Abstract
South Africa’s available freshwater resources are almost fully-utilised and under stress. Water will increasingly become the limiting resource in South Africa, and supply will become a major restriction for the future socio-economic development of the country, in terms of both the amount and quality of water available (Harris et al., 1999). At present, many water resources contain large amounts of pollutants, such as dissolved metal elements (metals and metal compounds that occur in a dissolved state in water), particulate-bound metal elements (metals and metal compounds that are attached to particles such as sand), and suspended, colloidal and volatile fractions of these particulates (Sansalone et al., 1996; Su & Mitchell, 2003).
Pollutants interfere with the normal life cycle functions of organisms living in or which are dependent on the water source. These pollutants originate from a variety of point and non-point sources. Among the most significant point sources are industrial waste disposal sites, municipal landfills, and leaking septic tanks. Non-point sources, on the other hand, include agricultural runoff, mine drainage, highway runoff, and runoff from lawns and natural areas. It is therefore necessary to monitor the levels of pollutants in the water to determine if the concentrations are within legal limits, and to implement strategies to mitigate the detrimental impacts of these pollutants on the environment (Miller, 2000).
The transport of anthropogenic pollutants by rainfall runoff from highways, in particular, has the potential to adversely impact the quality of adjacent receiving waters and soils (Su & Mitchell, 2003). These pollutants result from traffic activities (such as tire wear and brake wear), thus being dependent on the amount of traffic; and are accumulated on roadways before it is washed away by precipitation (McKenzie & Irwin, 1983; Sansalone et al., 1996; Su & Mitchell, 2003). During the rising limb of the runoff hydrograph, often disproportionately large amounts of both the dissolved and particulate-bound fractions of these heavy metal pollutants are washed off the highway surface, causing peak concentrations of contaminants in rainfall runoff. This phenomenon is commonly
referred to as the seasonal first flush, where the initial stage of a storm event contains a large percentage of total pollution (accumulated in the preceding dry season) in a relatively small percentage of runoff volume (Sansalone et al., 1996; Lee et al., 2004; Kim et al., 2005). Such large concentrations of heavy metals introduced into the environment in a short amount of time can have immediate toxic effects, but, since heavy metal elements do not degrade in the environment, they can also accumulate and have long-term toxic effects caused by mass accumulation in plants, animals and humans (Sansalone et al., 1996; Miller, 2000; Arthur, 2004; Kirkby & Römheld, 2004). However, a number of these heavy metals are also crucial in maintaining normal health in humans, animals and plants (Arthur, 2004; Kirkby & Römheld, 2004).
Therefore, the primary objectives of this study are to discuss the phenomenon of the seasonal first flush of highway runoff; describe possible environmental effects of too low and too high levels of heavy metals, with reference to relevant legislation concerning water pollution; describe possible best management practices (BMP’s) to lessen the effects of too high levels of heavy metals; describe the study area and why it was chosen; identify shortcomings of the study; collect and analyse relevant data for the last five years (1999 – 2005) to determine if the first flush phenomenon is present for selected heavy metals (aluminium, cadmium, cobalt, chromium, copper, iron, manganese, nickel, lead, and zinc); identify the main heavy metals present in runoff in this area; propose improvements to future highway monitoring projects; and recommend mitigation strategies to reduce the environmental impacts of heavy metals.
It was found, using water quality data from Rand Water, that cadmium and copper were not detected at the selected study sites; however it is possible that some aluminium, chromium and lead were present in particulate form. Seasonal first flush was detected for cobalt, manganese, nickel, iron and zinc during most rainy seasons. It was also found that the concentrations of these metals were usually high above the regulation limits for irrigation, aquacultural purposes, aquatic ecosystem health, livestock watering and domestic use throughout the season. Therefore, the results of this study have shown that roads provide a significant contribution of heavy metals to nearby rivers, consequently
justifying the need for further study. It is recommended that a monitoring system be designed to collect more accurate information so as to be able to research, plan and implement mitigation strategies, such as Best Management Practices (BMP’s) to improve water quality and lessen the negative effect these metals have on the environment.
Dr. J.M. Meeuwis
Dr. J.L. Fischer