metal contamination on soil micro-structure and, consequently, on the physical and chemical conditions of soils and water percolated through the unsaturated zone in the groundwater (Folkeson 2000). Literature reports on oil spills on Slovene karstic areas reveal that spilt oil sinks very fast into the karstified underground and very soon appears in springs used for water supply (Kogovsek, 1995 & 2007).
Some case studies recording the impact of alternative road construction materials on groundwater are reported in the literature. As an example of this, a road was constructed on a new alignment in Cumbria, Lake District, UK in 1975 over a somewhat acidic bog. Soft marshy material was removed and the area was back-filled with approximately 300,000 tones of Blast Furnace Slag from iron production. The slag was also used below the water table. The slag material contained large quantities of calcium sulphide. Soon after construction, sulphur related pollution began to be observed in surrounding water courses, apparently as a consequence of groundwater seepages from the construction. Sulphide has a directly toxic effect on many life forms but its oxidation to sulphate can be a more serious problem as this can cause severe oxygen depletion, effectively asphyxiating many life-forms within the affected water bodies. In winter the problem was not so severe since there was plenty of water in the local receiving streams to enable dilution to occur. However, as groundwater continued to flow into receiving streams whose flow rates had decreased during the summer, and as water temperature increased, excessive algal blooms occurred. After construction was finished, the records indicate that samples of water discharging from drains from the roadway were found to be contaminated with sulphides and hydrogen sulphide. Biological and fisheries investigations showed that there was a marked impact on the invertebrate fauna and fish populations in the two streams receiving the drainage from the new road (Taylor, 2004).
Many alternative materials are often mentioned by reference to their leaching potential as a potential risk regarding the pollution of surface and groundwater. However, this risk must also be considered regarding some natural materials. Acid drainage is one of the phenomena that can appear in some natural materials. As an example we can refer to the construction of a capping layer consist of hornfels metamorphic rock aggregates that resulted in percolating waters with a pH between 3 and 4 (Odie, 2003). From the construction phase fish-kill was observed in the neighbourhood of the road. Such an acidification was induced by the oxidation of solid and dissolved pyrite (FeS2) originally present in the aggregate, and resulted in the production of H2 SO4 and Fe(OH)3. The presence of the latter at the outlet of the drain was testified by a dense orange colour.
