Readily Wash-Off Road Dust and Associated Heavy Metals on Motorways

Road dust contains a wide range of potentially health-hazardous pollutant sources. In this study, road dust samples were collected from nine locations along the Sydney orbital motorway during wet weather events and analysed for their mineralogy and heavy metal contents. The aim of this study was to examine for the specific particle size fractions in road dust samples that can be associated with anthropogenic pollutant sources, mainly on the prevalence of heavy metals. Surface morphological and elemental composition of the road dust particles was analysed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). The heavy metal contents and degree of contamination were also investigated including in the two specific particle size fractions of <75 and 75–150 μm. It was found that the particle size fraction of <75 μm made up between 6 and 16% of the entire particle size distributions and contributed to more than 90% of the heavy metal contents. In addition, a moderate to high degree of heavy metal contamination was measured in the collected road dust samples, and this was correlated well with the local traffic volumes. The good correlation between heavy metals and traffic volumes in the finer road dust particle size fraction of <75 μm indicated that the finer road dust particles were not only important in terms of heavy metal attachment, accumulation and mobilisation during wet weather events but they could also provide evidence of potential anthropogenic pollution sources. These findings will facilitate our scientific understanding on the specific role and importance of particle size fractions on the mobilisation of pollutant sources, particularly heavy metals during wet weather events. It is anticipated that this study will assist in the development of best management practices for pollution prevention and control strategies on the frequency of road sweeping and retention pond design to trap fine road dust particles.