Heavy metals in road dust, gully pots and parkland soils in a highly urbanised sub-catchment of Port Jackson, Australia

Abstract

The current temporal and spatial investigation was undertaken to determine the nature and concentration of heavy metals in road dust and to assess whether road dust was a possible source of metals to adjacent soils and gully pots.

Three roads were selected in the Iron Cove catchment, Sydney Harbour, to investigate the influence of traffic volume on the heavy metal concentrations and accumulations rates with time. The mass of road dust increased irregularly with time since the last cleaning event and metal concentrations increased with traffic volume, but concentrations did not increase with antecedent time, possible due to variable particle size distributions. The mass of material accumulating on 1 m² of road surface over 5 days was 72.5, 41.7, and 4.8 g for high (Parramatta Road), medium (Marion Street), and low (National Street) traffic volume roads, respectively. The source area of road surface for the test sites (200–400 m of road) may provide, over the 5-day test period, maximum loads of (g): Cr 2, Cu 7, Ni 2, Pb 29, and Zn 28 for Marion Street; Cr 0.1, Cu 0.3, Ni 0.1, Pb 2, and Zn 3 for National Street; and Cr 2, Cu 8, Ni 1, Pb 26, and Zn 18 for Parramatta Road. To provide better spatial information than the 3 samples sites used in the temporal study, road dust (n = 171) and gully pot (n = 23) samples where taken across the entire Iron Cove catchment. Mean concentrations of road dust were 6, 34, 164, 28 000, 284, 27, 487, and 523 μg/g for Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn, respectively, for all road types, and concentrations in gully pot material were 7, 20, 112, 24 000, 316, 20, 199, and 257 μg/g, respectively. A large proportion of these metals is associated with the mobile, fine fraction (<62.5 μm), and the majority (54–100%) of these elements are extracted with weak leaching agents (EDTA and 1 M hydrochloric acid). However, more information on speciation is required to determine bioavailability. SEM/EDAX analysis suggests that metals in the fine fraction may be adsorbed, whereas particulate metal and inclusion phases are common in the coarser (62.5–125 μm) material.

Metal concentrations in parkland soils adjacent to the roads are highly enriched and increase with traffic volume. Concentrations decline markedly with distance from roads, but maximum values are at 30–50 m from the road edge, due to high-velocity wind generated by vehicles in proximity to the road. Metal-rich soils and gully pot deposits may be available for transport to the adjacent stormwater system.