- Road surface carry harmful heavy metals, PAHs and salts which will effect water quality;
- The very large portion of water harvested with roads, however, originates from the entire catchment – not from the road surface and are not affected by contamination form road surfaces;
- The water harvested with the roads improves water supply by augmenting the resource through recharge and feeding surface storage;
- Depending on geology, road development may also help develop new springs and seeps.
- Road water can be collected in cisterns and ponds (Ch 10) or feed into recharge structures (Ch 2);
- Springs along roads should be protected and in some cases even have their outflow managed (Ch 5);
- Grass strips can reduce pollution loads (this chapter).
This chapter focuses on opportunities for harvesting and managing water around roads to increase water availability for rural water supplu. Although access to water and sanitation has improved over the past decades, the World Health Organization (WHO) estimates that 748 million people still lack access to improved drinking water, and 2.5 billion people do not have improved sanitation (WHO 2014). Because it enriches the resource, road water harvesting can contribute to better access to domestic water supply. This chapter brings recommended practices together to use water from roads for this purpose, particularly in dry areas.
This chapter discusses several opportunities to make roads contribute to improved rural water supply, especially to better access to drinking water:
- Road water harvesting can recharge groundwater, and the development of shallow tube wells can subsequently serve to source drinking water.
- Water harvesting with road bodies can feed surface drinking water systems, but care should be taken to ensure acceptable water quality;
- Protecting and managing springs opened by road construction can provide a safe and reliable source of water (for the latter see Chapter 5).
In domestic water, water quality is always a concern, particularly when road surfaces and intensely used highways are used to collect drinking water. However, using such runoff is uncommon. Generally speaking, water collected from roads originates from the entire catchment. Water coming directly from road surfaces plays only a minor role. Moreover, most water will be harvested with low intensity unpaved roads, which are numerous. Precautions are necessary close to intensely used highways.
There is concern that, especially in case of intensely used highways, the water captured in road water harvesting may have high contaminant loads associated with the traffic on the roads. Surface and groundwater would then be susceptible to pollution from road runoff. Surface waters are particularly vulnerable, as they are directly exposed to the contaminants. Pollution of groundwater tends to occur gradually as some of the contaminants are intercepted before reaching the aquifer system, but the clean-up process is difficult and expensive.
Common contaminants in highway runoff are heavy metals, inorganic salt, aromatic hydrocarbons, and suspended solids on the road surface due to regular highway operation and maintenance activities (FHWA, 2016). In addition, road surface runoff may contain grease, oil, rust, and rubber particles due to vehicle wear and tear. These materials are often washed off the highway during rainstorms. Heavy metals like lead, zinc, iron, chromium, cadmium, nickel, and copper, generally undergo physical, chemical, and biological transformations as they reach adjacent ecosystems. They are either taken up by plants or animals or adsorbed by clay particles, or they settle as bottom sediments that may or may not leach metals depending on the condition and sensitivity of the receiving water.
Low pH levels (below 7) trigger metal solubility and leaching (Hanes et al. 1970). However, the leaching of copper, iron, chromium, and nickel is limited in natural waters where aerobic conditions are maintained (Granato et al. 1995). Heavy metals from highway runoff are not necessarily toxic, yet the form of metal and its availability to organisms determines the toxicity of water. For instance, ionic copper is more harmful to aquatic organisms than elemental copper (Yousef et al. 1985). This is similar for ionic zinc and cadmium.
The second group of contaminants are polycyclic aromatic hydrocarbons (PAHs). These originate from asphalt pavement leachate, tire wear, lubrication oils, and grease. Increased traffic activity will generally lead to higher levels of PAHs in road surface runoff. Low molecular weight PAHs in runoff is indicative of a petrogenic origin, while the presence of high molecular weight PAHs is associated with potential pyrolytic sources (vehicle exhaust emissions, burning organic matter, etc). The presence of PAHs in surface water and groundwater is an indication of source pollution. They are slowly biodegradable under aerobic conditions and are stable to hydrolysis (WHO 2003).
Typical concentrations of pollutants in road runoff in intensely used (>10000 vehicles/day) road sections are given in table 6.1.
Table 6.1: Typical concentrations of pollutants in highway run-off
|Contaminant||Typical concentration (mg/l)||WHO norm (mg/l)|
Source: (Buwal, 1996; Pfeifer, 1998; Heinzmann, 1993; Krauth et al. 1982; U.S. EPA, 1983; Dierkes, 1996) quoted by OWAV, 2002) WHO, 2017
The beginning of a rainstorm event usually has a much higher concentration of pollutants though. The highest pollution level is in the “first flush.” It is assumed that up to 70 percent of the pollution load is associated with sediments, as much of the oil adheres to such fine particles. The typical pollution levels as in table 6.1 can be compared with the acceptable levels: the main areas of conceen in =is the concentration of lead.
The third category of pollutants, de-icing salts, can be a problem in temperate and cold climates. The most common salts used are sodium chloride, magnesium chloride, calcium chloride, and special mixtures. Their harmful effects may be reduced by careful applications. Different types of cold weather conditions (sleet, ice, light snow, heavy snow, compacted snow, ice rain) require different applications of de-icing agents and methods, such as pre-wetting. Also, better understanding of the nature of road surfaces and their responses to different cold weather conditions will help to tailor the use of road salts to what is strictly required. Brick roads and wooden bridges, for instance, are much more prone to freezing than tarmac or earthen roads. There is a double benefit: less stress on the local water resources and the careful application of road salts will also help reduce the costs of de-icing operations.
There are several strategies to prevent or reduce the risk of water contamination from road surfaces, in particular close to intensely used pollution risk hot spots. A first strategy is to revisit road specifications: some PAHs, for instance, originate from the material chosen in constructing the road – coal, tar-based pavement sealants are a notorious source of PAHs (Valentyne et al, 2018). Second is to avoid the use of this category of road runoff, particularly near intensely used highways. To avoid accidental pollution, one can consider the safe removal of contaminated water from road drains. For example, close to the world famous mineral water resources in Vittel, France, very strict care is taken that no highway runoff will recharge the aquifer systems. All such water is collected and disposed of away from recharge zones. A third strategy is the better operation of roads, in particular in de-icing. (See Box 6.1.) A fourth method is to make use of natural remediation (Wilson, 1999).
An example of the latter is the use of roadside vegetation, in particular grass strips or vegetated drainage channels. This can improve the quality of water in two ways: by absorbing and adsorbing pollutants from water, and by stopping pollutants from the release of sediment. The effects of vegetation on contaminant removal depend on environmental conditions, the number and type of plants, and the nature and chemical structure of pollutants.
Vegetated channels along roads slow water runoff, trap sediment, and enhance infiltration. They are little artificial wetlands, engineered and planted to slow the flow of storm water runoff. The goal is to expose the dirty water to plants and soil, which absorb toxic metals, filter out water-clouding sediment, and neutralize noxious germs. According to the USDA Natural Resources Conservation Service, if properly installed and maintained, plants and soil have the capacity to:
- remove 50% or more of nutrients and pesticides
- remove 60% or more of certain pathogens
- remove 75% or more of sediment
Planting grass buffer strips along potential problem road sections can also decease the effects and costs associated with sediment deposition. The beneficial effects of grass strips in filtering nutrients, pesticides, and sediments from runoff has been demonstrated for instance by Morschel et al. (2004). Reduction rates fluctuate from about 50 percent to 95 percent depending on vegetation type, strip width, upslope inclination and area, and rainfall characteristics. Trials on high risk road sections suggest that a 12 m wide strip combined with a hedge might be enough to completely remove sediment deposits from the roadway.