On heavy metal pollution from a suburban road network
Pal, Sudip Kumar
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In the context of urban diffuse pollution a suburban road acts as a potential source of toxic pollutants among which heavy metals are very common, are found at elevated concentrations and are generally persistent. With a lack of detailed understanding of metal emission patterns on suburban roads, a detailed study is therefore essential for gaining an improved understanding to plug the knowledge gap in terms of urban diffuse pollution management. The present understanding of pollutant build-up and wash-off processes on road surfaces elucidates that these processes are highly site specific and are hard to generalise in certain aspects. Therefore, this study aimed to characterise heavy metal emissions and associated pollution levels at several road sites on the Riccarton Campus road network using road sediment collected during dry and wet weather periods. The heavy metal concentrations of Cd, Cr, Cu, Ni, Pb and Zn, are believed to be greatly toxic and are highly abundant in road traffic environments, were determined by strong nitric acid digestion and atomic absorption spectrometry. The study revealed that the pollutant build-up and wash-off processes were site specific and so also were the derived local build-up and wash-off parameters, as expected. However, these derived parameters were very different from those used in common urban drainage models suggesting that the default values are (significantly) inappropriate for the studied road network. The quantification of heavy metals in road sediments displayed significantly higher concentrations than local background concentrations. Their concentrations between weather types were found in the order of runoff, snow and dry road sediment, and also varied between sampling sites according to site-specific attributes, such as road lay-out inured traffic movement pattern, road surface condition and presence of road paint rather than due to traffic volume alone. The concentrations of all metals except Pb were significantly higher in finer sediment sizes than in larger sediment sizes. Correlation analyses revealed a similar pattern showing a greater number of statistically significant associations between metals in finer sediment sizes. An assessment of heavy metal contamination indicated that road sediment may likely pose a moderate to considerable level of ecological risk, if transported to the nearby water environment in the study area. The assimilation of the knowledge gained in this study should help to improve current understanding of environmental pollution from suburban roads and to provide better guidance for selecting appropriate control measures under the framework of sustainable urban drainage systems.